Kengo Oka

Colossal negative thermal expansion in BiNiO3 induced by intermetallic charge transfer

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.

Pressure-Induced Spin-State Transition in BiCoO3

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.

Magnetic Ground-State of Perovskite PbVO3 with Large Tetragonal Distortion

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.

Intermetallic charge transfer in A-site-ordered double perovskite BiCu3Fe4O12.

An A-site-ordered double perovskite BiCu(3)Fe(4)O(12) was synthesized at high-pressure and high-temperature conditions. Similar to the isostructural LaCu(3)Fe(4)O(12), the temperature-induced intermetallic charge transfer between the A-site Cu and the B-site Fe ions occurs but at higher temperature (428 K) than LaCu(3)Fe(4)O(12) (393 K) does. This charge transfer causes an isostructural phase transition with volume contraction by 0.6% as well as semiconductor-to-metal and antiferromagnetism-to-paramagnetism transitions. Although the Bi cation at the A site does not take part in the charge transfer, it appears to enhance the charge-transfer temperature by stabilizing the square-planar coordinated Cu(3+).

Polarization Rotation in the Monoclinic Perovskite BiCo1−xFexO3

A piezoelectric ceramic, Pb(Ti1 xZrx)O3 (PZT), is widely used for various applications, such as transducers and sensors. PZT shows a maximum piezoelectric effect with a composition of about x = 0.5, the boundary separating the tetragonal P4mm (Ti-rich) and the rhombohedral R3m (Zr-rich) phases in the phase diagram. This boundary is known as a morphotropic phase boundary (MPB). Investigation of the structural evolution at the MPB is indispensable for understanding the origin of the enhanced piezoelectric property in PZT. In 2000, Noheda reported the presence of a monoclinic phase at x = 0.50 and 0.52 below 200 and 250 K, respectively, on the basis of powder synchrotron X-ray diffraction (SXRD) studies. The monoclinic phase had a ffiffiffi

Pressure-induced transformation of 6H hexagonal to 3C Perovskite structure in PbMnO3.

A tetragonal perovskite PbMnO(3) was obtained by treating the 6H hexagonal perovskite phase at 15 GPa and 1273 K. Structural analysis using synchrotron X-ray diffraction suggested that PbMnO(3) crystallizes in the centrosymmetric space group P4/mmm, unlike PbTiO(3) and PbVO(3) which have a polar structure in space group P4mm. Iodometric titration revealed the presence of the oxygen deficiency of x = 0.06 for PbMnO(3-x). The hexagonal 6H and the 3C perovskite phases exhibited antiferromagnetic ordering at 155 and 20 K, respectively.

Hydrothermal Synthesis of a New Double Perovskite-Type Bismuthate, (Ba0.75K0.14H0.11)BiO3·nH2O

A new double perovskite-type bismuthate, (Ba0.75K0.14H0.11)BiO3nH2O was prepared by low-temperature hydrothermal reaction. This compound had the double perovskite-type structure with a cubic cell of a = 0.85444(3) nm, in which the A site was occupied by Ba atoms, K atoms, and water molecules, and Bi atoms occupied one crystallographic site. This compound exhibited superconductive diamagnetism with an onset temperature of about 8 K.

Charge and Magnetic Orderings in the Triangular-Lattice Antiferromagnet InFe2O4

The charge ordering and the magnetic ordering in InFe 2 O 4 were investigated using samples free of impurity Fe 3 O 4 . X-ray diffraction, magnetic susceptibility, magnetization, Mossbauer spectroscopy, and dielectric constant measurements consistently showed that both these orderings occurred between 230 and 250 K. This temperature range is much smaller than the corresponding one in the cooling of electronic ferroelectric LuFe 2 O 4 , where charge ordering occurs at 320 K and magnetic ordering occurs at 250 K.

Effect of Zn substitution for Cu on Ca2-xNaxCuO2Cl2 near the hole concentration of 18 per Cu

A weakening of superconductivity upon substitution of Cu by Zn (0.5–1%) is observed in a high-TcTc cuprate, Ca2-xNaxCuO2Cl2Ca2-xNaxCuO2Cl2, near the hole concentration of 18 per Cu. The superconducting transition temperature and its volume fraction, estimated by magnetic susceptibility, exhibit a sizable anomaly for x=0.12–0.14x=0.12–0.14, where the slowing down of Cu spin fluctuations below 5xa0K is demonstrated by muon spin relaxation experiments. These observations are in close resemblance to other typical cuprates including YBa2Cu3O7-δYBa2Cu3O7-δ, and Bi2Sr2Ca1-xYxCu2O8+δBi2Sr2Ca1-xYxCu2O8+δ, providing further evidence that Zn-induced “stripe” correlation is a universal feature of high-TcTc cuprate superconductors common to that of La2-xAxCuO4La2-xAxCuO4 (A=BaA=Ba, Sr).

Effect of Zn substitution for Cu on Ca2-xNaxCuO2Cl2Ca2-xNaxCuO2Cl2 near the hole concentration of 18 per Cu

A weakening of superconductivity upon substitution of Cu by Zn (0.5–1%) is observed in a high-TcTc cuprate, Ca2-xNaxCuO2Cl2Ca2-xNaxCuO2Cl2, near the hole concentration of 18 per Cu. The superconducting transition temperature and its volume fraction, estimated by magnetic susceptibility, exhibit a sizable anomaly for x=0.12–0.14x=0.12–0.14, where the slowing down of Cu spin fluctuations below 5xa0K is demonstrated by muon spin relaxation experiments. These observations are in close resemblance to other typical cuprates including YBa2Cu3O7-δYBa2Cu3O7-δ, and Bi2Sr2Ca1-xYxCu2O8+δBi2Sr2Ca1-xYxCu2O8+δ, providing further evidence that Zn-induced “stripe” correlation is a universal feature of high-TcTc cuprate superconductors common to that of La2-xAxCuO4La2-xAxCuO4 (A=BaA=Ba, Sr).