T.P. Sinha

Structural and dielectric studies of BaFe0.5Nb0.5O3

Ferroelectric BaFe0.5Nb0.5O3 (BFN) ceramic is synthesized by the solid-state reaction technique for the first time. The x-ray diffraction of the sample at room temperature shows a monoclinic phase. Dielectric studies of the sample show a frequency dependence of the temperatures at which the dielectric permittivity (real and imaginary) peaks. The temperature variations of the real and imaginary components of the dielectric permittivity show broad maxima. There is evidence for Vogel-Fulcher-type relaxational freezing. The analysis of the 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 as well as the scaling behaviour of the dielectric loss. All these observations clearly suggest that BFN is a relaxor ferroelectric. The Mossbauer spectrum of the sample at room temperature shows a symmetric doublet, with the iron being trivalent.

Dielectric relaxation in SrFe1∕2Nb1∕2O3

The strontium iron niobate SrFe1∕2Nb1∕2O3 ceramic is synthesized by a solid-state reaction technique. The x-ray diffraction of the sample at room temperature shows a monoclinic phase. The dielectric permittivity and the loss tangent of the sample are measured in a frequency range from 50Hzto2MHz and in a temperature range from 143to473K. The frequency dependence of the loss peak is found to obey an Arrhenius law with an activation energy of 0.272eV. An analysis of the loss factor with frequency is performed by using the scaling behavior of the dielectric loss spectra. The scaling behavior of the dielectric loss spectra shows its temperature-independent nature. The relaxation mechanism is discussed in the framework of conductivity and impedance spectroscopy. The variation of dielectric constant with temperature is explained considering the space-charge polarization.

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.

Structural and optical properties of paraelectric SrTiO3

The electronic energy band structure, site- and angular-momentum-decomposed densities of states (DOS) and charge-density contours of perovskite SrTiO3 in the paraelectric cubic phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows a direct band gap of ~1.4 eV at the gamma point in the Brillouin zone. The total DOS is compared with experimental x-ray photoemission spectra. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Sr and TiO3 is mainly ionic and that the TiO3 entities bond covalently. Using the projected DOS and band structure we have analysed the interband contribution to the optical properties of SrTiO3 . The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient and absorption coefficient) and the electron energy-loss spectrum are calculated. The calculated spectra are compared with the experimental results for SrTiO3 in the cubic phase and are found to be in good agreement with the experimental results in low-energy regions. The role of band-structure calculation as regards the optical properties of SrTiO3 is discussed.

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...

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.

Large electrostrictive effect in (Ba1-xGd2x/3)Zr0.3Ti0.7O3 relaxor towards moderate field actuator and energy storage applications

The need of lead-free high performance ceramics with large electrostrictive effect, minimum hysteresis loss and energy storage ability at room temperature has become indispensable. At room temperature one of the key challenges in ceramic materials is to enhance the electrostrictive and energy storage properties simultaneously. In this regards, lead-free gadolinium modified barium zirconate titanate (Ba1-xGd2x/3)(Zr0.3Ti0.7)O3 (x = 0.02, 0.04, 0.06, 0.08, 0.10) ceramic was experimentally investigated to gain the competent electromechanical parameters near room temperature. Dielectric measurements exhibit a diffuse type of phase transition of relaxor phenomena and slim hysteresis loop with low remnant polarization and low hysteresis loss were observed. A moderate electric field of 30 kV/cm, recoverable energy and storage efficiency increases with Gd content. Strain-electric field hysteresis curves such as S-E, S-E2, and S-P2 profiles indicate improved electrostrictive characteristic of the ceramics. Results...

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.