Ab initio modelling of intermetallics with CuAl₂ structure type: computed powder patterns from cell-and-type CRYSTMET entries

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 type C16 of AB₂ compounds, with CuAl₂ as its representative material, because of the variable x coordinate of its B site. Ab initio cell and coordinate relaxation for four key compounds with C16 type, all started from the same c/a = 1 and x = 1/6 configuration, converged on the correct axial ratios and fractional coordinates. A linear regression on a plot of x coordinates optimized ab initio at experimental cell data versus X-ray coordinates for all 48 C16-type compositions for pure-phase compounds with coordinates in the CRYSTMET database gave y = 0.998x, R² = 0.88. Credible calculated powder patterns were then produced for AlHf₂, AlZr₂, AuTl₂, BCr₂, GaHf₂, IrZr₂ and TlTh₂, all with cell, type and pure-phase composition information but no coordinates in this database, using their X-ray cell data and ab initio coordinates. 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 initio coordinates' category of full structure entries may be helpful for other applications, such as the simulation of EXAFS spectra or of backscattered electron patterns.