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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!
Intro to pseudo-Voigt peak profiles
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.
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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 = Finger-Cox Axial Divergence S term | H/L = Finger-Cox 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 = sublattice anisotropic broadening (stacking faults) |
| S/L = Finger-Cox Axial Divergence S term | H/L = Finger-Cox 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 |
