Structure and thermal expansion of Lu₂O₃ and Yb₂O₃ up to the melting points

Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and intense radiation encountered in nuclear reactors. Structure and thermal expansion of Lu₂O₃ and Yb₂O₃ were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb₂O₃ and Lu₂O₃ with temperature is linear within experimental error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10⁻⁶ K⁻¹ and (7.7 ± 0.6) · 10⁻⁶ K⁻¹, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb₂O₃ phase transformation to hexagonal phase prior to melting.