81 Br NQR measurements of the mixed-valence compound (NH 4 ) 4 Sb III Sb V Br 12 , which has Sb V Br − 6 and Sb III Br 3− 6 octahedra in different oxidation states in the tetragonal pseudo-K 2 PtCl 6 structure, were carried out by pulse method in the temperature range between 80 and 300 K. The phase transition temperature of T c = 212 K was redetermined. The T 1 values are quite different between Sb V Br − 6 and Sb III Br 3− 6 octahedra. For all resonance lines T 1 minima were observed at T c . The T 1 behavior at T c was explained by a softening of the rotary lattice mode around a principal axis of each octahedron along the c axis of the crystal. The distinctive feature in the temperature dependence of both 81 Br NQR frequencies and T 1 values for each anion indicates that the static rotation may occur for Sb V Br − 6 but not for Sb III Br 3− 6 at T c in the low temperature phase.
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