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 Y 2 O 3 . C–H transition enthalpies of Er 2 O 3 and Tm 2 O 3 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 Er 2 O 3 and Tm 2 O 3 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 Er 2 O 3 and Tm 2 O 3 obtained from ab initio molecular dynamic computations aligns with the experimentally measured temperature for the C–H transition.