A. K. Karnal

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.

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.

Effect of Mg doping on the growth aspects, crystalline perfection, and optical and thermal properties of congruent LiNbO3 single crystals

Mg-doped congruent lithium niobate single crystals were grown by the Czochralski technique. High-quality single crystals were grown using a novel seeding technique in a resistive heating furnace. Analysis of crystalline perfection carried out by a multi-crystal X-ray diffractometer revealed that the grown crystals do not contain any structural grain boundaries but do contain point defects. The transmission characteristics showed an enhancement of band gap with an increase in Mg concentration. Conoscopy patterns revealed that the grown crystals are homogeneous and the incorporation of Mg into the lattice does not affect the optical sign (negative uniaxial) of the crystal. The refractive index measurements carried out using a prism coupler showed an increase in the optical birefringence (Δn), while the refractive index was found to decrease with the increase in doping concentration. Further, thermal conductivity was found to decrease with Mg incorporation in the lattice owing to phonon scattering from the Mg ions and, as a consequence, at high concentrations (>4 mol%) crack formation occurred. However, optimization of growth conditions reveals that a slower pulling rate leads to crack-free lithium niobate crystals even at 6 mol% Mg doping.

A Comparative Study of Dielectric Relaxation in Sr0.61Ba0.39Nb2O6 and Sr0.75Ba0.25Nb2O6 Single Crystals

Dielectric behaviour of Sr0.61Ba0.39Nb2O6 (61SBN) and Sr0.75Ba0.25Nb2O6 (75SBN) single crystals has been studied. It is observed that 61SBN has relatively low diffuseness as well as smaller relaxation compared to 75SBN. Increase in the configurational entropy imposed by the change in defect structure is suggested to be the origin of this behaviour. Analysis using Vogel–Fulcher expression and modified Curie–Weiss law give a signature of long-range polar order in 61SBN. The value of γ for 61SBN is closer to unity (1.14), where as for 75SBN it is 1.52. The fluctuation (in the orientation) of dipoles near dielectric maxima in SBN system is mediated by a paraelectric state.

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.

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.

Crystal growth of Ru-doped congruent LiNbO3 and investigation of crystalline perfection and optical properties

Single crystals of undoped and Ru-doped congruent LiNbO3 (LN) were successfully grown by the Czochralski method. The axial and radial gradient of the radio frequency furnace was controlled in order to obtain crack-free single crystals. Wafers were cut from the grown Ru-doped single crystal at different axial positions along the growth direction and subjected to various characterization analyses. Good optical homogeneity and low residual strain in the grown crystal is confirmed by the conoscopy patterns. Further, it is confirmed that Ru doping does not influence the optical sign of the crystal. From a high-resolution X-ray diffractometry analysis, it is evident that the as-grown undoped LN crystal exhibited better crystalline perfection with a single and sharp diffraction curve in comparison to the Ru-doped crystals, which are characterized by a broader diffraction curve. Absorption coefficient and band-gap analysis across the axial length revealed a concentration variation of Ru across the length. Refractive index measurements carried out using a prism coupler showed variation in the optical birefringence, also due to the variation of Ru concentration at different positions of the grown single crystal.

Doped YVO4 single crystals: Growth issues in the OFZ technique and investigation of spectroscopic properties

Here issues related to the growth of doped YVO4 crystal by the optical floating zone (OFZ) technique have been discussed. Focus has been on the synthesis of the starting material, melt stability and the controllability of the oxidation state of Cr ion. It has been observed that the molten zone is more stable for the growth along [100] in comparison to growth along [001]. As a result crystals grown along [001] suffer from severe overflow problem. Spectroscopic parameters of absorption and emission characteristics for the Nd and Er doped crystals have been discussed in the light of Judd-Ofelt analysis.