Sorry about the confusion on the instrument configuration. I am new to the field of x-ray diffraction. Hopefully this helps. The Bruker D8 uses reflection geometry and a THETA : THETA goniometer, where the x-ray source and detector can be move simultaneously on the arms of the goniometer. The x-ray source is Cu and is directed at a Bruker multipurpose Si Gobel mirror which reflects a parallel beam of Cu K-alpha (1&2) radiation at a 2-bounce Ge(022) analyzer crystal. As I understand it, the analyzer crystal filters our the K-alpha 2 peak, producing monochromatic K-alpha 1radiation. There are no soller slits on the primary side of the instrument. The beam is directed at the specimen and the diffracted beam passes through a set of soller slits and then to the point detector. Thanks, Patrick
On Fri, Dec 4, 2009 at 5:59 AM, Cline, James Dr. <james.cl...@nist.gov> wrote: > Patrick, > > ________________________________________ > From: Patrick Price [patrickpric...@gmail.com] > Sent: Friday, December 04, 2009 6:30 AM > To: Rietveld_l@ill.fr > Subject: Rietveld refinement in TOPAS with parallel beam geometry > > Since this is my first post I will start with a brief introduction. My > name is Patrick Price and I am in my second year of graduate school. > My thesis work involves the investigation of phase equilibria in > perovskites. > > I am using a Bruker D8 Discover diffractometer with parallel beam > geometry. The diffractometer has a Cu K-alpha X-Ray source with a Si > Gobel mirror and a Ge monochromator giving a parallel beam > monochromatic x-ray source. > > This instrument description doesn't make sense. > > Regards, > > Jim > > > The receiving side has Soller slits and a > Tl-doped NaI point detector. I am trying to teach myself how to use > TOPAS to PROPERLY analyze my data using Rietveld refinement > techniques. > > I have recently taken a scan of the NIST line profile 660 LaB6 > standard followed by scans of my perovskite powders using a step size > of 0.02 degrees and scan time of 4 seconds. > > Most of the articles I have read are specific to convergent/divergent > beam geometries and I do not know how much of that information > transfers to parallel beam geometries. If anyone could help me answer > the following questions I would greatly appreciate it. These questions > mainly address which parameters should be refined with the LaB6 > standard when using parallel beam geometry. > 1. I need to use the scan of the LaB6 powders to characterize the > contributions of the instrument to the diffraction profile. Starting > with the emission profile, TOPAS asks for the wavelength, the Area, > and the Lorentz Half Width. First, I assume the wavelength I should be > the more recent Cu Ka wavelength of 0.154059 nm instead of 0.154056 > nm. Second, does Cu Ka have a definite Lorentz HW and “Area” or should > these parameters be refined with the LaB6 diffraction pattern? > 2. Since I have a Ge monochromater I assume the Lorentz polarization > factor should be fixed at 27.3 (Is this correct?). Obviously the > lattice parameters and atomic positions would be fixed. > 3. I read that you should NOT refine both the zero shift error and > sample displacement, and since it is parallel beam I only refine the > zero shift error. Should I refine surface roughness, absorption, or > sample tilt with the LaB6? (Currently I do not refine these) > 4. Am I correct in assuming that I do not have any EQUITORIAL > convolutions (e.g. from slits, FDS, beam spill, VDS) since it is > parallel beam geometry? What about TUBE TAILS? > 5. I am using the Finger_et_al method to refine the AXIAL > convolutions, however I often get a large error associated with the S > value (sample length), even when my GOF is decent (<1.45). Do any of > you know why this would happen? > 6. Should I refine the “Scale” or scale factor. (Currently I do) > 7. IMPORTANT: Originally I was refining the crystallite size but it > always refined to a very small value (~300nm), where as NIST claims > 660 LAB6 should have a mean grain size of a few microns or more. I > assume this happens because the TOPAS is accounting for instrument > caused peak broadening by making the crystallite size smaller than it > actually is in the software. However, when I do refine the grain size > I do get a better fit. Should I leave this unchecked, refine it, or > fix it at a reasonable value of ~2500 nm. > > In summary, currently I am only refining the Lorentz HW and “Area” in > the emission profile, zero shift error, the Finger parameters (S & H), > the scale factor, and nothing else. > I am unsure if I should be refining anything else such as the > crystallite size, tube tails and other forms of equatorial > convergence, or if there is something else that is important which I > am disregarding completely. I am also unsure if I am correct in > refining Lorentz HW and area in the emission profile. > Sorry if I got a little long winded; I just wanted to give enough > detail so people could answer. Thank you in advance for your help. > Patrick > > > > > James P. Cline > Ceramics Division > National Institute of Standards and Technology > 100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ] > Gaithersburg, MD 20899-8523 USA > jcl...@nist.gov > (301) 975 5793 > FAX (301) 975 5334 >
