• K B R Varma

      Articles written in Bulletin of Materials Science

    • Growth and electro-optic studies in mixed (NH4)x K1−xH2PO4 single crystals

      K B R Varma Kolluri V Ramanaiah

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      Single crystals and mixed crystals of KH2PO4 (KDP) and NH4H2PO4 (ADP) were grown with different dopant concentrations of NH4H2PO4 in KH2PO4 in solution by Holden’s rotary crystallizer technique. The effect of additives like Borax (Na2B4O7·10H2O), seed crystal rotation rate and qualities of the crystals were studied. The half-wave voltages (in longitudinal mode) for KDP mixed with 1% ADP (by weight) were found and hence the unclamped (low frequency) electro-optic coefficients (r63) were calculated for various wavelengths in the visible region of the spectrum. It was noted that the half-wave voltage increases with increase in wavelength and temperature.

    • Dispersion of photoelastic constants in doped KDP crystals

      K V Ramanaiah K B R Varma

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      The photoelastic behaviour ofkdp crystals with additives like borax,adp, nickel phosphate, manganese phosphate and sodium dihydrogen orthophosphate has been studied as a variable birefringent compensator. The dispersion of the stress birefringence with wavelength was studied for a single orientation.

    • Twin roller quenching technique and preparation of glasses of high melting ionic solids

      K B R Varma

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      A rapid quenching technique with a quenching rate of roughly 106°C/sec has been developed to prepare glassy samples of ABO3 type materials. Glasses of potassium lithium niobate have been prepared by this technique. These glasses have been characterized by x-ray diffraction, electron diffraction and differential scanning calorimetry techniques to assess the quality of the obtained glasses.

    • Structural and dielectric properties of a novel Bi2O3-(1 −x)WO3-xCuO solid solution

      Vandana Sharma K B R Varma A K Shukla

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      A novel solid solution in the system Bi-W-Cu-O has been synthesized and its structural and dielectric properties studied. The solid solution Bi2O3-(1 −x)WO3-xCuO exists up tox=0·7; the solid solutions up tox=0·65 are orthorhombic but stabilize in tetragonal structure in a narrow range aroundx=0·7. The solid solutions are non-centrosymmetric and exhibit ferroelectric behaviour similar to their parent phase Bi2WO6. The Curie point of the solid solution is found to decrease with increase inx.

    • SrB4O7:Bi2VO5·5 — A novel nanocomposite

      M V Shankar G N Subbanna K B R Varma

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      Glass composites of strontium tetraborate, SrB4O7(SBO) with bismuth vanadate, Bi2VO5·5 (BiV) of the composition (1 −x) SBO-x BiV (0 ≤x ≤ 0·75), have been synthesized. X-ray powder diffraction and electron microscopy indicate as-quenched composites to be amorphous and the annealed samples showed the presence of nanometer sized particles of BiV dispersed in the glassy matrix of SBO. The dielectric constant of these composites increases with increase in the volume fraction of BiV, at 300 K. The measured dielectric constant of the composite very nearly obeys the Maxwell’s relationship. Optical transmission studies confirm a steady shift in the optical absorption edge towards longer wavelengths with increase inx.

    • Dielectric dispersion and piezoelectric resonance in benzil single crystals grown by Bridgman-Stockbarger technique

      M V Shankar K B R Varma

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      Large single crystals of an organic nonlinear optical (NLO) material, benzil, have been grown by employing an indigenously built Bridgman-Stockbarger crystal grwoth system. The dielectric constant (εr), dielectric loss tangent (tanδ) and impedance (Z) of these crystals, monitored along the crystallographica-axis, as a function of frequency in the 100kHz-10MHz range, exhibit sharp resonance effects in the 100–300kHz region. These strong resonance effects are attributed to the piezoelectric nature of the crystal. The origin of the appearance of the sharp peaks in the frequency variation ofεr and tanδ is in response to the elastic resonances induced via the externally applied electric field. The resonance peak positions shift towards lower frequencies with increase in temperature as a consequence of the decrease in the stiffness coefficient (C11) of the crystal.

    • Influence of inherent strain on the curie temperature of rare earth ion-doped bismuth vanadate

      K Sooryanarayana T N Guru Row R Somashekar K B R Varma

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      X-ray line broadening is found to be an effective parameter to estimate the strain associated with rare earth ion (Gd3+)-doped polycrystalline bismuth vanadate(Bi2VO5·5). The strain increases with increasing Gd3+ concentration. It is anisotropic and found to be maximum in (111) plane. The Curie temperature which is known to decrease with increase in the rare earth ion concentration in these compounds is correlated with increase in strain.

    • Magnetic and ferroelectric characteristics of Gd$^{3+}$ and Ti$^{4+}$ co-doped BiFeO$_3$ ceramics


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      Polycrystalline BiFeO3 and Bi$_{0.9}$Gd$_{0.1}$Fe$_{1−x}$Ti$_x$O$_3$ ($x = 0$, 0.01, 0.05 and 0.1) samples were synthesized by solid-state reaction route. Structural, magnetic and ferroelectric properties of these samples were investigated. X-ray powder diffraction (XRD) results confirmed the presence of a significant amount of Bi$_2$Fe$_4$O$_9$ impurity phase in the undoped BiFeO$_3$ sample. Mössbauer spectroscopy studies corroborated the XRD studies to confirm the presence of impurity phase.We have observed that gadolinium (Gd$^{3+}$) and titanium (Ti$^{4+}$) doping, respectively, on Bi$^{3+}$ and Fe$^{3+}$ sites facilitated a significant reduction in the impurity phase formation in BiFeO$_3$. Interestingly, Gd$^{3+}$-doping significantly reduced the impurity phase formation as compared to the undoped BiFeO$_3$ sample. This impurity phase formation was further overcome by doping higher ($x \ge 0.05$) amounts of Ti in BiFeO$_3$. The crystallographicsite occupancies of Gd and Ti were confirmed by Rietveld refinement of XRD data,Mössbauer spectroscopy and magnetization measurements. An enhancement in ferromagnetic properties along with moderate ferroelectricproperties have been observed after co-doping. There was an increasing trend in remnant polarization (Pr) with the increase in Ti concentration besides an improvement in the characteristic saturation magnetization. Our resultsdemonstrate that Gd$^{3+}$ and Ti$^{4+}$ doping could be used to enhance multifunctional properties of BiFeO3 ceramics to enable them as potential material for various devices.

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      Posted on October 12, 2020

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      Prof. Surajit Dhara — School of Physics, University of Hyderabad, Hyderabad
      Physical Sciences 2020

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