The orthorhombic and rhombohedral phases of - a neutron powder diffraction study

Neutron powder diffraction has been used to examine the crystal structure of the perovskite , at room temperature, and then at elevated temperatures to 1673 K. The orthorhombic structure at room temperature ( Pbnm , a = 5.5245(1) Å, b = 5.4922(1) Å, c = 7.7740(2) Å) was confirmed. At temperatures from about 420 K the structure is rhombohedral, in space group . The rhombohedral structure is characterized by rotation of the oxygen-atom octahedra about the threefold axis, and compression of these octahedra parallel to the same axis. The rotation angle varies from 11.0 to , and the octahedral strain parameter (a measure of the compression) from 0.969 to 0.983 over the temperature range of this study. The question as to whether the two effects are coupled is considered but not yet resolved.