Crystallo-chemical evolution of the La 2ZnTiO 6 double perovskite upon reduction: A structural study

Abstract

A polycrystalline stoichiometric La 2ZnTiO 6 double-perovskite oxide, white in color, has been prepared by soft-chemistry procedures, followed by annealing in air at 1150 °C. This oxide has been treated in an H 2/N 2 flow at 900 °C to obtain a reduced specimen, blue in color, with a La 2Zn 1− x Ti 1+xO 6 composition, as demonstrated in a structural studied conducted from neutron powder diffraction data, very sensitive to the contrast between Zn and Ti. Both perovskites crystallize in the monoclinic P2 1/n space group, with unit-cell parameters related to a 0 (ideal cubic perovskite, a 0 ≈ 3.8 Å) as a ≈ 2a 0, b ≈ c ≈ √2 a 0 . Both oxides are fully oxygen-stoichiometric, as demonstrated from the refinement of the oxygen occupancy factors. The sample treated in air presents an anti-site Zn/Ti disordering of about 10%, meaning that 10% of Ti atoms occupy the 2 c Zn positions and viceversa. However, for the reduced sample, the amount of Zn at 2 d Ti sites is only 7.6(2)%, whereas there is 10.8(2)% of Ti at the Zn sites. This means that the reduced sample is, globally, Zn defective, with a Ti/Zn ratio of 1.066 and a composition La 2Zn 0.968Ti 1.032O 6. This fact implies the partial reduction of Ti 4+ to Ti 3+ upon ZnO segregation from the crystal structure. A certain electronic conductivity would be expected from the presence of Ti 3+–Ti 4+ mixed valence, although it could not be measured due to the reactivity of the reduced perovskite in the usual laboratory conditions. The magnetic susceptibility indicates a paramagnetic behavior, thus confirming the presence of Ti 3+ ions in the structure.