Structure and Phase Transition of GdBO3

The crystal structure and a phase transition of gadolinium orthoborate, GdBO 3, were studied by electron diffraction and X-ray powder diffraction. GdBO 3 crystallizes in the vaterite structure (LT), in a rhombohedral space group R 32 with the lattice constants of a = 6.6357(2) Å and c = 26.706(1) Å. The structure of the LT phase was derived from the hexagonal YBO 3 structure and refined using X-ray powder diffraction data. The structure consists of tetrahedral polyborate group B 3 O 9 9 - and the gadolinium atoms, located respectively on the 3-fold screw axis and a general position. The material undergoes a first-order phase transition with a large thermal hysteresis. The high-temperature (HT) phase crystallizes in a calcite related structure with the lattice constants of a = 4.1154(2) and c = 8.592(1) Å, which consists of planer triangular borate group BO 3 3 - . The observed large thermal hysteresis of the phase transition is mainly caused by a structural change of the borate group, from B 3 O 9 9 - in the LT phase to BO 3 3 - in the HT phase.