H. Fujishita

Incommensurate superlattice reflections in quasi one dimensional conductors, (MSe4)2I (M=Ta and Nb)

Abstract X-ray diffraction and conductivity measurements have been made on (MSE4)2I, (M=Ta and Nb). A phase transition of (NbSe4)2I was found and the transition temperature Tc was determined to be about 210K by the conductivity measurement. For (TaSe4)2I, Tc was about 260K as reported previously. Superlattice reflections due to the formation of charge density wave (CDW) in (TaSe4)2I were observed at points q =(±0.05, ±0.05, ±0.085) . The type of CDW of (NbSe4)2I is known to be identical to that of (TaSe4)2I. Temperature dependence of the threshold electric field of nonlinear conductivity is consistent with the result in previous reports.

Neutron scattering study on the structural transition of quasi-one-dimensional conductor K0.3MoO3

Preliminary results of the neutron diffraction and inelastic scattering studies on the quasi-one-dimensional conductor K0.3MoO3 with a metal-insulator transition are reported. The temperature dependence of the intensity of the superlattice reflection is found to be approximately described by a simple equation. A phonon mode seems to have an almost complete softening at the transition temperature Tc. The existence of the large central peak in the inelastic spectra above Tc suggest that the local correlation between electrons is very strong. The wavevector of the charge density wave smoothly approaches the commensurate one, as the temperature is lowered and lock-in transition does not occur down to T approximately=6K.

Phase transition of Ag3SX (X=I, Br)

Abstract The structural phase transitions of Ag 3 SI and Ag 3 SBr at low temperature were studied by dielectric measurements as well as by neutron scattering. Both substances show an anomalous increase of dielectric constant at the transition point but no peak at T c is seen due to a high ionic conductivity near and above T c . Neutron inelastic scattering measurement showed the existence of a local model of thermal vibration at low temperature similar to that found in RbAg 4 I 5 . Mechanisms of the phase transition in these crystals, as contrasted to the phase transition of CsPbCl 3 , are discussed in terms of the ionic motions and the order-disorder behavior of the cation.

Neutron inelastic scattering and X-ray structural study of the charge-density-wave state in K0.3MoO3

The Peierls transition in the quasi-one-dimensional conductor K0.3MoO3 has been studied to clarify its dynamical nature and the electron correlation problem. A giant Kohn anomaly and the central peak in S(q, omega ) were observed. The amplitude below the transition temperature Tc was also measured. An asymmetry of the elastic diffuse scattering above Tc around the reciprocal point which corresponds to the superlattice point can be explained by taking into account the structure factor of the charge-density wave. A detailed type of modulation of Mo atoms is presented by the X-ray study. No evidence for the correlation effect was observed and the transition of K0.3MoO3 seems to be explained by a simple band picture.

Structural phase transitions and superconducting transition temperatures of hexagonal MxWO3 compounds

Neutron scattering experiments have been performed on non-stoichiometric compounds of the hexagonal tungsten bronzes MxWO3 (M=Rb and K). A nearly dispersionless phonon branch has been observed in Rb0.33WO3 at an energy omega M approximately=5.5 meV. This branch reflects the local-mode character of the Rb atoms, which are loosely trapped at their sites within the large channel along the c direction formed by the linkage of WO6 octahedra. The x dependence of the energy omega M of this low-lying phonon (LLP) has been determined by measurements of the peak position in the powder inelastic scattering spectra. Since omega M depends only weakly on x, the variation in omega M appears not to be related to the anomalous x dependences of the superconducting transition temperature Tc observed previously in RbxWO3 and KxWO3. Two kinds of structural phase transitions were observed using powder diffraction measurements. One transition, which takes place above room temperature at Tc1, is due to a distortion of the WO3 host cage of MxWO3. It is accompanied by an optical phonon softening at the Gamma point. The second transition is an order-disorder transition of the M atoms. It produces for a certain value of x a long-wavelength incommensurate structure as the low-temperature phase. This is in contrast with the other incommensurate phases such as the CDW states of two-dimensional conductors, where the incommensurate structures generally appear at high temperatures. A local structural excitation model is adopted in the discussion of the superconductivity, and the x dependences of Tc in CsxWo3 and RbxWO3 are consistently explained in relation to the ordering schemes of the M atoms.

Structural transitions in Mo8O23

By means of neutron and X-ray four-circle diffractions, evidence for two structural transitions in Mo8O23 has been found. The incommensurate phase appears at a value of Tcl higher than room temperature and the incommensurate-commensurate transition takes place at Tc2 approximately=285K. The idea that the transitions are associated with the charge-density wave formation can explain the temperature dependences of the magnetic susceptibility and the electrical resistivity consistently. The characteristic features of the condensed mode in the commensurate phase are discussed.

X-ray diffraction study of the quasi-one-dimensional conductors (MSe4)2I (M=Ta and Nb)

Resistivity and X-ray diffraction measurements have been made for the quasi-one-dimensional conductors (MSe4)2I (M=Ta and Nb). (TaSe4)2I is known to undergo a phase transition at Tc approximately=260K. A phase transition of (NbSe4)2I was found to take place at Tc approximately=210K. Non-linear conduction was observed below Tc. Superlattice reflections due to the formation of a charge-density wave (CDW) were observed at eight points around fundamental Bragg spots in the reciprocal space. The CDW is known to be formed along only one direction; therefore the appearance of eight peaks instead of two is caused by the formation of domains. The condensed phonon was revealed by the measurements on (TaSe4)2I to be a transverse acoustic mode that polarizes in the c plane. Diffuse scattering shows that these compounds are not in the class of systems with extreme anisotropy.

Structure determination of low-dimensional conductor Mo8O23

Abstract The crystal structure of the new low-dimensional conductor Mo 8 O 23 has been determined at 370 and 100 K using a four-circle X-ray diffractometer. Mo 8 O 23 has an incommensurate structure at room temperature and the incommensurate-commensurate phase transition takes place at about 285 K. The structure of the normal phase at 370 K is essentially the same as that presented by Magneli for room temperature. The structure of commensurate phase at 100 K can be described mainly by the inhomogeneous rotations of MoO 6 octahedra.

Specific heat anomaly of BaPb1-xBixO3 at the superconducting transition

A specific heat anomaly in BaPb1-xBixO3 single crystals at the superconducting transition has been found by AC calorimetry, in contrast to the previous result of Methfessel et al. (1980) that no specific heat anomaly was observed. The present experimental result seems to support the idea that the superconductivity of the compounds can be described by the simple BCS theory.

A neutron scattering study of the quasi-one-dimensional conductor (TaSe4)2I

Neutron scattering experiments were carried out on the quasi-one-dimensional conductor (TaSe4)2I. At the superlattice point (0.05, 0.05, 0.085) a soft temperature-dependent transverse acoustic (TA) mode polarised in the c plane was observed. A spoon-like anomaly of the dispersion curve of the TA branch was observed around the superlattice point. In addition, an extremely low-lying TA mode was found.