Br NQR relaxation of the mixed-valence compound (NH4)4SbIIISbVBr12

⁸¹Br NQR measurements of the mixed-valence compound (NH₄)₄Sb^III Sb^VBr₁₂, which has Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra in different oxidation states in the tetragonal pseudo-K₂PtCl₆ 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^VBr₆ ^? and Sb^IIIBr₆ ^3? 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 ⁸¹Br NQR frequencies and T ₁ values for each anion indicates that the static rotation may occur for Sb^VBr₆ ^? but not for Sb^IIIBr₆ ^3? at T _c in the low temperature phase.