Structural and electrical characteristics of rare earth simple perovskite oxide La0.57Nd0.1Pb0.33Mn0.8Ti0.2O3

Polycrystalline La_0.57Nd_0.1Pb_0.33Mn_0.8Ti_0.2O₃ (LNPMT) is prepared by the solid-state reaction technique. The formation of single phase material was confirmed by X-ray diffraction studies, and it was found to be a rhombohedral phase at room temperature. The impedance plane plot shows semicircle arcs at different temperatures and an electrical equivalent circuit has been proposed to explain the impedance results. The frequency dependent conductivity spectra follow the universal power law. The activation energy deduced from analysis of the imaginary part of electric modulus and imaginary impedance is found to be ∼75 meV. Such a value of activation energy indicates that the conduction mechanism for the sample is due to electron hopping. The imaginary part of the electric modulus suggests that the relaxation describes the same mechanism at various temperatures.     

Electronic and magnetic properties of double perovskites (, Ca)

We have studied the magnetic and electronic properties of double perovskites A₂FeReO₆ with A=Ba and Ca using ab initio methods based on density functional theory. It has been shown that when changing Ba by Ca the system undergoes a metal–insulator transition coincidentally to a structural phase transition, even if conduction band fillings are not significantly changed. Our results suggest that only considering a quite large correlation term on the Re site could explain an insulating character for Ca₂FeReO₆. Instead, we found that small correlations could induce a bad metallic behavior as suggested in more recent experiments.     

Theoretical studies of structural and magnetic properties of cubic perovskites PrCoO3 and NdCoO3

The structural and magnetic properties of cubic perovskites, PrCoO₃ and NdCoO₃, are studied using the full potential linearized augmented plane wave (FP-LAPW) method within the frame work of density functional theory (DFT). The structural parameters are also investigated by analytical techniques. The calculated structural parameters are consistent with the experimental results. The strong hybridization of the O-2p, Co-3d and Pr/Nd-4 f states around the Fermi level reveals that these compounds are metallic. It is also found that the origin of ferromagnetism in these compounds is double-exchange interaction between Co-3d states via O-2p states (Co-O-Co).     

Facetting and (n × 1) reconstructions of SrTiO3(1 1 0) surfaces

(n × 1) reconstructions and facetting of the (1 1 0) polar surface of SrTiO₃ are studied by means of a combination of shell model and density functional calculations. The polarity compensation can be achieved through the formation of {1 0 0} nano-facets, which play a crucial role in the reconstruction process. The behaviors of various possible terminations (Sr, Ti, and O) are analyzed, as well as their atomic structure and energetics. Their stability in different chemical environments is discussed, with respect to previous formulations and experimental results. The Sr-terminated surface tends to expose large facets, while the TiO and O terminations are marginally stabilized or even destabilized by (n × 1) reconstructions, respectively. Trend to facetting results from a subtle competition between the thermodynamic stability of the ideal non stoichiometric (n × 1) surfaces, and huge atomic relaxations that contribute to the lowering of the surface energy differently for each termination.     

Microwave losses in ABO3 type perovskites

Using Green’s function method, microwave lossess were theoretically calculated for BaTiO₃, SrTiO₃ and KTaO₃ ferroelectric perovskites. In microwave range, an increase in frequency is followed by an increase in the dielectric loss. In the paraelectric phase, the dielectric loss decreases with increasing temperature showing the Curie-Weiss behaviour of the tangent loss.     

Structural characterization, magnetic behavior and high-resolution EELS study of new perovskites Sr2Ru2−xCoxO6−δ (0.5?x?1.5)

New oxides of general formula Sr₂Ru_2−xCo_xO_6−δ (0.5?x?1.5) have been synthesized as polycrystalline materials and characterized structurally by X-ray diffraction. For 0.5?x<0.67 the orthorhombic, Pnma, perovskite structure of the end member, SrRuO₃, is found. At x=0.67 a phase separation into an Ru-rich Pnma phase and a Co-rich I2/c phase occurs. The I2/c form is also found for x=1.0 but another orthorhombic phase, Imma, obtains for x=1.33 and 1.5. Reductive weight losses indicate negligible oxygen non-stoichiometry, i.e., δ∼0, for all compositions even those rich in Co. High-resolution electron energy loss spectroscopy (EELS) indicates that cobalt is high-spin Co³⁺ or high-spin Co⁴⁺ for all x. Appropriate combinations of Ru⁴⁺, Ru⁵⁺, HS Co³⁺ and HS Co⁴⁺ are proposed for each x which are consistent with the observed Ru(Co)-O distances. Significant amounts of Co⁴⁺ must be present for large x values to explain the short observed distances. Broad maxima in the d.c. susceptibilities are found between 78 and 97 K with increasing x, along with zero-field-cooled (ZFC) and field-cooled (FC) divergences suggesting glassy magnetic freezing. A feature near 155 K for all samples indicates a residual amount of ferromagnetic SrRuO₃ not detected by X-ray diffraction.     

Magnetic and electric properties of layered perovskites Nd2−2xSr1+2xMn2O7 (x=0.3–0.5)

Magnetic and electric properties of layered perovskites Nd_2−2xSr_1+2xMn₂O₇ (x=0.3, 0.4 and 0.5) are sensitive to the doping content x. The sample with x=0.5 is antiferromagnetic (AFM) and insulating. On decreasing x, the AFM ordering is suppressed and a canted AFM or weak ferromagnetic (FM) ordering appears, and the resistivity decreases. The sample with x=0.4 still shows insulating behavior, but a metal–insulator transition is observed for x=0.3. By suggesting the presence of a competition between AFM super-exchange interaction and FM double-exchange interaction, the doping dependence of magnetic and electric properties can be understood.     

Effect of disorder on the electronic structure of the double perovskite Sr2FeMoO6

Recently, Sr₂FeMoO₆ has been established as a new colossal magnetoresistance material with substantial low-field magnetoresistance at room temperature and has attracted much attention in the double perovskite family. This material always appears with a certain degree of miss-site disorder where Fe and Mo interchange their positions. Using renormalized perturbation expansion, we calculate the density of states and determine the variation of the critical temperature in the low disorder regime.     

Rattling in the Quadruple Perovskite CuCu3V4O12

Of particular interest is a peculiar motion of guest atoms or ions confined to nanospace in cage compounds, called rattling. While rattling provides unexplored physical properties through the guest–host interactions, it has only been observed in a very limited class of materials. Herein, we introduce an A‐site‐ordered quadruple perovskite, CuCu₃V₄O₁₂, as a new family of cage compounds. This novel AA′₃B₄O₁₂‐type perovskite has been obtained by a high‐pressure synthesis technique and structurally characterized to have cubic Im$\bar 3$ symmetry with an ionic model of Cu²⁺Cu²⁺₃V⁴⁺₄O₁₂. The thermal displacement parameter of the A‐site Cu²⁺ ion is as large as U_iso≈0.045 Ų at 300 K, indicating its large‐amplitude thermal oscillations in the oversized icosahedral cages. Remarkably, the presence of localized phonon modes associated with rattling of the A‐site Cu²⁺ ion manifests itself in the low‐temperature specific heat data. This work sheds new light on the structure–property relations in perovskites.