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[en] We report the impact of Cr ion doping on structural and dielectric properties of Mg0.5Zn0.5−xCrxCo2O4 (0.0 ≤ x ≤ 0.5). All the samples were prepared by high-temperature solid-state reaction route. X-ray diffraction patterns reveal that Mg0.5Zn0.5−xCrxCo2O4 (0.0 ≤ x ≤ 0.5) crystallizes into more than one phase. Rietveld analysis of Synchrotron-based X-ray diffraction also confirms the existence of secondary phases corresponding to the cubic structure with Fd-3m and Fm3m space group. Scanning electron microscopy (SEM) images confirm the growth in grain size as a result of high sintering temperature. The FTIR spectra of doped cobaltites show two strong peaks at ~ 665 and 563 cm− 1 due to the presence of metal–oxygen bond. Variation in the dielectric constant and dielectric loss with frequency have also been investigated. Both decreases with increasing frequency of the applied alternating field and becomes constant at high frequencies that signify the substantial role of interfacial polarisation. It is also noted that the material having the smallest crystallite size (~ 29.16 nm) has high dielectric constant (~ 1.07 × 105) value. Cole–Cole plot indicates that all the samples of cobaltite show dominant grain capacitance.
[en] We report the specific heat C (T) behaviour of Al doped Mg1-xAlxB2 (x = 0, 0.1, 0.2), in the temperature domain 0 ≤T≤ 300 K. Phonon specific heat is well estimated from the Debye and Einstein temperature for Mg1-xAlxB2 obtained following an (RIM) overlap repulsive potential. The Fermions constituent as the electronic specific heat is deduced using a suitable trial function above and below Tc. At Tc the discontinuity in specific heat of parent MgB2 is higher than that of 10 and 20 % Al doping, the transition is sharper than for both parent and doped superconductors. (author)
[en] A simplified two-band model for describing the longitudinal dielectric function for a layered MgB2 system has been developed to reveal the attractive pairing mechanism leading to the superconducting state. A model dielectric function for σ and π band carriers is set up that fulfils the appropriate sum rules on the electronic and ionic polarizabilities. The value of the in-layer electron-phonon coupling strength and the Coulomb screening parameter for σ (π) band carriers is obtained from the residues of the dielectric function. It is noticed that the σ-holes in the two-dimensional boron planes via the screened phonon coupling is responsible for the attractive interaction and the superconductor is in the strong coupling regime. Within this framework, the superconducting transition temperature Tc of MgB2 is estimated as 42 K and the pressure derivative of Tc is negative. The isotope effect, coherence length, magnetic penetration depth, and volume derivative of Tc are also estimated to be consistent with the earlier results. Within the two-band model of superconductivity in MgB2, we place particular emphasis on intraband channels, carriers either in σ or π bands, as interband effects are negligible. In such a situation the superconductivity originates from either σ or π band carriers. The implications of the effective interactive potential with both σ and π carriers and its analysis are discussed.
[en] The pressure and temperature dependent elastic properties such as melting temperature nature in REX; (RE = La, Pr, Eu; X = O, S, Se, Te) chalcogenides is computed with emphasis on charge transfer interactions and covalent contribution in the effective interionic interaction potential. The pressure dependent elastic constants and melting temperature confirms that REX chalcogens lattice get stiffened as a consequence of bond compression and bond strengthening, however thermal softening arose due to bond expansion and bond weakening is evidenced from temperature dependence of melting temperature (TM)
[en] The high-pressure structural phase transition and pressure as well temperature induced elastic properties in ReY; (Re = La, Sc, Pr; Y = N, P, As, Sb, Bi) pnictides have been performed using effective interionic interaction potential with emphasis on charge transfer interactions and covalent contribution. Estimated values of phase transition pressure and the volume discontinuity in pressure-volume phase diagram indicate the structural phase transition from NaCl to CsCl structure. From the investigations of elastic constants the pressure (temperature) dependent volume collapse/expansion, second order Cauchy discrepancy, anisotropy, hardness and brittle/ductile nature of rare earth pnictides are computed.
[en] Samples of ZnO, Zn0.5Mg0.5O and MgO were prepared by co-precipitation method. X-ray diffraction (XRD) pattern infers that the sample of ZnO is in single-phase wurtzite structure (hexagonal phase, space group P63mc), MgO crystallizes in cubic Fd3m space group and Zn0.5Mg0.5O represents mixed nature of ZnO and MgO lattices. Similar features were observed from Raman spectroscopy. The energy band gaps estimated from UV-Vis spectroscopy are found to be 4.21 and 3.42 eV for ZnO and Zn0.5Mg0.5O samples respectively
[en] The pressure and temperature dependent mechanical properties as Young modulus, Thermal expansion coefficient of rare earth REX (RE = La, Pr, Eu; X = O, S, Se, and Te) chalcogenides are studied. The rare earth chalcogenides showed a structural phase transition (B1–B2). Pressure dependence of Young modulus discerns an increase in pressure inferring the hardening or stiffening of the lattice as a consequence of bond compression and bond strengthening. Suppressed Young modulus as functions of temperature infers the weakening of the lattice results in bond weakening in REX. Thermal expansion coefficient demonstrates that REX (RE = La, Pr, Eu; X = O, S, Se, and Te) chalcogenides is mechanically stiffened, and thermally softened on applied pressure and temperature.
[en] Multiferroic CoCr2O4 is synthesized by low-temperature sol-gel auto combustion technique. High energy synchrotron x-ray diffraction pattern confirms the single-phase cubic structure with space group Fd3m. Average crystallite size (17.91 nm) and negative micro-strain (9.86 × 10−4) are estimated by Williamson–Hall (W-H) plot. FTIR confirms the formation of spinel metal oxide-based cobalt chromites. The strong decrease in ε′ and tan δ at higher frequency can be interpreted by Maxwell-Wagner-type interfacial polarization. Weak ferroelectricity is mainly attributed to the partial reversal of polarization. J-E characteristic reveals the Ohmic (I–V) and Child’s square law (I–V2) behavior observed in CoCr2O4 nanomaterials with slope value ~ 1.04 and ~ 1.6, respectively. The conduction process for the leakage current density is interpreted using the space-charge limited current (SCLC) mechanism.
[en] Thermoelectric power of MgB2 superconductors is theoretically investigated and carrier diffusion contribution to the thermoelectric power (Scdiff) for σ band carriers is estimated using Mott expression, which exhibits a linear behavior. The difference between experimental data and Scdiff is discussed in terms of phonon drag (Sphdrag) mechanism. Following relaxation time approximation, Sphdrag is obtained when heat transfer is limited by the scattering of phonons from defects, grain boundaries, phonons and carriers. Behaviour of the S(T) is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between carrier diffusion and phonon drag contributions. (author)
[en] The pressure and temperature dependent mechanical properties as melting temperature, hardness and brittle nature of XBi (X = Am and Cf) are studied. The rare earth actinides pnictides showed a structural phase transition (B1–B2) at a transition pressure (P_T) of 14.3 GPa (AmBi) and 10.8 GPa (CfBi). Pressure dependence of melting temperature (T_m) discerns an increase inferring the hardening or stiffening of the lattice as a consequence of bond compression and bond strengthening. Suppressed T_M as functions of temperature infers the weakening of the lattice results in bond weakening in XBi (X = Am, Cf). Vickers Hardness (H_V), Poisson’s and Pugh ratio of XBi (X = Am and Cf) demonstrates that XBi (X = Am and Cf) is mechanically stiffened, thermally softened and brittle on applied pressure and temperature.