Détermination du groupe d'espace des phases quadratiques de RbCdF₃ et TlCdF₃ par l'analyse des symétries des modes de vibration en centre de zone de Brillouin

The fluoroperovskites RbCdF₃ and TlCdF₃ undergo, at low temperature, a phase transition associated with small rotations of CdF₆ octahedra. It is shown how both experimental and theoretical study of the modes of vibration appearing at the Brillouin zone centre allows one to attribute the space group D¹⁸_4h to (I4/mcm) to the quadratic phase.