Modeling of hP3 intermetallics in space group P-3m1: Calculated powder patterns from CRYSTMET® X-ray cell data and ab initio coordinates

Abstract

There are 39 CRYSTMET® entries in the hexagonal space group P-3m1 (164) reporting both distinct pure phase compounds and atomic coordinates. Having the same Wyckoff positions in the same space group as the C6 structure type, all are isopointal with it. The range of observed c/a values extends from about 0.65 to 1.83. Three types are distinguished: Layered materials with CdI2 type, the CeCd2 type which is a slight distortion of the hexagonal AlB2 type, and the intermediate EuGe2 type made of the materials AuTe2, BaSi2, EuGe2, and SrGe2. Ab initio modeling of the 26 entries with CdI2 and EuGe2 type and atomic coordinates reproduces convincingly both their c/a axial ratios and z coordinates. For CoO2 and SiTe2, both c/a and z deviate to a degree from the reported values, indicating that those materials should be reexamined for superstructures, stoichiometry, etc. Ab initio modeling of the 11 cell-and-type entries with CdI2 type and no coordinates in CRYSTMET reproduced convincingly their reported axial ratios. The X-ray cell data and the ab initioz coordinates were then used in the production of reliable calculated powder patterns for CoTe2, CrSe2, HfS2, HfSe2, HfTe2, NbTe2, SnSe2, VS2, VTe2, ZrS2, and ZrTe2. All 11 patterns have been inserted in the intense diffraction line search system of CRYSTMET operated under the Materials Toolkit. Comparison of calculated patterns for SnSe2 and ZrTe2 with experimental entries in the PDF exposes the complementarity of calculated and experimental powder patterns and suggests that JCPDS pattern #15-223 should be reinterpreted in terms of the CdI2 structure type. The CeCd2⇔AlB2 type transformation is modeled and discussed on YCd2 using both ab initio methods and a hard-sphere model. For z<0.45, the ab initio solution is identical with that from the hard-sphere model while a quantum regime is predicted in the small region 0.45<z<0.467 beyond which YCd2 abruptly transforms to the AlB2 type. In spite of the new understanding gained, this modeling fell slightly short of allowing calculation of z values and powder patterns for the materials CaHg2, DyHg2, ErCd2, GdHg2, HoCd2, HoHg2, LuCd2, NdCd2, SmHg2, TbCd2, and TbHg2 with no coordinates in CRYSTMET.