Defect formation and transport in La_0.95Ni_0.5Ti_0.5O_3-δ

Deficiency in the A sublattice of orthorhombic perovskite-type La_1−xNi_0.5Ti_0.5O_3−δ, with maximum at x = 0.07–0.08, is compensated by the formation of trivalent nickel and oxygen vacancies. The atomistic computer simulations showed that these defects are trapped near the A-site cation vacancies, resulting in the stabilization of Ni³⁺ cations and low electronic and oxygen-ionic transport. The average thermal expansion coefficient of La_0.95Ni_0.5Ti_0.5O_3−δ ceramics, calculated from dilatometric data in air, increases from 8.6 × 10⁻⁶ K⁻¹ at 300–800 K to 12.0 × 10⁻⁶ K⁻¹ at 1300–1500 K. The data on Seebeck coefficient and total conductivity, predominantly p-type electronic, suggest a broadband mechanism of hole transport. The activation energies for the hole and ionic conductivities are 89 and 430 kJ/mol, respectively. The oxygen ion transference numbers determined by the faradaic efficiency measurements in air, vary in the range 9.5 × ¹⁰⁻⁵–8.1 × ¹⁰⁻⁴ at 1173–1248 K, increasing with temperature. Reducing oxygen partial pressure leads to a moderate decrease of the conductivity, followed by phase decomposition in the p(O²) range 9×¹⁰⁻¹¹ to 8× ¹⁰⁻⁹ atm at 1073–1223 K. The low-p(O₂) stability limit of La_0.95Ni_0.5Ti_0.5O_3−δ perovskite was found between that of La₃Ni₂O₇ and Ni/NiO boundary.