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

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.

arXiv comment6 pages, 5 Figures, supplemental material included

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• Soft phonon columns on the edge of the Brillouin zone in the relaxor PbMg1/3Nb2/3O3
• Magnetic and dielectric order in the kagome-like francisite Cu${}_3$Bi(SeO${}_3$)${}_2$O${}_2$Cl
• Antiferroelectric instability in kagome francisites Cu${}_3$Bi(SeO${}_3$)${}_2$O${}_2$X (X = Cl, Br)
• Longitudinal magnon, inversion breaking and magnetic instabilities in the pseudo-Kagome francisites Cu3Bi(SeO3)2O2X with X=Br, Cl

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• Linear magnetoelastic coupling and magnetic phase diagrams of the buckled-kagomé antiferromagnet Cu${}_3$Bi(SeO${}_3$)${}_2$O${}_2$Cl
• Dipole Ordering of Water Molecules in Cordierite: Monte Carlo Simulations
• Coexistence of ferroelectric and ferrielectric phases in ultrathin antiferroelectric PbZrO3 thin films
• Evidence for a soft-phonon-mode-driven Peierls-type distortion in Sc${}_3$CoC${}_4$