Shuji Kawasaki

Phase transitions in Fe4+ (3d4)-perovskite oxides dominated by oxygen-hole character

The electronic properties of CaFe 1- x Co x O 3 , a perovskite system dominated by oxygen-hole character, were studied experimentally. A partial Co-substitution has been found to induce a serial transformation from a charge-disproportionated, antiferromagnetic (AF), insulating phase (Phase I, x , while the length of wave vector changes from | k |=0.161 a * at x =0 to | k |=0.13 a * at x =0.1. We ascribe the serial transformation to broadening of the oxygen-hole band with increasing Co content.

Different routes to charge disproportionation in perovskite-type Fe oxides

Iron perovskites CaFeO 3 and La 0 . 3 3 Sr 0 . 6 7 FeO 3 show charge disproportionation, resulting in charge-ordered states with Fe 3 + :Fe 5 + = 1:1 and =2:1, respectively. We have made photoemission and unrestricted Hartree-Fock band-structure calculations of CaFeO 3 and compared it with La 0 . 3 3 Sr 0 . 6 7 (FeO 3 . With decreasing temperature, a gradual decrease of the spectral weight near the Fermi level occurred in CaFeO 3 as in La 0 . 3 3 Sr 0 . 6 7 FeO 3 although lattice distortion occurs only in CaFeO 3 . Hartree-Fock calculations have indicated that both the breathing and tilting distortions are necessary to induce the charge disproportionation in CaFeO 3 , while no lattice distortion is necessary for the charge disproportionation in La 0 . 3 3 Sr 0 . 6 7 FeO 3 .

Physical properties of Co-substituted iron perovskite systems

Abstract ABO 3 type perovskite systems, CaFe 1− x Co x O 3 (0≤ x ≤0.5) and Sr 2/3 La 1/3 Fe 1− x Co x O 3 (0≤ x ≤1) were prepared by using a high pressure technique in the presence of an oxidizer KClO 4 . The parent Fe 4+ -oxides, CaFeO 3 and Sr 2/3 La 1/3 FeO 3 , are both characterized by a charge disproportionation leading to an antiferromagnetic and insulating state, which is 2Fe 4+ →Fe 3+ +Fe 5+ for the former and 3Fe 11/3+ →2Fe 3+ +Fe 5+ for the latter. The Co-substitution leads to a switching to a ferromagnetic and metallic phase. In comparison with the cubic Sr-analogue, the ferromagnetic Curie temperature is lower and the saturation magnetization is smaller, while the coercive force is larger and the resistivity is higher.

Local spin moment of LaCoO3 probed by a core hole

Abstract The electronic structures of the ground state and the 1s −1 core-hole state of high-spin cobalt oxide clusters are calculated. The calculated results indicate that the high-spin Co 3+ oxide becomes low spin in the core-hole state. To check the validity of the present calculation, we have measured the high-resolution X-ray fluorescence spectra of the Co K–L 3 (Kα 1 ) line shape for various cobalt compounds using a double-crystal X-ray fluorescence spectrometer: Co(acac) 2 , Co(acac) 3 , CoF 3 , CoCl 2 , CoBr 2 , K 3 [Co(CN) 6 ], CoO, Co 3 O 4 , LaCoO 3 , and Co (metal), where acac denotes CH 3 COCHCOCH 3 − . We conclude from the comparison of the calculations with the experimental results that the cobalt ion in LaCoO 3 at room temperature is high-spin state in spite that the reported spin state of this perovskite is different among the probing methods. The implications of the spin-flip shake-down process for the interpretation of the core-level X-ray photoelectron spectra, X-ray absorption spectra, and X-ray fluorescence spectra are discussed.

High-pressure Mössbauer study of perovskite iron oxides

The perovskite oxides CaFeO3 and La1/3Sr2/3FeO3 have been investigated by high-pressure 57Fe Mossbauer spectroscopy. The critical temperatures of the charge disproportionation (CD) and the magnetic order (MO) have been determined as a function of pressure. In CaFeO3 the CD (2Fe4+ → Fe3+ + Fe5+) occurs at an almost constant temperature of 290 K in the pressure range of 0–17 GPa. Above 20 GPa, the CD is suppressed. The MO temperature of 125 K at an ambient pressure rises to 300 K at 34 GPa. In La1/3Sr2/3FeO3 the CD (3Fe11/3+ → 2Fe3+ + Fe5+) and the MO occur at the same temperature up to 21 GPa, which decreases from 207 to 165 K with increasing pressure. Above 25 GPa, however, the MO temperature rises above 400 K.

High-Pressure Mössbauer and X-Ray Powder Diffraction Studies of SrFeO3

The perovskite oxide SrFeO 3 has been investigated by high-pressure 57 Fe Mossbauer spectroscopy and X-ray powder diffraction (XRD) up to 62 GPa and 56 GPa, respectively. Magnetic ordering occurs below 134 K at 0.1 MPa and below 210 K at 15 GPa. Above 17 GPa, the magnetic ordering temperature increases markedly to above 400 K. The cubic crystal structure of SrFeO 3 has been confirmed to persist up to 56 GPa by using XRD with synchrotron radiation.

Enhancement of incoherent elastic scattering with magnetic ordering in the energy spectra of nuclear resonant scattering

We have observed the extreme enhancement of elastic peak intensity in the energy spectrum of nuclear resonant scattering by 57 Fe in CaFeO 3 following magnetic phase transition, while the inelastic scattering part of the spectrum hardly changed. This enhancement effect clearly indicates that the sum of the elastic and inelastic parts changes as the temperature changes, that is, it is not conserved. This effect is thought to be due to the change of the coherent scattering channel caused by the splitting of the nuclear energy level induced by magnetic phase transition without structural phase transition. This effect shows that the splitting in the energy ranges on the neV order emerges in the energy spectrum of the nuclear resonant scattering spectrum measured with the resolution function on the meV order. Therefore, the energy spectra of incoherent nuclear resonant scattering can be applied to the study of the phase transition of electronic states in addition to phonon states, and the origin of the phase t...

High-pressure M ssbauer study of perovskite iron oxides

The perovskite oxides CaFeO3 and La1/3Sr2/3FeO3 have been investigated by high-pressure 57Fe Mossbauer spectroscopy. The critical temperatures of the charge disproportionation (CD) and the magnetic order (MO) have been determined as a function of pressure. In CaFeO3 the CD (2Fe4+ → Fe3+ + Fe5+) occurs at an almost constant temperature of 290 K in the pressure range of 0–17 GPa. Above 20 GPa, the CD is suppressed. The MO temperature of 125 K at an ambient pressure rises to 300 K at 34 GPa. In La1/3Sr2/3FeO3 the CD (3Fe11/3+ → 2Fe3+ + Fe5+) and the MO occur at the same temperature up to 21 GPa, which decreases from 207 to 165 K with increasing pressure. Above 25 GPa, however, the MO temperature rises above 400 K.

Effects of Charge Disproportionation on the Phonon Density of States in Fe Perovskites

Iron perovskites CaFeO 3 and La 0.33 Sr 0.67 FeO 3 show charge disproportionation, resulting in charge-ordered states with “Fe 3+ ”:“Fe 5+ ” =1:1 and =2:1, respectively. We have performed nuclear resonant scattering measurements of phonon density of states in CaFeO 3 and La 0.33 Sr 0.67 FeO 3 . With decreasing temperature, we found changes in the phonon density of states in the region of 30–60 meV in CaFeO 3 , while only a small change was observed in La 0.33 Sr 0.67 FeO 3 . This finding gives support to the previous finding that the charge disproportionation is driven by electron–phonon interaction in CaFeO 3 , and electronically in La 0.33 Sr 0.67 FeO 3 .

Magnetic Susceptibility of Deintercalated Tunnel Compound of KxCr5Se8 and Mössbauer Spectra of Kx(Cr0.9557Fe0.05)5Se8

Abstract KCr5Se8 has a TIV5S8-type structure, containing K ions in one-dimensional tunnels. Deintercalated samples of KxCr5Se8 (0.32 ≤ x ≤ 0.93) were prepared by leaching method using Alc3/FeCl3 aqueous solution. These samples showed a broad peak of magnetic susceptibility at ca. 130 K. 57Fe-Mössbauer spectra of Kx(Cr0.95 57Fe0.05)5Se8 (x = 1.0, 0.49) showed a quadrupole doublet at 300 K. Magnetic sextets appeared at 4.2 K in both samples, indicative of magnetic ordering. The observed isomer shift indicated that the charge of Fe is +3 in both samples. It was proposed that Se2- was partially oxidized by the deintercalation.