PROFVAL: functions to calculate powder-pattern peak profiles with axial-divergence asymmetry

<h2>Journal of Applied Crystallography</h2><h3></h3><h3>0021-8898</h3> <h2>computer program abstracts</h2> Volume 31 Part 1 Page 111 February 1998 <h2> PROFVAL : functions to calculate powder-pattern peak profiles with axial-divergence asymmetry</h2> L. W. Finger J. Appl. Cryst. (1998). 31, 111 PROFVAL: functions to calculate powder-pattern peak profiles with axial- divergence asymmetry L. W. FINGER Bayerisches Geoinstitut, Universit~t Bayreuth, D-95440 Bayreuth, Germany, and Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015--1305, USA. E-maih [email protected] (Received 21 April 1997; accepted 28 April 1997) The crystallographic problem: In a powder diffractometer, the curvature of the Debye---Scherrer rings causes an asymmetry in the peak profiles as the Bragg angle approaches either 0 or 180°. For accurate Rietveld refinement, this asymmetry, which is known as axial divergence, must be modeled correctly. Traditionally, empirical ad hoc functions such as a split-Pearson-VII function or multiple overlapping Gaussian peaks have been employed. Finger eta/. (1994) extended the method of van Laar & Yellon (1984) for convolution of the ring shape with an intrinsic peak profile - a procedure that describes extremely asymmetric profiles and involves only physically meaningful parameters. To date, however, only the program GSAS (Larson &von Dreele, 1990) has incor- porated this method. This contribution makes available tested code for the convolution of the asymmetry function with the intrinsic line function in the hope that more Rietveld codes will incorpo- rate this method. Method of solution: As described by Finger eta/. (1994), the convolution is performed using a Gauss-Legendre quadrature procedure, with the weights...