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

TL;DR

This study interrogates whether Jahn–Teller distortions are active in the 5d^3 antifluorite K$_2$ReCl$_6$ by combining high-resolution single-crystal X-ray diffraction and neutron diffraction with a fully filled-d shell reference, K$_2$SnCl$_6$. Both compounds exhibit the same sequence of symmetry-lowering transitions (cubic Fm$\bar{3}$m → tetragonal P4/mnc → monoclinic C2/c → monoclinic P2_1/n) driven by octahedral rotations and tilts, with intermediate tetragonal and monoclinic distortions that relax at low temperature. The octahedral distortions are substantial only in intermediate phases and vanish in the low-temperature P2_1/n phase, indicating an absence of a static Jahn–Teller effect; this conclusion is reinforced by analogous results in Rb$_2$ReCl$_6$ and Cs$_2$ReCl$_6$. The magnetic order sets in at $T_N¡=12$ K for K$_2$ReCl$_6$ with a triclinic distortion linked to the magnetic structure, and the Rb/Cs analogs show similar magnetic behavior without structural JT activity. Overall, the work demonstrates that the structural transitions in these antifluorites are governed by bond-length mismatch and rotational/tilt modes rather than electronic JT distortions, providing insight into SOC-dominated 5d materials and guiding interpretation of potential spin-orbit entangled ground states.

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.

Figures (11)

• Figure 1: Crystal structure of K$_2$ReCl$_6$ in the three low temperature phases as determined with PND. Panel (a) shows the structure at 15 K in the monoclinic $P2_1/n$ phase. Panel (b) presents the monoclinic $C2/c$ phase at 90 K. In these two monoclinic phases staggered rotation occurs around the $c_m$ axis and the in-phase tilt around the monoclinic $b_m$ axis that, however, changes between the two phases. Panel (c) shows the crystal structure in the tetragonal phase with only the staggered rotation around $c_{tet}$. Here we show the smaller lattice corresponding to $P4/mnc$. In all panels the red-green-blue arrows indicate the orientation of the lattice, K sites are shown in purple spheres and ReCl$_6$ octahedrons in green. Red, green and blue arrows indicate the $\boldsymbol{a}$, $\boldsymbol{b}$ and $\boldsymbol{c}$ lattice vectors. Pictures were drawn with the visualization software VESTA3Momma2011.
• Figure 2: The average octahedral rotation angle $\phi$, as defined in the main text, is given in panel (a) for K$_2$ReCl$_6$, and in (b) for K$_2$SnCl$_6$. Tilt angles $\theta$ are shown in panels (c) and (d). Note that in the $P2_1/n$ phase, the tilt axis is no longer parallel to a cubic [100] but parallel to cubic [110]. Panels (e) to (h) present the ADPs for the Re (left) and Sn (right) compounds. For K and Re/Sn the ADP is isotropic (e,f), while for Cl the two ADPs $U_{\parallel}$ and $U_{\perp}$ are given in (g,h). The vertical dashed black lines indicate the structural transitions at $T_{\rm t}=111$ K and $T_{\rm m1}=103$ K, and $T_{\rm m2}=77$ K for K$_2$ReCl$_6$ , and at $T_{\rm t}=263$ K, $T_{\rm m1}=260$ K, and $T_{\rm m2}=$255 K for K$_2$SnCl$_6$ . The solid lines are linear fits, used as a guide to the eye. Open and filled symbols refer to SXD or PND measurements, respectively.
• Figure 3: High-resolution PND patterns measured on K$_2$ReCl$_6$ , (a) in the triclinic antiferromagnetic phase at $T=1.5$ K and (b) at $T=15~K$ in the monoclinic $P2_1/n$ phase. The red circles indicate the experimental data points, the solid black lines illustrate the Rietveld refinements with monoclinic space group $P2_1/n$ (b) , and with both a triclinic nuclear phase in space group $P\bar{1}$ and a magnetic phase described in $P_S\bar{1}$ (a). The corresponding Bragg-peak positions are indicated by the green markers. The blue curve denotes the difference between the fit and the experimental data. In the panels we give the $R$ values corrected for background and the insets present a zoom on the low-angle part, where magnetic Bragg peaks are most prominent.
• Figure 4: Temperature dependencies of the triclinic lattice parameters $a$, $b$ and $c$ (a), of the triclinic angles $\alpha$, $\beta$ and $\gamma$ (b) and of the ordered magnetic moments along the (almost perpendicular) directions $a$ and $b$ (c) that were deduced from Rietveld refinements with high-resolution PND patterns on HRPT. The magnetic transition at $T_{\rm N}=12$ K is indicated by the vertical blue dotted line.
• Figure 5: Precession maps computed in the $(h0l)$ scattering plane from SXD measurements on K$_2$SnCl$_6$ at 270 K in the cubic phase (left panel) and at 262 K in the tetragonal phase (right panel). The blue circles highlight the superstructure reflections emerging in the low-symmetry phase, and breaking the $F$ centering.