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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 Terms

Axial Divergence (i.e. low angle peak asymmetry) is modeled in GSAS profile types 3 & 4 with the Finger-Cox-Jephcoat model (see GSAS manual). The profile terms S/L & H/L describe the instrumental asymmetry. Most users of 11-BM will not need to refine these terms.

=== Sample Transparency=== (trns profile term)

=== Sample Transparency=== (trns profile term)

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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

1. stacking defects (see manual)