Saswata Halder

Time–temperature superposition in the grain and grain boundary response regime of A2HoRuO6 (A = Ba, Sr, Ca) double perovskite ceramics: a conductivity spectroscopic analysis

The pursuit for a universal scaling factor to satisfy the time–temperature superposition principle for grain and grain boundary responses has been explored in the ac conductivity domain for polycrystalline double perovskite oxides A2HoRuO6 (AHR; A = Ba, Sr, Ca). The samples show different structural phases, from cubic to monoclinic, with decreasing ionic radii. The degree of distortion in the materials is correlated to the strength of the bonding through the bond valence sum (BVS) formalism. The conductivity spectra for all of the samples obey the power law variation. The contribution of different microstructural domains to the conduction process is established. Thermal variation of the dc resistivity points towards a gradual crossover from nearest neighbour to variable range hopping. The activation energies obtained from the dc conductivity, hopping frequency and relaxation frequency show close correlation between the conduction and relaxation mechanisms. The scaled conductivity curves for AHR display the presence of two different conduction processes with dissimilar activation energies in the grain boundary and the grain response regimes. It is thus concluded that a single scaling parameter is insufficient to satisfy the time temperature superposition principle universally when two different thermally activated regions are present simultaneously in the materials.

Exploring the electronic structure and optical properties of double perovskite Ba2RESbO6 (RE = Ho, Er) from first-principles calculations

ABSTRACT First principles density functional theory (DFT) calculations are performed to explore the electronic structure and optical properties of Ba2RESbO6 (RE = Ho, Er) in the cubic symmetry using the experimental lattice parameters. The calculations are performed in two different DFT approaches, FP-LAPW method and plane wave pseudo potential method. The electronic band structures of these compounds were analyzed using the generalized gradient approximation (GGA). A direct band gap (Γ-Γ for Ba2HoSbO6 and Ba2ErSbO6 was obtained. The calculated optical properties were analyzed from band structure calculations. The Born effective charges and the static dielectric tensor of these materials were also calculated.

Understanding the photoluminescence characteristics of Eu3+-doped double-perovskite by electronic structure calculation

Europium-doped luminescent barium samarium tantalum oxide Ba2SmTaO6 (BST) has been investigated by first-principles calculation, and the crystal structure, electronic structure, and optical properties of pure BST and Eu-doped BST have been examined and compared. Based on the calculated results, the luminescence properties and mechanism of Eu-doped BST has been discussed. In the case of Eu-doped BST, there is an impurity energy band at the Fermi level, which is formed by seven spin up energy levels of Eu and act as the luminescent centre, which is evident from the band structure calculations.