I have an answer from Jeremy Cockcroft the author of the 'Birkbeck'
Tables. Actually Jeremy has been at UCL for a decade or so and they are
hosted from their
Nick
In answer to the question regarding the use of R and H for trigonal
space groups, the letters refer to two distinct types of lattice
centring symmetry operation, namely +(⅓,⅔,⅔), +(⅔,⅓,⅓) and +(⅓,⅔,0),
+(⅔,⅓,0), respectively. For the subset of rhombohedral space groups, the
symbol R should always be used.When the alternative unit cell with a=b=c
and α=β=γ is chosen, this cell corresponds to a primitive Bravais
lattice, but the label P is not used as this would result in confusion
with non-rhombohedral space groups. (The choice of symmetry operators
for rhombohedral space groups is wholly dependent of the choice of unit
cell, i.e. hexagonal versus rhombohedral, so there is no ambiguity if
the unit cell is specified.)
For the non-rhombohedral space groups, it may occasionally be convenient
to choose a larger H-centred unit cell that is not the usual primitive P
one.(The use of larger unit cells is quite common for systems that
undergo phase transformations as it may enable the crystallographer to
keep the contents of the unit cell the same in both phases.)Note that
the use of an H-centred lattice switches the order of the symmetry
elements in these space group symbols, e.g. P312 becomes H321.
I am not aware of any changes to this convention, which I believe has a
long history. However, it is possible that the letter H has been used
unwittingly for other purposes.
Jeremy Karl Cockcroft
--
NOTE NEW PHONE NUMBER JULY 2020
Prof Nicholas H. Keep
Executive Dean of School of Science
Professor of Biomolecular Science
Crystallography, Institute for Structural and Molecular Biology,
Department of Biological Sciences
Birkbeck, University of London,
Malet Street,
Bloomsbury
LONDON
WC1E 7HX
Office G54a
Dean Email; scid...@bbk.ac.uk
Dept email n.k...@mail.cryst.bbk.ac.uk
Telephone 020-3926-3475 (Will contact me at home if working as well as my
office)
If you want to access me in person you have to come to the crystallography
entrance
and ring me or the department office from the internal phone by the door
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