  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.
