Energetics of high pressure monoclinic Y2O3 and Er2O3 from experiment and computation

Many rare earth sesquioxides (RE2O3) undergo a series of structural transitions at high temperature and pressure. The high pressure phases are metastable at ambient conditions and exhibit different physical properties from their corresponding low pressure phases, making them candidates for new technological applications. Despite this potential, there is little experimental data available on the energetics of the high P - T forms of these materials, which is necessary to understand their stability. We used high P - T conditions to drive the C -> B transition in Y2O3 and Er2O3, conducted first principles calculations, and used high temperature oxide melt solution and scanning calorimetry to explore the fundamental energetics of the transformation reactions in both materials. The reaction enthalpy of the cubic C-type -> monoclinic B-type transformation from oxide melt solution calorimetry is 15.57 +/- 4.50 kJ/mol for Y2O3 and 15.89 +/- 4.00 kJ/mol for Er2O3. These values are similar to those predicted by density functional theory for both materials and to the transition enthalpy for Er2O3 determined by differential scanning calorimetry. For Y2O3, calorimetry with complementary in situ high temperature X-ray diffraction measurements indicate a sluggish reversal to the low pressure phase over an 873 K temperature interval when heated, suggesting that kinetics control the decomposition process. Ultimately both Y2O3 and Er2O3 revert to the cubic phase when heated to 623 - 723 K. This work provides experimental data on the energetics of the transformation from cubic to monoclinic Y2O3 and Er2O3. Similar data on other rare earth oxides and higher pressure phases of all RE2O3 are needed so that comprehensive stability models can be developed and implemented to support new applications.