Origin of ferroelectricity in cubic phase of Hf substituted BaTiO3

The origin of ferroelectricity in the cubic phase of BaTi_1-xHf_xO₃has been investigated. The presence of well-defined ferroelectric polarization versus electric field (PE) hysteresis loop in the samples with global cubic symmetry suggests the presence of 'local polar regions', induced possibly due to the huge difference in the electronegativity and also difference in the ionic radii of Hf⁺⁴and Ti⁺⁴ions, which may lead to local structural disorder. The presence of polar regions is also supported through the appearance of A₁(TO) polar mode in Raman spectra which in principle should be absent in the samples with cubic symmetry. The results are discussed in terms of disorder-induced local dipoles due to the electronegativity difference between Hf and Ti ions.

Ni-Doped ZnO Thin Films: Deposition, Characterization and Photocatalytic Applications

Root like structured Ni-doped zinc oxide [Zn_(1-x)Ni_xO (x = 0.09)] thin films were deposited on a non-conducting glass substrate by indigenously developed spray pyrolysis system at optimized substrate hotness of 573±5 K. Thus obtained Ni-doped ZnO thin films were characterized by UV-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Atomic Force Microscopy (AFM). XRD result revealed that Ni-doped ZnO has a polycrystalline nature with a hexagonal wurtzite structure. For pure ZnO and Ni-doped ZnO thin films, the particle sizes were 60.9 and 53.3 nm while lattice strain values were 1.56×10^-3 and 1.14×10^-3, respectively. The film surface showed characteristic root-like structure as observed by the SEM. It was observed that the Ni-doped ZnO thin films were grown in high density along with more extent of branching as compared to pure ZnO thin films but retained the root-like morphologies, however, the branches were more-thinner and of shorter lengths. AFM analysis showed that the surface grains of the Ni-doped samples are homogeneous with less RMS roughness values compared with the undoped ZnO samples. The photocatalytic activity of the prepared thin films was evaluated by the degradation of methyl orange (MO) dye under UV light irradiation. Pure ZnO and Ni-doped ZnO thin films took 150 min and 100 min to degrade about 60% MO dye, respectively.

Ultralow Thermal Conductivity and Large Figure of Merit in Low-Cost and Nontoxic Core-Shell Cu@Cu2O Nanocomposites

Identification of novel materials with enhanced thermoelectric (TE) performance is critical for advancing TE research. In this direction, this is the first report on TE properties of low-cost, nontoxic, and abundant core-shell Cu@Cu₂O nanocomposites synthesized using a facile and cheap solution-phase method. They show ultralow thermal conductivity of nearly 10^-3 of the copper bulk value, large thermopower of ∼373 μVK^-1, and, consequently, a TE figure of merit of 0.16 at 320 K which is larger than those of many of the potential TE materials such as PbTe, SnSe, and SiGe, showing its potential for TE applications. The ultralow thermal conductivity is mainly attributed to the multiscale phonon scattering from intrinsic defects in Cu₂O, grain boundaries, lattice-mismatched interface, as well as dissimilar vibrational properties. The large thermopower is associated with a sharp modulation in carrier density of states due to charge transfer between Cu and Cu₂O nanoparticles and carrier energy filtering. They are tuned by varying the trioctylphosphine concentration.

Low-temperature high magnetic field powder x-ray diffraction setup for field-induced structural phase transition studies from 2 to 300 K and at 0 to 8-T field

A low-temperature and high magnetic field powder x-ray diffractometer (XRD) has been developed at UGC-DAE CSR (UGC: University Grant Commission, DAE: Department of Atomic Energy, and CSR: Consortium for scientific research), Indore, India. The setup has been developed around an 18 kW rotating anode x-ray source delivering Cu-K_α x-rays coming from a vertical line source. It works in a symmetric θ-2θ parallel beam geometry. It consists of a liquid helium cryostat with an 8 T split-pair Nb-Ti superconducting magnet comprising two x-ray windows each covering an angular range of 65°. This is mounted on a non-magnetic type heavy duty goniometer equipped with all necessary motions along with data collection accessories. The incident x-ray beam has been made parallel using a parabolic multilayer mirror. The scattered x-ray is detected using a NaI detector through a 0.1° acceptance solar collimator. To control the motions of the goniometer, a computer programme has been developed. The wide-angle scattering data can be collected in a range of 2°-115° of 2θ with a resolution of ∼0.1°. The whole setup is tightly shielded for the scattered x-rays using a lead hutch. The functioning of the goniometer and the artifacts arising possibly due to the effect of stray magnetic field on the goniometer motions, on the x-ray source, and on the detector have been characterized by collecting powder XRD data of a National Institute of Standards and Technology certified standard reference material LaB₆ (SRM-660b) and Si powder in zero-field and in-field conditions. Occurrence of field induced structural-phase transitions has been demonstrated on various samples like Pr_0.5Sr_0.5MnO₃, Nd_0.49Sr_0.51MnO_3-δ and La_0.175Pr_0.45Ca_0.375MnO₃ by collecting data in zero field cool and field cool conditions.

Kinetic arrest of the first-order to R3c Pbnm phase transition in supercooled La(x)MnO(3+δ) (x = 1 and 0.9)

We report the occurrence of kinetic arrest of the first-order phase transition from R3c to Pbnm in supercooled La(x)MnO(3±δ) (x = 1 and 0.9, i.e. δ > 0.125). Structural studies have been done, employing low temperature transmission electron microscopy (LT-TEM) and low temperature x-ray diffraction (LT-XRD) techniques. No phase transformation was observed even in La(x)MnO(3±δ) aged for ~12 h at 98 K. The evidence of the occurrence of kinetic arrest was realized at low temperatures through in situ electron beam triggered nucleation and perpetual devitrification of the R3c phase into a Pbnm phase. It was clearly evidenced that the R3c structure of La(x)MnO(3±δ), below its ferromagnetic transition temperature, is metastable and prone to be transformed to a Pbnm orthorhombic structure following initiation by an electron beam trigger. The electron beam transformed Pbnm phase was found to transform back to the R3c phase through a first-order phase transition occurring close to the ferromagnetic to paramagnetic transition (T(c)) during heating. The glass-like kinetics of the arrested R3c phase has been investigated through resistance relaxation measurements, showing a decreasing logarithmic rate of decay of the arrested R3c phase towards the stable Pbnm phase with decreasing temperature, down to 5 K. On the basis of the correlations observed in the resistance-versus-temperature, magnetization-versus-temperature, magnetization-versus-field, resistance relaxation and LT-XRD measurements, the occurrence of kinetic arrest has been attributed to the suppression of Jahn-Teller distortion by double exchange across the insulator-metal transition.

Nanoparticle induced piezoelectric, super toughened, radiation resistant, multi-functional nanohybrids

We have developed multifunctional nanohybrids of poly(vinylidene fluoride-co-chlorotrifluoroethylene) (CTFE) with a small percentage of surface modified inorganic layered silicate showing dramatic improvement in toughness, radiation resistant and piezoelectric properties vis-à-vis pristine polymer. Massive intercalation (d(001) 1.8 → 3.9 nm) of polymer inside the nanoclay galleries and unique crystallization behavior of the fluoropolymer on the surface of individual silicate layer has been reported. Toughness in the nanohybrid increases more than three orders of magnitude as compared to pure CTFE. High energy radiation (80 MeV Si(+7)) causes chain session, amorphization and creates olefinic bonds in the pure polymer while the nanohybrids are radiation resistant at a similar dose. Nanoclay induces the metastable piezoelectric β-phase in CTFE, suitable for sensor and actuator application. Molecular level changes after irradiation and controlled morphology for smart membrane have been confirmed by using spectroscopy, sol-gel technique, surface morphology studies and in situ residual gas analysis.

High coercive field and magnetization reversal in core-shell cum nanotwin driven Ni/NiO nanospheres

We report here nanotwin-core-shell Ni(core)NiO(shell) spheres of average size 25 nm prepared through polyol method. They exhibit high coercive field at 2 K, sharp peak at approximately 20 K in magnetization curve and magnetization reversal. Interestingly, exchange bias due to antiferromagnetic NiO shell is absent. Among other possibilities, anisotropy variations due to particle size distribution and twinning associated with disorder appear to play an important role. Further, magnetic interactions of twinned bigger spheres, which may also act as superferrimagnetic-like Ni multilayer cores, with superparamagnetic Ni of smaller spheres, might be the additional causes. These nanostructures therefore seem to have potential interest in memory effect.

Synthesis and characterization of Ag-polymer nanocomposites

We report the synthesis of Ag nanoparticles in polyethylene terephthalate (PET) matrix using atom beam co-sputtering. Metal filling factor was evaluated by Rutherford backscattering spectrometry. Microstructural evolutions of the nanocomposites films were investigated by transmission electron microscopy, which confirmed the formation of irregular shaped Ag nanoparticles. The X-ray photoelectron spectroscopy measurements of the sputter deposited PET film and co-sputtered deposited Ag-PET as well as PET bulk foil (from Goodfellows) were performed to study chemical composition of the nanocomposite films. The optical properties of these nanocomposites were studied by light absorption/transmission, which revealed a narrow transmission of UV light approximately 320 nm and a broad surface plasmon resonance absorption extending up to infrared region (approximately 2400 nm). Swift heavy ion irradiation of Ag-PET nanocomposite resulted in narrowing the full width at half maximum of transmission band.

A facile and fast approach for the synthesis of doped nanoparticles using a microfluidic device

The microfluidic approach emerges as a new and promising technology for the synthesis of nanomaterials. A microreactor allows a variety of reaction conditions to be quickly scanned without consuming large amounts of raw material. In this study, we investigated the synthesis of water soluble 1-thioglycerol-capped Mn-doped ZnS nanocrystalline semiconductor nanoparticles (TG-capped ZnS:Mn) via a microfluidic approach. This is the first report for the successful doping of Mn in a ZnS semiconductor at room temperature as well as at 80 °C using a microreactor. Transmission electron microscopy and x-ray diffraction analysis show that the average particle size of Mn-doped ZnS nanoparticles is ∼3.0 nm with a zinc-blende structure. Photoluminescence, x-ray photoelectron spectroscopy, atomic absorption spectroscopy and electron paramagnetic resonance studies were carried out to confirm that the Mn(2+) dopants are present in the ZnS nanoparticles.

Cross-sectional transmission electron microscopic study of irradiation induced nano-crystallization of nickel in a W/Ni multilayer

The present study reports the cross-sectional transmission electron microscopic investigations of swift heavy ion-irradiation induced nano-size recrystallization of Ni in a nearly immiscible W/Ni multilayer structure. Multilayer structures (MLS) of [W(25 Å)/Ni(25 Å)](10BL) were grown on Si-(100) substrate by the ion-beam sputtering technique. The as-synthesized MLS were subjected to 120 MeV-Au(9+) ion-irradiation to a fluence of ∼5 × 10(13) ions cm(-2). Wide-angle x-ray diffraction studies of pristine as well as irradiated W/Ni multilayers show deterioration of the superlattice structure, whereas x-ray reflectivity (XRR) measurement reveals a nearly unaffected microstructure after irradiation. Analysis of the XRR data using 'Parratt's formalism' does show a significant increase of W/Ni interface roughness. Cross-sectional transmission electron microscopy (TEM) studies carried out in diffraction and imaging modes (including bright-field and dark-field imaging), show that at high irradiation dose the intralayer microstructure of Ni becomes nano-crystalline (1-2 nm). During these irradiation induced changes of the intralayer microstructure, the interlayer definition of the W and Ni layers still remains intact. The observed nano-recrystallization of Ni has been attributed to competition between low miscibility of the W/Ni interface and the ion-beam induced mixing kinetics.

Novel features of Sr1-xCaxTiO3 phase diagram: evidence for competing antiferroelectric and ferroelectric interactions

Structural and dielectric evidences are advanced to show that the Sr1-xCaxTiO3 (SCT) system undergoes an antiferroelectric (AFE) phase transition in the composition range 0.18</=x</=0.40. Stabilization of the AFE phase, instead of the ferroelectric (FE) phase known for 0<x<0.12, is shown to be intimately linked with the nonpolar antiferrodistortive phase transitions driven by the R and M point instabilities preceding the AFE transition. It is proposed that the disorder and frustration introduced by CaTiO3 substitution in the incipient FE matrix of SrTiO3 leads to the smearing of the varepsilon(')(T) response in SCT for 0.002<x<0.12 akin to dipole glasses and relaxor ferroelectrics.