Phase relations and exsolution phenomena in the system NiOTiO2

The phase relations in the system NiOTiO₂ were studied between 1000 and 1600°C using quenched powder specimens, DTA runs, and single crystal diffusion couples. Quenching experiments establish the stable phases TiO₂ (rutile), NiTiO₃ an ilmenite structure type, Ni_2(1+x)Ti_1?xO₄ (x ≥ 0.16), a cation-excess spinel, and Ni_1?2xTi_xO (rocksalt structure type). DTA runs reveal the existence of an additional nonstoichiometric ilmenite phase Ni_1?2xTi_1+xO₃ (x ≤ 0.03) above 1260°C. In quenched (1500, 1450°C) or slowly cooled single crystal diffusion couples, mutual oriented exsolutions occur in the rutile crystal and in the ilmenite diffusion zone. Orientation relations are: {101}_rutile{1120}_ilmenite; ∥010〉_rutile∥00.1〉_ilmenite. The cation-excess spinel decomposes below 1375°C into oriented intergrowth of NiTiO₃ (ilmenite) and NiO: {111}_NiO{0001}_NiTiO3; ∥110〉_NiO∥21.0〉_NiTiO3.