Bond thermal expansion and effective pair potential in crystals: the case of cadmiumselenide

Abstract

The local dynamics of cadmium selenide (CdSe) with wurtzite structure has beeninvestigated by molecular dynamics simulations, using a many-body Tersoff potential. Theradial distribution functions (i.e., the effective pair potentials) of the first sevencoordination shells have been determined as a function of temperature, as well as theirparallel and perpendicular mean-square relative atomic displacements. The bond thermalexpansion of the first coordination shell is mainly due to the asymmetry of theeffective pair potential. In contrast, the bond thermal expansion of the outer shells ismostly due to a rigid shift of the effective pair potential. This behavior, recentlyobserved also in simple cubic monoatomic crystals, can be generalized and related tothe correlation of atomic motion. Finally, a shift toward lower values of the firstSe–Cd effective pair potential has been observed when increasing the temperature,confirming previous findings by extended x-ray absorption fine-structure measurements.Differently from superionic conductors like AgI and CuBr, in which this anomalousnegative shift was tentatively explained by cluster distortion and cation diffusion,the negative shift of CdSe is related to the peculiar properties of the crystallinepotential.