摘要：

The crystal structure of β‐Ga2O3has been determined from single‐crystal three‐dimensional x‐ray diffraction data. The monoclinic crystal has cell dimensionsa=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 Ga2O3in the unit cell. The most probable space group to which the crystal belongs isC2h3—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‐Ga2O3is lower than that of α‐Ga2O3which has the α‐corundum structure. Also the closest approach of two Ga3+ions in β‐Ga2O3is 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‐Ga2O3structure are discussed and it is shown that a possible Fe2O3isomorph could be expected to be at least antiferromagnetic with a Néel temperature of about 700°K Furthermore, a knowledge of the β‐Ga2O3structure 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—xFexO3recently discovered by Remeika.

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