R.P. Sreekanth Chakradhar

Solution combustion derived nanocrystalline Zn2SiO4:Mn phosphors: A spectroscopic view

Manganese doped nanocrystalline willemite powder phosphors Zn(2-x)Mn(x)SiO(4) (0.1<or=x<or=0.5) have been synthesized by a low-temperature initiated, self-propagating, gas producing solution combustion process. The phosphors have been characterized by using x-ray diffraction (XRD), energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), and photo luminescence (PL) spectroscopic techniques. The lattice parameters calculated from XRD confirm that Zn(2-x)Mn(x)SiO(4) has a rhombohedral space group R3H. The XRD patterns confirm that Zn(2-x)Mn(x)SiO(4) phosphor samples undergo a phase transformation from beta-willemite to alpha-willemite phase at 950 degrees C. The EPR spectra of Mn(2+) ions exhibit resonance signals at g approximately = 3.24 and g approximately = 2.02, with a sextet hyperfine structure centered around g approximately = 2.02. The EPR signals of Mn(2+) give a clear indication of the presence of two different Mn(2+) sites. The magnitude of the hyperfine splitting (A) indicates that the Mn(2+) is in an ionic environment. The number of spins participating in resonance (N), the paramagnetic susceptibility (chi), and the zero-field splitting parameter (D) have been evaluated as function of x. It is interesting to observe that the variation of N with temperature obeys Boltzmann. The paramagnetic susceptibility is calculated from the EPR data at various temperatures and the Curie constant and Curie paramagnetic temperature was evaluated from the 1/chi versus T graph. The luminescence of Mn(2+) ion in Zn(2)SiO(4) shows a strong green emission peak around 520 nm from the synthesized phosphor particles under UV excitation (251 nm). The luminescence is assigned to a transition from the upper (4)T(1)-->(6)A(1) ground state. The mechanism involved in the generation of a green emission has been explained in detail. The effect of Mn content on luminescence has also been studied.

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 g\cong 2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with g\cong 4.64, with a shoulder around g\cong 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 g\cong 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.

Electron paramagnetic resonance and optical absorption studies of Cu2+ ions in alkali barium borate glasses

Abstract Electron Paramagnetic Resonance (EPR) and optical absorption studies have been made for Cu 2+ ions in alkali barium borate glasses. The EPR spectrum consists of four hyperfine lines characteristic of Cu 2+ ions. From the observed spectra the spin-Hamiltonian parameters have been evaluated. The values of spin-Hamiltonian parameters indicate that the Cu 2+ ions in alkali barium borate glasses were present in octahedral coordination with a strong tetragonal distortion. The linewidth of the parallel and perpendicular hyperfine peaks increases in order with magnetic quantum number of copper nuclei and this has been attributed to the microenvironmental fluctuations around Cu 2+ ion which is intrinsic to the glassy state. The optical absorption spectra of Cu 2+ ions doped glasses show a single broad absorption band which is attributed to 2 B 1g → 2 B 2g transition. By correlating the EPR and optical data, the molecular orbital coefficients have been evaluated. The theoretical values of optical basicity ( Λ th ) of these glasses have also been evaluated.

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.

Electronic absorption spectra and energy gap studies of Er3+ ions in different chlorophosphate glasses

Spectroscopic properties of Er3+ ions in different chlorophosphate glasses 50P2O5-30Na2HPO4-19.8RCl (R = Li, Na, K, Ca and Pb) are studied. The direct and indirect optical band gaps (Eopt) and the various spectroscopic parameters (E1, E2, E3, and zeta4f and alpha) are reported. The oscillator strengths of the transitions in the absorption spectrum are parameterized in terms of three Judd-Ofelt intensity parameters (omega2, omega4 and omega6). These intensity parameters are used to predict the transition probabilities (A), radiative lifetimes (tauR), branching ratios (beta) and integrated cross sections (sigma) for stimulated emission. Attention has been paid to the trend of the intensity parameters over hypersensitive transitions and optical band gaps. The lifetimes and branching ratios of certain transitions are compared with other glass matrices.

Electron paramagnetic resonance and luminescent properties of Mn2+:MgGa2O4 phosphor

Electron-paramagnetic-resonance EPR and photoluminescence PL studies on magnesium gallate

Solution combustion derived nanocrystalline macroporous wollastonite ceramics

(MgGa_{2}O_{4})

Mixed alkali effect in Li2O-Na2O-B2O3 glasses containing CuO - an EPR and optical study

phosphor activated with divalent manganese have been carried out. The EPR spectrum exhibits a six-line hyperfine structure centered at g=1.995, which is a characteristic of a

Low temperature synthesis, structural characterization, and zero-field resistivity of nanocrystalline La1−xSrxMnO3+δ (0.0 ≤ x ≤ 0.3) manganites

Mn^{2+}

EPR and optical investigations of manganese ions in alkali lead tetraborate glasses.

ion with a nuclear spin I=5/2. The spin-Hamiltonian parameters have been evaluated. The concentration and temperature 123-433 K dependences of EPR signals have been studied. The number of spins participating in resonance N with temperature obeys the Boltzman law and from the graph of log N versus 1/T the activation energy has been evaluated. The paramagnetic susceptibilities have been calculated from the EPR data at different temperatures and it is interesting to note that it obeys the Curie-Weiss behavior. The Curie constant has been evaluated from the 1/\chi versus T graph. A bright green photoluminescence according to the transition