Thermal Expansion of EuF2 + x Single Crystals and Their Thermal Stability

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) Å) 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.