EXTENT OF SOLID-SOLUTION IN PB-SN AND SB-BI CHALCOGENIDES

The systems of Pb-Sn and Sb-Bi chalcogenides were examined for the for mation of solid solutions. In the system PbS - PbSe - SnS - SnSe at 50 0 degrees C, both cubic (NaCl-type) and orthorhombic (GeS-type) solid solutions are extensive, whereas in the system PbSe - PbTe - SnSe - Sn Te, the cubic solid-solution is the dominant phase, and the orthorhomb ic solid-solution is limited to the SnSe comer. Between Pb-Sn sulfides and tellurides, ranges of solid solution are restricted. In the Sb-Bi chalcogenides, the systems Sb2S3 - Sb2Se3 - Bi2S3 - Bi2Se3 and Sb2Se3 - Sb2Te3 - Bi2Se3 - Bi2Te3 at 500 degrees C are characterized by exte nsive orthorhombic and hexagonal solid-solutions, respectively. Tetrad ymite is stable in the system Sb2S3 - Sb2Te3 - Bi2S3 - Bi2Te3 at 500 d egrees C. Several ranges of solid solutions that form in these systems cannot be successfully explained by using ionic size and electronegat ivity, the traditional parameters. R(pi) and R(sigma) of St. John and Bloch (1974) are used to differentiate between NaCl- and GeS-type stru ctures in the case of Pb-Sn chalcogenides and between stibnite- and te tradymite-type structures in the case of Sb-Bi chalcogenides. The two parameters are defined by r(s) and r(p), the radii of the s and p orbi tals. For a bond between atoms A and B, they can be calculated as foll ows: R(pi)(AB) = R(pi)(A) + R(pi)(B) = (r(p)(A) + r(s)(A)) + (r(p)(B) - r(s)(B)) and R(sigma)(AB) = R(sigma)(A) + R(sigma)(B) = (r(s)(A) + r (p)(A)) - (r(s)(B) + r(p)(B)).