Antiferroelectric instability in kagome francisites Cu$_3$Bi(SeO$_3$)$_2$O$_2$X (X = Cl, Br)

TL;DR

This study reveals antiferroelectric distortions in kagome francisite Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl below 115K, showing how atomic displacements influence magnetic properties and structural stability, with implications for related halogen compounds.

Contribution

First demonstration of antiferroelectric distortion in kagome francisite Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl using density-functional calculations and synchrotron diffraction.

Findings

Antiferroelectric distortion occurs below 115K in Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl.

Br analogue remains stable but shows local displacements near instability.

Distortion affects kagome spin lattice and magnetization.

Abstract

Density-functional calculations of lattice dynamics and high-resolution synchrotron powder diffraction uncover antiferroelectric distortion in the kagome francisite Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl below 115K. Its Br-containing analogue is stable in the room-temperature crystal structure down to at least 10K, although the Br compound is on the verge of a similar antiferroelectric instability and reveals local displacements of Cu and Br atoms. The I-containing compound is stable in its room-temperature structure according to density-functional calculations. We show that the distortion involves cooperative displacements of Cu and Cl atoms, and originates from the optimization of interatomic distances for weakly bonded halogen atoms. The distortion introduces a tangible deformation of the kagome spin lattice and may be responsible for the reduced net magnetization of the Cl compound compared to the Br one. The polar structure of Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl is only slightly higher in energy than the non-polar antiferroelectric structure, but no convincing evidence of its formation could be obtained.