CuSe₂, a Marcasite Type Superconductor

Abstract

CuSe₂ occurs in a room temperature form of the marcasite type¹ and a high-pressure high-temperature modification of the pyrite type². In both the marcasite and pyrite structures the cations are coordinated by six anions forming a distorted octahedron while the anions are bonded to three cations and another anion. In the pyrite structure the anion octahedra are connected by shared corners and in the marcasite structure the octahedra form chains by sharing edges. In the pyrites and the normal marcasites the deformation of the anion octahedra is not very marked. The marcasite family, however, has another branch, the loellingites, in which the deformation of the anion octahedra is much more pronounced and of opposite sign. The ratio of the octahedron edge parallel and perpendicular to the cation chains is roughly 1.1 in the normal marcasites and about 0.75 in the loellingites^3,4. Different electronic structures of the cations account for the different distortions. Loellingites are found only in non-metallic phases containing d² and d⁴ cations and distortions then result from the Jahn–Teller effect. Pyrites with dⁿ cations occur for n≧5, while normal marcasites are known only for n≧6. In both the pyrite and the normal marcasite phases non-metallic properties are possible only if the cations have completely occupied d subbands, such as dε³dγ², dε⁶, dε⁶dγ² and d¹⁰. Examples of these configurations are MnS₂, RuS₂, NiS₂ and ZnS₂(p) respectively. (The symbols (r), (h), (p) indicate room temperature, high temperature and high pressure modifications, respectively.) Because large distortions would be required to separate the energies of the degenerate dγ states, all d⁷ and d⁹ phases are metallic and adopt either the pyrite or the normal marcasite structure.