Unusual phase transition into frustrated antipolar phase in ferromagnetic EuAl12O19

Hexagonal EuAl12O19 exhibits a phase transition at Ts = 49 K visible in specific heat and thermal expansion. Pyroelectric measurements reveal polarization and temperature-dependent dielectric permittivity shows an unusual change of slope at Ts characteristic for improper ferroelectrics. At low temperatures, a dielectric dispersion resembling relaxor ferroelectrics is observed. However, second harmonic generation measurements did not show a breaking of center symmetry and synchrotron XRD did not reveal any structural change below Ts. Based on theoretical calculations on a similar system BaFe12O19 (Wang and Xiang, PRX 4, 011035 (2014)), we suggest that EuAl12O19 transfers into a frustrated antiferroelectric phase below Ts. This should be visible in diffuse X-ray scattering. In addition, phonon softening should be observed at the Brillouin zone boundary. Therefore, we propose to measure diffuse scattering (6 shifts) and phonons (12 shifts) at the BZ boundary as function of temperature.