Stability, cation ordering and oxygen non-stoichiometry of some perovskites and related layered oxides

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Matti Lehtimäki Name of the doctoral dissertation Stability, cation ordering and oxygen non-stoichiometry of some perovskites and related layered oxides Publisher School of Chemical Technology Unit Department of Chemistry Series Aalto University publication series DOCTORAL DISSERTATIONS 118/2013 Field of research Inorganic Chemistry Manuscript submitted 8 April 2013 Date of the defence 16 August 2013 Permission to publish granted (date) 18 June 2013 Language English Monograph Article dissertation (summary + original articles) Abstract Perovskites and perovskite-related layered oxides have been widely studied for various applications due to their interesting physical and chemical properties. This thesis investigates the thermal stability, cation ordering and oxygen non-stoichiometry of several perovskite-related materials. Effects of various cation substitutions on the cation ordering, thermal stability and physical properties of Sr2MgMoO6 − δ double perovskite were investigated. Replacing Mg with transition metal atoms enhanced the conducting properties of the phase but depressed its thermal stability. Substitution of Mo with Nb or W retained the good thermal stability of the parent phase but the conducting properties became significantly weaker. An entirely new series of double-perovskite derived layered transition metal oxides, (Sr1 − xLax)3(Mg1/3 + x/2Nb2/3 − x/2)2O7 (0.15 ≤ x ≤ 0.33), was synthesised. Like in the double perovskites, the Mg and Nb atoms were found to be ordered in these new Ruddlesden–Popper structured phases too. Many layered transition metal oxides have been found to react with atmospheric moisture even at room temperature forming new layered compounds. In this work the effect of oxygen non-stoichiometry on the water reactivity of An + 1BnO3n + 1 − δ Ruddlesden–Popper compounds was studied. A novel categorisation based on the crystal structure of the derivative phases was established, and the main factors affecting the reactivity were determined to be the A-site cation composition and oxygen stoichiometry of the compound. Reversible low-temperature oxygen absorption and desorption is needed in many important applications such as three-way catalysts. The layered perovskite-derived phases, Pb2CuSr2RCu2O8 + δ, are promising oxygen-storage materials as they have been found to exhibit large oxygen absorption and desorption at relatively low temperatures. In this work it was shown that both the oxygen-storage capability and phase stability can be influenced by the selection of the rare-earth constituent R.Perovskites and perovskite-related layered oxides have been widely studied for various applications due to their interesting physical and chemical properties. This thesis investigates the thermal stability, cation ordering and oxygen non-stoichiometry of several perovskite-related materials. Effects of various cation substitutions on the cation ordering, thermal stability and physical properties of Sr2MgMoO6 − δ double perovskite were investigated. Replacing Mg with transition metal atoms enhanced the conducting properties of the phase but depressed its thermal stability. Substitution of Mo with Nb or W retained the good thermal stability of the parent phase but the conducting properties became significantly weaker. An entirely new series of double-perovskite derived layered transition metal oxides, (Sr1 − xLax)3(Mg1/3 + x/2Nb2/3 − x/2)2O7 (0.15 ≤ x ≤ 0.33), was synthesised. Like in the double perovskites, the Mg and Nb atoms were found to be ordered in these new Ruddlesden–Popper structured phases too. Many layered transition metal oxides have been found to react with atmospheric moisture even at room temperature forming new layered compounds. In this work the effect of oxygen non-stoichiometry on the water reactivity of An + 1BnO3n + 1 − δ Ruddlesden–Popper compounds was studied. A novel categorisation based on the crystal structure of the derivative phases was established, and the main factors affecting the reactivity were determined to be the A-site cation composition and oxygen stoichiometry of the compound. Reversible low-temperature oxygen absorption and desorption is needed in many important applications such as three-way catalysts. The layered perovskite-derived phases, Pb2CuSr2RCu2O8 + δ, are promising oxygen-storage materials as they have been found to exhibit large oxygen absorption and desorption at relatively low temperatures. In this work it was shown that both the oxygen-storage capability and phase stability can be influenced by the selection of the rare-earth constituent R. 1