Job vacancies at ISIS - CORRECTION

[Radaelli, PG (Paolo)]

Hello again,
As clearly stated in the full Job Specs, the degree requirement for both posts 
is "PhD or equivalent in Physics, Chemistry or other relevant subject".  
Apologies to the non-physicists.
Paolo Radaelli
*

Hello,

There are two positions available in the ISIS Crystallography group:

FBU127 Instrument Scientist - Magnetic Diffraction
Department ISIS Department
Summary: We are looking for a highly motivated and independent scientist, who 
will take the lead in commissioning WISH, develop appropriate data analysis 
tools and build its science programme. The jobholder will also be required to 
provide an appropriate level of support for the ISIS User Programme, 
particularly but not exclusively on WISH, and will be strongly encouraged to 
build collaborations with UK and foreign visiting scientists.  
Salary Between £31100 and £35543 per annum 
Benefits 25 days holiday per year   
Date posted 10 Sep 2007   
Closing date 26 Oct 2007   
Requirements PhD in Physics  

FBU128 Postdoctoral Research Fellow - Crystallography
Department ISIS Department
Summary: The ISIS neutron and muon facility invites applications for a 
Post-Doctoral Research Fellow in the area of physical and materials 
crystallography. The project is to develop the scientific portfolio for a 
high-resolution neutron time-of-flight magnetic diffractometer (WISH). The 
Research fellow will also be part of the team developing analysis technique and 
commissioning the WISH instrument. In addition, the Research Fellow will join 
the existing ISIS research programmes in the field of transition-metal 
magnetism, the ordering of charges and orbitals and coupled degrees of freedom, 
and magnetism of geometrically frustrated lattices.  
Salary Between £24638 and £27998 per annum 
Benefits 25 days holiday per year   
Date posted 10 Sep 2007   
Closing date 26 Oct 2007   
Requirements PhD in Physics  

For additional information and application, please refer to the following link: 
 http://www.stfc.ac.uk/About/Vacs/Contents.aspx

Paolo G. Radaelli

Job vacancies at ISIS

[Radaelli, PG (Paolo)]

Hello,

There are two positions available in the ISIS Crystallography group:

FBU127 Instrument Scientist - Magnetic Diffraction
Department ISIS Department
Summary: We are looking for a highly motivated and independent scientist, who 
will take the lead in commissioning WISH, develop appropriate data analysis 
tools and build its science programme. The jobholder will also be required to 
provide an appropriate level of support for the ISIS User Programme, 
particularly but not exclusively on WISH, and will be strongly encouraged to 
build collaborations with UK and foreign visiting scientists.  
Salary Between £31100 and £35543 per annum 
Benefits 25 days holiday per year   
Date posted 10 Sep 2007   
Closing date 26 Oct 2007   
Requirements PhD in Physics  

FBU128 Postdoctoral Research Fellow - Crystallography
Department ISIS Department
Summary: The ISIS neutron and muon facility invites applications for a 
Post-Doctoral Research Fellow in the area of physical and materials 
crystallography. The project is to develop the scientific portfolio for a 
high-resolution neutron time-of-flight magnetic diffractometer (WISH). The 
Research fellow will also be part of the team developing analysis technique and 
commissioning the WISH instrument. In addition, the Research Fellow will join 
the existing ISIS research programmes in the field of transition-metal 
magnetism, the ordering of charges and orbitals and coupled degrees of freedom, 
and magnetism of geometrically frustrated lattices.  
Salary Between £24638 and £27998 per annum 
Benefits 25 days holiday per year   
Date posted 10 Sep 2007   
Closing date 26 Oct 2007   
Requirements PhD in Physics  

For additional information and application, please refer to the following link: 
 http://www.stfc.ac.uk/About/Vacs/Contents.aspx

Paolo G. Radaelli

VIII INTERNATIONAL SCHOOL OF NEUTRON SCATTERING "FRANCESCO PAOLO RICCI" : NEUTRON SCATTERING FROM MAGNETIC SYSTEMS

[Radaelli, PG \(Paolo\)]

Title: VIII INTERNATIONAL SCHOOL OF NEUTRON SCATTERING “FRANCESCO PAOLO RICCI” : NEUTRON SCATTERING FROM MAGNETIC SYSTEMS






Dear Rietvelders,

the registration deadline for the VIII INTERNATIONAL SCHOOL OF NEUTRON

SCATTERING“FRANCESCO PAOLO RICCI” : NEUTRON SCATTERING FROM MAGNETIC SYSTEMS

is 1st July 2006. 


- Detailed information and registration:  http://www.fis.uniroma3.it/sns_fpr

- E-mail contact: [EMAIL PROTECTED]

- Starting date : 25 Sept 2006  Ending date:  6 Oct 2006

- Location of school : Hotel Flamingo ,  Santa Margherita di Pula

(Sardegna), CA 09010 ITALY

- School Directors: 

Prof. Dante Gatteschi, Department of Chemistry, University of Florence,

Italy

Prof. Paolo G. Radaelli, ISIS Facility, RAL, UK

- Lecturers:

Jane Brown

Roberto Caciuffo

Juan Rodriguez Carvajal

Laurent Chapon

Arsene Gukasov

Gian Piero Felcher

Dante Gatteschi

Hans-Ulrich Guedel

Toby Perring

Paolo Radaelli

Roberto Triolo

Albrecht Wiedenmann

Marco Zoppi

- Aim of the School

This school, established in 1994, is primarily addressed to graduate

students or postdoctoral with an interest in Neutron Scattering. The School

will comprise lectures, tutorials and hands-on data analysis sessions,

covering diverse aspects of Neutron Scattering, but with an emphasis on

techniques and instrumentation designed to study the Structure and Dynamics

of Magnetic Systems. The School will commence on Monday, September 25th

2006, with a series of introductory lectures covering the fundamental

aspects of neutron scattering and neutron instrumentation. During the next

few days, a series of lectures will provide the basis to understand Magnetic

Symmetry, Magnetic Structure Description, Solution and Refinement from

powder and single crystals data, Spin Density Determination using polarised

neutron techniques, Neutron Polarimetry, Magnetic Small-Angle Neutron

Scattering,  Magnetic Neutron Reflectometry and Inelastic Neutron Scattering

from Single-Ion, Magnetic Clusters and Collective Magnetic Excitations. Each

of these topics will be expanded in a series of tutorials, to be held in

small groups, which will also include hands-on data analysis sessions.  The

combination of introductory lectures, scientific sessions and training in

scattering techniques will provide participants with a unique opportunity to

become familiar with neutron scattering methods and their applications to

current research topics.  

- There are 40 places available for students attending the school. Those who

have pre-registered will be offered places preferentially in the event that

the School is oversubscribed. Applicants will be notified if they have been

accepted for the School by 10th July 2006.

- Bursaries, provided from the sponsors, will be available for the purpose

of increasing the participation of international and national research

scientists. These may be used to cover attendant's expenses such as hotel,

travel, etc. Total travel and subsistence expenses are usually not provided.



Best Regards


Paolo Radaelli

XPRESS access mechanism

[Radaelli, PG \(Paolo\)]

Dear Colleague,

Starting from cycle 2004/03 (April-May 2005), ISIS has started to offer a 
"Measure-by-Courier" powder diffraction access mechanism to the GEM 
diffractometer (XPRESS).  This mechanism is aimed at new and infrequent users, 
research programmes requiring only limited and/or occasional neutron beamtime 
and chemistry programmes operating a tight loop between synthesis and 
structural characterisation. A total of 1 beam day per cycle on the GEM powder 
diffractometer will be devoted to this programme. 

Initially, samples will only be measured at room temperature and in standard 
containers, but access to sample environment is currently being planned.Fully 
reduced and corrected high-quality data, ready for Rietveld refinement, will be 
provided to the user, together with example files and guidelines for 
refinement. The user is expected to carry out the structural analysis with 
minimal assistance.
The next XPRESS run is scheduled for
TUESDAY, JULY 19 2005

To access XPRESS and read the full instructions, please follow the link on the 
ISIS proposal page http://www.isis.rl.ac.uk/applying/.  The system is designed 
to be self explanatory.  
Your Login ID (email address) and password is the same as for the normal 
proposal submission.  If you are not currently an ISIS user, you will be asked 
to register on line prior to submission of an XPRESS request.
I very much looking forward to receiving your first XPRESS requests.
Paolo Radaelli  
***
Prof. Paolo G. Radaelli
Crystallography Group Leader
ISIS Facility, Rutherford Appleton Laboratory
Chilton, Didcot, Oxfordshire

OX11 0QX, United Kingdom

Phone: +44-1235-445685
FAX:  +44-1235-445642
***

3 Position at ISIS in the area of Crystallography

[Radaelli, PG (Paolo)]

 
Employment Opportunities at ISIS
ISIS is in a period of major expansion, including the construction of a
second target station, with an initial phase of 7 new instruments to be
operational in 2008. 

We are looking for world-class scientists, instrument developers and
electrical engineers who can make major contributions to our current and
future research programmes.

Senior Scientists (2 positions) 
Scientists (8 positions) 
Electrical Engineers (9 positions) 
PA to the Director (1 position) 
Complete details and application forms can be found at the 
central CCLRC Job Vacancy Pages http://www.cclrc.ac.uk/Activity/Jobs





VN2624: Senior Scientist (Diffraction)
Type   Full-time 
Department/Centre   ISIS Department 
Location   RAL 
Office   RAL 

All application forms must be returned by 6 December 2004.

 Description

We are looking for a scientist with a proven research record to play a
leadership role in the development of diffraction techniques at ISIS, both
in terms of instrumentation and applications. The post will involve line
management, project management and budget management responsibilities. The
jobholder should be able to lead a team to achieve both short term and long
term goals. The successful candidate should demonstrate a broad experience
and outlook and an innovative approach.




VN2622: Instrument Scientist, Crystallography Group
Type   Full-time, 2 Posts 
Department/Centre   ISIS Department 
Location   RAL 
Office   RAL 

All application forms must be returned by 6 December 2004.

 Description

We are looking for two highly motivated and independent scientists, who will
assume responsibility for a world-leading scientific, instrument design and
instrument-operation programme within the ISIS crystallography group. The
jobholders will be required to provide an appropriate level of support for
the ISIS User Programme and will be strongly encouraged to build
collaborations with UK and foreign visiting scientists.

Applicants should have a PhD or equivalent qualification in a relevant
science or engineering discipline. The successful candidates will have a
proven track record in an area of neutron or x-ray crystallography
including, but not limited to, single-crystal neutron diffraction,
high-pressure research, magnetic neutron diffraction and cultural heritage
research. The ability to carry out independent research and an innovative
outlook towards instruments and technique development will be counted as
strong assets. Experience in the use of central facilities and the proven
ability to develop instrumentation and/or data analysis software would also
be an advantage.

  

bursaries available: Neutron Scattering from Biological Systems

[Radaelli, PG (Paolo)]

The Institute of Physics and the Faraday Division of the Royal Society
of Chemistry Neutron Scattering Group

Student and Young researcher bursaries available for

Neutron Scattering from Biological Systems
Cosener's House, Abingdon, OXON, 13-14 December 2004


The Neutron Scattering Group is pleased to offer a number of student and
young postdoctoral researcher bursaries to assist with the costs of
attending the British Crystallographic Association Physical Crystallography
Group autumn 2004 meeting on Neutron Scattering from Biological Systems.
Applicants are expected to be contributing at the meeting through either a
poster or oral presentation.

Applications should be submitted to the group chairman (Prof Don McKenzie
Paul) as soon as possible. The number and value of bursaries awarded will be
limited by available funds, something of the order of ten awards being
available. Successful applicants will be required to claim their travel and
subsistence awards against receipts.

Applications should take the form of a letter of introduction from the
student, outlining  their background and the contribution they expect to
make. Each application should also be supported by a letter of reference
from the student's supervisor. Informal applications by email or telephone
would also be welcomed.

Successful applicants are expected to acknowledge the support of the Neutron
Scattering Group in their presentation, and on their return, provide the
Neutron Scattering Group committee with a short written report on their
contribution and the meeting of 200 words or half a page of A4.

Eligibility: Applicants are expected to be student members of the Institute
of Physics or the Royal Society of Chemistry, and also to be a member of the
Neutron Scattering Group (or be in the process of joining). A maximum of two
bursaries can be awarded to a given student in the course of their PhD
studies, and, in general, no applicant is eligible to receive more than one
bursary in any academic year.


Prof Don McKenzie Paul
Chairman of the Neutron Scattering Group
Department of Physics
University of Warwick
Coventry CV4 7AL

Tel : 024 76 523603
Fax : 024 76 692016
Email : [EMAIL PROTECTED]

Meeting Announcement: Neutron Scattering from Biological Systems

[Radaelli, PG (Paolo)]

Autumn Meeting 2004 
BCA Physical Crystallography Group 
and the 
IoP Structural Condensed Matter Physics Group 
Neutron Scattering from Biological Systems
Cosener's House, Abingdon, OXON, 13-14 December 2004 
Hosted at Cosener's House, Abingdon, OXON and supported by the PCG and
Rutherford Appleton Laboratory.
Organising Committee: John R. Helliwell (Dept. of Chemistry, University of
Manchester), Jeff Penford (ISIS Facility, CCLRC), John S.O. Evans (Dept. of
Chemistry, University of Durham) and P.A. Thomas (dept. of Physics,
University of Warwick)
Local Organiser Paolo G. Radaelli
 ISIS Facility, RAL, CCLRC 

In this meeting we aim to give a broad overview of the contribution of
neutrons to biology, with particular emphasis on neutron protein
crystallography, neutron fibre diffraction, small-angle scattering and
reflectometry from systems such as enzymes, amyloids, membranes, proteins
absorbed on surfaces, drug delivery vehicles, biosensors and many more.
These topics will be introduced by a series of invited and contributing
speakers and by a poster session, but there will also be ample time for an
open discussion about the present and future of neutron techniques and
facilities. In particular, we will explore the opportunities for neutron
scattering experiments in biology at existing and future neutron sources,
such as the ISIS Second Target Station. We expect a stimulating discussion
on these topics to continue over our traditional Christmas Dinner. The level
of the presentations will be suitable for beginners and expert alike. The
meeting will start at 1:30 PM on December 13, 2004 and end at 12:30 PM on
December 14, 2004.

We encourage graduate students to attend, and to bring posters for display
at the meeting. 
Confirmed speakers: Mathew Blackley (EMBL - Grenoble) Giovanna Fragneto
(Institut Laue-Langevin), Hermann Heumann (Max Planck Institute of
Biochemistry), Jayne Lawrence (King's College London) Bob Thomas (University
of Oxford), Peter Timmins (Institut Laue-Langevin), Tim Wess (University of
Cardiff)
A registration form is available as an HTML document by clicking the link
below 
Registration Form . 

RE: Rietveld refinement and PDF refinement ?

[Radaelli, PG (Paolo)]

> do you really have the 
> resolution even on
> HRPD to see the diffuse scattering between Bragg peaks at 
> high Q ?

No we don't, but this is not the main point (by the way, we don't use HRPD
for PDF, it doesn't go to sufficiently short wavelengths).  The main reason
to go to high Q is to avoid truncation errors.  If you truncate S(Q), all
your G(r) peaks will be convoluted with the Fourier transform of a step
function, which is a sinx/x function.  The width of the central peak is
roughly 1/Qmax.  If you use a wavelength of 0.5 A, this corresponds to about
0.08 A, or an equivalent B of 0.5.  This in itself can be a problem when you
want to look at sharp correlation features.  Even worse, the "ripples" will
propagate to adjacent PDF peaks, generating unphysical features.  There are
ways to suppress the ripples by convoluting the data with an appropriate
smooth function rather than truncating them (these are extensively used in
disordered materials work), but they all tend to broaden the features.  You
can also fit a model including the ripples (as in PDFfit) but it is clearly
better not to have them if you are trying to exploit the model independence
of PDF.  Going to high Q does not solve all the problems. If the high-Q data
are noisy, your truncation function will have higher frequency but also
higher (and random) amplitude in the ripples, so there is always a
compromise Qmax, depending on statistics.  Finally, very high-Q data are
quite difficult to normalise, because of the epithermal background.   


> You may
> get better temperature factors with high-Q PDF refinement, 
> but you will
> also do that with high-Q Rietveld.

Generally, all crystallographic parameters come out worse from PDF
refinements than from Rietveld on the same data sets.  I think this is
because you are trying to fit an average structure to something that
contains correlations, so the fit is bound to be worse.  You could fit a
correlated model, but then you would not get "temperature factors" in the
usual sense. 

> I also doubt that just because PDF uses data between the 
> Bragg peaks, then
> it must be superior for seeing details not centered on atoms 
> in real space
> in a crystal, eg the split atom sites in (In/Ga)As). You 
> might do just as
> well with Bragg scattering if you use the result of Rietveld 
> refinement to
> construct a Fourier map of the structure. Happily, a sampling of
> reciprocal space (Bragg peaks) is sufficient to re-construct 
> the entire
> density of a periodic structure in real space, not just point 
> atoms, to a
> resolution limited only by Q.

You are right.  PDF is not always superior.  It is the interpretation of the
Fourier density in terms of correlated displacements that emerges uniquely
from PDF, although you can often guess it right from the Fourier map in the
first place.  The case of Jahn-Teller polarons in manganites (La,Ca)MnO3 is
quite illuminating.  Several groups noticed that the high-temperature phase
(above the CMR transition) has large DW factors for O.  We showed that this
affects primarily the longitudinal component along the Mn-O bonds, and
guessed that this was caused by an alternation of short and long Mn-O
distances.  Simon Billinge showed the same thing quite convincingly from PDF
data.  Only the latter can be considered "direct evidence" (with some
caveats).

> But you do agree that in a PDF experiment you integrate over 
> energy, so
> you only see an instantaneous snapshot of the structure...

Yes, I agree with this and the fact that inelasticity corrections are an
issue.  Sometimes they are exploited to obtain additional information, and
there is a claim that one can "measure" phonon dispersions with this method,
but the issue is quite controversial.

> 
> So while I am convinced of the interest of PDF for non-crystalline
> materials, with short or intermediate range order, I am not 
> yet convinced
> that you gain much from PDF refinement of crystalline 
> materials, where you
> can also apply Rietveld refinement.

I agree completely.  The directional information gained from phasing and the
fact of "locking in" to specific Fourier components is a major asset of
Rietveld analysis.  PDF is useful when correlated disorder is important (and
large), even if superimposed on an ordered structure.

Paolo Radaelli

RE: Rietveld refinement and PDF refinement ?

[Radaelli, PG (Paolo)]

Two very good points by Armel:

>all the very good PDF studies ...are made by using synchrotron data or
neutron data from spallation
>sources

This is because they are the only means to get to high Q (i.e., high
resolution in real space) and sufficiently high resolution (in reciprocal
space) simultaneously.  The RMC method is somewhat similar and does not
require such high Q, but it has the drawback of requiring a starting model
(arguably, there is also a uniqueness issue with RMC).  One nice feature of
the latest generation of TOF instruments is that one does not have to choose
in advance between PDF and crystallography, as long as one has appropriate
references (empty can, empty instrument etc.), which are collected as a
matter of course anyway.  PDF analysis requires better statistics, but, in
the context of a large phase diagram study, it is always possible to collect
a few data point to PDF accuracy.

>So, this PDF advantage does not impress me a lot

True, in most cases PDF=Rietveld + Common Sense.  However, there are some
exceptions.  For some nice cases see the work of Simon Hibble et al. (e.g.,
Hibble SJ, Hannon AC, Cheyne SM Structure of AuCN determined from total
neutron diffraction INORG CHEM 42 (15): 4724-4730 JUL 28 2003 and references
cited therein) and that by Simon Billinge (e.g. Petkov V, Billinge SJL,
Larson P, et al.
Structure of nanocrystalline materials using atomic pair distribution
function analysis: Study of LiMoS2 PHYS REV B 65 (9): art. no. 092105 MAR 1
2002 ).  Particularly, Simon Billinge makes the point that the future of PDF
is in the study of materials with short and intermediate-range order but no
long-range order ("nano-crystallography").  It is an interesting point of
view, although, at the moment, there are not very many examples of this in
the literature.

Paolo Radaelli

RE: Rietveld refinement and PDF refinement ?

[Radaelli, PG (Paolo)]

>> I would argue that the Bragg &
>> diffuse scattering both reflect the average instantaneous atomic
>> structure.

>Yes. If you integrate over energy, the scattering function factors to a
>delta function in time, corresponding to an instantaneous snapshot of the
>spatial correlations. It is not a question of Bragg or or diffuse
scattering.

Your statement about integrating over energy is correct regardless of Bragg
or diffuse scattering, but Brian's statement is not, at least in the context
of the PDF/Bragg discussion, so "it is and it isn't" would have been a more
appropriate statement on my part.

It is true that in diffraction you measure the intermediate scattering
function S(Q,t=0), but this is not the same thing as saying that you can
then Fourier-transform any part of it you like to a G(x, t=0).  To get a
real-space function G(x) you have to integrate over the *whole* Q domain,
and in doing so for Bragg scattering you set to zero everything that is
outside the nodes of the RL.  However, it is easy to see that this can be
equivalent to setting an energy cut-off.  This is because fluctuations in
time and space are usually correlated, so by selecting an integration range
in Q for you Bragg peaks, you also effectively select an integration range
in energy.  Your superstructure example shows it clearly:  if you are far
from the phase transition and the correlation length of your tilt
fluctuation is 10 A,  you would not see a Bragg peak there and you would get
the time-average structure (without the superstructure).  Clearly there is
the limiting case of critical scattering very near the phase transition,
where the fluctuating regions are so large that you effectively take a
snapshot of each of them.  There could even be a deeper point here to do
with ergodicity, whereby you could show that coherent space average and
coherent time average are effectively the same (I am not positive about
this, though). 

The point I was trying to make is a different one. We are discussing about
the difference between Bragg and PDF.  If all the scattering is near the
Bragg peaks, so that you integrate it all in crystallography, there is and
there cannot be any difference between the two techniques.  I am sure we are
not discussing this case.  The interesting case is when there is additional
diffuse scattering. What I am saying is that if this diffuse scattering is
inelastic, then PDF will reflect istantaneous correlations in a way that is
missed by Bragg scattering. Let's look at the case of two bonded atoms
again, with a bond length L, and lets this time assume that they vibrate
harmonically in the transverse  direction, and that the semi-axis of the
thermal ellipsoid is a.  The possible istantaneous bond lengths range from L
to sqrt(L^2+4a^2) for an uncorrelated or anti-correlated motion, but is
always L for a correlated motion.  Bragg scattering will give you a distance
of L between the two centres, which is only correct for correlated motion.
It also provides information about the two ellipsoids, which is the same you
would get by averaging the scattering density over time, regardless of
correlations.  Istantaneous correlations are only contained in the diffuse
scattering, and are in principle accessible by PDF.

Paolo Radaelli

RE: Rietveld refinement and PDF refinement ?

[Radaelli, PG (Paolo)]

>> I would argue that the Bragg &
>> diffuse scattering both reflect the average instantaneous atomic
>> structure.

>Yes. If you integrate over energy, the scattering function factors to a
>delta function in time, corresponding to an instantaneous snapshot of the
>spatial correlations. It is not a question of Bragg or or diffuse
scattering.

Your statement about integrating over energy is correct regardless of Bragg
or diffuse scattering, but Brian's statement is not, at least in the context
of the PDF/Bragg discussion, so "it is and it isn't" would have been a more
appropriate statement on my part.

It is true that in diffraction you measure the intermediate scattering
function S(Q,t=0), but this is not the same thing as saying that you can
then Fourier-transform any part of it you like to a G(x, t=0).  To get a
real-space function G(x) you have to integrate over the *whole* Q domain,
and in doing so for Bragg scattering you set to zero everything that is
outside the nodes of the RL.  However, it is easy to see that this can be
equivalent to setting an energy cut-off.  This is because fluctuations in
time and space are usually correlated, so by selecting an integration range
in Q for you Bragg peaks, you also effectively select an integration range
in energy.  Your superstructure example shows it clearly:  if you are far
from the phase transition and the correlation length of your tilt
fluctuation is 10 A,  you would not see a Bragg peak there and you would get
the time-average structure (without the superstructure).  Clearly there is
the limiting case of critical scattering very near the phase transition,
where the fluctuating regions are so large that you effectively take a
snapshot of each of them.  There could even be a deeper point here to do
with ergodicity, whereby you could show that coherent space average and
coherent time average are effectively the same (I am not positive about
this, though). 

The point I was trying to make is a different one. We are discussing about
the difference between Bragg and PDF.  If all the scattering is near the
Bragg peaks, so that you integrate it all in crystallography, there is and
there cannot be any difference between the two techniques.  I am sure we are
not discussing this case.  The interesting case is when there is additional
diffuse scattering. What I am saying is that if this diffuse scattering is
inelastic, then PDF will reflect istantaneous correlations in a way that is
missed by Bragg scattering. Let's look at the case of two bonded atoms
again, with a bond length L, and lets this time assume that they vibrate
harmonically in the transverse  direction, and that the semi-axis of the
thermal ellipsoid is a.  The possible istantaneous bond lengths range from L
to sqrt(L^2+4a^2) for an uncorrelated or anti-correlated motion, but is
always L for a correlated motion.  Bragg scattering will give you a distance
of L between the two centres, which is only correct for correlated motion.
It also provides information about the two ellipsoids, which is the same you
would get by averaging the scattering density over time, regardless of
correlations.  Istantaneous correlations are only contained in the diffuse
scattering, and are in principle accessible by PDF.

Paolo Radaelli

RE: Rietveld refinement and PDF refinement ?

[Radaelli, PG (Paolo)]

>Yes. If you integrate over energy, the scattering function factors to a
>delta function in time, corresponding to an instantaneous snapshot of the
>spatial correlations. It is not a question of Bragg or diffuse scattering.

Actually it is.  Bragg scattering is equivalent to projecting all the
scattering density in the crystal onto a single unit cell divided by the
number of unit cells in the crystal and replicate that average unit cell.
In other words, you do a coherent configurational average (over the
correlation length of the probe).  Furthermore, because Bragg scattering is
elastic, you also get a coherent time average.  If you Fourier transform all
of the scattering function you get the instantaneous local pair-correlation
function averaged incoherently over time and space (actually, this is not
quite true because you can never measure Q in a diffraction experiment).

Paolo Radaelli

RE: Rietveld refinement and PDF refinement ?

[Radaelli, PG (Paolo)]

The only truly unique PDF information is about *correlations*.  Let's say
you have two bonded sites, both with anisotropic thermal ellipsoids along
the bond, and let's assume that the motion is purely harmonic.  A sharp PDF
peak will indicate that the atoms move predominanly in-phase, a broad PDF
peak that the atoms move predominantly out-of-phase.  The two scenarios will
give identical Fourier maps as reconstructed from the Bragg peaks, whatever
the Qmax, so the additional width (or additional narrowness) of the peak
with respect to an uncorrelated model arises purely from the non-Bragg
scattering.  You can make the same argument for static correlations.
Ga(1-x)In(x)As is a typical case.  It is not a split-site problem, in that
both Ga/In and As will be slightly displaced locally depending on their
surrounding, but the displacements are correlated in such a way as to give a
shorter bond length for Ga-As and a longer one for In-As.
Of course, PDF is also used to look at more general issues of static/dynamic
disorder that could also be examined using Bragg scattering, and in many
case it does quite well.  PDF is not (yet) very good for structural
refinements (so it is to be used only in desperate cases of highly
disordered systems) and is pretty hopeless for weak ordered displacement
patters, since the extra Bragg peaks in crystallography "lock-in" on the new
modulation even in the presence of large unrelated displacements.   For this
very reason, PDF tends to miss phase transitions, particularly at higher
temperatures, which led to some very wierd claims in the past literature.
There is a lot of controversy about PDF being able to say something about
weak disordered displacement patters (e.g., dynamic stripes), but I am
personally very skeptical.  PDF requires exquisite data and a true passion
for data analysis.  If you have a good problem, you can get (probably) the
best PDF data worldwide "almost" routinely on my instrument GEM at the ISIS
facility (see also the cited paper by Billinge).

Paolo Radaelli

Sequential refinement with GSAS

[Radaelli, PG (Paolo)]

Dear Doroteya,

there is no systematic way in GSAS to make sequential Rietveld refinements
on a large amount of data files, but it is possible to write a sort of
script in a separate language, and couple it with the GSAS scripting to
achieve your goal.  I routinely run through tens or hundreds of files using
this method. Here are the steps that are needed (at least the way I do it):

1) Refine a 'starting' file, e.g., the lowest temperature file, say
sam_001.gss.
2) Copy that file to a new file, sam_002.gss.
3) Edit the new file to change the input file names to the new data for the
next temperature.
4) Run POWPREF and GENLES on the new file. 
5)Check for convergence.  If not converged, run them again etc. If a
divergence occurs, stop and issue a warning.  Clearly, the process will fail
through structural phase transitions.
6) When convergence is achieved, write out parameters with PUBTBL, DISAGL,
BIJCALC etc. 
7) Repeat step 2 with the next file, and so on.
8) At the end of the process, one needs to extract all the parameters and
put them in tables.  This is relatively easy for the .tbl files, since the
variable parameters are all followed by a bracket containing the errors, so
one can extract all of those and worry about what they are later.  For
distances and angles and magnetic moments, one has to search through the
.lst files using some keywords (e.g., CU_O for Cu-O distances).
9) After running through tens or hundreds of files, one typically discovers
that he would like to change something in the structural model.  Therefore,
it is important to be able to edit the .exp files and re-run them in a
sequential way. 

The way this is implemented depends very much on the operating system.  For
example,  points 2), 4) and 6) reqire a system call, e.g. of the form
'POWPREF sam_002.gss'.  Point 9) can similarly be achieved (in DOS) through
a system call of the type 'EXPEDT sam_002.gss < macro.mac', where macro.mac
is a GSAS macro file that implements the changes.  I believe there is a
similar syntax for other operating systems.  Points 3),5) and 8) require
physically editing files.  This can easily be done in some operating system
shells (e.g. UNIX), less so in others.  Alternatively, one can use scipting
languages (e.g. PEARL) or higher-level languages (e.g. FORTRAN).

I wrote a couple of working packages of this type, one in UNIX and the other
for Windows, using IDL as a scripting/GUI language, but I have got nothing
polished enough to be distributed.  It takes something like 3 or 4 days to a
modestly competent programmer (like myself) to write the whole thing, and it
is well worth the effort, because you recuperate the time immediately on the
first big project.

I very much wish that somebody (Bob?, Brian?) will take the time to do this
once and for all.  I suspect that Bob will see the light as soon as HIPPO
will start producing data.

I hope this helps.

Paolo


-Original Message-
From: Doroteya (details omitted)
Sent: Tuesday, August 21, 2001 10:34 AM
To: [EMAIL PROTECTED]
Subject: 


Dear Mr. Radaelli,

I am a PhD student in the ... and I am working with GSAS to make Rietveld
refinement on powder diffraction data.
May I ask you, please, for an advice:
I need to make Rietveld analysis with big amount of data files and I would
like to ask you if you know is it possible to make GSAS refinement for many
files at the same time or is there some procedure to do analysis
automatically
for some consequence of files.

Thank you very much in advance,

Sincerely yours,
Doroteya

RE: Debye Temperature & Lattice Thermal Expansion

[Radaelli, PG (Paolo)]

This is to answer the question from Alexandros:
 
The simplest approximation to the thermal expansion coefficient uses the
Debye or Einstein specific heat through the so-called Gruneisen formula
 
alpha=gamma*cv/(3*B*Vm)
 
where alpha is the linear expansion coefficient (1/3 of the volume TEC), B
is the Bulk modulus, cv is the lattice specific heat, Vm is the molar volume
and gamma is the Gruneisen coefficient.  Note that the high-temperature
limit for both the Debye and Einstein formulas (cfr. Ashcroft & Mermin,
chapter 23) is cv->3*n*Kb, where n is the number of atoms in a molecule and
Kb is the Boltzmann constant (8.31 J/mol/K).  For oxides with the perovskite
structure, B~150-180 GPa, Vm~3x10^-5 m^3/mol and n=5, so
alpha->gamma*7x10^-6 at high temperatures.  Typical values of gamma are of
the order of 2.
 
The problem with using this formula to extract the Debye temperature
(thetad) from thermal expansion data is that it invariably yields
irrealistically low values of Thetad.  The reason of this is that gamma is
an anharmonicity coefficient, which is not the same for different phonons.
So, strictly speaking, one gets an anharmonicity-weighted Debye temperature.
This problem has been addressed for alkali halides and perovskites in a
useful paper by H. Inaba (J. Cer. Soc. Japan, 106, 1988, p 272), which also
contains references to other materials.  Inaba proposes a semi-empirical
model which works well for non-pathological systems, and (in principle)
should allow one to extract the true Debye temperature.  In practice, I
think this is more useful when Thetad is known and one wants to fit the TE
data.
 
By the way, getting Debye tempeatures from Debye-Waller factors is also
risky, because one gets a complicated average over the phonons that project
on that particulat site and direction.
 
Paolo

Job Opening

[Radaelli, PG (Paolo)]

ARGONNE NATIONAL LABORATORY/ISIS NEUTRON FACILITY
POSTDOCTORAL POSITION

The Materials Science Division at Argonne National Laboratory is seeking
candidates for a postdoctoral research associate position in neutron
diffraction.  The successful candidate will be located predominantly at the
ISIS pulsed neutron facility at the Rutherford Appleton Laboratory in the
UK.  The research involves using the new high-intensity powder
diffractometer GEM and other state-of-the-art facilities at ISIS to study
the structural and magnetic phase diagrams of materials displaying subtle
transitions driven by parameters such as composition, temperature, magnetic
field and pressure.  With the advent of GEM, it has become possible for the
first time to obtain high-precision structural parameters, often comparable
to those from single crystals, with a few minutes' data acquisitions.
Therefore, multi-dimensional phase diagrams can be constructed using
internal structural parameters, such as anisotropic Debye-Waller factors.
Because of the absolute accuracy of these parameters, close comparisons with
lattice dynamics models are also possible.  Areas of science that will be
the focus of this work include existing and new research programs at both
ISIS and Argonne, namely: magnetoresistive materials, including both
perovskite and layered compounds, magnetoelastic materials, ferroelectric
materials, ionic conductors including battery and fuel-cell materials and
ceramic membranes.  Candidates must have a Ph.D. in Physics, Chemistry,
Materials Science or related subject.  Experience in neutron scattering or
diffraction and computer analysis/programming is preferred.  Postdoctoral
appointees at Argonne much have received their Ph.D. within the last three
years.  The deadline for applications is July 10, 2001 or until the position
is filled.  Please send applications, including a resume and publication
list to Dr. James D. Jorgensen, Materials Science Division, Argonne National
Laboratory, Argonne, IL 60439 ([EMAIL PROTECTED]). Further information can
also be obtained from Dr. Jorgensen or Dr. Paolo Radaelli
([EMAIL PROTECTED]). Information on GEM can be found on the following
web site: http://www.isis.rl.ac.uk/disordered/gem/gem_home.htm
Argonne National Laboratory is an affirmative action/equal opportunity
institution.  Women and minorities are especially encouraged to apply.

RE: One GSAS question.

[Radaelli, PG (Paolo)]

Dear Xiangyun Qiu,

the easiest thing is to create a new purely magnetic phase containing only
Mn and with P1 symmetry.  Since Mn is in a special position, you don't need
to refine the coordinates.  You'll have to constrain the reciprocal lattice
tensor parameters and the phase fraction between the two phases.  The latter
are inversely proportional to the cell volume.  Tricky but a good
exercise...

Paolo Radaelli

RE: Most cited powder diffraction papers

[Radaelli, PG (Paolo)]

Armil,

the Jorgensen paper is mostly about powder diffraction, and I'm sure the
word is in the abstract, but unfortunately abstracts are not available on
WOS for older papers.  For other papers like the one mentioned by Alan, I'm
a bit puzzled.  There is more: the following paper:

SIMULTANEOUS STRUCTURAL, MAGNETIC, AND ELECTRONIC-TRANSITIONS IN
LA1-XCAXMNO3 WITH X=0.25 AND 0.50
RADAELLI PG, COX DE, MAREZIO M, CHEONG SW, SCHIFFER PE, RAMIREZ AP
PHYSICAL REVIEW LETTERS
75: (24) 4488-4491 DEC 11 1995

has 235 citations, and should be on your list, because the abstract mentions
powder diffraction  and is listed on WOS (sorry for being parochial, but it
is my most cited work).

Paolo

RE: Most cited powder diffraction papers

[Radaelli, PG (Paolo)]

Armil:

What criterion was adopted in the search? The following famous paper,
clearly of structural subject, is not on your list, but has 768 citations:

STRUCTURAL-PROPERTIES OF OXYGEN-DEFICIENT YBA2CU3O7-DELTA
JORGENSEN JD, VEAL BW, PAULIKAS AP, NOWICKI LJ, CRABTREE GW, CLAUS H, KWOK
WK
PHYSICAL REVIEW B-CONDENSED MATTER
41: (4) 1863-1877 FEB 1 1990

Paolo

post-doc position at MSU-ISIS

[Radaelli, PG (Paolo)]

MICHIGAN STATE UNIVERSITY/ISIS NEUTRON FACILITY
POSTDOCTORAL POSITION

The Department of Physics and Astronomy at Michigan State University is
looking to fill a postdoctoral research associate position in the
local-structure-property relationship of complex oxides.  The successful
candidate will be located predominantly at the ISIS pulsed neutron
facility at the Rutherford Appleton Laboratory in the UK.  The research
involves using neutron diffraction, including Pair Distribution Function
analysis, and scattering to study the relationsip of structure and local
structure to the properties of such materials as colossal
magnetoresistant manganites and high-temperature superconductors. 
Candidates must have a Ph.D. in Physics, Chemistry, Materials Science or
related subject.  Experience in neutron scattering or diffraction and
computer analysis/programming is preferred.  Deadline for applications
is April 16th, 2001 or until the position is filled.  Please send
applications, including a vita, statement of research, and at least two
letters of recommendation, to Prof. Simon J.L. Billinge, Department of
Physics and Astronomy, Michigan State University, East Lansing, MI 
48824-1116 USA (www.pa.msu.edu/cmp/billinge-group). Further information
can also be obtained from Dr. Paolo Radaelli ([EMAIL PROTECTED]).
Michigan State University is an affirmative action/equal opportunity
institution.  Women and minorities are especially encouraged to apply.

RE: Combined XRD and ND refinements using GSAS

[Radaelli, PG (Paolo)]

Dear Caroline,

in general, it is very difficult to calibrate a neutron diffractometer well
enough to trust the absolute lattice parameters (the relative ones are
usually excellent).  This is especially true for TOF diffractometers.  There
are many reasons for this, including sample position and transparency
effects.  The latter are particularly annoying, because they are
wavelength-dependent, and introduce a non-linear relation between TOF and
d-spacing.  Also, the asymmetric pulse shape does not help.  En passant (are
you listening Bob?), I notice that there is currently no Rietveld code I'm
aware of capable of correcting neutron data for transparency.  Ideally, the
refineable parameter should be mu*R, exactly as for the absorption
correction, and it should be possible to set a constraint between the two
values.  Also, a correction for sample position errors would be nice.  Note
that, for modern scintillator-based diffractometers, a 0.5mm positioning
error (forward or backwards) is equivalent to a peak shift of 10% of the
FWHM at 90 degrees.
My usual advice to my users is that if they want to know the absolute
lattice parameters, they should measure them on their x-ray diffractometer.
Having done that, they should fix the lattice constants and refine the
neutron instrument parameter constants for all banks on the room temperature
data.  These constants should than be fixed for the other temperatures.
This advice should apply equally well to your case, especially if you have
synchrotron data, which are usually very precise.  

The situation is not that bad for constant-wavelength diffractometers,
especially of the multi-soller type (like D2B), provided that they are
recalibrated every time the monochromator is moved.  
You should also consider the possibility that the difference you observe is
due to a compositional gradient from the inside to the outside of the grain.

Paolo Radaelli
GEM instrument scientist

RE: Thermal Parameters and Occupancies

[Radaelli, PG (Paolo)]

One important consideration on the issue of thermal parameters vs
occupancies is the Q-range (Q=4pisin(theta)/lambda) where useful information
exists.  This is a function of the wavelength used, the resolution, the
complexity of the structure and the radiation (x-rays vs neutrons).  The
best results are obtained for simple structures using good-resolution TOF
neutron diffraction.  I include a comparison between single-crystal
synchrotron x-ray and TOF neutron powder data on Y2O3 (3g sample, Q up to 25
A-1).  Clearly, one obtains very good *anisotropic* D-W factors within one
or a few error bars of the SX values.  Also, notice the error bars on oxygen
coordinates, which are 1/10 of the x-ray ones.  Short data acquisition times
also yield sensible results.  The oxygen occupancy was also refined at the
same time, yielding 1.013(1) for the long run, so there is a little problem
at the 1% level.
Another issue is that of the absolute value of the D-W factors, which is
critical to distinguish between static and dynamic disorder.  In order to
get accurate values, one has to perform a careful attenuation correction.

Paolo


  SXXD* GEM-8 hours GEM-1 min GEM-10
sec
Y1 U11*100 0.32(1)   0.357(6) 0.36(2)
0.34(2)
  U12*100 0.056(7) 0.063(9) 0.04(2)
-0.08(5)
Y2 X 0.96764(3) 0.96745(2) 0.96750(4)
0.9676(8)
 U11*100 0.27(1)   0.285(7)  0.33(2)
0.34(4)
 U22*100 0.27(1)   0.282(9)  0.29(2)
0.28(4)
 U33*100 0.27(1)   0 .28(1)   0.40(3)
0.22(5)
 U23*100 -0.026(6)  -0.035(8) -0.02(2)
-0.01(4)
O X 0.3907(2)   0.39065(2)  0.39065(6)
0.3908(1)
Y  0.1518(2)   0.15187(2)  0.15188(6)
0.1520(1)
Z  0.3801(2)   0.38009(2)  0.38017(6)
0.3802(1)
 U11*1000.51(5)0.49(1)0.44(2)
0.32(5)
 U22*1000.53(5)0.457(9)0.41(2)
0.45(5)
 U33*1000.41(5)0.348(9)0.25(2)
0.25(4)
 U12*100   -0.03(4)   -0.034(7)  -0.01(2)
-0.04(3)
 U13*100   -0.06(4)   -0.060(6)  -0.07(2)
-0.09(3)
 U23*100   -0.05(4)   -0.050(8)   -0.08(2)
-0.03(4)

*E.N. Maslen et al. Acta Cryst. B52,(1996) P. 414-422  Photon Factory data

RE: Riet_L: Scale factor in Rietveld (with a question for Bob and Juan)

[Radaelli, PG (Paolo)]

Hi everybody:

Here is a useful couple of formulas for you neutron lot.  They can be used
to predict in advance the Rietveld scale factor S for a TOF powder pattern.
I remind you that the profile intensity Y is defined as

Y=S|F|^2*H(T-Thkl)*L*A*E*O/Vo (see old GSAS manual, page 122).  The profile
H(T-Thkl) is normalised so that its TOF integral (in mmsec) is 1.

The following formula defines the scale factor S of a TOF powder pattern
normalised to an equivalent amount of vanadium (corrected for attenuation):

[1] S=K*Ltot*f/Vo [mmsec/Angstrom/barns], where

K= 1365 [Angstrom^2*mmsec/barns/m]
Vo   = Unit cell volume [Angstrom^3]
Ltot = Total flightpath [m]
f= Fractional density [dimensionless] = mass/volume/theoretical density

For the more curious, K=252.8*(2Vv/sigmaV/Zv), where

Vv   = Vanadium unit cell volume=27.54 A^3
SigmaV = Vanadium total neutron cross section = 5.1 barns
Zv = Number of vanadium atoms in a unit cell = 2
252.8  = wavelength-velocity conversion constant for neutrons.

>From this, it is easy to deduce the second formula:

[2] S=505.56*Ltot*Sinf/sigmas, where

Sinf   = Q->infinity limit of the scattered intensity S(Q)
sigmas = Total neutron cross section for a unit cell of the sample (Just the
sum of the individual sigmas of the atoms).

For the novices, I remind you that the scattered intensity flatens out at
high Q (or it should if all the corrections are done propertly).

I verified both formulas using my diffractometer GEM, which has detectors
from 15 degrees to 170 degrees 2th.  Needless to say that the refined scale
factors for the different banks are equal with an uncertaintly of about 3%.
[2] is extremely accurate, better than 1% at high angle.  [1] is slightly
less accurate at the moment (~10%), but I plan to improve my corrections to
reach a 1-2% level.  If these levels of accuracy can be reached, these
formulas could be valuable to obtain absolute |F|^2 for problems with
multi-site substitutions/vacancies.

I'll leave to the reactor people as an exercise to derive the equivalent of
this formulas.  Note that, for CW data, you rearly if ever to S(Q)
saturation.

Finally, here is a question for Bob and Juan.  To me, it would be much more
natural to remove Vo from the scale factor, that is to redefine a new S' so
that


Y=S'*L*A*E*|F|^2/Vo^2 and S'=K*Ltot*f

This way, the scale factor will only depend on the sample effective density
and not its crystal structure.  This is very useful in phase transitions
involving a change in the size of the unit cell, as you can imagine.  Is
there any rationale in doing it the way it's currently done?

Best

Paolo

RE: Lorentz Factor in TOF neutron diffraction

[Radaelli, PG (Paolo)]

To answer Nail's question:

The Lorentz factor can be deduced from the expression of the integrated
intensity of a single reflection of a TOF powder pattern (in the absence of
attenuation):

I=[e(lam)*Omega]*[Vs/(32*pi*Vu^2)]*[lam^4*i(lam)]*[1/sin^3(theta)]*[Mhkl*|Fh
kl|^2]=
=[e(lam)*Omega]*[Vs/(2*pi*Vu^2)]*[i(lam)]*[Mhkl*|Fhkl|^2]*[d^4*sin(theta)]

in this formula,Omega is the detector solid angle, e(lam) its efficiency, Vs
is the sample volume, Vu is the unit cell volume, lam is the wavelength,
i(lam) the incident spectrum (neutrons/cm^2/Angstrom), theta is the Bragg
angle, Mhkl is the reflection multiplicity and Fhkl is the structure factor.
The second formula is deduced from the first, keeping in mind that
lam^4/sin^3(theta)=16d^4*sin(theta).  The reference to this formula is given
in B. Buras and L. Gerward, Acta Cryst A31 (1975) p372, and also reported in
the book by R. A. Young, "The Rietveld method" IUCr-Oxford  University Press
(1995) p. 214.  In there, the expression for the integrated intensity over
the full Debye-Scherrer cone is given. The expression I quote is easily
deduced by noting that for such a cone

Omega=8*pi*sin(theta)*cos(theta)*Dtheta

I hope this answers your question.

Paolo

Vacancy in neutron powder diffraction

[Radaelli, PG (Paolo)]

Dear All,

please note the following open position in powder diffraction.  Regards

JOB VACANCY AT CLRC RUTHERFORD APPLETON LABORATORY
 VN Reference and title
  VN1978: Instrument Scientist for Powder Diffraction

 Description
  ISIS is the most powerful source of pulsed neutrons and muons in
the world and is one of the
  UK's leading centres for experimental condensed matter research.
The facility accommodates
  approx. 20 neutron and muon spectrometers which are used by UK and
foreign scientists from
  a broad range of disciplines: physics, chemistry, biology and
materials science. The
  experiments are performed under the direction of a dynamic and
innovative team of about 100
  experimental scientists and the science programme is divided into
six experimental groups.
  There is an ongoing and successful programme to upgrade and expand
the instrument suite at
  ISIS. Further details may be found at http://www.isis.rl.ac.uk.

  We have recently completed construction of a major new instrument
GEM, the General
  Materials Diffractometer, which forms part of a top-class suite of
powder diffraction instruments
  at the facility. We now have a vacancy in the Crystallography
Group for an experimental scientist
  in powder diffraction to become part of the instrument scientist
team operating the GEM
  instrument. The appointee will also be expected to undertake a
recognised programme of
  personal and collaborative research, preferably in a field related
to the activities of the
  Crystallography Group, and will also have the opportunity to
undertake work on the other powder
  diffractometers at ISIS. Duties of the post include supporting the
operation of the scheduled
  instruments, assisting and advising users, assisting with the
scheduling of the GEM instrument,
  contributing to the development of techniques, instrumentation and
analysis software and
  stimulating a wider use of the experimental facilities. In
particular for this post, there will be an
  emphasis on contributing to the new opportunities offered by the
high flux/high resolution GEM
  instrument, both in experiment design and in advanced data
processing and analysis
  techniques.

  The post is suitable for either graduate or post-doctoral
scientists who have shown an aptitude
  for original research. Candidates should hold a 1st or 2nd Class
BSc (Hons) degree or
  equivalent qualification, in an appropriate physical science. A
background in crystallography and
  a familiarity with computing would be advantageous for this post.
Previous experience in
  applying neutron techniques in condensed matter research is not
essential and applicants with
  experience in other experimental methods are welcome. 

 Further information
  For more details contact: Dr. C. C. Wilson, Group Leader,
Crystallography Group, phone: +44
  (0)1235 445137, e-mail: [EMAIL PROTECTED] ,
http://www.isis.rl.ac.uk/Crystallography.

 Salary
  The salary range is between £23,240 and £33,200 (pay award
pending). A non-contributory
  pension scheme and a generous leave allowance are also offered.

 Application forms
  Application forms can be obtained from:

  Operations Group
  HR Division
  Rutherford Appleton Laboratory
  Chilton
  Didcot
  Oxfordshire
  OX11 0QX.

  Telephone (01235) 445435 (answerphone) quoting reference VN1978,
or e-mail [EMAIL PROTECTED]

  All application forms must be returned by 8 September 2000. 

 More information about CLRC is available from CCLRC's World Wide Web
pages at
 http://www.cclrc.ac.uk/. 

 The CCLRC is committed to Equal Opportunities and to achieving the
Investors In People standard. A
 no smoking policy is in operation.