Temp

Intro

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

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.

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 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