K2ReCl6 belongs to the antifluorite family and exhibits a sequence of structural transitions above the onset of magnetic order at TN = 12 K. Because of its 5d3 electronic configuration in an octahedral coordination, the ground state is a pure spin state without orbital degeneracy within the LS coupling scheme, but it can become Jahn-Teller active in the strong spin-orbit coupling limit described by the jj coupling [S. Streltsov and D. I. Khomskii, Phys. Rev. X 10, 031043 (2020)]. While the structural transitions in K2ReCl6 are understood in terms of octahedral rotation and tilting, the possible impact of a Jahn-Teller distortion remains an open issue. We report on comprehensive crystalstructure studies by means of powder neutron and single-crystal x-ray diffraction on K2ReCl6 and on K2SnCl6. The latter material is used as a reference, because it exhibits the same sequence of structural transitions as K2ReCl6, but possesses a filled 4d shell ruling out a Jahn-Teller distortion. While the ReCl6 octahedron in K2ReCl6 presents sizable distortions at intermediate temperatures, there is no such distortion persisting to low temperatures excluding a sizable Jahn-Teller effect. Studies on polycrystalline samples of Rb2ReCl6 and Cs2ReCl6, in which the structural transitions are suppressed due to the larger alkaline ionic radius, also do not find any indications for a Jahn-Teller distortion.
@article{arxiv.2602.08665,
title = {Structural studies on $A_2$ReCl$_6$ ($A$=K, Rb, Cs): absence of Jahn-Teller distortion},
author = {A. Bertin and L. Kiefer and V. Pomjakushin and O. Fabelo and P. Becker and L. Bohaty and M. Braden},
journal= {arXiv preprint arXiv:2602.08665},
year = {2026}
}