Growth and X-ray diffraction study of YCa4O(BO3)(3): Ce,Er,Yb crystals

YCa4O(BO3)(3) crystals (sp. gr. Cm), nominally pure and activated with Yb, Er, and/or Ce, were grown by the Czochralski technique. The crystal structu re of YCa4O(BO3)3 was refined by single-crystal x-ray diffraction, and the site compositions were determined: (Y0.792Ca0.208(6))(Ca0.938Y0.062(4))(2)C a2O0.96(2)[](0.04)(BO3)(3) ([] = vacancy). The data set for YCa4O(BO3)(3) w as found to contain additional reflections which were inconsistent with spa ce group Cm but could be indexed in a cell with doubled a. No superlattice reflections were detected in the case of the activated crystals. Structural data for LnCa(4)O(BO3)(3) were used to derive relations for the lattice pa rameters and average cation-anion distances as functions of r(Ln) and weigh ted-average cation radii on the mixed-occupancy sites, which made it possib le to assess the distributions of Y, Yb, Er, Cc, and Ca over the mixed-occu pancy sites. Absolute structure determination showed that the pure and Yb3-activated YCa4O(BO3)(3) crystals differed in polarity.