Glances at Ternary Compounds

— In the large family of ternary compounds one could expect to find systematic trends for the various physical properties. For instance the geometrical arrangement of the atoms is, in most ternaries, tetrahedral. Beyond qualitative laws very few quantitative relationships have been established between crystallographic structures and other physical and chemical properties Topics such as tetrahedral distortion, structural stability, amorphism, ionicity, anionic compounds. are reviewed. Several questions are outlined which have not yet been given satisfactory answers. Introduction. — In this opening talk of the Second International Conference on Ternary Semiconducting Compounds I would like first to recall some historical steps in the growth of our knowledge of these materials. I will then discuss several points related with the structural properties of ternary compounds and their chemical bonds. I will show that very simple questions have not yet received satisfactory answers and emphasize some problems which remain unsolved. 1. Brief historical recall [1,2]. — Elementary crystals and binary compounds exhibiting semiconducting properties have been known long time ago. The invention of the transistor in the late fourties boosted the era of semiconductors and stimulated the search for new semiconducting materials. In 1962 the discovery of laser action in semiconductors gave a new incentive to the investigation of new materials. In the early fifties it was shown by Hahn et al. [3] and by Goodman et al. [4] that the 2452 compounds were a natural extension of the 35's and that the 1362 compounds were a natural extension of the 26's. Although minerals belonging to these classes of ternary compounds were known a systematic investigation in fact started only after this pioneering work. This was a part of a systematic analysis of tetrahedral structures, primarily carried out by Parthe [1]. The Leningrad group under the late Goryunova [5] made significant contribution to the field. This group was the first to observe optical nonlinear properties [6] and to demonstrate laser action in a ternary compound [7]. More recently Chemla, Kupecek, Robertson and Smith [8 ] demonstrated phase-matched second harmonic generation in a chalcopyrite crystal. In the last five years several groups have been very active in various countries mainly on linear and nonlinear optical properties, on semiconducting properties and luminescence. Laser action has been studied in several ternary compounds by Bell Telephone Laboratories' scientists. Shay and coworkers [2] have shown that the quasi-cubic model of band structure is very adequate to derive band structure of 2452 from the band structure of 35's. They also showed that a somewhat similar situation hold for the 1362 which can be derived from the 26, if provision is made for the additional perturbation due to the d-band of the monovalent atom (Cu or Ag). 2. Valence and structure. — Most of the ternary compounds have tetrahedral structures. Fourfold coordinated compounds (binary, ternary quaternary, etc..) have a remarkable position among anorganic crystals. Parthe [1] has shown that there are precise relationship between the electronic valence of the atoms and the structure. In a tetrahedral structure each atom has four nearest neighbours which form a more or less regular tetrahedron. Two nearest neighbours are bonded by the overlap of two valence electrons' wave functions. This overlapping of sp3 hybrids accounts for the cohesive energy just like in purely covalent crystals, like diamond, silicon and germanium. Besides these normal tetrahedral structures one can consider the bond defect tetrahedral structures in which some atomic sites are JOURNAL DE PHYSIQUE Colloque Ci, supplément au n° 9, Tome 36, Septembre 1975, page C3-1