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.
Anonymous et al. (Tue,) studied this question.