Thermal Expansion of EuF 2 + x Single Crystals and Their Thermal Stability

摘要

Thermal expansion of an EuF~(2.136)nonstoichiometric crystal with the fluorite structure type (Eu ~(0.864)_(2+) Eu ~(0.136)_(3+) F~(2.136), lattice parameter 5.82171(5) ?) has been experimentally investigated in the temperature range of 9–500 K. The coefficient of thermal expansion is α = 15.8 × 10_(–6)K_(–1)at T = 300 K. The observed anomalies in the behavior of the coefficient of thermal expansion at T > 400 K are related to the oxidation processes with partition of Eu_(2+)ions. It is established by differential scanning calorimetry that the onset temperature of EuF~(2 + x )oxidation in air is 430 K and that this process occurs in three stages. X-ray diffraction analysis shows that the oxidation is accompanied by the formation of a phase mixture based on two modifications of the Eu ~(1– y )_(3+) Eu ~( y )_(2+) F~(3– y )solid solution with the structure types of tysonite (LaF~(3)), orthorhombic β-YF~(3)phase, and europium oxyfluorides of variable composition EuO~(1– x )F~(1 + 2 x ), with dominance of the latter.