Phase relations in the MgO-Ga2O3-B2O3 system and the crystal structure of MgGaBO4

In this paper, the subsolidus phase relations of the ternary system MgO-Ga2O3-B2O3 were investigated. There are five binary compounds and one ternary compound. The ternary phase diagram consists of eight ternary phase regions. The ternary compound, GaMgBO4, is isostructural to Y2BeO4. It crystallizes in the orthorhombic system with the space group Pnam and lattice parameters a=9.1847(1) Angstrom, b=9.2942(1) Angstrom, c=3.0779(1) Angstrom, Z=4 and rho (cal.) = 4.267 g/cm(3). The Mg2+ and Ga3+ ions are distributed between the two cation sites: 77.8% Ga3+ and 22.2% Mg2+ occupy the cation site I, others (22.2% Ga3+ and 77.8% Mg2+) occupy the cation site II. In this structure the Mg2+ and Ga3+ ions are surrounded octahedrally by oxygen atoms, the berate ion is a single triangle (BO3). The most important feature of this structure is the way these octahedra are linked together by sharing edges to form ribbons, which are one octahedron thick and four octahedra wide in the ab plane and extend infinitely along the direction of the c axis. The XRD results show no phase transition occurs between -173 degreesC and 227 degreesC. The lattice parameters and the volume expansion coefficient were calculated. (C) 2001 Elsevier Science B.V. Al rights reserved.