The unusual property of negative thermal expansion is of fundamental interest and may be used to fabricate composites with zero or other controlled thermal expansion values. Here we report that colossal negative thermal expansion (defined as linear expansion <−10−4 K−1 over a temperature range ~100 K) is accessible in perovskite oxides showing charge-transfer transitions. BiNiO3 shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer that is shifted to ambient pressure through lanthanum substitution for Bi. Changing proportions of coexisting low- and high-temperature phases leads to smooth volume shrinkage on heating. The crystallographic linear expansion coefficient for Bi0.95La0.05NiO3 is −137×10−6 K−1 and a value of −82×10−6 K−1 is observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet. Colossal negative thermal expansion materials operating at ambient conditions may also be accessible through metal-insulator transitions driven by other phenomena such as ferroelectric orders.

The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.

Mixed anion compounds such as oxynitrides and oxychalcogenides are recognized as potential candidates of visible-light-driven photocatalysts since, as compared with oxygen 2p orbitals, p orbitals of less electronegative anion (e.g., N, S) can form a valence band that has more negative potential. In this regard, oxyfluorides appear unsuitable because of the higher electronegativity of fluorine. Here we show an exceptional case, an anion-ordered pyrochlore oxyfluoride PbTiOF that has a small band gap (ca. 2.4 eV). With suitable modification of PbTiOF by promoters such as platinum nanoparticles and a binuclear ruthenium(II) complex, PbTiOF worked as a stable photocatalyst for visible-light-driven H evolution and CO reduction. Density functional theory calculations have revealed that the unprecedented visible-light-response of PbTiOF arises from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb-O bond in the pyrochlore lattice due to the fluorine substitution.

The crystal structure change of the solid solution BiCo1-xFexO3 was investigated in order to determine the phase boundary between tetragonal BiCoO3 and rhombohedral BiFeO3. It was found that BiCo1-xFexO3 with x = 0 to 0.6 had tetragonal BiCoO3 structures, while those with x = 0.8 to 1 had rhombohedral BiFeO3 structures at room temperature. The monoclinic √2a ×√2a ×a phase was found for the x = 0.7 sample. The tetragonal-to-cubic phase transition was first observed at around 700–850 °C in Bi-based perovskite for the x = 0.8 sample.

The magnetic properties of PbVO 3, a PbTiO 3-type perovskite with a large tetragonal distortion ( c/a = 1.229), were investigated. The temperature dependence of the measured magnetization of multidomain single-crystal samples showed a broad maximum centered around 180 K, indicating a two-dimensional antiferromagnetism. muSR measurement revealed the presence of a long-range order below 43 K. The two-dimensional magnetism is due to the ordering of d xy orbitals, which is thought to also be related to the large tetragonal distortion of PbVO 3.

Perovskite PbCoO synthesized at 12 GPa was found to have an unusual charge distribution of PbPbCoCoO with charge orderings in both the A and B sites of perovskite ABO. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to PbPbCoCoO quadruple perovskite structure. It is shown that the average valence distribution of PbCoO between PbCrO and PbNiO can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Double-perovskite Bi oxides are a new series of superconducting materials, and their crystal structure and superconducting properties are under investigation. In this paper, we describe the synthesis and characterization of a new double-perovskite material that has an increased superconductive transition temperature of 31.5 K. The structure of the material was examined using powder neutron diffraction (ND), synchrotron X-ray diffraction (SXRD), and transmission electron microscopy (TEM). Rietveld refinement of the sample based on ND and SXRD data confirmed an A-site-ordered (K1.00)­(Ba1.00)3­(Bi0.89Na0.11)4O12 double-perovskite-type structure with the space group Im3̅m (No. 229). This structural analysis revealed the incorporation of Na with Bi in the structure and a bent bond between (Na, Bi)–O–(Na, Bi). TEM analyses also confirmed a cubic double-perovskite structure. This hydrothermally synthesized compound exhibited a large shielding volume fraction, exceeding 100%, with onset of superconductivity at ∼31.5 K. Its electrical resistivity dropped near onset at ∼28 K, and zero resistivity was confirmed below 13 K. The calculated band structure revealed that the metallicity of the compound and the flatness of the conduction bands near the Fermi level (E F) are important for the appearance of superconductivity.

Perovskite-type structures (ABO3) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double-perovskite-type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A-site-ordered double perovskite structure, (Na(0.25)K(0.45))(Ba(1.00))3(Bi(1.00))4O12, with a maximum T(c) of about 27 K.