No abstract

High-resolution x-ray diffraction (XRD), Raman spectroscopy and total scattering XRD coupled to atomic pair distribution function (PDF) analysis studies of the atomic-scale structure of archetypal BaZrxTi(1-x)O3 (x = 0.10, 0.20, 0.40) ceramics are presented over a wide temperature range (100-450 K). For x = 0.1 and 0.2 the results reveal, well above the Curie temperature, the presence of Ti-rich polar clusters which are precursors of a long-range ferroelectric order observed below TC. Polar nanoregions (PNRs) and relaxor behaviour are observed over the whole temperature range for x = 0.4. Irrespective of ceramic composition, the polar clusters are due to locally correlated off-centre displacement of Zr/Ti cations compatible with local rhombohedral symmetry. Formation of Zr-rich clusters is indicated by Raman spectroscopy for all compositions. Considering the isovalent substitution of Ti with Zr in BaZrxTi1-xO3, the mechanism of formation and growth of the PNRs is not due to charge ordering and random fields, but rather to a reduction of the local strain promoted by the large difference in ion size between Zr(4+) and Ti(4+). As a result, non-polar or weakly polar Zr-rich clusters and polar Ti-rich clusters are randomly distributed in a paraelectric lattice and the long-range ferroelectric order is disrupted with increasing Zr concentration.

It is shown that the tetragonal (T) and rhombohedral (R) structures of (1−x)BiFeO3–xPbTiO3 are stable for xT⩾0.31 and xR⩽0.27, respectively, giving the narrowest width of 0.03, reported so far, for the morphotropic phase boundary region in this system. The Rietveld refined structure of the T phase for x=0.31 reveals partial covalent character of the Ti∕Fe–O and Pb∕Bi–O bonds which may be responsible for the unusually large tetragonality.

PACS 72.15.Eb -Electrical and thermal conduction in crystalline metals and alloys PACS 71.20.Eh -Electron density of states and band structure of crystalline solids: Rare earth metals and alloys PACS 61.05.cp -X-ray diffraction Abstract -Resistivity measurements and temperature-dependent X-ray structural analyses are reported for the crystalline compounds GdPd3BxC1−x. We show that a controlled tuning of the temperature coefficient of resistance (TCR) can be done by modifying the structural parameters and chemical environment of the compounds. We have achieved the result of negative TCR in an ordered, non-Kondo crystalline compound. Electronic-structure calculations have been carried out to elucidate some of our observations.

We have observed negative thermal expansion (NTE) in a metallic, polycrystalline, and structurally ordered cubic compound GdPd3B0.25C0.75. Our analysis suggest that the NTE observed in this compound does not stems from valence or magnetic instability of lattice ions, which is in general the case of metallic compounds exhibiting such an anomaly. We propose a possible alternative mechanism, namely, the transverse vibrations at low temperatures arising from site anisotropy, that induce the lattice contraction thereby resulting in isotropic NTE. The observed NTE also reflects its effect on the electrical transport properties of this compounds.