${\mathrm{K}}_2{\mathrm{ReCl}}_6$ belongs to the antifluorite family and exhibits a sequence of structural transitions above the onset of magnetic order at $T_{\mathrm{N}}=12\mathrm{K}$. Because of its $5d^3$ 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. V. Streltsov and D. I. Khomskii, Phys. Rev. X 10, 031043 (2020)]. While the structural transitions in ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ 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 crystal-structure studies by means of powder neutron and single-crystal x-ray diffraction on ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ and on ${\mathrm{K}}_2{\mathrm{SnCl}}_6$. The latter material is used as a reference, because it exhibits the same sequence of structural transitions as ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ but possesses a filled $4d$ shell ruling out Jahn-Teller distortion. While the ${\mathrm{ReCl}}_6$ octahedron in ${\mathrm{K}}_2{\mathrm{ReCl}}_6$ 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 ${\mathrm{Rb}}_2{\mathrm{ReCl}}_6$ and ${\mathrm{Cs}}_2{\mathrm{ReCl}}_6$, in which the structural transitions are suppressed due to the larger alkaline ionic radius, also do not find any indications of Jahn-Teller distortion.

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• Electronic structure
• Magnetic order
• Structural properties
• Mott insulators
• Crystallography
• Neutron diffraction
• X-ray diffraction