Transformation of a layered perovskite to a defect perovskite via cooperative Li-insertion and O/N substitution

Abstract

Defect perovskite oxynitrides containing lithium on the octahedral sites, i.e., Sr_1−x(Li_xTa_1−x)(O_{3.5−3x−3y}N_2y), were produced by the ammonolytic heating of a layered perovskite, Sr₂Ta₂O₇, with Li₂CO₃. The above phase evolution involved the thermal diffusion of Li⁺ into the Sr₂Ta₂O₇ lattice and concurrent 3O²⁻ → 2N³⁻ substitution. The concentrations of Li, N, and anion vacancies in the product oxynitride could be controlled by adjusting the Li₂CO₃ to Sr₂Ta₂O₇ ratio in the reactant mixture. This study identified three different compositions, Sr_0.92Li_0.08Ta_0.92O_1.91N_0.89, Sr_0.83Li_0.17Ta_0.83O_1.88N_0.74 and Sr_0.73Li_0.27Ta_0.73O_1.82N_0.58, where an increase in the Li composition accompanied decreases in the a- and c-parameters of the tetragonal cell, along with an increase in the band gap. Neutron diffraction and solid state ⁷Li nuclear magnetic resonance spectroscopy of Sr_1−x(Li_xTa_1−x)(O_{3.5−3x−3y}N_2y) showed that Li occupied the octahedral site, and these oxynitrides exhibited enhanced bond covalency compared to similar oxides.