We present temperature-dependent single-crystal diffraction results on seven antifluorite-type A2MeX6 compounds with Me=Os or Ir: K2OsCl6, A2OsBr6 with A=K, Rb, Cs and NH4, and K2IrX6 with X=Cl and Br. The structural transitions in this family arise from MeX6 octahedron rotations that generate a rich variety of symmetries depending on the rotation axis and stacking schemes. In order to search for local distortions in the high-symmetry phase we perform refinements of anharmonic atomic displacement parameters with comprehensive data sets. Even at temperatures close to the onset of structural distortions, these refinements only yield a small improvement indicating only small anharmonic effects. The phase transitions in these antifluorites are essentially of displacive character. However, some harmonic displacement parameters are very large reflecting soft phonon modes with the softening covering large parts of the Brillouin zone. The occurrence of the rotational transitions in the antifluorite-type family can be remarkably well analyzed in terms of a tolerance factor of ionic radii.
@article{arxiv.2402.15358,
title = {Rotational phase transitions in antifluorite-type osmate and iridate compounds},
author = {A. Bertin and L. Kiefer and P. Becker and L. Bohatý and M. Braden},
journal= {arXiv preprint arXiv:2402.15358},
year = {2024}
}