Experimental thermodynamics of high pressure rare earth sesquioxides

Rare earth sesquioxides (RE₂O₃) are commonly used in dielectrics, catalysis, and barrier coating applications. Of these, Dy₂O₃ is used in magneto-optical materials and radiation shielding, and Ho₂O₃ is used in lasers and optical coatings. The rare earth sesquioxides undergo structural changes under high pressure and temperature. For Y₂O₃ and RE₂O₃ containing RE heavier than Nd, the ambient cubic C-type phase transforms into the monoclinic B-type phase at high pressure and temperature. At ambient conditions, the monoclinic phase is metastable, possessing unique physical properties that may make it appealing for new technological applications. However, only limited experimental thermodynamic data are available for the B-type RE₂O₃, which are needed for a complete understanding of their stability.

We present experimental measurements of the energetics of the C → B transition in Dy₂O₃ and Ho₂O₃. For both materials, the phase transformation was driven at 4 GPa pressure and 1100 °C in a multianvil press, and energetics were studied with high temperature oxidative melt solution calorimetry with complementary differential scanning calorimetry. Measured transformation enthalpies from drop solution calorimetry were 7.76 ± 2.64 kJ/mol for Dy₂O₃ and 9.88 ± 4.07 kJ/mol for Ho₂O₃, in agreement with literature estimates. Dy₂O₃ reverts to the cubic phase on heating at 600 °C, higher than the values observed for Y₂O₃ and Er₂O₃. Decomposition conditions and P-T boundaries for the C → B transition determined from experimental reaction enthalpies will be reported for all four materials. Results of current work on Gd₂O₃ and preliminary results on other RE₂O₃ will also be presented, enabling evaluation of systematic variation in transformation energetics of RE₂O₃.