Yoshiyuki Inaguma

Perovskites with Bi or Pb on the A site host a number of interesting and yet to be understood phenomena such as negative thermal expansion in BiNiO3. We employ hard x-ray photoemission spectroscopy of Ni 2p core level as well as valence band to probe the electronic structure of BiNiO3 and PbNiO3. The experimental results supported by theoretical calculations using dynamical mean-field theory reveal essentially identical electronic structure of the Ni-O subsystem typical of Ni2+ charge-transfer insulators. The two materials are distinguished by filling of the Bi(Pb)-O antibonding states in the vicinity of the Fermi level, which is responsible for the Bi disproportionation in BiNiO3 at ambient pressure and absence of similar behavior in PbNiO3. The present experiments provide evidence for this conclusion by revealing the presence/absence of Bi/Pb 6s states at the top of the valence band in the two materials.

We prepared high-purity Sr1-xBaxFeO2F by a low-temperature fluorination method, investigated the average structure by synchrotron X-ray diffraction (SXRD), and measured second harmonic generation (SHG) in this material. For BaFeO2F, the average and local structures were investigated by time-of-flight (TOF) neutron diffraction and the X-ray pair distribution function (PDF), respectively. In addition, the magnetic moment under a magnetic field was measured. SHG was observed for Sr1-xBaxFeO2F (x = 1.0, 0.9, and 0.7) and was below the detection limit for x = 0.5. The average crystal structure of BaFeO2F was determined by the refinement of the TOF neutron diffraction data to be a cubic structure in which Fe3+ ions are displaced to (x, x, 1/2) off-site positions with a 1/12 occupancy. The magnetic structure was a G-type antiferromagnetic structure, and the magnetic moments of the Fe3+ ion were 3.84 and 3.50 μB at 3 and 300 K, respectively. According to the X-ray PDF analysis, the local structure was not a cubic structure with the equivalent displacement of the Fe3+ ion to the off-site positions but a noncentrosymmetric structure in which Fe3+ ions are displaced in various directions in distorted BX6 octahedra. These local polar domains induce the SHG. The coexistence of polar domains with different polarization directions results in a cubic average structure. The formation of local polar domains was supported by the canted ferromagnetic components observed in magnetic moment measurements carried out under an applied magnetic field.

We synthesized a perovskite-type RbNbO3 at 1173 K and 4 GPa from non-perovskite RbNbO3 and investigated its crystal structure and properties towards ferroelectric material design. Single-crystal X-ray diffraction analysis revealed an orthorhombic cell in the perovskite-type structure (space group Amm2, no. 38) with a = 3.9937(2) Å, b = 5.8217(3) Å, and c = 5.8647(2) Å. This non-centrosymmetric space group is the same as the ferroelectric BaTiO3 and KNbO3 but with enhanced distortion. Structural transition from orthorhombic to two successive tetragonal phases (Tetra1 at 493 K, Tetra2 at 573 K) was observed, maintaining the perovskite framework before reverting to the triclinic ambient phase at 693 K, with no structural changes between 4 and 300 K. The first transition is similar to that of KNbO3, whereas the second to Tetra2, marked by c-axis elongation and a significant cp/ap ratio jump (from 1.07 to 1.43), is unique. This distortion suggests a transition similar to that of PbVO3, where an octahedron's oxygen separates along the c-axis, forming a pyramid. Ab initio calculations simulating negative pressure like thermal expansion predicted this phase transition (cp/ap = 1.47 at -1.2 GPa), aligning with experimental findings. Thermal analysis revealed two endothermic peaks, with the second transition entailing a greater enthalpy change and volume alteration. Strong second harmonic generation signals were observed across Ortho, Tetra1, and Tetra2 phases, similar to BaTiO3 and KNbO3. Permittivity increased during the first transition, although the second transition's effects were limited by thermal expansion-induced bulk sample collapse. Perovskite-type RbNbO3 emerges as a promising ferroelectric material.