# Archetypal soft-mode driven antipolar transition in francisite Cu3Bi(SeO3)2O2Cl

## TL;DR

This study investigates the lattice dynamics of a structural phase transition in francisite Cu3Bi(SeO3)2O2Cl at 115 K, revealing a soft antipolar phonon mode as the transition driver, combining experimental and theoretical insights.

## Contribution

It provides a detailed analysis of a rare antipolar soft-mode driven transition in francisite, combining inelastic X-ray, Raman scattering, and first-principles calculations.

## Key findings

- Soft mode at (0,0,0.5) observed at high temperature
- Mode energy squared follows linear temperature dependence
- Discrepancy between calculated phonon band and experimental displacive behavior

## Abstract

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)2O2Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at hightemperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic X-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this softmode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behaviour over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.