Soft-mode dynamics and sublattice polarization in an archetypal antiferroelectric

Terahertz and far-infrared techniques are combined with complementary Raman and inelastic neutron scattering results to probe the low-energy phonon dynamics of archetypal antiferroelectric candidate - francisite (Cu
3
Bi(SeO
3
)
2
O
2
Cl). The combined results characterize the relationship between in-phase (polar) and out-of-phase (antipolar) sublattice dynamics across an antiferroelectric phase transition. Applying a modified Cochran description of these soft-modes allows us to extract an experimental signature that is highly suggestive of the illusive sublattice polarization that is the antiferroelectric order parameter.