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GSAS offers 5 different Constant Wavelength (CW) X-ray profile functions. They are described in detail within the GSAS technical manual (see page 156). 11-BM users are encouraged to use either profile type 3 or type 4.
A quick reference for the various profile type 3 and 4 terms is given below. Users are strongly encouraged to read this section of the GSAS manual at least once before (or after) blindly using this guide!
Pseudo-Voigt Profile Functions
These peak profile functions are a pseudo-Voigt type, combining Gaussian (G) and Lorentzian (L) components.
The general Gaussian shape (as a function of angle θ) is described by the Cagliotti function
Gaussian Profile ≈ U*tan2θ + V*tanθ + W + P/cos2θ
These U, V, W, and P variables match the GU, GV, GW, and GP profile terms you see below.
The Lorentzian shape is more complex (check the manual), but includes size and strain broadening terms.
Asymmetry, Zero-Shift and Related
Low angle asymmetry (aka Axial Divergence) is modeled in GSAS profile types 3 & 4 with the Finger-Cox-Jephcoat model (see GSAS manual). This S/L & H/L
GSAS offers two ways to treat low-angle asymmetry poor: asym (profile functions #1 & #2) – works only if small or no low-angle data excellent: Finger-Cox-Jephcoat (profile #3 & #4) – two terms: S/L & H/L
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Suggested Profile Types & Terms for Fitting 11-BM Data
11-BM users are encouraged to the GSAS constant wavelength (CW) profile type 3 or type 4. Profile #4 is best for cases in which anisotropic terms are required.
11-BM users will not (usually!) need to change or refine the default 'G' terms given in the instrumental parameter file. For the high-resolution synchrotron powder data collected at 11-BM, the instrumental resolution is well described by Gaussian terms.
On the other hand, sample effects in 11-BM data, such as size and strain broadening are (usually!) best fit and refined using Lorentzian terms. Gaussian sample size broadening is *rarely* observed; this requires a very tight mono-disperse size distribution rarely encountered in powder samples (solid metal samples may be an exception).
Many 11-BM diffraction patterns can then be well fit by refining only the LX (size), LY (strain) and
Type 3
Constant Wavelength X-ray GSAS Profile Type 3
| GU = Gaussian U term | GV = Gaussian V term | GW = Gaussian W term |
| GP = Gaussian crystallite size broadening | LX = Lorentzian crystallite size broadening | LY = Lorentzian strain broadening |
| S/L = Axial Divergence S term | H/L = Axial Divergence H term | trns = Sample 'Transparency' |
| shft = Sample 'Displacement' | stec = Lorentzian anisotropic strain broadening ** | ptec = Lorentzian anisotropic crystallite size broadening ** |
| sfec = Lorentzian sublattice anisotropic broadening ** | LXX = Anisotropic Lorentzian microstrain ** | LYY = Anisotropic Lorentzian microstrain ** |
- NOTE
Type 4
Constant Wavelength X-ray GSAS Profile Type 4
| GU = Gaussian U term | GV = Gaussian V term | GW = Gaussian W term |
| GP = Gaussian crystallite size broadening | LX = Lorentzian crystallite size broadening | ptec = anisotropic crystallite size |
| trns = Sample 'Transparency' | shft = Sample 'Displacement' | sfec = Lorentzian sublattice anisotropic broadening # |
| S/L = Axial Divergence S term | H/L = Axial Divergence H term | eta = mixing factor, from pure Gaussian (0) to pure Lorentzian (1) |
| SXXX = Stephens anisotropic microstrain broadening | SYYY = Stephens anisotropic microstrain broadening | SZZZ = Stephens anisotropic microstrain broadening |
- stacking defects (see manual)
