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Soft rotatory node and structural phase transition in the rare-earth bromo-elpasolites Cs 2 NaREBr 6

1987; IOP Publishing; Volume: 20; Issue: 25 Linguagem: Inglês

10.1088/0022-3719/20/25/010

1747-3802

W. Bührer, H.U. Güdel,

Inorganic Fluorides and Related Compounds

The structural and dynamical properties of Cs2NaREBr6 (RE=Dy, Ho, Tm) are investigated by neutron scattering and differential scanning calorimetry. Low-energy phonon dispersion curves are measured and analysed with a rigid-ion model. A cubic-to-tetragonal phase transition is observed, and the temperature dependence of the driving rotatory soft mode is determined with a damped harmonic oscillator analysis. The transition temperatures are 101, 126 and 139 K for the Tm, Ho, and Dy elpasolites, respectively. The low-temperature space group is presumably I4/m with a rotation angle of the REBr6 octahedron of approximately 4 deg in Cs2NaHoBr.