N. K. Gaur

Dielectric, magnetic, and thermodynamic properties of Y1−xSrxMnO3 (x = 0.1 and 0.2)

We report the effect of strontium (Sr) doping on dielectric, magnetization, and thermodynamic properties of polycrystalline Y1−xSrxMnO3 (x = 0.1, 0.2) samples prepared by conventional solid-state reaction method. The temperature dependent dielectric permittivity and specific heat curves noticeably show the anomalies near its magnetic transition point, i.e., Neel temperature (TN), which are probably generated by the frustrated Mn3+ spins (S = 2) on a triangular Mn lattice showing the magneto-electric coupling between the electric and magnetic orders. However, the magnetic transition is not clearly evident in the magnetic susceptibility due to the frustration on the Mn triangular lattice and the dominating paramagnetic susceptibility of the Y3+ spins. The electronic transport mechanism in these materials was analyzed within the framework of conventional Arrhenius Law, i.e., ρ(T) = C exp(Ea/kBT) applied to low frequency ac resistivity data, and through the peak method employed to the permittivity and loss ta...

Structural and elastic properties of barium chalcogenides (BaX, X=O, Se, Te) under high pressure

In the present paper we have investigated the high-pressure, structural phase transition of Barium chalcogenides (BaO, BaSe and BaTe) using a three-body interaction potential (MTBIP) approach, modified by incorporating covalency effects. Phase transition pressures are associated with a sudden collapse in volume. The phase transition pressures and associated volume collapses obtained from TBIP show a reasonably good agreement with experimental data. Here, the transition pressure, NaCl-CsCl structure increases with decreasing cation-to-anion radii ratio. In addition, the elastic constants and their combinations with pressure are also reported. It is found that TBP incorporating a covalency effect may predict the phase transition pressure, the elastic constants and the pressure derivatives of other chalcogenides as well.

Lattice dynamics of manganites RMnO3 (R =Sm, Eu or Gd): instabilities and coexistence of orthorhombic and hexagonal phases

The lattice dynamical and allied properties of the multiferroic manganites SmMnO3, EuMnO3 and GdMnO3 were investigated in this work by means of a shell model with transferable pairwise interionic interaction potential. This shell-model potential is able to reproduce the available crystal structure and phonon frequencies. A zone center imaginary Au mode is observed in these lattice dynamics calculations that indicates metastability of the perovskite phase. Comparison of the Gibbs free energies in the orthorhombic and hexagonal phases points to the possible coexistence of the two phases of these manganites under ambient conditions.

LOW TEMPERATURE SPECIFIC HEAT OF (KBr)1-x(KCN)x MIXED CRYSTAL

We have investigated the cohesive and thermodynamic properties of (KBr)1-x(KCN)x using an extended three-body force shell model (ETSM), which has been found to be adequately suitable for the description of orientationally disordered mixed cyanide-halide crystals. The specific heat of (KBr)1-x(KCN)x for compositions (x=0.53, 0.65, 0.73, 0.84 and 0.93) at temperature 10 K≤T≤150 K have been computed using the ETSM for the first time. The paper also reports the calculated results on Debye temperature (ΘD), cohesive energy (Φ), compressibility (β), molecular force constant (f) and Restrahlen frequency (ν0) of (KBr)1-x(KCN)x. The results on specific heat and some other thermal properties are in good agreement with their available experimental data.

Lattice dynamics of orthorhombic perovskite yttrium manganite, YMnO3

The lattice dynamics of yttrium manganite (YMnO(3)) has been investigated by means of a shell model with pair-wise interionic interaction potential. The experimental data of crystal structure and Raman and infrared frequencies compare well with the lattice dynamical calculations. The phonon dispersion curves found along three high symmetry directions and the density of states of YMnO(3) have also been calculated from this model. The computed phonon density of states is used to derive the macroscopic thermodynamic quantities like the Debye temperature and specific heat. The crystal structure data computed from this model are in good agreement with the available experimental data measured by neutron powder diffraction. We have made a comparative study of the structures derived from the potential model calculations for both LaMnO(3) and YMnO(3). Symmetry vectors obtained through group theoretical analysis at the zone centre point were employed to classify the phonon frequencies obtained into their irreducible representations. The computed Raman and infrared frequencies have shown good agreement with the measured data.

Thermodynamic properties of SmMnO3, Sm0.55Sr0.45MnO3 and Ca0.85Sm0.15MnO3

We have investigated thermodynamic properties of the perovskite Sm(Ca,Sr)MnO3 manganites, probably for the first time, using the rigid ion model (RIM) after modifying its framework to incorporate the van der Waals attraction and the short-range Hafemeister–Flygare type overlap repulsion operating between the first and second neighbour ions. We have found that the evaluated thermodynamic properties reproduce the corresponding experimental data well, implying that the modified RIM properly represents the elastic nature of these perovskite manganite systems. The results of the present investigation can be further improved by including the ferromagnetic spin wave contribution and Jahn–Teller distortion effect in the framework of the modified RIM.

LOW TEMPERATURE SPECIFIC HEAT OF INTERMETALLICS (Ti/Be)B2

We have applied the Rigid Ion Model (RIM) to study the cohesive and thermal properties of binary intermetallic BeB2 and TiB2. The paper reports the calculated results on cohesive energy (ϕ), compressibility (β), molecular force constant (f), Restrahalen frequency (ν0), Debye temperature (ΘD) and Gruneisen parameter (γ) for the temperature range 50 K ≤ T ≤ 300 K and the effect of van der Waal interaction on these properties are also shown. Our results on Debye temperature are closer to the experimental data. In addition, we have computed the specific heats for BeB2 and TiB2 and compared them with the available experimental data.

Effect of Hg addition on synthesis of Bi-based superconductors

Abstract We have investigated the effect of Hg addition on the superconducting properties of BiSrCaCuO system. Polycrystalline samples with nominal composition Bi 2 Sr 2− x Hg x Ca 1 Cu 2 O y and Bi 2 Sr 2− x Hg x Ca 2 Cu 3 O y ( x =0.3) were synthesized and used to investigate the phase evolution by XRD, superconducting behaviour by R – T measurement and the structural grain boundary effects by SEM. From these measurements, it has been noticed that the phases obtained with both types of compositions are the same as Bi2212 but the T c values are different. With additional annealing, T c zero values were raised from 60 to 72 K in Bi 2 Sr 2− x Hg x Ca 1 Cu 2 O y and 64 to 92 K in Bi 2 Sr 2− x Hg x Ca 2 Cu 3 O y . Also, an improved grain boundary linkage has been observed by SEM for the 92 K sample.

Investigations of Thermal Properties of Earth Mantle Silicates: ASiO3 (A = Mg, Ca)

We have investigated the thermal properties and specific heat of earth mantle silicates ASiO3 (A = Mg, Ca) by the means of rigid ion model (RIM) and compared them with the earlier experimental reports. The calculated specific heat of ASiO3 (A = Mg, Ca) obeys T3 law at low temperature (T < θD/50). The temperature dependence of the specific heat of MgSiO3 and CaSiO3 (0 K ≤ T ≤ 2000 K) is presented along with the elastic, cohesive and thermal properties, like Bulk modulus (B), cohesive energy (φ), molecular force constant (f), reststrahlen frequency (ν), Debye temperature (θD) and Gruneisen parameter (γ) at T = 300 K.

Structural properties of silver iodide and copper iodide

The structural changes within the Silver iodide (AgI) and Copper iodide (CuI) induced by pressure have been investigated using an effective interaction potential. CuI and AgI in their parent zinc blende (ZnS) to rock salt (NaCl) through an intermediate structure have been reported. The calculated values for the phase transition pressures and associate volume collapses are generally in good agreement with measured data.