Phase equilibria and ordering in the system HfO2-Yb2O3

The system HfO₂-Yb₂O₃ was investigated in the 0 to 100 mol % Yb₂O₃ range using X-ray diffraction analysis, linear thermal expansion measurements and melting point studies. At high temperatures, the system is dominated by wide regions of solid solutions based on HfO₂ and Yb₂O₃ separated by a two-phase field which appears to extend to the solidus. The extent of the cubic hafnia and ytterbia C-type solid solution fields was established using the precision lattice parameter method. At low temperature (< 1800° C) two ordered phases were found in the system, one at 40 mol % ytterbia with ideal formula Yb₄Hf₃O₁₂, and another at 70 mol % ytterbia with formula Yb₆HfO₁₁. Four eutectoid reactions and a peritectic reaction cubic ytterbia solid solution cubic hafnia solid solution + liquid at 67 mol % and 2380° C have been established in the system. By incorporating the known tetragonal-cubic hafnia and C-type-hexagonal ytterbia transition temperatures, and the melting points data in the system, a tentative phase diagram is given for the system HfO₂-Yb₂O₃.