Estimates of electronic interaction parameters for LaMMO3_3 compounds (MM=Ti-Ni) from ab-initio approaches

We have analyzed the ab-initio local density approximation band structure calculations for the family of perovskite oxides, La$M$O$_3$ with $M$=Ti-Ni within a parametrized nearest neighbor tight-binding model and extracted various interaction strengths. We study the systematics in these interaction parameters across the transition metal series and discuss the relevance of these in a many-body description of these oxides. The results obtained here compare well with estimates of these parameters obtained via analysis of electron spectroscopic results in conjunction with the Anderson impurity model. The dependence of the hopping interaction strength, t, is found to be approximately $r^{-3}$.Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct 15,1996

Electrocaloric effect and energy storage in lead free Gd0.02Na0.5Bi0.48TiO3 ceramic

International audienceThe electrocaloric effect (EC) was investigated in the ferroelectric lead-free Gd0.02Na0.5Bi0.48TiO3 (GdNBT) ceramics. A pure perovskite structure was observed by the X-ray diffraction in GdNBT ceramics synthesized by a solid-state reaction method. Dielectric measurements revealed the existence of different ferroelectric and antiferroelectric transformations in this material. Based on the temperature dependence of P-E loops, reversible electrocaloric temperature changes of about Delta T = 0.75 K at the electric field of 90 kV/cm and the EC responsivity Delta T/Delta E approximate to 0.08 x 10(-6) K m V-1 were obtained in the GdNBT ceramics. The energy-storage density W = 0.85 J cm(-3) and the associated efficiency (eta = 65%) at 413 K were determined from the P-E loops data. The simultaneous existence of EC responsivity and energy-storage density makes GdNBT very promising lead-free material and opens a great potential for its application in electronic devices. (C) 2017 Elsevier Masson SAS. All rights reserved

Electrostatic energy storage in antiferroelectric like perovskite

International audienceLead-free ferroelectric Na0.5Bi0.5TiO3 (NBT) and NBT-doped Ln (Ln = Er and Ho) in both bulk and thin films forms were processed via solid-state reaction and solution deposition respectively. SEM investigations showed dense samples with fine-grained and smooth microstructure for thin films. A pure perovskite structure was observed by the X-ray diffraction for all specimens. The frequency dependences of the dielectric constant and loss tangent were investigated at room temperature and revealed a decrease in the dielectric losses with the introduction of the lanthanide element. The energy storage density (Wrec) was calculated using the obtained P–E loops. The stored and delivered energies were found depending on the form and composition of the sample

Effect of Fock exchange on the electronic structure and magnetic coupling in NiO

The effect of Fock exchange on the periodic description of the geometrical structure, elastic constants, and electronic and magnetic properties of NiO is analyzed. Hybrid density functionals which combine a portion of "exact" Fock exchange with conventional local density approximation (LDA) or generalized gradient approximation (GGA) functionals remedy a number of serious inconsistencies with the traditional LDA or GGA descriptions of this prototypical "Mott" insulator. For example, the hybrid B3LYP functional (which mixes ∼20% Fock exchange with GGA functionals) introduces a significant insulating gap and yields antiferromagnetic Heisenberg coupling constants between Ni sites (J 2 ) in semiquantitative agreement with experiment. Closer inspection shows that while the B3LYP orbital band gap is in excellent agreement with experiment, the magnitude of the antiferromagnetic coupling is overestimated by slightly more than 50%. This has led us to examine a simplified model which combines Fock exchange with the LDA exchange and correlation functionals. This combination allows us to study the magnitude and nature of the band gap, the magnitude of the unpaired spin densities in the different magnetic phases, and the two most important magnetic coupling constants as a function of the fraction of Fock exchange included. It is concluded that ∼35% Fock exchange gives a reasonably balanced description of all properties, including structural parameters, magnetic form factors, the antiferromagnetic Ni-Ni exchange constant, and the character and magnitude of the band gap