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The structural and dynamical properties of Cs 2 NaREBr 6 (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 REBr 6 octahedron of approximately 4 deg in Cs 2 NaHoBr.
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Bührer et al. (1987) studied this question.
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