Masaichiro Mizumaki

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.

HELICITY-MODULATION TECHNIQUE USING DIFFRACTIVE PHASE RETARDER FOR MEASUREMENTS OF X-RAY MAGNETIC CIRCULAR DICHROISM

A diamond X-ray phase retarder has been combined with a phase-sensitive detection method to improve the accuracy of X-ray magnetic circular dichroism (XMCD) measurements. Fluctuation of the absorption coefficient of a magnetized sample was induced by photon-helicity switching at 40 Hz by flipping the phase retarder, and directly detected through a lock-in amplifier referring to the helicity-modulation frequency. This new technique for XMCD measurements was applied to the Fe K-edge in pure Fe foil. XMCD spectra of higher signal-to-noise ratio were obtained in shorter measuring time.

CaFeO2: a new type of layered structure with iron in a distorted square planar coordination.

CaFeO(2), a material exhibiting an unprecedented layered structure containing 3d(6) iron in a high-spin distorted square-planar coordination, is reported. The new phase, obtained through a low-temperature reduction procedure using calcium hydride, has been characterized through powder neutron diffraction, synchrotron X-ray diffraction, Mossbauer spectroscopy, XAS experiments as well as first-principles DFT calculations. The XAS spectra near the Fe-K edge for the whole solid solution (Sr(1-x)Ca(x))FeO(2) supports that iron is in a square-planar coordination for 0 </= x </= 0.8 but clearly suggests a change of coordination for x = 1. The new structure contains infinite FeO(2) layers in which the FeO(4) units unprecedentedly distort from square-planar toward tetrahedra and rotate along the c-axis, in marked contrast to the well-studied and accepted concept that octahedral rotation in perovskite oxides occurs but the octahedral shape is kept almost regular. The new phase exhibits high-spin configuration and G-type antiferromagnetic ordering as in SrFeO(2). However, the distortion of the FeO(2) layers leads to only a slight decrease of the Neel temperature with respect to SrFeO(2). First-principles DFT calculations provide a clear rationalization of the structural and physical observations for CaFeO(2) and highlight how the nature of the cation influences the structural details of the AFeO(2) family of compounds (A = Ca, Sr, Ba). On the basis of these calculations the driving force for the distortion of the FeO(2) layers in CaFeO(2) is discussed.

The Mixed Valence States in the Unconventional Heavy Fermion Compound SmOs4Sb12

The X-ray absorption spectra of the heavy fermion compounds SmFe 4 P 12 and SmOs 4 Sb 12 have been measured near the Sm L 3 -edge at selected temperatures below 300 K. The spectra show the valence of Sm ions in SmFe 4 P 12 to be trivalent between 14 and 300 K with no significant temperature dependence. In contrast, that in SmOs 4 Sb 12 is 2.83 at 300 K and shows a significant decrease below 150 K, indicating an important role of the nonmagnetic Sm 2+ ions in the heavy fermion state that are insensitive to an applied magnetic field.

Single-crystal epitaxial thin films of SrFeO2 with FeO2 “infinite layers”

Single-crystal thin films of SrFeO2, which is an oxygen-deficient perovskite with “infinite layers” of Fe2+O2, were prepared by using CaH2 for low-temperature reduction of epitaxial SrFeO2.5 single-crystal films deposited on KTaO3 substrates. This reduction process, removing oxygen ions from the perovskite structure framework and causing rearrangements of oxygen ions, topotactically transforms the brownmillerite SrFeO2.5 to the c-axis oriented SrFeO2.

Glitch-free X-ray absorption spectrum under high pressure obtained using nano-polycrystalline diamond anvils

Nano-polycrystalline diamond (NPD) [Irifune et al. (2003), Nature (London), 421, 599] has been used to obtain a glitch-free X-ray absorption spectrum under high pressure. In the case of conventional single-crystal diamond (SCD) anvils, glitches owing to Bragg diffraction from the anvils are superimposed on X-ray absorption spectra. The glitch has long been a serious problem for high-pressure research activities using X-ray spectroscopy because of the difficulties of its complete removal. It is demonstrated that NPD is one of the best candidate materials to overcome this problem. Here a glitch-free absorption spectrum using the NPD anvils over a wide energy range is shown. The advantage and capability of NPD anvils is discussed by a comparison of the glitch map with that of SCD anvils.

Temperature dependence of Sm valence in SmB6 studied by X-ray absorption spectroscopy

X-ray absorption spectra of a valence-fluctuated Kondo insulator SmB6 have been studied near the Sm L3-edge at temperatures below 300 K. The average valence of Sm ions in SmB6 was deduced as 2.59 at 300 K; it shows a characteristic temperature behaviour with three anomalies at T = 15, 50 and 150 K. These temperatures correspond to the characteristic energies of SmB6 in macroscopic measurements. The present results imply that the Sm-4f electrons strongly hybridize with the conduction electrons and play an important role in the gap formation.

A Possible Novel Magnetic Ordering in SmRu4P12

We have carried out X-ray absorption spectroscopy, 149 Sm nuclear resonant forward scattering, and 99 Ru Mossbauer spectroscopy of SmRu 4 P 12 . The X-ray absorption spectra indicate that the Sm ions are trivalent up to 14 K. The quantum beats due to magnetic dipolar ordering are observed at 4.5 K in 149 Sm nuclear resonant forward scattering. The temperature dependence of time-integrated intensity in 149 Sm nuclear forward scattering suggests that the magnetic dipolar ordering occurs just below the metal–insulator transition temperature. No significant changes in the parameters are observed between 77 and 5 K in 99 Ru Mossbauer spectroscopy. The present result suggests a magnetic ordering accompanied by a magnetic dipole moment at the metal–insulator transition temperature in SmRu 4 P 12 in spite of the proposal of the magnetic ordering accompanied by a T β -type octupole.

Insulator-to-Superconductor Transition upon Electron Doping in a BiS2-Based Superconductor Sr1−xLaxFBiS2

Effects of electron doping on the BiS2-based superconductors Sr1−xLaxFBiS2 (0 ≤ x ≤ 0.6) have been investigated using the systematically synthesized polycrystals by means of x-ray diffraction, x-ray absorption spectroscopy, transport and thermodynamic measurements. The pristine compound is a band insulator with the BiS2 layer, which accommodates electron carriers through the La substitution for Sr, as evidenced by the change in x-ray absorption spectra reflecting the occupancy of Bi 6p orbitals. With increasing the carrier density, the resistivity progressively decreases and a bad metallic state appears for x ≥ 0.45, where bulk superconductivity manifests itself below approximately 3 K. The value of Tc gradually increases with decreasing x from 0.6 to 0.45 and immediately decreases down to zero at the critical concentration of x ∼ 0.4, resulting in an insulator–superconductor transition highly sensitive to the carrier density. Thermodynamic measurements furthermore have revealed the possible enhancement o...