Ti02-Doped Zirconia: Crystal Structure, Monoclinic-Tetragonal Phase Transition, and the New Tetragonal Compound Zr3Ti08

Abstract

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.