A primitive tetragonal intermediate in the orthorhombic–cubic phase transition of perovskite-type strontium niobate Sr 0.92NbO 3

Abstract

The Sr deficient perovskite Sr 0.92NbO 3 was synthesized from Sr 5Nb 4O 15 and Nb and its crystal structure was determined using powder neutron diffraction. At room temperature the structure is orthorhombic in space group Pnma with both in-phase and out-of-phase tilting of the NbO 6 octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→ P4/ mbm→ Pm3¯ m. The intermediate tetragonal phase has only in-phase tilts of the NbO 6 octahedra, rather than the out-of-phase tilts present in the more commonly observed I4/ mcm structure, due to initial softening at the M point rather than R point. The tetragonal phase exists only over a very narrow temperature range. The importance of M– M and M–O bonding in controlling the transition temperatures in Sr MO 3 perovskites is discussed.