High-pressure synthesis and characterization of the alkaline earth borate beta-BaB_4O_7

摘要

The new orthorhombic barium borate beta-BaB_4O_7 was synthesized under high-pressure/high-temperature conditions in a Walker-type multianvil apparatus at 7.5 GPa and 1100 deg C, starting from stoichiometric mixtures of the binary oxides. beta-BaB_4O_7 crystallizes in space group Pmnb with Z = 2 and lattice parameters a = 1099.4(2), b = 901.7(2), c = 430.73(9) pm, R1 = 0.0199, and wR2 = 0.0406 (all data). The network-structure is built up exclusively from BO_4-tetrahedra, linked via common corners. Its structural differences to the ambient-pressure phase alpha-BaB_4O_7 and structural agreements with the isotypic high-pressure phases beta-MB_4O_7 (M = Ca, Hg, Sn) and the ambient-pressure phases MB_4O_7 (M = Sr, Pb, Eu) are discussed. beta-BaB_4O_7 and a hypothetical BaB_4O_7 in the beta-MB_4O_7 (M = Ca, Hg, Sn) structure were studied as high-pressure phases of alpha-BaB_4O_7, using density functional calculations. The transition pressure of alpha-BaB_4O_7 into the structure of beta-BaB_4O_7 was calculated to 1.5 GPa; the transition pressure of beta-BaB_4O_7 into BaB_4O_7 in the beta-MB_4O_7 (M = Ca, Hg, Sn) structure to 7.5 GPa.