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

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 ½{odd,odd,odd} type reflections in all compositions, complicating the crystallographic characterization

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

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.

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

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 Ba₂Ln_2/3TeO₆ (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu³⁺ Phosphor Hosts

Ba₂Ln_2/3TeO₆ (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 LnO₆ and TeO₆ octahedra. In accordance with observed number of bands and group theoretical predictions, the most likely symmetry of all compounds in the Ba₂Ln_2/3TeO₆ system was found to be monoclinic with <i>P</i>2₁<i>/n</i> space group. Rietveld refinement of the XRD patterns further confirmed the <i>P</i>2₁<i>/n</i> space group and also the 1:1 rock salt ordering of the B-site cations. Diffuse reflectance spectra of Ba₂Ln_2/3TeO₆ 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 Ba₂La_{2/3–<i>x</i>}Eu_<i>x</i>TeO₆ (<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 Sr₂Si₅N₈:Eu²⁺, which points toward the possible applicability of these new red phosphors in solid-state lighting industry. Finally, Judd–Ofelt intensity parameters Ω_λ (λ = 2 and 4) were calculated, which indicate that Eu³⁺ ions occupy the symmetric octahedral B-site of the Ba₂La_2/3TeO₆

Microwave and infrared dielectric properties of Sr1−3x/2CexTiO3(x = 0.154–0.400) incipient ferroelectrics at cryogenic temperatures

Sr1−3x/2CexTiO3 (x = 0.154–0.400) or Sr2+nCe2Ti5+nO15+3n (n ≤ 8) ceramics were prepared by the mixed oxide route. The microwave (MW) dielectric properties of the compounds were investigated in the temperature range from 8 to 295 K. The permittivity increases for decreasing temperatures and saturates below 30 K, following Barrett's equation, demonstrating the incipient ferroelectric nature of the investigated materials. The dielectric loss tangent decreases for decreasing temperatures, reaching a minimum at about 80–120 K, and again increases with further cooling due to the rotations of TiO6 octahedra. Infrared-reflectivity data show that the dielectric response of the system is driven by the lowest-frequency polar (soft) mode, particularly at lower temperatures, where the phonons become practically uncoupled. The results help us to understand why Sr1−3x/2CexTiO3 materials present more appropriate dielectric properties for MW tunable applications, compared with pure SrTiO3

Sr₂FeO₃ with Stacked Infinite Chains of FeO₄ Square Planes

The synthesis of Sr₂FeO₃ through a hydride reduction of the Ruddlesden–Popper layered perovskite Sr₂FeO₄ is reported. Rietveld refinements using synchrotron and neutron powder diffraction data revealed that the structure contains corner-shared FeO₄ square-planar chains running along the [010] axis, being isostructural with Sr₂CuO₃ (<i>Immm</i> space group). Fairly strong Fe–O–Fe and Fe–Fe interactions along [010] and [100], respectively, make it an <i>S</i> = 2 quasi two-dimensional (2D) rectangular lattice antiferromagnet. This compound represents the end-member (<i>n</i> = 1) of the serial system Sr_<i>n</i>+1Fe_<i>n</i>O_2<i>n</i>+1, together with previously reported Sr₃Fe₂O₅ (<i>n</i> = 2) and SrFeO₂ (<i>n</i> = ∞), thus giving an opportunity to study the 2D-to-3D dimensional crossover. Neutron diffraction and Mössbauer spectroscopy show the occurrence of <i>G</i>-type antiferromagnetic order below 179 K, which is, because of dimensional reduction, significantly lower than those of the other members, 296 K in Sr₃Fe₂O₅ and 468 K in SrFeO₂. However, the temperature dependence of magnetic moment shows a universal behavior