Thermal Expansion and Enthalpies of Phase Transformation and Fusion of Er₂O₃ and Tm₂O₃ from Experiment and Computation

Erbium and thulium sesquioxides are the only lanthanide oxides that exhibit premelting cubic-hexagonal (C–H) phase transformation. The enthalpies of phase transitions and fusion were measured for the first time using a differential thermal analyzer calibrated by melting Y2O3. C–H transition enthalpies of Er2O3 and Tm2O3 are 48 ± 7 and 69 ± 7 kJ/mol at 2301 ± 10 and 2384 ± 14 °C, with enthalpies of fusion 59 ± 9 and 69 ± 8 kJ/mol at 2419 ± 12 and 2416 ± 20 °C, respectively. The mean linear thermal expansion of cubic phases up to transformation temperature was determined from synchrotron diffraction on laser heated aerodynamically levitated Er2O3 and Tm2O3 as (8.80 ± 0.02) and (8.74 ± 0.01) × 10–6/K, respectively. The experimental results were used to benchmark ab initio molecular dynamic computations which provided an isobaric heat capacity of 178 ± 5 J/mol/K for both liquid oxides at 2427–3027 °C. The comparable enthalpy values for premelting C–H transformation and fusion corroborate a high degree of dynamic disorder in the H phase suggesting superplasticity and ionic conductivity. The melting temperature for cubic Er2O3 and Tm2O3 obtained from ab initio molecular dynamic computations aligns with the experimentally measured temperature for the C–H transition.