Crystal Growth, Crystal Structure, Optical Properties, and Phase Transition of BaCaBO₃F

Abstract

A crystal of BaCaBO3F was grown by the Czochralski method. It shows a phase transition at 242.7 °C upon cooling where the hexagonal high-temperature modification is transformed to a monoclinic low-temperature form (space group C2, a = 17.8779(8) Å, b = 9.0596(4) Å, c = 12.9735(5) Å, β = 118.875(13)°, V = 1840.03(22) Å3, Z = 18). The monoclinic phase has a symmetry lowered by an index of 18 relative to the hexagonal supergroup. This represents a higher symmetry than the rhombohedral model which has a symmetry of index 36 lower than the hexagonal form determined in a previous study (Li and Zeng, J. Cryst. Growth 2013, 382, 47). The monoclinic system is confirmed by optical investigations with a polarizing microscope showing biaxial negative character and refractive indices nx = 1.615(1), ny = 1.664(1), and nz = 1.665(1), 2Vx = 10.0(2)° as measured on a microrefractometer spindle stage in immersion liquids. Electron microprobe analysis yielded a slight excess of Ba resulting in the composition Ba1.04Ca0.96BO3F corresponding to [7]Ba4.7[8]Ba6[9]Ba4[10]Ba4[7]Ca17.3[3]B18O54F18 in a crystal-chemical notation for the unit-cell content with the cations in their respective coordination. On the basis of this composition, the mean refractive index <n> can be calculated from individual electronic polarizabilities of the ions resulting in <ncalc> = 1.638 which is close to the observed value of <nobs> within an error of 0.5%