The structural phase transition (PT) of elpasolite‐type crystal Cs 2 NaBiCl 6 is investigated by neutron scattering. At first the acoustic phonon dispersion is measured at room temperature for the highsymmetric directions Δ, ∑, and Λ of the Brillouin zone. The transverse rotary mode is studied for phonon wave vectors q parallel as well as perpendicular to Δ and the dispersion shows a quasitwo‐dimensional behaviour of the Cl‐octahedra rotation. At cooling down to the PT, this branch shows a softening along Δ, reaching the lowest energies at the zone centre. The PT, which is found to be of weakly first order, seems to be driven and triggered by the ferrorotative octahedra mode.
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Prokert et al. (1984) studied this question.
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