Crystal Structure of β-Ga2O3

The crystal structure of β-Ga2O3 has been determined from single-crystal three-dimensional x-ray diffraction data. The monoclinic crystal has cell dimensions a=12.23±0.02, b=3.04±0.01, c=5.80±0.01 A and β=103.7±0.3° as originally reported by Kohn, Katz, and Broder [Am. Mineral. 42, 398 (1957)]. There are 4 Ga2O3 in the unit cell. The most probable space group to which the crystal belongs is C2h3—C2/m; the atoms are in five sets of special positions 4i: (000, ½½0)±(x0z). There are two kinds of coordination for Ga3+ ions in this structure, namely tetrahedral and octahedral. Average interionic distances are: tetrahedral Ga–O, 1.83 A; octahedral Ga–O, 2.00 A; tetrahedron edge O–O, 3.02 A; and octahedron edge O–O, 2.84 A. Because of the reduced coordination of half of the metal ions, the density of β-Ga2O3 is lower than that of α-Ga2O3 which has the α-corundum structure. Also the closest approach of two Ga3+ ions in β-Ga2O3 is 3.04 A which is considerably larger than the closest approach of metal ions in the sesquioxides with the α-corundum-type structure and, in agreement with the results of thermodynamic measurements, the β phase appears to be the structurally more stable one. The average Ga–O distances in the structure seem to account for the fact that although the Ga3+ ion is substantially larger than the Al3+ ion its quantitative preference for tetrahedrally coordinated sites when substituted for Fe3+ ion in the iron garnets is very nearly the same as that of the Al3+ ion. The structure accounts for a recent result obtained by Peter and Schawlow from paramagnetic-resonance measurements on Cr3+-ion-doped β-Ga2O3, namely that the Cr3+ ion substitutes for the Ga3+ ions in a single set of equivalent octahedral sites. The magnetic aspects of the β-Ga2O3 structure are discussed and it is shown that a possible Fe2O3 isomorph could be expected to be at least antiferromagnetic with a Néel temperature of about 700°K Furthermore, a knowledge of the β-Ga2O3 structure and of the nature of site preferences of the Ga3+ and Fe3+ ions in the garnets lead to a prediction regarding the structure of the ferrimagnetic crystals of formula Ga2—xFexO3 recently discovered by Remeika.