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arXiv · 2602.08665

Structural studies on $A_2$ReCl$_6$ ($A$=K, Rb, Cs): absence of Jahn-Teller distortion

A. Bertin·L. Kiefer·V. Pomjakushin·O. Fabelo·P. Becker·L. Bohaty·M. Braden
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Abstract

K$_2$ReCl$_6$ belongs to the antifluorite family and exhibits a sequence of structural transitions above the onset of magnetic order at $T_N$ = 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 K$_2$ReCl$_6$ 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 K$_2$ReCl$_6$ and on K$_2$SnCl$_6$. The latter material is used as a reference, because it exhibits the same sequence of structural transitions as K$_2$ReCl$_6$, but possesses a filled 4d shell ruling out a Jahn-Teller distortion. While the ReCl$_6$ octahedron in K$_2$ReCl$_6$ 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 Rb$_2$ReCl$_6$ and Cs$_2$ReCl$_6$, 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.

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A. Bertin, L. Kiefer, V. Pomjakushin, O. Fabelo, P. Becker, L. Bohaty, M. Braden. 2026-02-09. Structural studies on $A_2$ReCl$_6$ ($A$=K, Rb, Cs): absence of Jahn-Teller distortion. https://doi.org/10.1103/jhcn-vj32

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