19 research outputs found

    ChemInform Abstract: LaPd 2

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    Crystallographic Characterization of (Sr\u3csub\u3e2-x\u3c/sub\u3eCa\u3csub\u3ex\u3c/sub\u3e)(MgTe)O\u3csub\u3e6\u3c/sub\u3e Double Perovskite via Electron Diffraction

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    Complex perovskite oxides [(A,A’)(B,B’)O3] are ubiquitous in electronic applications. An understanding of such structures and the mechanisms which affect them is essential in predicting and controlling the material properties. Structural distortions away from the ideal cubic aristotype may include tilting or deformation of the oxygen octahedra, chemical ordering or displacement of cations, or defect-induced lattice strain. Each of these mechanisms can alter the symmetry of the perovskite, resulting in a doubled supercell, with a corresponding effect on properties. By investigating the appearance of superlattice reflections in electron diffraction patterns, a summary of the octahedral tilting (tilt system) of the structure can be identified, cation ordering inferred, and space group symmetry established. In the case of the (Sr2-xCax)(MgTe)O6 system, a phase transformation occurs from a tetragonal I4/m symmetry at x = 0 (Sr2MgTeO6) to monoclinic P21/n symmetry for x ≄ 0.5; however, B-site cation ordering causes Âœ{odd,odd,odd} type reflections in all compositions, complicating the crystallographic characterization

    Structure of Compounds in the Sr1–3x/2CexTiO3 Homologous Series

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    Four compositions in the Sr1–3x/2CexTiO3 homologous series, corresponding to x = 0.1333, 0.1667, 0.25, and 0.4, have been produced by conventional solid-state processing. The structure of these compounds was analyzed by X-ray, electron, and neutron diffraction. While no superlattice can be observed via X-ray diffraction, both electron and neutron diffraction show evidence of a noncubic supercell caused by antiphase tilting of oxygen octahedra. The most likely space group is R3̅c, corresponding to an a−a−a− tilt system, except for the composition x = 0.4, for which an even more complex superstructure is observed. The degree of tilt increases with increasing x

    Vibrational spectroscopic study of Sr2ZnTeO6 double perovskites.

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    Sr2ZnTeO6 ceramics were prepared by the solid-state route and their vibrational phonon modeswere investigated using optical spectroscopic techniques, for the first time. X-ray diffraction (XRD) and Raman and infrared spectroscopies were employed to investigate the structures of these perovskite materials and the results analysed together with group-theoretical predictions. The number and behaviour of the first-order modes observed in both spectroscopic techniques are in agreement with the calculations for a tetragonal I4/mspace group. The complete set of the optical phononmodeswas determined, and the intrinsic dielectric properties of the materials were evaluated, allowing us to discuss their potential application in microwave (MW) circuitry

    Crystal Structure of Sr\u3csub\u3e0.4\u3c/sub\u3eCe\u3csub\u3e0.4\u3c/sub\u3eTiO\u3csub\u3e3\u3c/sub\u3e Ceramics

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    A cerium-doped SrTiO3 compound with the composition Sr0.4Ce0.4TiO3 has been produced by conventional solid-state processing. The structure of this compound was analyzed by X-ray, electron, and neutron diffraction. While no superlattice can be observed via X-ray diffraction, both electron and neutron diffraction show evidence of a noncubic supercell caused by antiphase tilting of oxygen octahedra. The most likely space group is C2/c, corresponding to an a-b-b- tilt system. Octahedra are tilted by ~5° about the pseudo-cubic a-axis and ~1.5° about the pseudo-cubic b- and c-axes

    Structural Characterization of B‑Site Ordered Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu<sup>3+</sup> Phosphor Hosts

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    Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> (Ln = La, Pr, Nd, Sm, and Eu) double perovskites were synthesized via solid-state ceramic route. Preliminary X-ray diffraction studies indicated a pseudocubic structure with lattice parameters ranging from 8.55 to 8.44 Å for the substitution of rare earths from La to Eu. Raman spectra show the frequency dependence of various Raman bands with respect to rare-earth substitution and exhibit a significant shift in peaks to higher wavenumber region, which was observed only for symmetric stretching modes of LnO<sub>6</sub> and TeO<sub>6</sub> octahedra. In accordance with observed number of bands and group theoretical predictions, the most likely symmetry of all compounds in the Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> system was found to be monoclinic with <i>P</i>2<sub>1</sub><i>/n</i> space group. Rietveld refinement of the XRD patterns further confirmed the <i>P</i>2<sub>1</sub><i>/n</i> space group and also the 1:1 rock salt ordering of the B-site cations. Diffuse reflectance spectra of Ba<sub>2</sub>Ln<sub>2/3</sub>TeO<sub>6</sub> showed the optical bandgaps of these compounds between 3.9 and 4.8 eV, indicating the suitability as luminescent host material. The reduction in bandgap energy with lanthanide contraction of rare-earth ions is attributed to the widening of conduction band with octahedral tilting. Photoluminescence (PL) spectra and PL excitation spectra of Ba<sub>2</sub>La<sub>2/3–<i>x</i></sub>Eu<sub><i>x</i></sub>TeO<sub>6</sub> (<i>x</i> = 0.025, 0.05, 0.075, 0.1, 0.125, 0.15) were investigated and found to exhibit bright orange-red emission under UV excitation. Chromaticity coordinates closely resemble those of commercial red phosphor Sr<sub>2</sub>Si<sub>5</sub>N<sub>8</sub>:Eu<sup>2+</sup>, which points toward the possible applicability of these new red phosphors in solid-state lighting industry. Finally, Judd–Ofelt intensity parameters Ω<sub>λ</sub> (λ = 2 and 4) were calculated, which indicate that Eu<sup>3+</sup> ions occupy the symmetric octahedral B-site of the Ba<sub>2</sub>La<sub>2/3</sub>TeO<sub>6</sub>
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