Binary and Pseudobinary Invar Materials based on Intermediate Valence Compounds

A new type of binary and pseudobinary invar materials based on intermediate valence samarium compounds Sm0.8B6, Sm1 – xLaxB6 (x = 0, 0.1, 0.22, and 0.5), Sm0.67Y0.33S, Sm0.55Y0.45S, and Sm2.75C60 are developed. The Weiss two-level invar model is used to describe the anomalous contribution to the coefficient of thermal expansion (CTE) for Sm0.8B6 and Sm1 – xLaxB6 (x = 0, 0.1, 0.22, and 0.5). The minimum position of the CTE of samarium hexaboride as a function of the degree of lanthanum doping is determined. The obtained dependence allows one to predict the CTE minimum position for Sm1 – xLaxB6 compounds in the temperature range from 40 to 180 K and to optimize the developed composite invar materials, where the positive CTE of the functional material is compensated by the negative CTE of the valence-unstable system.