Computationally Led High Pressure Synthesis and Experimental Thermodynamics of Rock Salt Yttrium Monoxide

Yttrium monoxide (YO) is a possible member of a large family of rare earth monoxides having the rock salt structure. It was predicted to be stable above 10 GPa and superconducting with higher critical temperatures at lower pressures. However, no syntheses of bulk YO have been reported. Using first-principles calculations, we predicted the stability of yttrium monoxide at pressures above 8.6 GPa and at high temperature. Guided by these predictions, we successfully synthesized bulk YO in the rock salt structure (Fm3̅m) at 15 GPa and 1600 °C. YO is very metastable (both thermodynamically and kinetically) under ambient conditions and decomposes rapidly on heating. Accordingly, this work focuses solely on theoretical prediction, high-pressure synthesis, and experimental thermodynamic measurements. Detailed structural analysis and physical property measurements will be published in future work. Our combined experimental and computational approach enabled us to obtain consistent results for the formation enthalpy and lattice constant of bulk YO. The predicted enthalpy of formation for the reaction Y + Y₂O₃ = 3YO is 32.7 kJ/mol, and experiments yield a value of 35.7 kJ/mol, with an estimated uncertainty of ±5%. YO in the rock salt structure has a refined lattice constant of 4.878 ± 0.010 Å and a molar volume of 17.47 ± 0.11 cm³ mol^–1. From these, we calculated the entropy and P–T slope of the reaction. Through this comprehensive investigation, we explored the synthesis and decomposition of a challenging metastable phase, which is stabilized under high pressure conditions. Moreover, we have gained valuable insights into the thermodynamics and physical properties of YO. These findings highlight the importance of leveraging pressure as an additional dimension in materials synthesis and underscore the potential of using first-principles calculations to guide experiments involving highly metastable materials.