journal menu
abstracts
Mono Phosphate Tungsten Bronzes (MPTB), (PO2)4(WO3)2m (m ranging from 4 to 14), are a large family of conductors of low dimensionality[1]. Their structure may be described as a regular stacking of WO3-type slabs with a thickness function of m, joined by slices of tetrahedral PO4 phosphate. Successive Peierls transitions towards charge-density wave (CDW) or spin-density wave (SDW) states are observed below different critical temperatures (TC1, TC2 ...). Structural transitions toward incommensurate modulated phases are associated with these complex electronic states. Recently an original member of MPTB, the member m=11.5 with formula (PO2)4(WO3)11(WO3)12, resulting in a regular intergrowth between m=11 and m=12 member, has been synthetized. An accurate investigation of the reciprocal space versus the temperature using single crystal X-ray diffraction shows the existence of two phase transitions at Tc1=4300C and Tc2=2800C. The ground state structure may be described with the following cell parameters a=5.3431(7) b=6.5901(9) c=40.884(6) α=93.096(2) β=93.734(2) γ=90.000(2) and the SG P-1. Both transitions associated with the occurrence of CDW are characterized by the appearance of modulation vectors (q1 and q2 associated to Tc1 and Tc2 respectively) and an increasing of the dimension of the superspace group (see figure 1). A structural study of the m=11.5 member is performed above Tc1 and below Tc1 and Tc2; an interpretation of the CDW state is then proposed. The evolution of the intensity of the satellite and main reflections around Tc1 and Tc2 is analyzed for characterizing the order parameter of the transitions. Finally, resistivity and magnetoresitance measurements of a large single crystal sample of m=11.5 MPTB are performed in a temperature interval from 2K to 300K. These measures are revealing at ~50K a possible SDW transition due to electron-electron interactions playing significant role in that material[2]. Further neutron diffraction experiment at very low temperature shall be performed to clarify this point.
abstracts
Bi2Sr2CaCu2O8+δ HTSC superconductor is characterized by a very strong normal-state resistivity anisotropy, with ρc/ρab typically above 10E4. The aim of this study is to use Quantitative Texture Analysis from x-ray diffraction measurements to estimate the orientation effect on the anisotropic macroscopic resistivity in melt-cast bulk Bi2Sr2CaCu2O8+δ superconductors. Our approach uses the geometric mean [1] of the single crystal resistivity tensor weighted by the Orientation Distribution Function (ODF) to quantitatively estimate the macroscopic resistivity tensor of the samples. The ODF is obtained from x-ray Combined Analysis [2], using the E-WIMV algorithm of the MAUD software. The GMA applies to the rank-two resistivity tensor of the orthorhombic space group considered tetragonal due to the small difference of a- and b-axes of the phase, with only two independent tensor components. We relate a relatively good agreement between measured and calculated macroscopic anisotropic resistivity ratios. Even with ρc/ρab between 10E4 and 10E5 for Bi2212 at room temperature in single crystals [3], we experiment macroscopic ratio in our bulk samples of around only 2. This small ratio is explained by the weak planar- or fiber-like (Figure) texture achieved in the melt-cast samples, characterized by maxima of orientation distributions not larger than 10 mrd. Calculated resistivities, based on homogeneous crystallites, perfect grain boundaries and no secondary phases, are 10 times larger than the observed ones. This suggests that the observed minor phases positively affect conductive pathways between grains. Calculated and measured anisotropic resistive ratios are coherent with one another, and Combined Analysis gives good predictions of these former.
