Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)... more Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)R 2 OÁ70B 2-O 3 and xTiO 2 Á(70 À x)B 2 O 3 Á30R 2 O (R = Li, K; 0 6 x 6 7, in mol%) glasses containing 2 mol% V 2 O 5 have been studied. Spin Hamiltonian parameters, g k , g \ , A k , A \ , the dipolar hyperfine coupling parameter (P) and the Fermi contact interaction parameter (K) have been evaluated. Decrease in Dg k /Dg \ ratio with increase in TiO 2 content in TiO 2 ÁLi 2 OÁB 2 O 3 glasses show that the octahedral symmetry of V 4+ O 6 complex is improved; whereas in TiO 2 ÁK 2 OÁB 2 O 3 glasses, it is reduced with increase in x. The decrease in P, in both systems, indicates that the 3d xy orbit expands with increase in mol% of TiO 2 . The molecular orbital coefficients, a 2 and c 2 have been calculated by recording the optical transmission spectra in the range 400-850 nm. a 2 and c 2 increase with increase in x in all glasses expect KB glasses, which indicate that, the covalency of the vanadium oxygen bonds decreases. The dc conductivity (r) decreases and activation energy (W) increases with increase in TiO 2 :R 2 O ratio in TiO 2 ÁR 2 OÁB 2 O 3 glasses whereas with increase in TiO 2 :B 2 O 3 ratio in TiO 2-ÁB 2 O 3 ÁR 2 O glasses the variation in r and W is within experimental error.
Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been ... more Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been prepared over the range 0 ≤ x ≤ 20 (in mol%). The electron paramagnetic resonance (EPR) spectra of VO 2+ ions of these glasses have been recorded in the X-band (≈9.13 GHz) at room temperature. Spin Hamiltonian parameters (SHP), g || , g ⊥ , A || , A ⊥ , dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α 2 and γ 2 , have been calculated by recording the optical transmission spectra in the wavelength range of 500-850 nm. It has been observed that the octahedral symmetry around V 4+ ion is improved, and the 3d xy orbit of the unpaired electron of the vanadium ion expands with increase in Bi 2 O 3 :B 2 O 3 ratio. Values of the theoretical optical basicity, Λ th , have also been reported. The dc conductivity increases with an increase in temperature. The dc conductivity variation with Bi 2 O 3 content shows mixed glass-former effect at x = 8 mol%. For x = 10 mol%, the conductivity decreases with increase in Bi 2 O 3 content because of the decrease in the mobility and the number of Li + ions available for the conduction. (A. Agarwal).
Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been ... more Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been prepared over the range 0 ≤ x ≤ 20 (in mol%). The electron paramagnetic resonance (EPR) spectra of VO 2+ ions of these glasses have been recorded in the X-band (≈9.13 GHz) at room temperature. Spin Hamiltonian parameters (SHP), g || , g ⊥ , A || , A ⊥ , dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α 2 and γ 2 , have been calculated by recording the optical transmission spectra in the wavelength range of 500-850 nm. It has been observed that the octahedral symmetry around V 4+ ion is improved, and the 3d xy orbit of the unpaired electron of the vanadium ion expands with increase in Bi 2 O 3 :B 2 O 3 ratio. Values of the theoretical optical basicity, Λ th , have also been reported. The dc conductivity increases with an increase in temperature. The dc conductivity variation with Bi 2 O 3 content shows mixed glass-former effect at x = 8 mol%. For x = 10 mol%, the conductivity decreases with increase in Bi 2 O 3 content because of the decrease in the mobility and the number of Li + ions available for the conduction. (A. Agarwal).
Solid solutions of bismuth layered (Bi 2 O 3 )(Ba x Mo 1 À x O 3 ) (0.2 r x r 0.8, x is in step o... more Solid solutions of bismuth layered (Bi 2 O 3 )(Ba x Mo 1 À x O 3 ) (0.2 r x r 0.8, x is in step of 0.2) ceramics were prepared by conventional solid-state reaction of the constitutive oxides at optimized temperatures with a view to study its electrical properties. Powder X-ray diffraction has been employed for physical characterization and an average grain size of $ 16 to 22 nm was obtained. XRD study reveals the single phase structure of the samples. Dielectric properties such as dielectric constant (e 0 ), dielectric loss (tand) and ac electrical conductivity (s ac ) of the prepared ceramics sintered at various temperatures in the frequency range 10 1 -10 7 Hz have been studied. A strong dispersion observed in the dielectric properties shows the relaxor type behavior of the ceramic. The presence of maxima in the dielectric permittivity spectra indicates the ferroelectric behavior of the samples. Impedance plots (Cole-Cole plots) at different frequencies and temperatures were used to analyze the electric behavior. The value of grain resistance increases with the increase in Ba ion concentration. The conductivity mechanism shows a frequency dependence, which can be ascribed to the space charge mainly due to the oxygen vacancies. The relaxation observed for the M 00 (o) or Z 00 (o) curves is correlated to both localized and long range conduction. A single 'master curve' for the normalized plots of all the modulus isotherms observed for a given composition indicates that the conductivity relaxation is temperature independent.
Pottasium nioborate glasses of composition xNb 2 O 5 ⋅(30-x)K 2 O⋅69B 2 O 3 containing 1 mol % of... more Pottasium nioborate glasses of composition xNb 2 O 5 ⋅(30-x)K 2 O⋅69B 2 O 3 containing 1 mol % of V 2 O 5 were prepared by melt quench technique (1473K, 1h). The electron paramagnetic resonance spectra of VO 2+ in these glasses have been recorded in X-band (ν ≈ 9.14 GHz) at room temperature (RT). The spin Hamiltonian parameters, dipolar hyperfine coupling parameters, P and Fermi contact interaction parameter, K have been calculated. It is found that V 4+ ions in these glasses exist as VO 2+ in octahedral coordination with a tetragonal distortion. The tetragonality of V 4+ O 6 complex decreases with increasing Nb 2 O 5 : K 2 O ratio and also there is an expansion of 3d XY orbit of unpaired electron in the vanadium ion. The study of IR transmission spectra over a range 400-4000 cm -1 depicts the presence of both BO 3 and BO 4 structural units and Nb 5+ ions are incorporated into the glass network as NbO 6 octahedra, substituting BO 4 groups.
The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 i... more The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 ions, have been studied by means of electron paramagnetic resonance (EPR) and dierential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu 2 ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, T g , have also been measured for these borate glasses and it is found that the T g decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, K th , of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP. Ó TX 77204 5641, USA.
Glasses with composition x Bi 2 O 3 ·(30 2 x)M 2 O·70B 2 O 3 (M ¼ Li, Na) containing 2 mol% V 2 O... more Glasses with composition x Bi 2 O 3 ·(30 2 x)M 2 O·70B 2 O 3 (M ¼ Li, Na) containing 2 mol% V 2 O 5 have been prepared over the range 0 # x # 15 (x is in mol%). The electron paramagnetic resonance spectra of VO 2þ of these glasses have been recorded in the X-band (<9.3 GHz) at room temperature (RT < 300 K). Spin Hamiltonian parameters, g k ; g ' ; A k ; A ' ; dipolar hyperfine coupling parameter, P; and Fermi contact interaction parameter, K; have been calculated. The molecular orbital coefficients, a 2 and g 2 ; have been calculated by recording the optical transmission spectra. In x Bi 2 O 3 ·(30 2 x)Li 2 O·70B 2 O 3 glasses there is decrease in the tetragonality of the V 4þ O 6 complex for x up to 6 mol% whereas for x $ 6 mol%, tetragonality increases. In x Bi 2 O 3 ·(30 2 x)Na 2 O·70B 2 O 3 glasses there is increase in the tetragonality of the V 4þ O 6 complex with increasing x: The 3d xy orbit expands with increase in Bi 2 O 3 :M 2 O ratio. Values of the theoretical optical basicity, L th ; have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10 25 ohm 21 m 21 at low temperature and 10 23 ohm 21 m 21 at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi 2 O 3 :M 2 O ratio. q
Electron paramagnetic resonance, optical transmission spectra and DC conductivity of the glasses ... more Electron paramagnetic resonance, optical transmission spectra and DC conductivity of the glasses 2xMX Á (0.30Àx)M 2 O Á 0.70B 2 O 3 (M=Na or K; X=Cl or Br) (0:01pxp0:10) containing 2.0 mol% of V 2 O 5 have been studied. The spin-Hamiltonian parameters (SHP) of the VO 2+ ions, the dipolar hyperfine parameter, P, the Fermi contact interaction parameter, K, and the molecular orbital coefficients (a 2 and g 2 ) have been calculated. It is observed that in KX Á K 2 O Á B 2 O 3 (X=Cl or Br) glasses, the tetragonal nature of V 4+ O 6 complex decreases with KBr for xX0:05: An increase in the 2KX:K 2 O ratio (xX0:05) also results in the contraction of 3dxy orbit of the unpaired electron in the vanadyl ion, whereas in case of NaX Á Na 2 O Á B 2 O 3 (X=Cl or Br) glasses, the SHP are independent of change in 2NaX:M 2 O ratio. It is observed that the SHP in these alkali halide borate glasses are independent of the theoretical optical basicity, L th : It is also observed that the DC conductivity increases with increase in temperature. The order of conductivity is 10 À6 O À1 m À1 at low temperatures and 10 À4 O À1 m À1 at high temperatures. In MCl Á M 2 O Á B 2 O 3 (M=Na or K) glasses, the conductivity increases and the activation energy decreases with an increase in mol% of the NaCl or KCl content whereas, in MBr Á M 2 O Á B 2 O 3 (M=Na or K) glasses, the conductivity first increases and then decreases with mol% of the MBr content. r
The mixed alkali effect (MAE) has been investigated in the glass system (30 À x)Li 2 OÁxK 2 OÁ20B... more The mixed alkali effect (MAE) has been investigated in the glass system (30 À x)Li 2 OÁxK 2 OÁ20Bi 2 O 3 Á50B 2 O 3 (0 V x V 30, mol%) through optical properties. The optical absorption and reflection spectra were recorded at room temperature in the wavelength range 400 -800 and 450 -800 nm, respectively. From the absorption edge studies, the values of optical band gap (E opt ) and Urbach energy (DE) have been evaluated. The values of E opt lie between 2.21 and 2.55 eV for indirect allowed transitions, and for indirect forbidden transitions, the values vary from 1.86 to 2.31 eV. The values of E opt show nonlinear behavior upon substitution of one alkali ion by another with a minima at x = 15 mol% because the glass structure in the mixed alkali region may be having more nonbridging oxygens than in the dilute foreign alkali regions. The theoretical analysis of optical absorption indicates that the present glass system behaves as an indirect gap semiconductor. The refractive index (n) and optical dielectric constant (e) have been evaluated from the reflection spectra. n and e also show nonlinear variation in their values with maxima at x = 15 mol%, which supports the existence of MAE in the optical properties of the present glass system. D
Glass system xNb 2 O 5 ·(30-x)BaO·70B 2 O 3 (0 ≤ x ≤ 12, mol%) has been prepared using normal mel... more Glass system xNb 2 O 5 ·(30-x)BaO·70B 2 O 3 (0 ≤ x ≤ 12, mol%) has been prepared using normal melt-quench technique (1473K, 1h). In the present work the influence of Nb 2 O 5 on density, molar volume, optical band gap and electrical conductivity of barium borate glasses has been reported. The density and molar volume increases with increasing concentration of Nb 2 O 5 . Increase in V M indicates that the volume of non-bridging oxygens (NBOs) sites produced by Nb 2 O 5 is greater than that produced by BaO in the present glasses. The optical band gap energy (E g ) has been calculated from the Tauc's plots for both direct and indirect transitions. The decrease in E g with increase in Nb 2 O 5 content is again due to increasing concentration of the NBO ions which results in the shifting of valence band maximum (VBM) to higher energies. The value of Urbach energy (ΔE) is in the range 0.16-0.22 eV for all the glasses. The dc electrical conductivity has been measured in the temperature range 300K to 623K and is found to increase with increasing temperature. The increase in conductivity with increase in Nb 2 O 5 content in these glasses indicates the presence of NbO 6 units.
Electronic paramagnetic resonance (EPR) spectra of xLi 2 O⅐(30 Ϫ x)MoO 3 ⅐ 70B 2 O 3 glasses (0 Յ... more Electronic paramagnetic resonance (EPR) spectra of xLi 2 O⅐(30 Ϫ x)MoO 3 ⅐ 70B 2 O 3 glasses (0 Յ x Յ 25) containing 2 mol% V 2 O 5 were studied between room and liquid-helium temperatures at X-band frequencies. The spectra are typical of V 4ϩ ions present in vanadyl (VO 2ϩ ) form in the glass. The spin-Hamiltonian parameters (SHP), g g Ќ , A , and A Ќ , the dipolar hyperfine coupling parameter, P, and the Fermi constant interaction parameter, k, were evaluated. It was found that these parameters are independent of the temperature. Results suggest that the tetragonal distortion of the V 4ϩ O 6 complex increases with increase in the Li 2 O:MoO 3 ratio. It was also found that the increase in this ratio results in an expansion of the 3d xy orbit of unpaired electrons in the vanadium ion and that the spin-Hamiltonian parameters do not depend on the value of theoretical optical basicity.
Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have bee... more Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have been prepared over the range 0 ≤ x ≤ 15 (in mol%). The electron paramagnetic resonance spectra of VO 2+ in these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters, dipolar hyperfine coupling parameter and Fermi contact interaction parameter have been calculated. It is observed that, with increase in Bi 2 O 3 :K 2 O ratio, tetragonal distortion of the V 4+ O 6 complex increases and there is an expansion of the 3d xy orbit of unpaired electron in the vanadium ion. The values of theoretical optical basicity have also been reported. The molecular orbital coefficients have been calculated by recording the optical transmission spectra in the wavelength range 500-850 nm. The direct-current electrical conductivity, σ, has been measured in the temperature range 443-523 K. σ decreases with increase in Bi 2 O 3 :K 2 O ratio because of the decrease in the mobility and number of mobile K + ions and there is no electronic contribution to the conductivity by Bi 2 O 3 .
The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 i... more The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 ions, have been studied by means of electron paramagnetic resonance (EPR) and dierential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu 2 ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, T g , have also been measured for these borate glasses and it is found that the T g decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, K th , of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP. Ó TX 77204 5641, USA.
The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni-Cu ferrites wi... more The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni-Cu ferrites with general formula Ni 0.5 Cu 0.5 À x Mg x Fe 2 O 4 (0.0 r x r 0.5) have been investigated as a function of frequency, temperature and composition. ac resistivity of all the samples decreases with increase in the frequency exhibiting normal ferrimagnetic behavior. The frequency dependence of dielectric loss tangent showed a maximum in between 10 Hz and 1 kHz in all the ferrites. The conductivity relaxation of the charge carriers was examined using the electrical modulus formulism, and the results indicate the presence of the non-Debye type of relaxation in the prepared ferrites. Similar values of activation energies for dc conduction and for conductivity relaxation reveal that the mechanism of electrical conduction and dielectric polarization is the same in these ferrites. A single 'master curve' for normalized plots of all the modulus isotherms observed for a given composition indicates that the distribution of relaxation time is temperature independent. The saturation magnetization and coercivity as calculated from the hysteresis loop measurement show striking dependence on composition.
Glasses with composition xC00. (0.3-x)Na20.0.70B203 have been prepared over the range 0 5 x 5 0.1... more Glasses with composition xC00. (0.3-x)Na20.0.70B203 have been prepared over the range 0 5 x 5 0.12 containing 1 mo1%/2mol% of V z 0 5 . Electron paramagnetic resonance spectra of V 0 2 + of the glasses have been measured at X-band (v -9.3 GHz) at room temperature. Spin
Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have bee... more Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have been prepared over the range 0 ≤ x ≤ 15 (in mol%). The electron paramagnetic resonance spectra of VO 2+ in these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters, dipolar hyperfine coupling parameter and Fermi contact interaction parameter have been calculated. It is observed that, with increase in Bi 2 O 3 :K 2 O ratio, tetragonal distortion of the V 4+ O 6 complex increases and there is an expansion of the 3d xy orbit of unpaired electron in the vanadium ion. The values of theoretical optical basicity have also been reported. The molecular orbital coefficients have been calculated by recording the optical transmission spectra in the wavelength range 500-850 nm. The direct-current electrical conductivity, σ, has been measured in the temperature range 443-523 K. σ decreases with increase in Bi 2 O 3 :K 2 O ratio because of the decrease in the mobility and number of mobile K + ions and there is no electronic contribution to the conductivity by Bi 2 O 3 .
The frequency and temperature dependent conductivity investigations for alkali niobium borate gla... more The frequency and temperature dependent conductivity investigations for alkali niobium borate glasses of composition xNb 2 O 5 Á (30 À x)M 2 O Á 70B 2 O 3 (where M ¼ Li, Na; x ¼ 0, 4, 8 mol%) have been carried out using impedance spectroscopy (IS). The complex impedance data have been analyzed by using both the conductivity and the electric modulus formalisms. The conductivity decreases with the decrease in M 2 O:Nb 2 O 5 ratio. The effect of temperature on the scaling of dielectric modulus indicates that the conductivity relaxation mechanism is temperature independent. The overlapping of the normalized peaks corresponding to impedance and electric modulus and the identical values of thermal activation energy for conduction and relaxation suggest the single mechanism for the dynamic processes occurring in the present glasses. The variation of density, molar volume and glass transition temperature with glass composition has also been reported to supplement the conductivity results.
Lithium vanadoxide bismuth borate glasses with composition (30-x)Li 2 O · xV 2 O 5 · 50B 2 O 3 · ... more Lithium vanadoxide bismuth borate glasses with composition (30-x)Li 2 O · xV 2 O 5 · 50B 2 O 3 · 20Bi 2 O 3 (x=5, 10, 15) has been prepared with the traditional melt-quenching technique. The density measurements were performed using Archimedes Principal and using density data molar volume was calculated. To determine the oxygen covalency theoretical optical basicity was calculated. To get an idea about the structure FTIR spectroscopy was carried out in the mid-IR region. The spectra revealed absence of boroxol ring and presence of absorption bands corresponding to the combined contributions of tri and tetra borate stretching vibrations. The increasing basicity reveals decrease in the covalence nature of oxygen as we replace the Li 2 O content with V 2 O 5 . The decrease in molar volume may be due to the increase in compactness of the network structure.
Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)... more Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)R 2 OÁ70B 2-O 3 and xTiO 2 Á(70 À x)B 2 O 3 Á30R 2 O (R = Li, K; 0 6 x 6 7, in mol%) glasses containing 2 mol% V 2 O 5 have been studied. Spin Hamiltonian parameters, g k , g \ , A k , A \ , the dipolar hyperfine coupling parameter (P) and the Fermi contact interaction parameter (K) have been evaluated. Decrease in Dg k /Dg \ ratio with increase in TiO 2 content in TiO 2 ÁLi 2 OÁB 2 O 3 glasses show that the octahedral symmetry of V 4+ O 6 complex is improved; whereas in TiO 2 ÁK 2 OÁB 2 O 3 glasses, it is reduced with increase in x. The decrease in P, in both systems, indicates that the 3d xy orbit expands with increase in mol% of TiO 2 . The molecular orbital coefficients, a 2 and c 2 have been calculated by recording the optical transmission spectra in the range 400-850 nm. a 2 and c 2 increase with increase in x in all glasses expect KB glasses, which indicate that, the covalency of the vanadium oxygen bonds decreases. The dc conductivity (r) decreases and activation energy (W) increases with increase in TiO 2 :R 2 O ratio in TiO 2 ÁR 2 OÁB 2 O 3 glasses whereas with increase in TiO 2 :B 2 O 3 ratio in TiO 2-ÁB 2 O 3 ÁR 2 O glasses the variation in r and W is within experimental error.
Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)... more Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)R 2 OÁ70B 2-O 3 and xTiO 2 Á(70 À x)B 2 O 3 Á30R 2 O (R = Li, K; 0 6 x 6 7, in mol%) glasses containing 2 mol% V 2 O 5 have been studied. Spin Hamiltonian parameters, g k , g \ , A k , A \ , the dipolar hyperfine coupling parameter (P) and the Fermi contact interaction parameter (K) have been evaluated. Decrease in Dg k /Dg \ ratio with increase in TiO 2 content in TiO 2 ÁLi 2 OÁB 2 O 3 glasses show that the octahedral symmetry of V 4+ O 6 complex is improved; whereas in TiO 2 ÁK 2 OÁB 2 O 3 glasses, it is reduced with increase in x. The decrease in P, in both systems, indicates that the 3d xy orbit expands with increase in mol% of TiO 2 . The molecular orbital coefficients, a 2 and c 2 have been calculated by recording the optical transmission spectra in the range 400-850 nm. a 2 and c 2 increase with increase in x in all glasses expect KB glasses, which indicate that, the covalency of the vanadium oxygen bonds decreases. The dc conductivity (r) decreases and activation energy (W) increases with increase in TiO 2 :R 2 O ratio in TiO 2 ÁR 2 OÁB 2 O 3 glasses whereas with increase in TiO 2 :B 2 O 3 ratio in TiO 2-ÁB 2 O 3 ÁR 2 O glasses the variation in r and W is within experimental error.
Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been ... more Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been prepared over the range 0 ≤ x ≤ 20 (in mol%). The electron paramagnetic resonance (EPR) spectra of VO 2+ ions of these glasses have been recorded in the X-band (≈9.13 GHz) at room temperature. Spin Hamiltonian parameters (SHP), g || , g ⊥ , A || , A ⊥ , dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α 2 and γ 2 , have been calculated by recording the optical transmission spectra in the wavelength range of 500-850 nm. It has been observed that the octahedral symmetry around V 4+ ion is improved, and the 3d xy orbit of the unpaired electron of the vanadium ion expands with increase in Bi 2 O 3 :B 2 O 3 ratio. Values of the theoretical optical basicity, Λ th , have also been reported. The dc conductivity increases with an increase in temperature. The dc conductivity variation with Bi 2 O 3 content shows mixed glass-former effect at x = 8 mol%. For x = 10 mol%, the conductivity decreases with increase in Bi 2 O 3 content because of the decrease in the mobility and the number of Li + ions available for the conduction. (A. Agarwal).
Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been ... more Glasses with composition xBi 2 O 3 ·(70 −x)B 2 O 3 ·30Li 2 O containing 2 mol% V 2 O 5 have been prepared over the range 0 ≤ x ≤ 20 (in mol%). The electron paramagnetic resonance (EPR) spectra of VO 2+ ions of these glasses have been recorded in the X-band (≈9.13 GHz) at room temperature. Spin Hamiltonian parameters (SHP), g || , g ⊥ , A || , A ⊥ , dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α 2 and γ 2 , have been calculated by recording the optical transmission spectra in the wavelength range of 500-850 nm. It has been observed that the octahedral symmetry around V 4+ ion is improved, and the 3d xy orbit of the unpaired electron of the vanadium ion expands with increase in Bi 2 O 3 :B 2 O 3 ratio. Values of the theoretical optical basicity, Λ th , have also been reported. The dc conductivity increases with an increase in temperature. The dc conductivity variation with Bi 2 O 3 content shows mixed glass-former effect at x = 8 mol%. For x = 10 mol%, the conductivity decreases with increase in Bi 2 O 3 content because of the decrease in the mobility and the number of Li + ions available for the conduction. (A. Agarwal).
Solid solutions of bismuth layered (Bi 2 O 3 )(Ba x Mo 1 À x O 3 ) (0.2 r x r 0.8, x is in step o... more Solid solutions of bismuth layered (Bi 2 O 3 )(Ba x Mo 1 À x O 3 ) (0.2 r x r 0.8, x is in step of 0.2) ceramics were prepared by conventional solid-state reaction of the constitutive oxides at optimized temperatures with a view to study its electrical properties. Powder X-ray diffraction has been employed for physical characterization and an average grain size of $ 16 to 22 nm was obtained. XRD study reveals the single phase structure of the samples. Dielectric properties such as dielectric constant (e 0 ), dielectric loss (tand) and ac electrical conductivity (s ac ) of the prepared ceramics sintered at various temperatures in the frequency range 10 1 -10 7 Hz have been studied. A strong dispersion observed in the dielectric properties shows the relaxor type behavior of the ceramic. The presence of maxima in the dielectric permittivity spectra indicates the ferroelectric behavior of the samples. Impedance plots (Cole-Cole plots) at different frequencies and temperatures were used to analyze the electric behavior. The value of grain resistance increases with the increase in Ba ion concentration. The conductivity mechanism shows a frequency dependence, which can be ascribed to the space charge mainly due to the oxygen vacancies. The relaxation observed for the M 00 (o) or Z 00 (o) curves is correlated to both localized and long range conduction. A single 'master curve' for the normalized plots of all the modulus isotherms observed for a given composition indicates that the conductivity relaxation is temperature independent.
Pottasium nioborate glasses of composition xNb 2 O 5 ⋅(30-x)K 2 O⋅69B 2 O 3 containing 1 mol % of... more Pottasium nioborate glasses of composition xNb 2 O 5 ⋅(30-x)K 2 O⋅69B 2 O 3 containing 1 mol % of V 2 O 5 were prepared by melt quench technique (1473K, 1h). The electron paramagnetic resonance spectra of VO 2+ in these glasses have been recorded in X-band (ν ≈ 9.14 GHz) at room temperature (RT). The spin Hamiltonian parameters, dipolar hyperfine coupling parameters, P and Fermi contact interaction parameter, K have been calculated. It is found that V 4+ ions in these glasses exist as VO 2+ in octahedral coordination with a tetragonal distortion. The tetragonality of V 4+ O 6 complex decreases with increasing Nb 2 O 5 : K 2 O ratio and also there is an expansion of 3d XY orbit of unpaired electron in the vanadium ion. The study of IR transmission spectra over a range 400-4000 cm -1 depicts the presence of both BO 3 and BO 4 structural units and Nb 5+ ions are incorporated into the glass network as NbO 6 octahedra, substituting BO 4 groups.
The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 i... more The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 ions, have been studied by means of electron paramagnetic resonance (EPR) and dierential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu 2 ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, T g , have also been measured for these borate glasses and it is found that the T g decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, K th , of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP. Ó TX 77204 5641, USA.
Glasses with composition x Bi 2 O 3 ·(30 2 x)M 2 O·70B 2 O 3 (M ¼ Li, Na) containing 2 mol% V 2 O... more Glasses with composition x Bi 2 O 3 ·(30 2 x)M 2 O·70B 2 O 3 (M ¼ Li, Na) containing 2 mol% V 2 O 5 have been prepared over the range 0 # x # 15 (x is in mol%). The electron paramagnetic resonance spectra of VO 2þ of these glasses have been recorded in the X-band (<9.3 GHz) at room temperature (RT < 300 K). Spin Hamiltonian parameters, g k ; g ' ; A k ; A ' ; dipolar hyperfine coupling parameter, P; and Fermi contact interaction parameter, K; have been calculated. The molecular orbital coefficients, a 2 and g 2 ; have been calculated by recording the optical transmission spectra. In x Bi 2 O 3 ·(30 2 x)Li 2 O·70B 2 O 3 glasses there is decrease in the tetragonality of the V 4þ O 6 complex for x up to 6 mol% whereas for x $ 6 mol%, tetragonality increases. In x Bi 2 O 3 ·(30 2 x)Na 2 O·70B 2 O 3 glasses there is increase in the tetragonality of the V 4þ O 6 complex with increasing x: The 3d xy orbit expands with increase in Bi 2 O 3 :M 2 O ratio. Values of the theoretical optical basicity, L th ; have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10 25 ohm 21 m 21 at low temperature and 10 23 ohm 21 m 21 at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi 2 O 3 :M 2 O ratio. q
Electron paramagnetic resonance, optical transmission spectra and DC conductivity of the glasses ... more Electron paramagnetic resonance, optical transmission spectra and DC conductivity of the glasses 2xMX Á (0.30Àx)M 2 O Á 0.70B 2 O 3 (M=Na or K; X=Cl or Br) (0:01pxp0:10) containing 2.0 mol% of V 2 O 5 have been studied. The spin-Hamiltonian parameters (SHP) of the VO 2+ ions, the dipolar hyperfine parameter, P, the Fermi contact interaction parameter, K, and the molecular orbital coefficients (a 2 and g 2 ) have been calculated. It is observed that in KX Á K 2 O Á B 2 O 3 (X=Cl or Br) glasses, the tetragonal nature of V 4+ O 6 complex decreases with KBr for xX0:05: An increase in the 2KX:K 2 O ratio (xX0:05) also results in the contraction of 3dxy orbit of the unpaired electron in the vanadyl ion, whereas in case of NaX Á Na 2 O Á B 2 O 3 (X=Cl or Br) glasses, the SHP are independent of change in 2NaX:M 2 O ratio. It is observed that the SHP in these alkali halide borate glasses are independent of the theoretical optical basicity, L th : It is also observed that the DC conductivity increases with increase in temperature. The order of conductivity is 10 À6 O À1 m À1 at low temperatures and 10 À4 O À1 m À1 at high temperatures. In MCl Á M 2 O Á B 2 O 3 (M=Na or K) glasses, the conductivity increases and the activation energy decreases with an increase in mol% of the NaCl or KCl content whereas, in MBr Á M 2 O Á B 2 O 3 (M=Na or K) glasses, the conductivity first increases and then decreases with mol% of the MBr content. r
The mixed alkali effect (MAE) has been investigated in the glass system (30 À x)Li 2 OÁxK 2 OÁ20B... more The mixed alkali effect (MAE) has been investigated in the glass system (30 À x)Li 2 OÁxK 2 OÁ20Bi 2 O 3 Á50B 2 O 3 (0 V x V 30, mol%) through optical properties. The optical absorption and reflection spectra were recorded at room temperature in the wavelength range 400 -800 and 450 -800 nm, respectively. From the absorption edge studies, the values of optical band gap (E opt ) and Urbach energy (DE) have been evaluated. The values of E opt lie between 2.21 and 2.55 eV for indirect allowed transitions, and for indirect forbidden transitions, the values vary from 1.86 to 2.31 eV. The values of E opt show nonlinear behavior upon substitution of one alkali ion by another with a minima at x = 15 mol% because the glass structure in the mixed alkali region may be having more nonbridging oxygens than in the dilute foreign alkali regions. The theoretical analysis of optical absorption indicates that the present glass system behaves as an indirect gap semiconductor. The refractive index (n) and optical dielectric constant (e) have been evaluated from the reflection spectra. n and e also show nonlinear variation in their values with maxima at x = 15 mol%, which supports the existence of MAE in the optical properties of the present glass system. D
Glass system xNb 2 O 5 ·(30-x)BaO·70B 2 O 3 (0 ≤ x ≤ 12, mol%) has been prepared using normal mel... more Glass system xNb 2 O 5 ·(30-x)BaO·70B 2 O 3 (0 ≤ x ≤ 12, mol%) has been prepared using normal melt-quench technique (1473K, 1h). In the present work the influence of Nb 2 O 5 on density, molar volume, optical band gap and electrical conductivity of barium borate glasses has been reported. The density and molar volume increases with increasing concentration of Nb 2 O 5 . Increase in V M indicates that the volume of non-bridging oxygens (NBOs) sites produced by Nb 2 O 5 is greater than that produced by BaO in the present glasses. The optical band gap energy (E g ) has been calculated from the Tauc's plots for both direct and indirect transitions. The decrease in E g with increase in Nb 2 O 5 content is again due to increasing concentration of the NBO ions which results in the shifting of valence band maximum (VBM) to higher energies. The value of Urbach energy (ΔE) is in the range 0.16-0.22 eV for all the glasses. The dc electrical conductivity has been measured in the temperature range 300K to 623K and is found to increase with increasing temperature. The increase in conductivity with increase in Nb 2 O 5 content in these glasses indicates the presence of NbO 6 units.
Electronic paramagnetic resonance (EPR) spectra of xLi 2 O⅐(30 Ϫ x)MoO 3 ⅐ 70B 2 O 3 glasses (0 Յ... more Electronic paramagnetic resonance (EPR) spectra of xLi 2 O⅐(30 Ϫ x)MoO 3 ⅐ 70B 2 O 3 glasses (0 Յ x Յ 25) containing 2 mol% V 2 O 5 were studied between room and liquid-helium temperatures at X-band frequencies. The spectra are typical of V 4ϩ ions present in vanadyl (VO 2ϩ ) form in the glass. The spin-Hamiltonian parameters (SHP), g g Ќ , A , and A Ќ , the dipolar hyperfine coupling parameter, P, and the Fermi constant interaction parameter, k, were evaluated. It was found that these parameters are independent of the temperature. Results suggest that the tetragonal distortion of the V 4ϩ O 6 complex increases with increase in the Li 2 O:MoO 3 ratio. It was also found that the increase in this ratio results in an expansion of the 3d xy orbit of unpaired electrons in the vanadium ion and that the spin-Hamiltonian parameters do not depend on the value of theoretical optical basicity.
Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have bee... more Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have been prepared over the range 0 ≤ x ≤ 15 (in mol%). The electron paramagnetic resonance spectra of VO 2+ in these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters, dipolar hyperfine coupling parameter and Fermi contact interaction parameter have been calculated. It is observed that, with increase in Bi 2 O 3 :K 2 O ratio, tetragonal distortion of the V 4+ O 6 complex increases and there is an expansion of the 3d xy orbit of unpaired electron in the vanadium ion. The values of theoretical optical basicity have also been reported. The molecular orbital coefficients have been calculated by recording the optical transmission spectra in the wavelength range 500-850 nm. The direct-current electrical conductivity, σ, has been measured in the temperature range 443-523 K. σ decreases with increase in Bi 2 O 3 :K 2 O ratio because of the decrease in the mobility and number of mobile K + ions and there is no electronic contribution to the conductivity by Bi 2 O 3 .
The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 i... more The structure and thermal behaviour of lithium potassium borate glasses, containing 2 mol% Cu 2 ions, have been studied by means of electron paramagnetic resonance (EPR) and dierential scanning calorimetry (DSC). From the observed EPR spectra, the spin-Hamiltonian parameters have been evaluated which are independent of the temperature from room temperature to liquid helium temperature and it is observed that the spin-Hamiltonian parameters (SHP) are dependent on the concentration of alkali ions present in the glass system. The observed SHP shows that Cu 2 ions in lithium potassium borate glasses are present in a distorted octahedral environment. Characteristic glass transition temperatures, T g , have also been measured for these borate glasses and it is found that the T g decreases with increasing lithium content upto 10 mol% and then increases for further lithium content. The theoretical optical basicity, K th , of the glasses has also been evaluated and it is observed that the changes in optical basicity values are independent of the changes in SHP. Ó TX 77204 5641, USA.
The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni-Cu ferrites wi... more The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni-Cu ferrites with general formula Ni 0.5 Cu 0.5 À x Mg x Fe 2 O 4 (0.0 r x r 0.5) have been investigated as a function of frequency, temperature and composition. ac resistivity of all the samples decreases with increase in the frequency exhibiting normal ferrimagnetic behavior. The frequency dependence of dielectric loss tangent showed a maximum in between 10 Hz and 1 kHz in all the ferrites. The conductivity relaxation of the charge carriers was examined using the electrical modulus formulism, and the results indicate the presence of the non-Debye type of relaxation in the prepared ferrites. Similar values of activation energies for dc conduction and for conductivity relaxation reveal that the mechanism of electrical conduction and dielectric polarization is the same in these ferrites. A single 'master curve' for normalized plots of all the modulus isotherms observed for a given composition indicates that the distribution of relaxation time is temperature independent. The saturation magnetization and coercivity as calculated from the hysteresis loop measurement show striking dependence on composition.
Glasses with composition xC00. (0.3-x)Na20.0.70B203 have been prepared over the range 0 5 x 5 0.1... more Glasses with composition xC00. (0.3-x)Na20.0.70B203 have been prepared over the range 0 5 x 5 0.12 containing 1 mo1%/2mol% of V z 0 5 . Electron paramagnetic resonance spectra of V 0 2 + of the glasses have been measured at X-band (v -9.3 GHz) at room temperature. Spin
Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have bee... more Glasses with composition xBi 2 O 3 ·(30 − x)K 2 O·70B 2 O 3 doped with 2 mol% of V 2 O 5 have been prepared over the range 0 ≤ x ≤ 15 (in mol%). The electron paramagnetic resonance spectra of VO 2+ in these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters, dipolar hyperfine coupling parameter and Fermi contact interaction parameter have been calculated. It is observed that, with increase in Bi 2 O 3 :K 2 O ratio, tetragonal distortion of the V 4+ O 6 complex increases and there is an expansion of the 3d xy orbit of unpaired electron in the vanadium ion. The values of theoretical optical basicity have also been reported. The molecular orbital coefficients have been calculated by recording the optical transmission spectra in the wavelength range 500-850 nm. The direct-current electrical conductivity, σ, has been measured in the temperature range 443-523 K. σ decreases with increase in Bi 2 O 3 :K 2 O ratio because of the decrease in the mobility and number of mobile K + ions and there is no electronic contribution to the conductivity by Bi 2 O 3 .
The frequency and temperature dependent conductivity investigations for alkali niobium borate gla... more The frequency and temperature dependent conductivity investigations for alkali niobium borate glasses of composition xNb 2 O 5 Á (30 À x)M 2 O Á 70B 2 O 3 (where M ¼ Li, Na; x ¼ 0, 4, 8 mol%) have been carried out using impedance spectroscopy (IS). The complex impedance data have been analyzed by using both the conductivity and the electric modulus formalisms. The conductivity decreases with the decrease in M 2 O:Nb 2 O 5 ratio. The effect of temperature on the scaling of dielectric modulus indicates that the conductivity relaxation mechanism is temperature independent. The overlapping of the normalized peaks corresponding to impedance and electric modulus and the identical values of thermal activation energy for conduction and relaxation suggest the single mechanism for the dynamic processes occurring in the present glasses. The variation of density, molar volume and glass transition temperature with glass composition has also been reported to supplement the conductivity results.
Lithium vanadoxide bismuth borate glasses with composition (30-x)Li 2 O · xV 2 O 5 · 50B 2 O 3 · ... more Lithium vanadoxide bismuth borate glasses with composition (30-x)Li 2 O · xV 2 O 5 · 50B 2 O 3 · 20Bi 2 O 3 (x=5, 10, 15) has been prepared with the traditional melt-quenching technique. The density measurements were performed using Archimedes Principal and using density data molar volume was calculated. To determine the oxygen covalency theoretical optical basicity was calculated. To get an idea about the structure FTIR spectroscopy was carried out in the mid-IR region. The spectra revealed absence of boroxol ring and presence of absorption bands corresponding to the combined contributions of tri and tetra borate stretching vibrations. The increasing basicity reveals decrease in the covalence nature of oxygen as we replace the Li 2 O content with V 2 O 5 . The decrease in molar volume may be due to the increase in compactness of the network structure.
Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)... more Electron paramagnetic resonance (EPR), optical properties and dc conductivity of xTiO 2 Á(30 À x)R 2 OÁ70B 2-O 3 and xTiO 2 Á(70 À x)B 2 O 3 Á30R 2 O (R = Li, K; 0 6 x 6 7, in mol%) glasses containing 2 mol% V 2 O 5 have been studied. Spin Hamiltonian parameters, g k , g \ , A k , A \ , the dipolar hyperfine coupling parameter (P) and the Fermi contact interaction parameter (K) have been evaluated. Decrease in Dg k /Dg \ ratio with increase in TiO 2 content in TiO 2 ÁLi 2 OÁB 2 O 3 glasses show that the octahedral symmetry of V 4+ O 6 complex is improved; whereas in TiO 2 ÁK 2 OÁB 2 O 3 glasses, it is reduced with increase in x. The decrease in P, in both systems, indicates that the 3d xy orbit expands with increase in mol% of TiO 2 . The molecular orbital coefficients, a 2 and c 2 have been calculated by recording the optical transmission spectra in the range 400-850 nm. a 2 and c 2 increase with increase in x in all glasses expect KB glasses, which indicate that, the covalency of the vanadium oxygen bonds decreases. The dc conductivity (r) decreases and activation energy (W) increases with increase in TiO 2 :R 2 O ratio in TiO 2 ÁR 2 OÁB 2 O 3 glasses whereas with increase in TiO 2 :B 2 O 3 ratio in TiO 2-ÁB 2 O 3 ÁR 2 O glasses the variation in r and W is within experimental error.
Uploads
Papers by satish khasa