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 degrees C, with enthalpies of fusion 59 +/- 9 and 69 +/- 8 kJ/mol at 2419 +/- 12 and 2416 +/- 20 degrees 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) x 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 degrees 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.