Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates

Using X‐ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO 4 , Hf 1.26 Ti 0.74 O 4 , and ZrTiO 4 (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO 4 is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are α a =+(8.7±0.5)×10 −6 °C −1 , α b =−(5.2±0.5)×10 −6 °C −1 , and α c =+ (5.3±0.5)×10 −6 °C −1 . The thermal expansion of Hf 1.26 Ti 0.74 O 4 was similar to that of HfTiO 4 but that of ZrTiO 4 was markedly less anisotropic. The compressibilities of HfTiO 4 and ZrTiO 4 also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high‐pressure phase having the monoclinic baddeleyite (ZrO 2 ) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.