Koji Kosuge

The phase diagram and phase transition of the V2O3-V2O5 system

Abstract The phase relation in the V 2 O 3 −V 2 O 5 system and the homogeneity range of each Magneli phases were re-examined by magnetic and differential thermal analyses, in addition to the usual X-ray analyses. In particular, this report aimed to clarify the nature of the phase transition in the homologous series V n O 2 n −1 . The Mossbauer effect measurements revealed that the phase transformation in V 4 O 7 and V 5 O 9 is a transition from paramagnetic to paramagnetic, namely merely a crystallographic transformation. The origin of phase transition of vanadium oxides including VO 2 and V 2 O 3 is discussed on the basis of the d -electron pairing model proposed by Goodenough.

Field-Induced Magnetic Transitions in the One-Dimensional Compound Ca3Co2O6.

Magnetization measurements on the one-dimensional oxide Ca 3 Co 2 O 6 having a triangular net of Co 2 O 6 chains have been carried out both in static and pulsed high magnetic fields. The M / H vs. T curve obeys the Curie-Weiss law at high temperatures. Below 25 K, however, M / H increases abruptly, and a plateau is observed at 1/3 of the full moment in the M - H curve, suggesting a ferrimagnetic state of the ferromagnetic chains due to the antiferromagnetic interchain interaction. At low H , this system is considered to be in a partially disordered antiferromagnetic state for 10 K< T <25 K and in a ferrimagnetic state below 10 K. The observed plateau in the M - H curve for T ≤5 K is broader than for 10 K≤ T ≤20 K, indicating spin freezing at lower temperatures. The results can be discussed in terms of the triangular Ising spin systems.

Anomalous temperature dependence of Cu nuclear spin-lattice relaxation in YBa2Cu3O6.91

An anomalous temperature dependence of Cu nuclear spin-lattice relaxation time T 1 has been observed for both the Cu1 chain and Cu2 plane sites of YBa 2 Cu 3 O 6.91 ( T c =90 K) and discussed in connection with spin fluctuations and an anisotropic energy gap which nucleates in the vicinity of Cu2 nuclei.

Iron configuration and superconducting properties of YBa2(Cu1-xFex)3Oy prepared by various thermal treatments

Abstract YBa 2 (Cu 1− x Fe x ) 3 O y (0 ≦ x ≦ 0.20) was prepared by three kinds of heat treatment: (a) slow cooling from 850°C in flowing O 2 gas, (b) quenching into liquid N 2 from 930°C in air followed by reoxidation in flowing O 2 gas below 400°C and (c) reducing at 800°C in flowing N 2 gas followed by reoxidation in flowing O 2 gas below 400°C. The crystal structure and superconducting properties of the samples were examined by X-ray powder diffraction, TG, electrical resistivity and 57 Fe Mossbauer effects measurements. The region of orthorhombic phase and superconducting properties strongly depend on the thermal treatments. The results were discussed from the standpoint of Fe distribution.

Observation of Antiferromagnetic Nuclear Resonance of Cu in YBa2Cu3O6

The zero-field antiferromagnetic nuclear resonance (AFNR) of Cu in the high- T c related oxide YBa 2 Cu 3 O 6 has been observed at 89.89±0.05 MHz for 63 Cu and 96.29±0.05 MHz for 65 Cu with well articulated quadrupole splittings at 1.3 K. These frequencies correspond to a hyperfine field of 79.65±0.05 KOe. From the analysis of the AFNR spectrum and the observation of NQR at 30.11±0.2 MHz for 65 Cu and 27.88±0.02 MHz for 63 Cu, both of which have no Zeeman splitting, it is concluded that the antiferromagnetic mounents reside only on the CuO 2 plane sites and the direction of moments is perpendicular to the c -axis. This is consistent with the spin structure proposed by neutron diffraction studies. The nuclear relaxation behavior is also discussed briefly.

Metal-insulator transition in VnO2n−1

Abstract Research on phase relationships and structure studies by electron diffraction confirm V n O 2 n −1 ( n = 3–9) phases between V 2 O 3 and VO 2 . Metal-insulator phase transitions have been found in all phases but V 3 O 5 and V 7 O 13 . Electrical, magnetic and thermodynamic properties associated with the transitions are reported for sintered samples or for single crystals prepared by a vapor-transport method. The results are collated and reviewed in summarized form.

NMR Studies of 17O in the Normal State of YBa2Cu3O6+x

The temperature dependence of both the Knight shift ( K ) tensor and the nuclear spin-lattice relaxation rate (1/ T 1 ) in the normal state have been studied, by using the magnetically oriented powered samples of YBa 2 Cu 3 O 6+ x with x =0.96, 0.8 and 0.6. From the K -χ plot for the planer oxygen sites (O(2, 3)), we have determined the principal values of the hyperfine field tensor and found that those hardly depend on the hole concentration. The remarkable feature has emerged from the T -dependence of the 1/( T 1 T ) at the O(2, 3) sites; i.e. it reveals almost the same T -dependence of the spin parts of the Knight shift. This observation suggests that the energy width of the spin excitations around q ∼0 is T -independent in the normal state. The Knight shift tensor at the apical oxygen sites has also been measured and is discussed.

Thermoelectric properties of (Zn1−yMgy)1−xAlxO ceramics prepared by the polymerized complex method

(Zn1−yMgy)1−xAlxO powders were synthesized by the polymerized complex method and then consolidated by spark plasma sintering apparatus. The microscopic structure and thermoelectric properties were examined comparing with the experimental results of the samples prepared by the conventional solid-state reaction method. A small amount of ZnAl2O4 spinel phase as the second phase was observed in the sintered samples with x⩾0.02 by x-ray diffraction and a scanning electron microscope. The grain size of the samples prepared by the polymerized complex method is much smaller than that of the samples prepared by the conventional solid-state reaction method. The absolute values of the Seebeck coefficient and electrical resistivity decrease with increasing x up to about x=0.01, but above x=0.01 they are almost independent of x. This result indicates that the solubility limit of Al in Zn1−xAlxO is about x=0.01, which is also confirmed by 27Al nuclear magnetic resonance spectroscopy. At a fixed composition of x, the ab...

Soft X-ray absorption spectroscopy of vanadium oxides

Abstract We present the V2p and O1s X-ray absorption spectra of V 2 O 3 , VO 2 and V 2 O 5 . The V2p spectra show strong multiplet effects caused by large 2p-3d and 3d-3d Coulomb and exchange interactions. The spectra exhibit noticeable chemical shifts and completely different multiplets. The shapes of the multiplet are related to the symmetry and spin of the ground state. The V2p spectrum of V 2 O 3 can be simulated by an atomic-multiplet calculation projected in the appropriate crystal field. The O1s spectra are related to unoccupied Op character mixed in the conduction band. The spectra reflect V3d bands near the edge and V4sp bands at higher energies. The V3d bands are split by crystal field effects. Changes in the relative intensities are related to differences in the metal-ligand hybridization. The extra dispersion of the V3d bands in V 2 O 3 is attributed to larger metal-metal interactions. The O1s spectrum of VO 2 compares favorably to a symmetry-projected band structure calculation. Finally, the O1s spectra of VO 2 taken at room temperature (insulator phase) and at T ≈ 120°C (metallic phase) show clearly the splitting of the V3d | band across the metal-insulator transition.

The Phase Transition in VO2

Magnetic properties in VO 2 have been studied in the temperature range from 100 to 380°K with the Mossbauer effect measurement of Fe 57 doped in VO 2 . Above and below the transition temperature, no internal field is observed. The observation indicates that VO 2 is paramagnetic even below the transition temperature and then it was concluded that the drastic transition in VO 2 from semiconductor to metal is caused by some origin other than the magnetic ordering. The nature of the chemical bond in VO 2 has been also discussed from the impurity effect on the magnetic susceptibility. The origin of the phase transition which appeared in V 2 O 3 and VO 2 was systematically discussed in the light of Goodenoughs model.