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

Typical titanium-based perovskite oxides Eu_1-xBa_xTiO₃ (x = 0.6-0.8), EU_1-xK_xTiO₃ (x = 0.2,0.32), and La_0.7(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 EU_1-xBa_xTiO₃ by varying Ba content [i.e., from cubic (x = 0.6,0.7) to tetragonal (x = 0.8)], and that samples Eu_1-xK_xTiO₃ and La_0.7(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 Eu_1-xBa_xTiO₃ and Eu_1-xK_xTiO₃, while Ti ions were present in pure Ti⁴⁺ state. Cubic Eu_1-xK_xTiO₃ 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 La_0.7(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.