Compound Formation and Melting Behavior in the AL? Compound and Rare Earth Oxide Systems

The compounds BeO, AlN, and Sic (AB type) belong to the family of “GrimmSommerfeld compounds.” These compounds crystallize in the same hexagonal wurtzite (84) structure. Such a structure can be considered as being composed of two interpenetrating hexagonal closepacked sublattices, one of atoms A, the other of atoms B (I, 2). A and B form interpenetrating tetrahedra ABJ and BA4, respectively. Increasing the valency of both elements increases the covalency of the bonds in the order Be0 < AlN < Sic. This structure can be regarded as a tetrahedron network sharing corners, which leads to the possibility of forming salts such as acidic groups like SiOZ with basic metal oxides. Some crystal chemistry data of these three compounds are listed in Table I, indicating the similarities and the differences between them. In a previous paper, one of the authors (T. S. Yen) reported (3) the phase relationships in a series of the BeO-R203 systems (where R = La, Nd, Sm, Gd, Dy, Ho, Er, Yb, Lu, and Y) showing the formation of incongruently melting compounds or metastable compounds with a BeO: R203 mole ratio of 2 : 1 in all systems, and 2 : 3 in BeOLa,O, and BeO-Nd203 systems, except 2BeO * La,O,, which is congruent. Subsolidus phase relationships in the systems AlN-R203 (where R = Nd, Sm, Gd, Dy, Er, Yb, and Y) have been reported by these authors (4). The binary compounds with AlN : R203 mole ratio of 1: 1 were found in the systems containing Nd and Sm. No compound, however, was found in the systems containing rare earth oxides with cations smaller than that of Gd. The