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.