THERMODYNAMICS OF HIGH PRESSURE RARE EARTH OXIDES AT FORCE AND MOTU

High pressure research is a major emphasis at Arizona State University. The Facility for Open Research in a Compressed Environment (FORCE) houses a suite of high pressure instruments previously unavailable in the western hemisphere, including a 6000 ton large volume press, a DIA type press capable of pressures up to 70 GPa, an internally heated gas vessel for very large volume experiments up to 0.6 GPa, and a 6 GPa torsional press. FORCE is complemented by the Navrotsky laboratory (Thermotu) which is part of the Center for Materials of the Universe (MotU), and provides unparalleled access to experimental thermochemistry, including high temperature melt solution calorimetry, differential thermal analysis, fast-scanning chip calorimetry, and a 10 MPa high pressure calorimeter. These complementary facilities enable synthesis, characterization, and thermochemical measurements of high pressure materials. We present an overview of FORCE and Thermotu, with recent results from work on rare earth oxides at high pressure, an important class of materials with applications from minerals to ceramics to semiconductors.

Rare earths usually form sesquioxides at ambient conditions, several of which transform from cubic to monoclinic (C to B type) structures at high P and T. Monoxides of rare earths from La to Sm, and Yb, have also been synthesized at similar conditions. We used a combination of computation (DFT), high P-T synthesis in a multi-anvil press, and experimental thermochemistry to predict, synthesize, and characterize B type Er₂O₃ and Y₂O₃, and rare earth monoxides YO and NdO. Monoclinic Y₂O₃ and Er₂O₃ were synthesized at 4 GPa and 1400 °C and 1100 °C, respectively. Y₂O₃ exhibits a sluggish decomposition, but the experimental transition enthalpy of –21.63 ± 1.77 kJ/mol, agrees with previous measurements and DFT. For B type Er₂O₃, the measured transition enthalpy of –11.76 ± 1.47 kJ/mol agrees well with values from DFT. The experimental reaction enthalpy for Y + Y₂O₃ = 3YO (synthesized at 15 GPa and 1600 °C) is 35.7 kJ/mol ± 5 %, which agrees with the DFT value of 32.7 kJ/mol. For the reaction Nd + Nd₂O₃ = 3NdO (synthesized at 5 GPa and 1200 °C), the reaction enthalpy was 27.7 ± 8.65 kJ/mol. From these results, P-T slopes were calculated for both YO and NdO. Our results illustrate the range of exciting new science which can exist at the intersection of high pressure and experimental thermodynamics.