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Model materials are precious test cases for elementary theories and providebuilding blocks for the understanding of more complex cases. Here, we describethe lattice dynamics of the structural phase transition in francisiteCu3Bi(SeO3)2O2Cl at 115 K and show that it provides a rare archetype of atransition driven by a soft antipolar phonon mode. In the high-symmetry phaseat hightemperatures, the soft mode is found at (0,0,0.5) at the Brillouin zoneboundary and is measured by inelastic X-ray scattering and thermal diffusescattering. In the low-symmetry phase, this softmode is folded back onto thecenter of the Brillouin zone as a result of the doubling of the unit cell, andappears as a fully symmetric mode that can be tracked by Raman spectroscopy. Onboth sides of the transition, the mode energy squared follows a linearbehaviour over a large temperature range. First-principles calculations revealthat, surprisingly, the flat phonon band calculated for the high-symmetry phaseseems incompatible with the displacive character found experimentally. Wediscuss this unusual behavior in the context of an ideal Kittel model of anantiferroelectric transition.

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