P. K. Bajpai

Raman spectroscopic study of alanine doped triglycine sulphate ferroelectric single crystals

Abstract Single crystals of triglycine sulphate doped with different concentrations of alanine were studied by Raman spectroscopy. Bands due to the subspecies modes from the three crystallographically distinct glycine ions in the lattice were identified. The anisotropy measurement in specific Raman scattering geometries as a function of concentration of alanine allow us to infer that alanine preferentially replaces glycine-I ions and induces realignment of other glycine units in the lattice.

Preparation, characterization and dielectric behaviour of some yttrium doped strontium stannates

Samples of Sr(1− 3x/2)YxSnO3 are prepared by usual solid state reaction route. X- ray diffraction studies confirm the formation of single cubic perovskite single phase. The dielectric constant and dielectric loss at 1 kHz were measured in the temperature range from room temperature up to -150†C. The dielectric constant decreases and losses increase with increased yttrium content in the samples. The percentage porosity and unit cell parameters are also calculated for the samples.

A fast technique for investigating the effect of biocides on lichens

A new method, based on ultra-weak photon emission, for estimating the physiological activity of lichen samples is described. Both Parmelia tinctorum and Peltigera polydactyla have been studied using this method. It has also been applied to study the effect of a number of biocides on Parmelia tinctorum. The minimum concentrations required for complete eradication of this species are 2% of Paraquat dichloride, 1·3% of Polycide, 1·2% of Diuran, 0·25% of benzylkonium chloride and 0·09% of sodium pentachlorophenol.

Relaxor behavior and dielectric relaxation in Pb(Ba1/3Nb2/3)O3: A phase pure new relaxor material

Pb(Ba1/3Nb2/3)O3 was prepared by two steps solid state reaction route. Material stabilized in orthorhombic perovskite phase with lattice constants a=4.0849(3) A, b=11.8469(4) A, and c=10.6818(9) A. The scanning electron micrograph exhibits heterogeneous grain distribution with average grain size of 1–3 μm. Temperature dependent dielectric constant exhibits a broad peak at 316 K (em=2250) that shows frequency dependent shifts toward higher temperature—typical relaxor behavior. Modified Curie–Weiss law is used to fit the dielectric data that exhibits almost complete diffuse phase transition characteristics. The dielectric relaxation obeys the Vogel–Fulcher relationship with the freezing temperature 286 K. In addition to relaxation observed due to transformation of the material into ergodic relaxor phase below Burn temperature, significant dielectric dispersion is observed in low frequency regime in both components of dielectric response and a small dielectric relaxation peak is observed in the temperature r...

Recent Advances in the Synthesis and Characterization of Chalcogenide Nanoparticles

Chalcogenide semiconductor nanoparticles and their self-assembly structures have become the most explored group of semiconductor nanomaterials due to the interesting physics involved in quantum confinement, surface chemistry and variety of applications. In the last couple of decades, facile routes for their synthesis and strategies for controlling the size, shape and morphology have been reported. In the present review, synthesis strategies of size and shape controlled nanoparticles belonging to II-VI group of semiconductor chalcogenides are presented and each method for preparation of nanoparticles is critically analysed. Role of various factors that affect the nucleation and growth of nanoparticles is discussed at length. Nanoparticles and self-assemblies of CdSe, CdTe, HgTe and ZnSe are synthesized using new and facile single molecular precursor based noble route by our group that uses non-pyrophoric, low temperature and non-toxic chemicals, their properties and synthesis scheme are discussed as future development in this field. Some recent applications of chalcogenides QDs in the fields of solar cell, optical fibre amplifiers, biosensing and bo-imaging are discussed and reviewed.

Synthesis and structural characterization of some Pb(B 1 3/t’ Nb2/3)O3 type materials by two-stage solid-state route

Two-stage columbite solid state reaction route has been used for the preparation of Pb(B13/t’Nb2/3)O3 materials (B′ = Mg, Ni and Cd). The columbite precursor phase was structurally characterized using diffraction data. MgNb2O6, NiNb2O6 and CdNb2O6 show orthorhombic structures i.e. pure columbite phase. Final phase materials get stabilized in mixed phase. The diffraction pattern shows that it is a mixture of cubic pyrochlore and perovskite phase. Percentage of perovskite phase was calculated using the band intensities of (110) perovskite and (222) pyrochlore peaks. The calculated percentages show the dominant perovskite phase. Possible reasons for mixed phase are discussed.

Phase transition and temperature dependence of the molecular distortion of ions in ammonium sulphate

The temperature dependence of the molecular distortion of ions in ammonium sulphate (AS) is discussed in order to ascertain the correct microscopic mechanism of phase transition (PT) in this crystal. It is reported that the transition in AS is triggered by distortion arising in the structure of the SO42- ion. This has been confirmed from a detailed analysis of the crystal structural data available at different temperatures around Tc. It is concluded that the type of transition occurring in AS is different from the well known displacive/order-disorder type and it could be given a name such as molecular distortion type. Such a transition can only take place in crystals having one or more than one kind of molecular unit. It leads to a change mainly in the structure and symmetry of such units, rather than in their location and orientation. This model is found to be consistent with the phenomenological theory of Ikeda and co-workers (1973) which explains reasonably well the temperature variation of dielectric constant, elastic compliances, etc.

The phase transition in ammonium sulphate: dynamics of deuterated NH4+ ions and the heat of transition

The temperature dependence of the D modes of partially deuterated NH+ ions in ammonium sulphate has been investigated with a view to examining the nature of the changes in these ions. It is inferred that the changes in these ions as well as those in the H bonds at Tc are minor and gradual. In contrast, a similar study of the nu 1(SO42-) mode reveals that the SO42- ion undergoes a significant and sudden change. These observations have been used to estimate the heat of transition, giving the result 4.2+or-0.4 kJ mol-1; this agrees well with the experimentally observed values ((3.89-4.27)+or-0.02 kJ mol-1). These inferences support the assertion that a molecular-distortion-type (MD-type) microscopic mechanism is responsible for the transition.

Diffuse Phase Transition and Electrical Conductivity of Pb(Ca1/3Nb2/3)O3

The lead-based relaxor ceramic Pb(Ca1/3Nb2/3)O3 was prepared by two-step solid-state reaction. The material stabilizes in the orthorhombic phase with refined lattice parameters a = 3.4814 Å, b = 12.9480 Å, and c = 14.2483 Å. The scanning electron micrograph is indicative of heterogeneous grain distribution with average grain size ~0.8–2.0 μm. The temperature-dependent dielectric response has a broad peak at 233.5°C (at 1 kHz, ε′ = 14523). A frequency-dependent shift toward higher temperature with increasing frequency is attributed to relaxor behaviour. Deviation from the Curie–Weiss law is observed at temperatures higher than the temperature, Tm, at which the dielectric constant is maximum. The modified Curie–Weiss law was used to fit the dielectric data; the results were indicative of almost complete diffuse phase transition characteristics. The dielectric relaxation obeys the Vogel–Fulcher relationship with freezing temperature Tf = 214.1°C, activation energy Ea = 0.16 eV, and relaxation frequency ν0 = 3.4 × 107 Hz. Electrical conduction is mainly attributed to the hopping mechanism.

Phenomenological theory of the phase transition in improper ferroelectric ammonium sulphate

A phenomenological theory accounting for the phase transition in improper ferroelectric ammonium sulphate has been discussed that is based on a molecular distortion-type microscopic mechanism (reported earlier). The theory uses the SO42--ion distortion as an order parameter and considers its linear coupling with the spontaneous polarisation to account for the ferroelectric behaviour. It has been used to account for the temperature dependence of the order parameter and dielectric constants. The dielectric susceptibility and the coefficient of linear coupling between the order parameter and the spontaneous polarisation in the ferroelectric phase are found to be different from those in the para-electric phase. The temperature dependence of the spontaneous polarisation and its possible origin have also been discussed.