| Popis: |
Searchable calculated powder patterns of crystalline substances are commonly used nowadays to complement experimental patterns for the identification of materials by diffraction methods. Reliable such patterns are often calculable from the known cell, type and chemistry data on types with fixed Wyckoff positions, like NaCl, but not on typeC16 ofAB2compounds, with CuAl2as its representative material, because of the variablexcoordinate of itsBsite.Ab initiocell and coordinate relaxation for four key compounds withC16 type, all started from the samec/a= 1 andx= 1/6 configuration, converged on the correct axial ratios and fractional coordinates. A linear regression on a plot ofxcoordinates optimizedab initioat experimental cell dataversusX-ray coordinates for all 48C16-type compositions for pure-phase compounds with coordinates in the CRYSTMET database gavey= 0.998x,R2= 0.88. Credible calculated powder patterns were then produced for AlHf2, AlZr2, AuTl2, BCr2, GaHf2, IrZr2and TlTh2, all with cell, type and pure-phase composition information but no coordinates in this database, using their X-ray cell data andab initiocoordinates. Those patterns, now inserted in the search system of the database, will help users identify those phases from powder diffraction angle and intensity data. The corresponding entries in a new `X-ray cell-and-type,ab initiocoordinates' category of full structure entries may be helpful for other applications, such as the simulation of EXAFS spectra or of backscattered electron patterns. |
