High-Pressure Synthesis, Crystal Structures, and Properties of Perovskite-like BiAlO₃ and Pyroxene-like BiGaO₃

New oxides, BiAlO 3 and BiGaO 3, were prepared using a high-pressure high-temperature technique at 6 GPa and 1273−1473 K. BiAlO 3 is isotypic with multiferroic perovskite-like BiFeO 3 and has octahedrally coordinated Al 3+ ions. Structure parameters of BiAlO 3 were refined from laboratory X-ray powder diffraction data (space group R 3 c; Z = 6; a = 5.37546(5) Å and c = 13.3933(1) Å). BiGaO 3 has the structure closely related to pyroxene-like KVO 3 . Structure parameters of BiGaO 3 were refined from time-of-flight neutron powder diffraction data (space group Pcca; Z = 4; a = 5.4162(2) Å, b = 5.1335(3) Å, and c = 9.9369(5) Å). The GaO 4 tetrahedra in BiGaO 3 are joined by corners forming infinite (GaO 3 ) 3 - chains along the a axis. Bi 3+ ions in BiGaO 3 have 6-fold coordination. Both BiAlO 3 and BiGaO 3 decompose at ambient pressure on heating above 820 K to give Bi 2 M 4 O 9 and Bi 25 MO 39 (M = Al and Ga). Vibrational properties of BiAlO 3 and BiGaO 3 were studied by Raman spectroscopy. In solid solutions of BiAl 1- x Ga x O 3, a C -centered monoclinic phase structurally related to PbTiO 3 with lattice parameters of a = 5.1917(4) Å, b = 5.1783(4) Å, c = 4.4937(3) Å, and β = 91.853(3)° was found.