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

Typical titanium-based perovskite oxides Eu1-xBaxTiO3 (x = 0.6-0.8), Eu1-xK xTiO3 (x = 0.2,0,32), and La-0.7(Na,K)(0.3)TiO3 were synthesized by high pr essure and temperature using RE2O3 (RE = La,Eu), TiO2, alkaline, or alkalin e earth carbonates as the starting materials. X-ray diffraction data analys is showed that there was a structural transformation in Eu1-xBaxTiO3 by var ying Ba content [i.e., from cubic (x = 0.6,0.7) to tetragonal (x = 0,8)], a nd that samples Eu1-xKxTiO3 and La-0.7(Na,K)(0.3)TiO3 crystallized in the c ubic perovskite structure. Eu-151 Mossbauer spectroscopy and electron param agnetic resonance measurements revealed mixed valence of Eu2+/Eu3+ in sampl es Eu1-xBaxTiO3 and Eu1-xKxTiO3, while Ti ions were present in pure Ti4+ st ate. Cubic Eu1-xKxTiO3 was metastable, which decomposed into a mixture of p erovskite and pyrochlore phases at high temperatures as accompanied by an o xidation process from Eu2+ to Eu3+ For samples La-0.7(Na,K)(0.3)TiO3, Ti3signals were clearly observed. The reduction mechanisms for Eu ions at A si te 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.