Sadhan Chanda

Magnetic ordering and conduction mechanism of different electroactive regions in Lu2NiMnO6

The magnetodielectric response of a double perovskite oxide Lu2NiMnO6 (LNMO) synthesised by the sol–gel process has been investigated. The Rietveld refinement of the X-ray diffraction data indicates that the room-temperature crystal structure of LNMO is monoclinic with the space group P21/n, which contains an ordered array of alternate MnO6 and NiO6 octahedra. X-ray photoelectron spectroscopy studies confirm the mix valence state of Mn (4+ and 3+) and Ni (2+ and 3+). The optical band gap (Eg = 1.56 eV) obtained from the UV-Visible absorption spectrum suggests that LNMO is a semiconductor. The field cooled and zero-field cooled measurements show the ferromagnetic behaviour of the sample with the transition temperature (Tc) = 45 K and a saturation magnetization of 5.2 μB/f.u. is observed at 2.5 K. The temperature and frequency dependent dielectric measurements reveal colossal values of the dielectric constant, which are interpreted by the Maxwell-Wagner interfacial polarization. The temperature dependence o...

Electronic structure and transport properties of antiferromagnetic double perovskite Y2AlCrO6

The antiferromagnetic G-type magnetic ordering in Y2AlCrO6 (YAC) has been investigated by electronic band structure calculations. The material is synthesised by a sol–gel technique and the electronic structure calculations are initiated by the experimental lattice parameters, obtained from the Rietveld refinement of the X-ray diffraction data. The Rietveld refinement shows that the room-temperature crystal structure of YAC is monoclinic with the space group P21/n, and contains an ordered array of alternating AlO6 and CrO6 octahedra tilted along the three pseudocubic axes according to the Glazer notation a−a−b+. The Raman spectrum of the sample is observed for P21/n symmetry. The field cooled and zero field cooled measurements of the sample are performed at a magnetic field of 100 Oe in the temperature range from 5 to 300 K. The temperature dependent magnetization shows the anti-ferromagnetic ordering of Cr ions in YAC. The calculated magnetic moment is well matched with the experimental magnetic moment and suggests the 3+ oxidation state of Cr with the canted alignment of its spin. The octahedral co-ordination of Cr3+ ions in YAC is confirmed from the photoluminescence spectrum. The band gap obtained from the diffuse reflectance measurements shows the semiconducting nature of the material. To observe the effect of grains, grain-boundaries and electrodes in the conduction process, the dielectric relaxation of YAC has been investigated using alternating current impedance spectroscopy in the frequency range from 50 Hz to 5 MHz as a function of temperature. An electrical equivalent circuit consisting of the resistance and the constant phase element is used to explain the impedance data. The observed results are used to discuss the effect of substitution of Cr by Al in the parent compound YCrO3.

Investigation of light induced charge transport properties in Dy2NiMnO6 perovskite based Schottky diode

ABSTRACT Here, we have discussed the charge transport phenomena through the interface formed by silver and double perovskite oxide Dy2NiMnO6 (DNMO). The charge carrier transport mechanism in Ag/DNMO junction under dark and illuminated conditions was analyzed on the basis of the thermionic emission theory of metal-semiconductor junction. The Schottky diode parameters like ideality factor, barrier potential height and series resistance of the freshly fabricated device were evaluated under dark and illuminated conditions. The results show significant improvements in the rectification ratio, barrier potential height and ideality factor of the device upon light illumination.

Structural, Raman spectroscopy and dielectric relaxation study of nanoceramics NdFeO3

The nanoceramic NdFeO3 was synthesized by the sol–gel citrate method. The Rietveld refinement of X-ray diffraction pattern at room temperature (30°C) shows the orthorhombic Pnma phase of the perovskite type with lattice parameters a = 5.576 A, b = 7.756 A, c = 5.447 A. The TEM image shows that the sample is made of nano-sized crystallites ranging from 45 to 75 nm. The room temperature Raman spectrum is fitted with the sum of 18 Lorentzian peaks. Eigen frequencies of vibrational modes are calculated by density functional theory in its generalized gradient approximation as implemented in Vienna Ab-initio Simulation Package (VASP). Dielectric spectroscopy (ACIS) study of NdFeO3 has been performed in the frequency range from 50 Hz to 1 MHz and in the temperature range from 50° C to 240° C. The frequency dependence of real and imaginary parts of dielectric permittivity is analyzed using modified Cole-Cole equation.

Vibrational modes and electronic structure of Sr2GdTaO6

The electronic structure of Sr2GdTaO6 (SGT) synthesized by solid state reaction technique, has been investigated by first-principles calculations and the eigen frequencies of different phonon modes have been calculated. The calculated DOS indicates a band gap of 3.30 eV between the top most valence band of O-2p character and the bottom of conduction band of Ta-5d character. The interaction between O-p state and Ta-d state is responsible for conduction mechanism in SGT as observed in DOS spectrum. Fourier transform infrared (FTIR) spectrum shows two primary phonon modes of the sample at around 373 cm−1 and 562 cm−1. Raman spectrum of the sample taken at 488 nm excitation wavelength shows four primary strong peaks at 150, 420, 560 and 790 cm−1. Lorentzian lines with 18 bands have been used to fit the Raman spectrum.

Structural and dielectric properties of Y1/2Er1/2FeO3

The complex perovskite oxide yttrium erbium iron oxide, Y1/2Er1/2FeO3 is synthesized by the solid-state reaction technique. The Rietveld refinement of X-ray diffraction pattern at room temperature (30°C) shows the orthorhombic Pnma phase with lattice parameters a = 5.582 A, b = 7.593 A, c = 5.267 A. Impedance spectroscopy study of Y1/2Er1/2FeO3 has been performed in the frequency range from 50 Hz to 1 MHz and in the temperature range from 303 K to 503 K. The frequency dependence of real and imaginary parts of dielectric permittivity is analyzed using modified Cole-Cole equation. The complex impedance plane plots of the sample shows that both grain and grain-boundary effects are present in the relaxation (conduction) process. The frequency dependent conductivity spectrum is found to follow the power law.