International
Tables for Crystallography Volume H Powder diffraction Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk © International Union of Crystallography 2018 |
International Tables for Crystallography (2018). Vol. H, ch. 3.4, p. 278
Section 3.4.4.5. Two commercial programs^{a}Institute of Crystallography – CNR, Via Amendola 122/o, Bari, I-70126, Italy |
This commercial indexing program (Coelho, 2003a), which uses the Monte Carlo method, is part of the TOPAS (Coelho, 2003b) suite from Bruker AXS. The reciprocal-cell parameters in equation (3.4.2) are found by using, in an iterative way, the singular value decomposition (SVD) approach (Nash, 1990) to solve linear equations relating (hkl) values to d spacings. The method is particularly useful in cases for which there are more equations than variables. All the observed lines in the powder pattern are involved in the indexing procedure. It is claimed that the program is relatively insensitive to impurity peaks and missing high d spacings; it performs well on data with large diffractometer zero errors.
More recently, two indexing methods have been introduced in TOPAS: LSI (least-squares iteration), an iterative least-squares process which operates on the d-spacing values extracted from reasonable-quality powder diffraction data, and LP-Search (lattice parameter search), a Monte Carlo based whole-powder-pattern decomposition approach independent of the knowledge of the d-spacings (Coelho & Kern, 2005).
This commercial program is part of the Materials Studio suite from Accelrys (Neumann, 2003). To perform an exhaustive search, like DICVOL, the program uses the successive-dichotomy approach. Its principal features are:
The program is described as `virtually exhaustive'; it is expected to work well when faced with missing lines, impurities and errors.
References
Coelho, A. A. (2003a). Indexing of powder diffraction patterns by iterative use of singular value decomposition. J. Appl. Cryst. 36, 86–95.Google ScholarCoelho, A. A. (2003b). TOPAS. Version 3.1 User's Manual. Bruker AXS GmbH, Karlsruhe, Germany.Google Scholar
Coelho, A. A. & Kern, A. (2005). Discussion of the indexing algorithms within TOPAS. IUCr Commission on Powder Diffraction Newsletter, 32, 43–45.Google Scholar
Nash, J. C. (1990). Compact Numerical Methods for Computers: Linear Algebra and Function Minimisation, 2nd ed. Bristol: Adam Hilger.Google Scholar
Neumann, M. A. (2003). X-cell: a novel indexing algorithm for routine tasks and difficult cases. J. Appl. Cryst. 36, 356–365.Google Scholar