The System HfO ₂ ‐TiO ₂

The system HfO 2 ‐TiO 2 was studied in the 0 to 50 mol% TiO 2 region using X‐ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO 2 was found at 1750°± 20°C. The definite compound HfTiO 4 melts incongruently at 1980°± 10°C, 53 mol% TiO 2 . A metatectic at 2300°± 20°C, 35 mol% TiO 2 was observed. The eutectoid decomposition of HfO 2,ss ) ( T ) → HfO 2,ss ( M ) + HfTiO 3 4,ss ss occurred at 1570°± 20°C and 22.5 mol% TiO 2 . The maximum solubility of TiO 2 in HfO 2,ss ,( M ) is 10 mol% at 1570°± 20°C and in HfO 2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO 2 ‐rich side and in the 10 to 30 mol% TiO 2 range a second monoclinic phase M of HfO 2 ( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO 2 region, which are based on thermal analysis techniques.