Changement de phase structural dans RbCdf3 et TlCdf3

Abstract
We have shown that a tetragonal distortion with c/a > 1 occurs at 124 K in RbCdF3 and at 191 K in TlCdF3. The anomalous behaviour of the elastic constant C44 is consistent with a slight rotation of the CdF6 octahedra. The study of both X-ray superstructure reflexions and Raman scattering spectrum allows us to retain the space group D184h for the low temperature phase. Two low frequency modes show the behaviour of soft modes

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