Anisotropic resistivity tensor of melt-cast Bi2212 superconductors from QTA

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.

Freshwater pearls as near single crystals for vaterite structure resolution

There has been lots of controversies about vaterite structure in the past decades. Extra peaks occurring out of the hexagonal structure and best described by Kamhi [1] still resist any indexing. Lower space group symmetries, superspace groups, microtwinning, and first-principle calculations [2], all failed in taking account of these minor peaks, surprisingly always present in all synthetic and biogenic vaterite formations. Recently, secondary interspersed domains observed in high-resolution TEM images indicated their incoherence and rather incompatible character with the vaterite matrix [3]. One of the major difficulty in resolving the vaterite structure lies in the absence of single crystals. Powder diffraction patterns are always composed of hexagonal and extra, but small, peaks, and temptation to index the pattern as a single phase is large, particularly since x-ray fluorescence invariably probes for CaCO3. We used Hyriopsis cumingii freshwater mussel pearls to help proving that vaterite is definitely crystallizing within the original hexagonal space group. Some of these pearls suffer defective growth toward vaterite. In such cases the hexagonal peaks clearly exhibit a strong texture while the extra peaks look more random. This is an invaluable evidence of the existence of clearly separated phases, though the minor phase (or phases) still resist indexing. The hexagonal structure refinement, thanks to the strong vaterite texture, is obtained with larger resolution than before.