High-pressure synthesis and characterization of the alkaline earth borate β-BaB4O7

The new orthorhombic barium borate β-BaB4O7 was synthesized under high-pressure/high-temperature conditions in a Walker-type multianvil apparatus at 7.5 GPa and 1100 °C, starting from stoichiometric mixtures of the binary oxides. β-BaB4O7 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 BO4-tetrahedra, linked via common corners. Its structural differences to the ambient-pressure phase α-BaB4O7 and structural agreements with the isotypic high-pressure phases β-MB4O7 (M = Ca, Hg, Sn) and the ambient-pressure phases MB4O7 (M = Sr, Pb, Eu) are discussed. β-BaB4O7 and a hypothetical BaB4O7 in the β-MB4O7 (M = Ca, Hg, Sn) structure were studied as high-pressure phases of α-BaB4O7, using density functional calculations. The transition pressure of α-BaB4O7 into the structure of β-BaB4O7 was calculated to 1.5 GPa; the transition pressure of β-BaB4O7 into BaB4O7 in the β-MB4O7 (M = Ca, Hg, Sn) structure to 7.5 GPa.

Graphical abstractβ-BaB4O7 was synthesized under high-pressure/high-temperature conditions using a multianvil high-pressure device.Figure optionsDownload full-size imageDownload as PowerPoint slide