Alo Dutta

AC conductivity and dielectric relaxation in CaMg1/3Nb2/3O3

The complex perovskite oxide calcium magnesium niobate, CaMg1/3Nb2/3O3CaMg1/3Nb2/3O3 (CMN) is synthesized by a solid-state reaction technique. Dielectric spectroscopy is applied to investigate the electrical properties of CMN in a temperature range from room temperature to 503 K and in a frequency range of 100 Hz–1 MHz. An analysis of the dielectric constant ϵ′ϵ′ and loss tangent tan⁡δtan⁡δ with frequency is performed assuming a distribution of relaxation times. The low-frequency dielectric dispersion corresponds to the DC electrical conductivity. The frequency dependence of the loss peak is found to obey an Arrhenius law with an activation energy of 1.8 eV. The frequency dependent electrical data are analyzed in the framework of the conductivity and modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary part of electric modulus M″M″ suggests that the relaxation describes the same mechanism at various temperatures in CMN.

Dielectric relaxation and electronic structure of double perovskite Sr2FeSbO6

The dielectric property and the electronic structure of a double perovskite, Sr2FeSbO6 (SFS) synthesized by solid state reaction technique are investigated. The x-ray diffraction of the sample taken at room temperature shows cubic phase. The scanning electron micrograph of the sample also confirms the formation of the single phase of the material. We have measured the capacitance and conductance of SFS in a frequency range from 50 Hz to 1 MHz and in a temperature range from 163 to 463 K. A relaxation is observed in the entire temperature range as a gradual decrease in ϵ′(ω) and as a broad peak in ϵ″(ω). The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with an activation energy of 0.74 eV. The Cole–Cole model is used to study the dielectric relaxation of SFS. The scaling behavior of imaginary part of el...

Electronic Structure of Sr2AlNbO6: Dielectric and X-ray Photoemission Spectroscopy Study

The dielectric property and the electronic structure of double perovskite oxide strontium aluminium niobate, Sr2AlNbO6 (SAN), synthesised by a solid state reaction technique are investigated. The Rietveld refinement of the X-ray diffraction data of the sample shows the cubic phase with the Fm3m structure (lattice parameter, a = 7.7789 A). The compound shows significant frequency dispersion in its dielectric properties. The Cole–Cole model is used to study the dielectric relaxation of SAN. The scaling behavior of loss tangent suggests that the relaxation describes the same mechanism at various temperatures. The electronic structure of SAN is studied by the first principles full potential linearized augmented plane wave method. Its valence band consists mainly of the O 2p states hybridised with the Nb 4d states. The calculated density of states is used to compare the valence band X-ray photoemission spectra of the sample.

Dielectric relaxation and electronic structure of BaAl1/2Nb1/2O3: x-ray photoemission and nuclear magnetic resonance studies

The frequency-dependent dielectric relaxation in barium–aluminium–niobate, BaAl1/2Nb1/2O3 (BAN), at low temperatures (103–443 K) is investigated by alternating-current impedance spectroscopy in the framework of conductivity and electric modulus formalisms. The Havriliak–Negami expression is used to analyse the electric modulus data. The scaling behaviour of the imaginary part of the electric modulus suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent conductivity spectra follow the power law. The electronic structure of BAN is studied using x-ray photoemission spectroscopy (XPS). The XPS data are analysed by the first-principles full potential linearized augmented-plane-wave method using density functional theory under the generalized gradient approximation. The electronic structure calculation reveals that the electrical properties of BAN are dominated by the interaction between niobium d-states and oxygen p-states. The 27Al and 93Nb nuclear magnetic resonance (NMR) studies of the sample are performed at 78 and 73 MHz, respectively, in the temperature range 4–295 K to understand the transport properties of charge carriers in terms of their dynamics on a microscopic level. The description of the NMR lineshape is given on the basis of analytical formulae. The NMR investigation confirms the chemical ordering of 1:1 Al/Nb in BAN.

Structural and Ferroelectric Properties of Complex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 (x = 0.00, 0.05, 0.1, 0.15)

We have synthesized the complex perovskite oxides Pb(1-x)Bax(Fe1/2Ta1/2)O3 (where x = 0.0, 0.05, 0.10 and 0.15) by two steps Coulombite precursor process in cubic phase and studied the effect of Ba substitution in Pb(Fe1/2Ta1/2)O3 through the dielectric and ferroelectric properties in the frequency range from 100 Hz to 1 MHz and in the temperature range from 118 to 363 K. The temperature dependence of the dielectric constant at different frequencies gives diffuse peaks which have been attributed to the occurrence of relaxor ferroelectric behaviour in Pb(1-x)Bax(Fe1/2Ta1/2)O3. The magnitudes of ϵ′m, the maximum value of dielectric constant and Tm, the temperature corresponding to ϵ′m are decreased with an increase of Ba2+ ion in the materials. There is evidence of Vogel-Fulcher type relaxational freezing in the samples. The analysis of real and imaginary parts of the dielectric permittivity with frequency has been performed assuming a distribution of relaxation times as confirmed by Cole-Cole plots.

Synthesis and electric characterization of rare earth double perovskite Ho2CdZrO6 ceramics

A new double perovskite oxide holmium cadmium zirconate Ho2CdZrO6 (HCZ) is synthesized by solid state reaction technique. The crystal structure has been determined by powder X-ray diffraction (XRD) which shows monoclinic phase at room temperature. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The scaling behavior of imaginary electric modulus suggests that the relaxation describe the same mechanism at various temperatures. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions. The conduction mechanism is explained by Mott’s theory. At the high temperature range, conductivity data satisfy the variable range hopping (VRH) model. In this regime, the conductivity of sample obeys Mott’s T1/4 law indicating 3D charge transport in HCZ compound. High temperature data indicates the formation of thermally activated small polarons.

Dielectric Relaxation and Electronic Structure of Double Perovskite Ca2AlNbO6

The dielectric property and the electronic structure of calcium aluminium niobate, Ca2AlNbO6 (CAN) synthesized by solid state reaction technique are investigated. The field dependence of dielectric response, loss tangent and conductance is studied in the temperature range from 303 to 443 K and in a frequency range from 50 Hz to 1 MHz. The Cole-Cole model is used to analyze the distribution of relaxation times. The temperature dependence of the most probable relaxation time obtained from imaginary part of dielectric function follows the Arrhenius law with the activation energy of 0.50 eV. The electronic structure of CAN is investigated using the first principles full potential linearized augmented plane wave method. A direct band gap of 3.3 eV is obtained between the top most valence band of O-2p character and the bottom of conduction band of Nb-4d character. The x-ray photoemission spectroscopy (XPS) study of the sample is performed to examine the electronic structure calculation. The calculated density of states is compared with the experimental XPS valence band spectrum.

Electronic, optical and thermoelectric properties of PrMO3 (M = Al, Ga, In) from first-principles calculations

We have systematically investigated the electronic structure, optical properties, Born effective charges and the thermoelectric properties of PrMO3 (M = Al, Ga, In) compounds using first-principles density functional theory calculations. Two different density functional approaches, the full potential linearized augmented plane wave method (FP-LAPW) and the plane wave pseudo potential method were used for the present study. A direct band gap at a Γ point of 3.8 eV, 3.07 eV and 2.9 eV is observed for PrAlO3 (PAO), PrGaO3 (PGO) and PrInO3 (PIO). The optical anisotropy of PAO and PGO is revealed from the computed optical properties such as complex dielectric function, refractive index, and absorption coefficient whereas PIO is found to be optically isotropic in the low energy range making it suitable for use in the development of ceramic scintillators. The inter-band transitions to the optical properties are analyzed with the help of calculated band structures. The Born effective charge and the static dielectric tensor of these materials have also been calculated.

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...

Electrical properties of the double perovskite oxide Ho2CuZrO6

The double perovskite oxide holmium copper zirconate, Ho2CuZrO6 (HCZ), was synthesized by a solid-state reaction technique. The crystal structure of HCZ shows a monoclinic phase. The dielectric relaxation of HCZ was investigated in the frequency range 44 Hz–1 MHz and in the temperature range 40–360 °C by using impedance spectroscopy. The complex impedance data were analysed by the Cole–Cole model. The ac conductivity follows the power law. The value of activation energy obtained from the temperature dependence of the dc conductivity plot indicates a hopping-type conduction mechanism. The scaling behaviour of the imaginary part of impedance indicates that relaxation in HCZ describes the same mechanism at various temperatures.