PHASE RELATIONS AND ORDERING IN THE SYSTEM ERBIA‐HAFNIA

The system erbia-hafnia was investigated using thermal expansion measurements and room-temperature X-ray diffraction of quenched and annealed samples. The range of existence of the solid solutions based on HfO2 and Er2O3 was determined by the precision lattice parameter method. Three compounds with hexagonal structure are reported: the first is an ordered fluorite-related phase with the ideal formula Er4Hf3O12 and decomposes with increasing temperature at ∼1500°C into a solid solution of the fluorite type; the second is formed at ∼55 mol% Er2O3, corresponding closely to the formula Er5Hf2O11.5, and undergoes a transformation at ∼1650°C into cubic solid solutions to fluorite and C type. The third compound is formed within the concentration range between 60 and 90 mol% Er2O3. This compound, Er6HfO11, decomposes at ∼1700°C into a cubic solid solution of the C type. A proposed phase diagram for the system erbia-hafnia is presented.