arXiv subjects

A. K. Himanshu

Publications and source records attributed to A. K. Himanshu.

8 recordsLinked to original sources

arXiv2022

Multiple low-energy excitons and optical response of $d^0$ double perovskite Ba$_2$ScTaO$_6$

Large bandgap insulators are considered promising for applications such as photocatalysts, dielectric resonators and interference filters. Based on synchrotron X-ray diffraction, diffuse reflectance measurement and density functional theory, we report the crystal structure, optical response, and electronic properties of the synthesized $d^0$ double perovskite Ba$_2$ScTaO$_6$. In contrast to earlier prediction, the electronic bandgap is found to be large, $\sim 4.66$ eV. The optical response is characterized by the presence of multiple exciton modes extending up to the visible range. A detailed investigation of the direct gap excitons based on the Elliot formula is presented. Density functional theory based investigation of the electronic properties within generalized gradient approximation severely underestimates the electronic gap. To reach a quantitative agreement, we consider different available flavors of the modified-Becke-Johnson exchange-correlation potential and discuss their effects on the electronic and optical properties.

cond-mat.mtrl-sci↗

arXiv2021

Revised crystal structure and electronic properties of high dielectric Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ ceramics

Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_3$ (BFN) ceramics are considered to be promising for technological applications owing to their high dielectric constant over a wide range of temperatures. However, there exists considerable discrepancy over the structural details. We address this discrepancy through a combined x-ray diffraction at room temperature and neutron powder diffraction measurements in the range from 5K up to room temperature, supplemented by a comparative analysis of the earlier reported structures. Our study reveals a cubic structure with space group Pm$\bar{3}$m at all measured temperatures. Further, the x-ray near edge structure and the extended x-ray absorption fine structure studies on the local environment of the Fe ions is consistent with the cubic symmetry. An appropriate value of $U$ for DFT+$U$ calculations is obtained by comparison with x-ray absorption spectroscopy, which agrees well with the earlier reported electronic properties.

cond-mat.str-el↗

arXiv2019

The X-ray photoemission and Co K -Edge X-ray absorption of Ba2CoWO6

The structural, electronic and optical properties of cubic double perovskite BaCoWO6 have been studied. Neutron powder diffraction data is collected on this sample from 6K to 300K. The crystal structure is face centered cubic, space group being Fm3m (No. 225). We did not find evidence for long range magnetic ordering in this system in this temperature range. The band-gap is estimated using Uv-vis spectroscopy. The Co-K edge X-ray absorption (XAFS) spectra of Ba2CoWO6 was analysed together with those Co-foil, which was used as reference compounds. X-ray photoemission spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies give the insight of the electronic and structural information on the Co local environment for Ba2CoWO6.

cond-mat.mtrl-sci↗

arXiv2019

Electronic properties of A2Zr2O7 (A= Gd, Nd) ceramic

The density functional theory with generalized gradient approximation has been used to investigate the electronic structure of gadolinium pyrochlore A2Zr2O7 (A=Gd, Nd) ceramic synthesized in polycrystalline form by solid state reaction. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis of the XRD pattern. The Zr-K edge X-ray absorption (XAFS) spectra of A2Zr2O7 (A=Gd, Nd) were analysed together with those Zr-foil, which was used as reference compounds. X-ray photoemission spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for A2Zr2O7 (A=Gd, Nd) has been employed to obtain quantitative structural information on the Zr-local environment. The band gap is estimated using UV-Vis spectroscopy. The crystal structure is face centered cubic, space group being Fd-3m (No. 227). The total energies in this work were calculated using the generalized gradient approximation to DFT plus on-site repulsion (U) method.

cond-mat.mtrl-sci↗

arXiv2013

Structural and Optical Studies on Manganese doped, Modified Sodium Potassium Lithium Niobate Lead-free Piezoelectric Ceramics

Structural and Optical properties of the (Na0.5 K0.5)y Li 1-y NbO3 (y = 0.934 & 0.936) ceramics have been explored in terms of the X-Ray diffraction patters, Rietveld analysis, UV-Vis spectroscopy, and Kubelka-Munk functions. Rietveld analysis of both these structures reveals them to be tetragonal with space group P4mm. It is found that increase in Li doping increases the optical band gap. Presence of additional hardening dopant MnO2 is found to have a uniform behavior for both these composition. With increasing MnO2 concentration, the band gap decreases indicative of increase in B-site cation and O-p orbitals. Different valences of the Mn ions and relative difference in the ionic radii of the Mn ions and Nb ion are most likely the reason for these optical properties.

cond-mat.mtrl-sci↗

arXiv2013

Dielectric Investigation, Piezoelectric Measurement and Structural Studies of Strontium-doped, modified PMS-PZT Piezoelectric Ceramics

The structure and dielectric properties of 6% $\rm{Sr}^{2+}$-substituted, modified Lead Zirconium Titanate (PZT) piezoelectric ceramic, with composition $[\rm{Pb}_{0.94}\rm{Sr}_{0.06}][(\rm{Mn}_{1/3}\rm{Sb}_{2/3})_{0.05}(\rm{Zr}_{0.54}\rm{Ti}_{0.46})_{0.95}]O_3$ and lead manganese antimonite as an additional dopant, synthesized by ceramic route have been investigated in a frequency range from 50Hz to 1MHz and in a temperature range between room temperature and 633K. The scanning electron micrograph of the sample taken at the room temperature confirms the formation of a fairly homogenous structure and well-formed grains with sharp boundaries. From the Rietveld analysis, it was found that the structure of the material is tetragonal with space group P4mm. The scaling behavior of imaginary part of the electric modulus suggests that the relaxation is described by the same mechanism at various temperatures. The values of the electromechanical coupling factor ($\rm{k_p}$), the high mechanical quality factor ($\rm{Q_m}$) and the piezoelectric coefficient ($\rm{d}_{33}$) of the synthesized sintered & poled ceramics gives a good set of values for the piezoelectric properties when compared with related materials, useful for various interesting piezoelectric device applications.

cond-mat.mtrl-sci↗

arXiv2013

High values of Ferroelectric polarization and magnetic susceptibility in BFO nanorods

Remarkably high values of polarization as well as a significant magnetic susceptibility have been observed in multiferroic Bismuth Ferrite (BFO) in the form of nanorods protruding out. These were developed on porous Anodised Alumina (AAO) templates using wet chemical technique. Diameters of nanorods are in the range of 20-100 nm. The high values of polarization and magnetic susceptibility are attributed to the BFO nanorod structures giving rise to the directionality. There is no leakage current in P-E loop examined at various frequencies. Magnetocapacitance measurements reflect a significant enhancement in magnetoelectric coupling also.

cond-mat.mtrl-sci↗

arXiv2013

Remarkably high value of Capacitance in BiFeO3 Nanorod

A remarkably high value of specific capacitance of 450 F/g has been observed through electrochemical measurements in the electrode made of multiferroic Bismuth Ferrite (BFO) in the form of nanorods protruding out. These BFO nanorods were developed on porous Anodised Alumina (AAO) templates using wet chemical technique. Diameters of nanorods were in the range of 20-100 nm. The high capacitance is attributed to the nanostructure. The active surface charge has been evaluated electrochemically by cyclic voltammetry (CV) at different scanning rates and charge-discharge studies. The specific capacitances were constant after several cycles of charge-discharge leading to their useful application in devices. The mechanism of accumulation of charge on the electrode surface has been studied.

cond-mat.mtrl-sci↗