TiO₂-doped zirconia: Crystal structure, monoclinic-tetragonal phase transition, and the new tetragonal compound Zr₃TiO₈

Zirconia samples with up to 27 mol% TiO₂ were synthesized at low and elevated pressures, significantly extending the range of ZrO ₂-TiO₂ solid solution compared with previous crystal structure studies of monoclinic and tetragonal zirconia (<17.5 mol%). Crystal structure data collected by powder X-ray diffraction (XRD) at ambient conditions reveal that the solid solution of TiO₂ in monoclinic zirconia (baddeleyite structure, P₂₁/c) is limited to about 25 mol%, where the tetragonal compound Zr₃TiO₈ forms. Changes in the monoclinic crystal structure with increasing TiO₂ can be understood in the light of the nearing monoclinic-tetragonal phase change. Unit-cell dimensions a and b become near identical, cell angle β starts to drop toward 90°, and atom positions start to change toward those of the tetragonal cell. The cation coordination polyhedron in the monoclinic structure becomes increasingly distorted, and bond-valence sums worsen. The transition from the monoclinic to the tetragonal lattice with increasing TiO₂ is not smooth however, but shows a significant jump in many of the parameters, in line with the first-order character of this transition. The previously unknown inorganic compound Zr₃TiO₈ has a structure related to that of tetragonal zirconia (P4₂/nmc), whereby weak superstructure reflections indicate that the structure is ordered and has a doubled c-dimension. Zr₃TiO₈ has the unit-cell dimensions a = 5.0317(4) Å and c = 10.4273(7) Å, space group (I-42m), and is isostructural with the compound Zr₃GeO₈, with Ti ⁴⁺ in tetrahedral coordination and Zr⁴⁺ in eightfold coordination. The stabilization of tetragonal Zr₃TiO₈ at ambient conditions depends strongly on the cooling rate, which has to be slow enough (e.g., 1°C/min) to allow for ordering to occur. Rapid quenching (e.g., ∼140°C/s) of the same composition results in monoclinic zirconia. The lowering of the tetragonal-monoclinic phase transition to ambient conditions near 25 mol% TiO₂ is in good agreement with extrapolations from previous studies. The relatively coarse grain size (< 20 μm) of the Zr₃TiO₈ crystals suggest that the tetragonal structure was not retained because of grain-size effects, but was stabilized predominantly by crystal structure adjustments based on cation size. Broadening and shortening of selected XRD peaks especially upon grinding suggests that tetragonal Zr₃TiO₈, depending on cooling rate and degree of ordering, may be retained metastably at room temperature, and is thus a potential candidate for TZP ceramics.