Indranil Bhaumik

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal has been investigated and electro-caloric coefficient of ∼0.38 K mm/kV has been obtained near the tetragonal-to-cubic phase transition. The value obtained is significantly higher than that for the lead-free ferroelectric materials reported so far.

Nanostructured Bi(1−x)Gd(x)FeO3 – a multiferroic photocatalyst on its sunlight driven photocatalytic activity

The photocatalytic activity of sol–gel synthesized nanostructured Bi1−xGdxFeO3 (x = 0, 0.05, 0.1, 0.15) particles on the degradation of methylene blue (MB) was demonstrated for the first time under sunlight. The X-ray diffraction (XRD) studies showed that the substitution induced structural changes in 10 and 15% Gd substituted BiFeO3 (BFO). The morphology analysis, by field emission scanning (FESEM) and high resolution transmission electron microscopy (HRTEM), presented the composition driven particle size reduction and morphology changes in BFO from irregular to spherical shape. The band gap estimation by UV-visible diffuse reflectance spectroscopy revealed that increasing concentration of Gd significantly reduced the band gap of BFO from 2.38 eV to 2.29 eV. An anomalous magnetic enhancement was observed in Bi0.90Gd0.10FeO3 nanoparticles due to the manifestation of antiferromagnetic (AFM) core–ferromagnetic (FM) shell-like structure, revealed by its M–H hysteresis curve. An increasing trend in the photocatalytic activity of BFO was observed with increasing concentration of Gd. In this case, enhanced photocatalytic activity observed in Bi0.85Gd0.15FeO3 could be due to its increased ferroelectric domains that drive the charge carriers to the catalyst surface–dye interface, leading to more effective degradation of the dye. Conversely, an anomalous photocatalytic activity was observed in Bi0.90Gd0.10FeO3 that should be attributed to its AFM–FM core–shell-like structure.

Temperature-dependent index of refraction of monoclinic Ga 2 O 3 single crystal

We present temperature-dependent refractive index along crystallographic b[010] and a direction perpendicular to (100)-plane for monoclinic phase (β) Ga(2)O(3) single crystal grown by the optical floating zone technique. The experimental results are consistent with the theoretical result of Litimein et al.1. Also, the Sellmeier equation for wavelengths in the range of 0.4-1.55 μm is formulated at different temperatures in the range of 30-175 °C. The thermal coefficient of refractive index in the above specified range is ~10(-5)/°C.

Dielectric and ac conductivity studies on undoped and MgO-doped near-stoichiometric lithium tantalate crystals

Dielectric and ac impedance studies have been carried out on undoped and Mg (0.5 and 1mol%) doped near-stoichiometric lithium tantalate (SLT) single crystals. No particularly significant qualitative change is observed in the nature of the ferroelectric phase transition on doping by Mg. However, an anomaly is observed in the ac impedance data: the ac conductivity is found to be low for 0.5mol% doping of Mg, compared to 1.0mol% doping. Also, the activation energy, calculated from the Arrehenius equation, is highest for 0.5mol% doping, which suggests that hopping of Li+ ions becomes more hindered in 0.5 Mg SLT. The Nyquist plot shows a single semicircle above 100Hz. The electrical behavior of lithium tantalate can be modeled as a resistance parallel to a constant-phase element. The calculated impedance from this equivalent circuit is also found to be high for the 0.5mol% doping. This feature is explained in terms of a Li-vacancy model.

The ferroelectric phase transition in lithium tantalate single crystals: a composition-dependence study

The dielectric behavior of lithium tantalate (LT) single crystals has been investigated in the vicinity of the ferroelectric phase transition for nonstoichiometric and near-stoichiometric compositions. Fitting of the experimental data to a modified Curie-Weiss equation indicates the occurrence of a mildly relaxor-ferroelectric-like phase transition in congruent LT single crystals (with the Curie-Weiss exponent γ having a value 1.2, rather than the value unity for a regular ferroelectric). As the Li concentration of these crystals approaches the stoichiometric value, the phase transition gets sharper, indicative of an increasing amount of long-range ordering. The bulk concentration of cation vacancies has also been estimated.

Applicability of Langmuir equation to oxygen pressure dependent photoluminescence from β-Ga2O3 nanostructures

β-Ga2O3 nanostructures were synthesized via vapor transport method on gold coated Silicon substrate in N2 ambient. The as synthesized products were investigated by grazing incident X-ray diffraction, scanning electron microscopy and photoluminescence (PL) spectroscopy. It is shown that the intensity of photoluminescence from the ensemble of β-Ga2O3 nanostructures in oxygen gas ambience is correlated with the oxygen pressure through the Langmuir equation. This correlation is found to be reversible and reproducible. This phenomenon, which was not observed in the bulk β-Ga2O3 single crystal, is attributed to the oxygen related shallow trap surface states of the nanostructures with energies at about 4.2 eV above the valance band. Based on the changes in the PL intensity with the oxygen pressures, a possible mechanism for the observed photoluminescence is suggested. The present results provide a route for room-temperature response of oxygen in the gallium oxide nanostructures.

Bipolar electro-caloric effect in SrxBa(1−x)Nb2O6 lead-free ferroelectric single crystal

Here we report the anomalous electro-caloric effect (ECE) observed in lead-free SrxBa(1−x)Nb2O6 (, 0.61 and 0.75) ferroelectric single crystals. Temperature-dependent hysteresis measurement revealed that the spontaneous polarization of SrxBa(1−x)Nb2O6 single crystals with all the compositions under investigation decreases with a decrease in the temperature below the temperature of the dielectric maxima . As a consequence, these crystals exhibited negative electro-caloric effect at lower temperature along with the usual positive ECE above . The EC coefficient obtained near is 0.21, 0.43 and 0.28 K mm/kV for , 0.61 and 0.75 samples, respectively. The maximum values of in the negative ECE region are −1.4, −0.81 and for 50SBN, 61SBN and 75SBN, respectively.

Effect of Oxygen Annealing on Domain Structure and Dielectric Properties of 0.91PZN-0.09PT Single Crystal

Single crystals of 0.91PZN-0.09PT were grown by flux method. Annealing of the crystals under oxygen atmosphere leads to stripe like domains. Dielectric properties were found to be enhances after oxygen annealing. P-E hystersis show increase in the remnant polarization and coercive field. Heat flow measurement does not reflect any significant change in the properties after oxygen annealing. Phase transition temperature remains the same for as grown and oxygen annealed samples. However, a little increase of the phase transition enthalpy is observed in oxygen annealed crystals. The change in properties due to oxygen annealing has been explained in terms of defect movement in the crystals.

Growth of 0.91PZN-0.09PT Single Crystal and Effect of Poling on Their Properties

Single crystals of 0.91PZN-0.09PT were grown by flux method. (001) oriented crystal plates were poled under different electric fields. Dielectric measurements show a phase transition peak at 460 K in unpoled samples. Poled samples show an additional anomaly around 360 K. Heat flow measurements also reflect a hump near 450 K in unpoled crystals. The hump gets diffused as we increase the poling field. As we pole the sample at 6kV/cm, before carrying out the heat flow measurement, the hump disappears.

Growth of Relaxor Ferroelectric Single Crystals PbZn1/3Nb2/3O3 (PZN) by High Temperature Solution Growth

The effects of Lead oxide (as solvent) content in the perovskite phase formation in spontaneous growth of PZN single crystal have been studied. In spite of adopting columbite route, 50 wt% of solvent leads to the growth of undesirable pyrochlore phase. With the increase of solvent content, the probability of formation of perovskite phase was found to increase. Growth run performed with PZN/PbO weight ratio 35:65 resulted the growth of crystals with > 98% perovskite phase present in it. With the help of oscillating thermal profile PZN single crystal was grown of the size 10× 10× 8 mm 3 .