A primitive tetragonal intermediate in the orthorhombic-cubic phase transition of perovskite-type strontium niobate Sr0.92NbO3

The Sr deficient perovskite Sr0.92NbO3 was synthesized from Sr5Nb4O15 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 NbO6 octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→P4/mbm→Pm3-bar m. The intermediate tetragonal phase has only in-phase tilts of the NbO6 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 SrMO3 perovskites is discussed. - Graphical abstract: Variable temperature powder neutron diffraction measurements reveal that Sr0.92NbO3 undergoes a sequence of phase transitions Pnma→P4/mbm→Pm3m with unanticipated softening of the M point rather than R point resulting in the formation of the primitive tetragonal intermediate.