Valence variations in titanium-based perovskite oxides by high-pressure and high-temperature method

Abstract

Typical titanium-based perovskite oxides Eu1−xBaxTiO3 (x = 0.6−0.8), Eu1−xKxTiO3 (x = 0.2,0.32), and La0.7 (Na,K)0.3TiO3 were synthesized by high pressure and temperature using RE2O3 (RE = La,Eu), TiO2, alkaline, or alkaline earth carbonates as the starting materials. X-ray diffraction data analysis showed that there was a structural transformation in Eu1−xBaxTiO3 by varying Ba content [i.e., from cubic (x = 0.6,0.7) to tetragonal (x = 0.8)], and that samples Eu1−xKxTiO3 and La0.7(Na,K)0.3TiO3 crystallized in the cubic perovskite structure. 151Eu Mössbauer spectroscopy and electron paramagnetic resonance measurements revealed mixed valence of Eu2+/Eu3+ in samples Eu1−xBaxTiO3 and Eu1−xKxTiO3, while Ti ions were present in pure Ti4+ state. Cubic Eu1−xKxTiO3 was metastable, which decomposed into a mixture of perovskite and pyrochlore phases at high temperatures as accompanied by an oxidation process from Eu2+ to Eu3+. For samples La0.7 (Na,K)0.3TiO3, Ti3+ signals were clearly observed. The reduction mechanisms for Eu ions at A site and Ti ions at B site in the perovskite oxides are discussed in terms of the chemical nature of the framework ions and substitution ions under high pressure and temperature.