N. Kojima

Semiconductor-to-metal and metal-to-metal transitions in the three-dimensional mixed-valence compound Cs2Au2I6 under high pressures

Electrical resistivity of three-dimensional Au mixed-valence compound Cs2Au2I6 was investigated under hydrostatic pressures up to 7.0 GPa and temperatures between room temperature and 480 K, where a semiconductor-to-metal transition (P=4.5 GPa, T=room temperature) and a metal-to-metal transition (P=6.5 GPa, T≈330 K) were observed. The metallic phase appeared at P=6.5 GPa and T⪆ 330 K could be obtained at room temperature and ambient pressure as a metastable phase, whose structure was found to be a cubic perovskite structure from the X-ray diffraction pattern. In connection with these results, the Au valence state of Cs2Au2I6 under high pressures was discussed.

Magnetic and Transport Properties of Ce-Ta-S Ternary System

Synthesis, transport and magnetic properties of a new ternary system Ce-Ta-S have been investigated. (CeS) 0.6 TaS 2 and (CeS) 1.2 TaS 2 order antiferromagnetically with Neel temperatures of 2.3 K and 2.7 K, respectively. In the former compound, a diverging anomaly in the susceptibility at T N , a small spontaneous magnetization below T N and a magnetic phase transition at 2.5 kOe are observed. From these results, (CeS) 0.6 TaS 2 is supposed to be a quasi-2D antiferromagnet with a weak ferromagnetism composed of ferromagnetic layers. The fractional superconductivity is observed with the magnetic ordering. Ce 4 f electrons are well localized and the correlation between Ce 4 f and conduction electrons is weak in these compounds.

Metallization and phase transitions of the three-dimensional halogen-bridge mixed-valence complex Cs2Au2I6 under high pressure

Abstract We have recently shown that insulating mixed-valence complex Cs 2 Au 2 I 6 transforms gradually toa metallic state between ambient pressure and 4.5 GPa at room temperature. Also shown that Cs 2 Au 2 I 6 undergoes a metal-to-metal transition in a process of increasing temperature at 6.5 GPa. The present energy-dispersive X-ray diffraction study has revealed that Cs 2 Au 2 I 6 undergoes a pressure-induced first-order tetragonal-to-tetragonal phase transition at about 5.5 GPa and room temperature. At the transition pressure, the Au-Au distance elongates along the c -axis, which is due to the ‘band Jahn-Teller’ transition. The Au 11 valence state is considered to be realized at this pressure. This phase transition would relate to the minimum in the pressure-resistance curve near 5.5 GPa, previously reported. It was also found that Cs 2 Au 2 I 6 undergoes a first-order tetragonal-to-cubic phase transition at 6.8 GPa and T ∾ 350 K.

Three photo-induced absorption bands of one-dimensional halogen-bridged mixed-valence complex, [Pt(en)2][PtBr2(en)2](SO4)2.6H2O

Abstract Weak absorption bands below the low-energy absorption edge of the intervalence charge-transfer (IVCT) transition in one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2 ][PtBr 2 (en) 2 ](SO 4 ) 2 .6H 2 O have been investigated at 4.2K for the light polarized parallel to the Pt chain axis. In this complex, we found that three weak absorption bands (A-, B-, C-bands) were observed below the IVCT band. It is the first example of a non-doped halogen-bridged mixed-valence complex having three weak absorption bands. The absorption intensity of the C-band is much weaker than those of the A- and B-bands, but it is remarkably enhanced by the Au-doping. The A-, B- and C-bands have the photo-induced absorption effect, and its effect depends sensitively on the crystal quality and on the condition of the crystal growth, and is enhanced by the Au-doping. The photo-induced absorption decays during several minutes since stopping the irradiating light in the region of the IVCT band. From these behaviors of the A-, B-, and C-bands, we discuss the relation between the origin of these bands and the mismatch of the valence alternation.

Raman and Infrared Spectra of Misfit Layer Compounds MNbS3 (M=Sn, Pb, La, Ce)

Raman scattering and infrared reflection spectra of misfit layer compounds MNbS3 (M=Sn, Pb, La, Ce) have been measured to study the interlayer interaction and the charge transfer from the MS layer to the NbS2 layer. Analysis of the Raman spectra indicates that the interlayer interaction is considerably stronger for the La and Ce compounds than for the Sn and Pb compounds, which may be related to the difference in the amount of the charge transfer.

Effects of alkali substitution and pressure on the charge-density wave transitions of two-dimensional metals K 3 Cu 8 S 6 and Rb 3 Cu 8 S 6

K 3 Cu 8 S 6 is a two-dimensional metal which has two phase transitions at T 1 (153 K) and T 2 (55 K), accompanied by an incommensurate and a commensurate superstructure, respectively. In order to investigate these phase transitions, the mixed crystals (K 1- x Rb x ) 3 Cu 8 S 6 have been synthesized and the effect of the alkali substitution and that of pressure have been studied by the measurements of their transport properties. With increasing x , T 1 significantly decreases, reaching T 1 of Rb 3 Cu 8 S 6 (=110 K). Pressure also reduces T 1 of K 3 Cu 8 S 6 and Rb 3 Cu 8 S 6 . On the other hand, the T 2 transition becomes obscure with increasing x , and disappears thoroughly for x ≧0.50. In Rb 3 Cu 8 S 6 , the T 2 transition is absent at ambient pressure but induced by the pressure of at most 0.5 GPa. Around x =0.06, a remarkable rise in the resistivity appears below T 2 . Its mechanism is discussed.

Electrical conductivity of the perovskite-type mixed-valence compound Cs2Au2I6 under high pressures and low temperature

Abstract We have investigated the electrical conductivity of single crystal Cs 2 Au 2 I 6 under high pressures (0–8 GPa) and low temperature (4.2–300 K). We have observed strongly pressure-dependent resistivity anomalies under hydrostatic pressure condition. We have also observed drastic resistance jump around 5 GPa which may be associated with the “band Jahn-Teller” transition or the CDW transition.

Hydrostatic pressure effect on the charge-density wave transition in the low-dimensional metal K3Cu8S6

Abstract The electrical resistivity of K3Cu8S6 was measured between 290K and 1.5K under hydrostatic pressure up to 1.1GPa. The two phase transitions, which are related to the formation of charge density waves (CDWs), were strongly influenced by pressure; the temperature of the first-order transition (50K under ambient pressure) rose and that of the second-order transition (152K under ambient pressure) fell with increasing pressure. The non-metallic behavior of the resistivity due to the CDWs was almost suppressed by the pressure of 1.1GPa.

Behaviour of the electrical conductivity of the three-dimensional mixed-valence compounds Cs2Au2X6 (X=C1, Br, I) under high pressures

Abstract Electrical resistivity of the three-dimensional Au mixed-valence compounds Cs2Au2X6 (X=Cl, Br, I) was investigated under hydrostatic pressures up to 7.0 GPa and temperatures between room temperature and 480K. The resistivities and the activation energies of these compounds decrease rapidly with increasing pressure. The activation energies of Cs2Au2Cl6, Cs2Au2Br6 and Cs2Au2I6 are estimated by extrapolation as zero at 4.4 GPa, 7.2 GPa and 7.5 GPa, respectively. In the case of Cs2Au2I6, a sonductor-to-metal transition (P= Gpa, T= room temp.) and a metal-to-metal transition (P=6.5 GPa, T≈330 K) were observed. The metallic phase appeared at P =6.5 GPa and T ⪆ 330 K could be obtained at room temperature and ambient pressure as a metastable phase, whose structure was found to be a cubic perovskite structure from the X-ray diffraction pattern.

Temperature-pressure phase diagrams of hydrogenated and deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Cl

Abstract The dc magnetization at ambient pressure and rf field screening under hydrostatic pressure are measured for hydrogenated and deuterated κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl to investigate the isotope effects on the temperature-pressure phase diagram. A possible weak ferromagnetic spin ordering is discussed based on the crystallographic symmetry of this compound.