J. Ramakrishna

Mixed alkali effect in borate glasses-EPR and optical absorption studies in xNa2O.(30-x)K2O-70B2O3 glasses doped with Mn2+

The mixed alkali borate xNa2O–(30−x)K2O–70B2O3 (5≤x≤25) glasses doped with 1 mol% of manganese ions were investigated using EPR and optical absorption techniques as a function of alkali content to look for ‘mixed alkali effect’ (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Mn2+ ions. The resonance signal at gcong 2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with gcong 4.64, with a shoulder around gcong 4.05 and 2.98, was also observed. From the observed EPR spectrum, the spin-Hamiltonian parameters g and A have been evaluated. It is interesting to note that some of the EPR parameters do show MAE. It is found that the ionic character increases with x and reaches a maximum around x=20 and thereafter it decreases showing the MAE. The number of spins participating in resonance (N) at gcong 2.02 decreases with x and reaches a minimum around x=20 and thereafter it increases showing the MAE. It is also observed that the zero-field splitting parameter (D) increases with x, reaches a maximum around x=15 and thereafter decreases showing the MAE. The optical absorption spectrum exhibits a broad band around ~20,000 cm−1 which has been assigned to the transition 6A1g(S)→4T1g(G). From ultraviolet absorption edges, the optical bandgap energies and Urbach energies were evaluated. It is interesting to note that the Urbach energies for these glasses decrease with x and reach a minimum around x=15. The optical band gaps obtained in the present work lie in the range 3.28–3.40 eV for both the direct and indirect transitions. The physical parameters of all the glasses were also evaluated with respect to the composition.

The effect of host glass on optical absorption and fluorescence of Nd3+ in xNa2O?(30? x)K2O?70B2O3 glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Nd3+ has been studied in mixed alkali borate glasses of the type xNa2O?(30? x)K2O?69.5B2O3?0.5Nd2O3 (x = 5, 10, 15, 20 and 25). Various spectroscopic parameters such as Racah (E1, E2 and E3), spin?orbit (?4f) and configuration interaction (?,?) parameters have been calculated. The Judd?Ofelt intensity parameters (??) have been calculated and the radiative transition probabilities (Arad),radiative lifetimes (?r), branching ratios (?) and integrated absorption cross sections (?) have been obtained for certain excited states of the Nd3+ ion and are discussed with respect to x. From the fluorescence spectra, the effective fluorescence line widths (??eff) and stimulated emission cross sections (?p) have been obtained for the three transitions , and of Nd3+. The stimulated emission cross section (?p) values are found to be in the range (2.0?4.8) ? 10?20?cm2 and they are large enough to indicate that the mixed alkali borate glasses could be potential laser host materials.

NMR study of molecular dynamics and phase transitions in dimethyl ammonium hexabromo selenate [NH2(CH3)2]2SeBr6

Abstract 1 H spin lattice relaxation study in the temperature range 439–77xa0K has been carried out in dimethyl ammonium (DMA) hexabromo selenate. A high temperature phase transition is observed as a dip in T 1 around 360xa0K, followed by a slope change at 222xa0K, and a discontinuous jump in T 1 at 154xa0K, which are also attributed to phase transitions. DMA diad axis motion and spin–rotation interaction are found to be the dominant mechanisms responsible for the observed T 1 behaviour above 222xa0K, while between 222 and 154xa0K, DMA torsional oscillations contribute significantly. Below 154xa0K the relaxation is governed by the methyl group dynamics. The motional effects are also reflected in the second moment variation with temperature.

MIXED ALKALI EFFECT IN BORATE GLASSES — EPR AND OPTICAL BAND GAP STUDIES IN xNa2O-(30-x)K2O-70 B2O3 GLASSES DOPED WITH Gd3+ IONS

Mixed alkali borate xNa2O-(30 - x)K2O-70 B2O3 (5 leq x leq 25) glasses doped with 0.5 mol% of gadolinium ions have been investigated by using electron paramagnetic resonance (EPR) and optical absorption techniques, as a function of alkali content, to look for the mixed alkali effect on the spectral properties of the glasses. The EPR spectrum consists of three prominent features with effective g-values, g approx 5:6, 2.8 and 2.0, and two weak features at g approx 3:3 and 4.3. The three EPR signals at g approx 2:0, g approx 2:8 and g approx 5:6 are attributed to Gd3+ ions located at sites with weak, intermediate and strong cubic symmetry fields, respectively. In principle these sites may be of network forming and network modifying types. Ionic radius considerations suggest that gadolinium ions cannot substitute the much smaller boron ions and thus only the network modifier site is acceptable. The number of spins (N) participating in resonance and its paramagnetic susceptibility (chi) for g approx 5:6 resonance line have been calculated. It is interesting to note that N and chi increase with x and reach a maximum around x = 15 and thereafter decrease showing the mixed alkali effect in these glasses. From ultraviolet absorption edges, the optical band gap energies were evaluated. It is interesting to note that the optical band gap energies for these glasses decrease slightly with increasing x and reach a minimum around x = 10, and thereafter increase showing the mixed alkali effect. Optical band gap energies (Eopt) obtained in the present work vary from 2.20-3.35 Ev for both the direct and indirect transitions. The physical parameters of the glasses have been evaluated with respect to the composition.

Proton NMR study of molecular dynamics in ammonium tribromo stannate (NH4SnBr3)

The proton second moment M2 and spin-lattice relaxation time T1 have been measured in ammonium tribromo stannate (NH4SnBr3) in the temperature range 77–300 K, to determine the ammonium dynamics. The continuous wave signal is strong and narrow at 77 and 300 K but has revealed an interesting intensity anomaly between 210 and 125 K. T1 shows a maximum (13 s) around 220 K. No minimum in the T1 vs 1000/T plot was observed down to 77 K. M2 and T1 results are interpreted in terms of NH+4 ion dynamics. The activation energy Ea for NH+4 ion reorientation is estimated to be 1.4 kcal mol−1.

Pulsed NMR study of molecular motions and phase transitions in [N(CH3)4]PbX3 (XCl, Br, I)

Abstract Proton spin—lattice relaxation time ( T 1 ) is measured in [N(CH 3 ) 4 ]PbX 3 (Xue5fbCl, Br, I) from 300-77 K at 9.75 MHz. All the compounds show discontinuous changes in T 1 values (at 256, 270 and 277 K, respectively), indicating phase transitions. Single T 1 minimum is observed in all the cases and the T 1 variation is explained in terms of [N(CH 3 ) 4 ] and CH 3 group dynamics. The activation energy E α decreases from chloride to iodide (from 4 to 2 kcal/mol). In bromide and iodide, T 1 is found to decrease with increase in temperature at higher temperatures, indicating the presence of spin—rotation interaction.

NMR study of molecular dynamics in hydrazinium phosphates

Abstract The proton spin-lattice relaxation time T 1 (at 5.4, 10 and 15 MHz) and second moment M 2 (at 9.8 MHz) were measured in the temperature range 77–300 K in hydrazinium dihydrogen phosphate N 2 H 5 H 2 PO 4 and dihydrazinium hydrogen phosphate (N 2 H 5 ) 2 HPO 4 . In N 2 H 5 H 2 PO 4 a well-defined single proton T 1 , minimum and three M 2 transitions have been observed. The T 1 minimum has been interpreted in terms of the NH 3 group reorientation with activation energy of 7.69 kcal mol −1 , relaxing all protons by spin-diffusion. In (N 2 H 5 ) 2 HPO 4 a single T 1 minimum and a small hysteresis behaviour have been observed in the temperature dependence of T 1 . Two M 2 transitions were observed. The T 1 minimum has been interpreted in terms of the NH 3 group reorientation with an activation energy of 4.4 kcal mol −1 . The M 2 behaviour in both phosphates is explained in terms of NH 3 /NH 2 motions.

Proton NMR T 1 Studies in Methylammonium TrichlorO Stannate(II) (CH 3 NH 3 SnCl 3 )

Proton spin lattice relaxation time ( T 1 ) measurements have been carried out in methylammonium trichloro stannate(II) (CH 3 NH 3 SnCl 3 ) as a function of temperature in the range 317-5 K at a Larmor frequency of 10 MHz. The temperature dependence of T 1 shows a phase transition around 220 K and four T 1 minima (294 K, 62 K, 32 K and 12 K). The results are discussed in terms of proton dynamics, namely, uncorrelated reorientation of NH 3 and CH 3 groups at high temperatures and tunnelling of NH 3 and CH 3 protons at low temperatures.

Proton NMR study of molecular dynamics and phase transitions in methylammonium hexachloro plumbate, [NH3CH3]2PbCl6

Abstract Proton spin lattice relaxation time (T1), measured as a function of temperature in the range 375–77 K, shows slope changes at 333, 221 and 111 K, in addition to a first order phase transition at 150K. The observed T1 behaviour and second moment (M2) variation with temperature are explained on the basis of the different possible motions of CH3 and NH3 groups.

1H NMR study of molecular dynamics in dihydrazinium sulphate

The proton spin-lattice relaxation time