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.