Transition-metal oxynitrides with perovskite-type structures are an emerging class of materials with optical, photocatalytic, dielectric and magnetoresistive properties that may be sensitive to oxide-nitride order, but the anion-ordering principles were unclear. Here we report an investigation of the representative compounds SrMO(2)N (M = Nb, Ta) using neutron and electron diffraction. This revealed a robust 1O/2(O(0.5)N(0.5)) partial anion order (up to at least 750 °C in the apparently cubic high-temperature phases) that directs the rotations of MO(4)N(2) octahedra in the room-temperature superstructure. The anion distribution is consistent with local cis-ordering of the two nitrides in each octahedron driven by covalency, which results in disordered zigzag M-N chains in planes within the perovskite lattice. Local structures for the full range of oxynitride perovskites are predicted and a future challenge is to tune properties by controlling the order and dimensionality of the anion chains and networks.

Motivated by the colossal negative thermal expansion recently found in BiNiO3, the valence transition accompanied by the charge transfer between the Bi and Ni sites is theoretically studied. We introduce an effective model for Bi-6s and Ni-3d orbitals with taking into account the valence skipping of Bi cations, and investigate the ground-state and finite-temperature phase diagrams within the mean-field approximation. We find that the valence transition is caused by commensurate locking of the electron filling in each orbital associated with charge and magnetic orderings, and the critical temperature and the nature of the transitions are strongly affected by the relative energy between the Bi and Ni levels and the effective electron-electron interaction in the Bi sites. The obtained phase diagram well explains the temperature-and pressure-driven valence transitions in BiNiO3 and the systematic variation of valence states for a series of Bi and Pb perovskite oxides. PACS numbers: 71.10.Fd , 71.30.+h , 75.25.Dk, 75.30.Kz Perovskite transition metal (TM) oxides (general formula: ABO 3) have been providing central issues of phase transitions and strong electron correlations in condensed matter physics [1, 2]. They exhibit a wide range of novel magnetic , dielectric, and transport properties: for example, the large negative magnetoresistance in La 1−x Sr x MnO 3 [3-5], the spin-state transition in La 1−x Sr x CoO 3 [6, 7], the metal-to-insulator transition in RNiO 3 (R: rare earth element) [8], and the ferroelectric to quantum paraelectric transition in Ba 1−x Sr x TiO 3 [9, 10]. In these phenomena, the central players are the electrons in 3d orbitals of the B-site TMs hy-bridized with oxygen 2p orbitals. The A-site cations, on the other hand, are usually inert and have been regarded as "stagehands": they control the electron filling and bandwidth through their valence state and ionic radius, respectively. Peculiar exceptions to the above standards have recently been found in several perovskite TM oxides, in which the A-site cations play an active role as "valence skipper". In these compounds, not only the B-site 3d electrons but also the va-lence s electrons in the A-site cations significantly contribute to the electronic properties. In the valence skippers, the outer-most s orbital prefers closed-shell configurations s 0 or s 2 , and tends to skip the intermediate valence s 1. This is attributed to the effective attractive interaction between s electrons [11-13], and hence the A-site valence state can be actively controlled through electronic degrees of freedom. Owing to the multiple electronic instabilities in both A-and B-site cations, the TM oxides with the A-site valence skipper have a potential of new electronic phases and functions. The colossal negative thermal expansion (CNTE) material BiNiO 3 [14] is one of such candidates; both Bi-6s and Ni-3d electrons are expected to play a key role in the large volume change [15, 16]. At ambient pressure, BiNiO 3 has a unique valence state, where the average valence of ...

An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.