Neutron powder diffraction study of the low-temperature phases of

Abstract

We made neutron powder diffraction measurements for phase III (at 4, 120, 180, 220, and 250 K) and phase II (at 280 K) of . The structure of phase III was determined by the Rietveld method using the data obtained at 4 K; the initial structure parameters were obtained by a systematic trial-and-error method. Phase III is monoclinic (space group: ) with a = 4.4010(2) Å, b = 9.5967(3) Å, c = 6.9817(2) Å, , and Z = 4. Both and ions are located at general positions, and the orientations of the ions are completely ordered. The structures of phase III at the higher temperatures were also examined on the basis of the structure at 4 K. The atomic displacement parameters of the ions increased anomalously with increasing temperature, indicating gradual but remarkable development of the orientational disorder of ions. The structure of phase II was determined to be rhombohedral (space group: ) with a = 5.027 90(4) Å, c = 10.055 87(8) Å, and Z = 3. The orientations of the ions were highly disordered about the axis.