Ternary crystals Sr1−yEuy−x2+Eux3+F2+x of fluorite phases with a variable europium valence and their thermal conductivity (50–300 K)

Abstract

Crystals of new fluorite materials: ternary nonstoichiometric Sr_0.9Eu_0.086Eu_0.014F_2.014 and Sr_0.5Eu_0.432Eu_0.068F_2.068 phases with a structure of fluorite (CaF₂) type, containing europium ions in different oxidation states (Eu²⁺, Eu³⁺), have been grown for the first time from a melt by the Bridgeman method. The variation in their thermal conductivity k(T) in a temperature range of 50–300 K has been experimentally studied. The equivalent cation positions with a large difference in cation masses (m_Sr = 87.62 amu and m_Eu = 151.96 amu) lead to low k(T) values of ternary (Sr_1-yEu_y-x²⁺Eu_x3+F_2+x) and binary (Eu_0.864²⁺Eu_0.136³⁺F_2.136) crystals. One can obtain crystals with thermal characteristics specified in a wide range by adjusting the ratio of strontium and europium (regardless of the valence). Since the thermal conductivity of Sr_0.5Eu_0.432Eu_0.068F_2.068 and Eu_0.864Eu_0.136F_2.136 is independent of temperature in the range of 200–300 K, they can be recommended as reference materials with k(T) = const.