About XANSONS for COD
XANSONS for COD is a research project aimed to create an open access database of simulated x-ray and neutron powder diffraction patterns for nanocrystalline phase of the materials presented in the Crystallography Open Database (COD).The calculations for this project are completed. The database of simulated powder diffraction patterns can be found at http://database.xansons4cod.com.
This project uses original open source (GPLv3 license) software XaNSoNS (X-ray and Neutron Scattering on Nanoscale Structures) to simulate the diffraction patterns on CPU and GPU.
XANSONS for COD is a privately managed BOINC project. It was supported by the Russian Foundation for Basic Research in 2015 - 2017 (project RFBR #15-07-07901-a).
Scientific Problem
The conventional technique used to recover the structural properties of the crystalline samples by their powder diffraction pattern is the Rietveld refinement method. In this method, the theoretical powder diffraction pattern is refined until it fits the experimental one. The computation of the angles and intensities of the Bragg peaks can be made almost instantaneously in the approximation of the infinite size of the crystallite. To adjust for the finite size of the crystallites in the samples or finite resolution of the measurement device, these peaks are broadened artificially with the broadening function (usually Gaussian). This artificial broadening works great until the size of the crystallite in the sample is something below few tens of nanometers. For the such small crystallites, it is very hard to get the right broadening function which works fine for all Bragg peaks. Fortunately, for the such small crystallites, it is not a problem to calculate the powder diffraction patterns using the Debye equation (with the distance-histogram approximation, such as that proposed by Marcin Wojdyr and implemented in his Debyer code). This project is aimed to calculate the x-ray and neutron powder diffraction patterns for the nanocrystallites with the size varying from 6 nm to 30 nm for the most of the entries of the Crystallography Open Database. The two different types of materials are considered: (a) isolated spherical crystalline nanoparticle of a given size (diameter), (b) crystalline material with long-range order broken on the distances greater than a given value. The obtained database may simplify the diagnostics of the nanocrystalline samples and complement the Full Profile Search Match method in the crystallite size analysis of the nanocrystalline samples.In addition to the above, the calculation of the powder diffraction pattern using the Debye equation allows to account the lattice defects such as site vacancies, atom replacements and displacements. So, if the Crystallography Information File (CIF) for the given structure provides the site occupancy numbers and atomic displacement parameters, the application will use them to calculate the diffraction pattern.
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