LHPM-Rietica

There is a variety of freely available software, which enables one to deconvolute a powder diffraction pattern and determine either or all individual intensities, lattice and peak shape function parameters, and observed structure factors of all possible Bragg reflections. Freeware codes include EXPO, FullProf, GSAS, LHPM-Rietica, and others. In addition to free programs, nearly all manufacturers of commercial powder diffractometers offer software for sale either as a package with the sale of the equipment or as stand-alone products. ... 

LHPM-Rietica (authors B.A. Hunter and C.J. Howard) may be downloaded from ftp //ftp.ansto.gov.au/pub/physics/neutron/rietveld/Rietica LHPM95/ or via a link at http //www.ccpl4.ac.uk. 

In this example, we will use LHPM-Rietica. For readers that have no experience with the program, we suggest using both the manual, which is available for download with the software, and a Web-based tutoriaP in combination with the experimental data found on the CD. Ideally, the results presented below should be reproduced nearly exactly, although small deviations may occur due to the nature of the non-linear least squares. 

Brett Hunter, LHPM-Rietica Rietveld for Win95/NT. Tutorial is accessible via http //www.ccpl4.ac.uk/, then Tutorials on the site map, then LHPM-Rietica Rietveld . 

Just as in the case of full pattern decomposition, we will use two freely available software codes (LHPM-Rietica and GSAS ) to carry out Rietveld refinements using either or both x-ray and neutron diffraction data. Many... 

To account for the presumably statistical distribution of Ni and Sn atoms in the 2(c) and 3(g) sites in this crystal structure, the initial distribution of atoms in the unit cell has been assumed as listed in Table 7.2. The initial profile and structural parameters are found in the input file for LHPM-Rietica on the CD, the file name is Ch7Ex01a.inp. Experimental diffraction data, collected on a Rigaku TTRAX rotating anode powder diffractometer using Cu Ka radiation in a continuous scan mode, are located on the CD in the file Ch7Ex01 CuKa.dat. 

The last column in Table 7.2 contains site occupancies by all atoms in the format required by LHPM-Rietica. The occupancy of each site ( ) is given as a product of the population parameter (g) and site multiplicity (m) divided by the multiplicity of the general site position (A ) ... 

This is usually not the case in the majority of Rietveld refinements because the default value of the scale factor selected in LHPM-Rietica (K = 0.01) is arbitrary. For example, see Table 7.5, below, where initial residuals are much higher due to the inadequacy of the default phase scale. 

In LHPM-Rietica and almost every other commonly available software product, any dependent parameter, Pdependem, may be constrained to any free... 

In the hexagonal crystal system, a and b unit cell parameters are constrained by symmetry, (a = b). In LHPM-Rietica, lattice parameters are constrained automatically for all crystal systems. 

After the individual isotropic atomic displacement parameters were replaced by the properly constrained individual anisotropic displacement parameters (LHPM-Rietica uses Pij, see Eq. 2.93), the refinement converges to the residuals listed in row 8 of Table 7.3. The parameters of the fully refined structure are found in the file Ch7Ex01e.inp on the CD. 

Structure refinements are described here in a way so that they can be repeated, to a certain extent, using LHPM-Rietica, FullProf, Rietan, and other computer codes mentioned in this book. Some examples (relatively complex structures coupled with rather low quality diffraction patterns), however, may not be easily reproduced even when using the same version of the GSAS unless every step is followed exactly as described in this text. 

Other computer programs handle default settings in a similar way. In LHPM-Rietica, for example, these can be specified for a variety of diffractometers/experimental setups and then chosen to represent initial parameters of every data set, which is included in the processing. 

This is the profile (histogram) file, which contains experimental powder pattern in a standard GSAS format. Note that this format is also suitable for LHPM-Rietica. 

The coordinates of atoms taken from Table 6.48 along with the unit cell dimensions and all profile parameters determined from Le Bail s full pattern decomposition are found in the input file, Ch7Exl0a.inp for LHPM-Rietica, and the diffraction data are in the file Ch7ExlO MoKa.dat, located on the CD. The progress of Rietveld refinement is illustrated in Table 7.28. 

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