Phase Relations and Thermal Expansion in the System HfO2‐TiO2

The system HfO2‐TiO2 was investigated in the 0 to 60 mol% TiO2 region using X‐ray diffraction analysis, differential thermal analysis, melting‐point studies, and dilatometry. For samples quenched from 1500° and 1250°C, single‐phase HfTiO4 is present ∼36 to 53% TiO2, with HfO2 coexisting as a second phase below 36% TiO2 and TiO2 coexisting as a second phase above 53% TiO2. Room‐temperature lattice parameters of the hafnium titanate phase decreased linearly with composition for samples quenched from 1500°C and furnace‐cooled from 1600°C. High‐temperature lattice parameter studies confirmed the expansion anisotropy of the hafnium titanate phase in the single‐phase region. Linear thermal‐expansion measurements revealed very low coefficients (< 1 × 10−6/°C) for compositions in the 30 to 40% TiOs range and relatively high coefficients (∼4× 10−6/°C) for the 25, 45, 50, and 60 mol% TiO2 compositions. The low expansion was attributed to microcracking.