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

Thermal expansion of an EuF2.136 nonstoichiometric crystal with the fluorite structure type (Eu0.8642+Eu0.1363+F2.136, 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 Eu2+ ions. It is established by differential scanning calorimetry that the onset temperature of EuF2 + 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 Eu1– y3+Euy2+F3–y solid solution with the structure types of tysonite (LaF3), orthorhombic β-YF3 phase, and europium oxyfluorides of variable composition EuO1–xF1 + 2x, with dominance of the latter.