Faisal Amir

Date of Award

January 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics & Astrophysics

First Advisor

Kanishka Marasinghe

Abstract

The atomic-scale structure of a series of (〖RE〗_2 O_3 )_x (〖Na〗_2 O)_y (P_2 O_5 )_(1-x-y) glasses (RE = Pr, Nd, Er) where has been characterized by high-energy X-ray diffraction technique (HEXRD). In addition, differential thermal analysis (DTA), Fourier transform infrared (FTIR) spectroscopy, and absorption and emission spectroscopy in visible and near IR ranges have been used as supplementary tools to validate structural features obtained from HEXRD techniques.Structural features such as inter-atomic distances and coordination numbers and their dependence on the concentration of 〖RE〗_2 O_3 have been obtained by analyzing pair distribution functions (PDF) extracted from diffraction data.

Coordination numbers for P-O, Na-O, O-O, and P-P were found to be independent of the 〖RE〗_2 O_3 concentration. In contrast, the RE-O coordination number varies between  8 and 7.2 as the 〖RE〗_2 O_3 concentration increases from 0.005 to 0.05. The variation of the bond distance between large rare-earth ions (Pr, Nd) and small rare-earth ion (Er) is approximately 0.2 Ŭ which is attributed to lanthanide contraction. The Na-O coordination number in these glasses was observed to  5.0 as the 〖RE〗_2 O_3 content increases. The overlapping correlation of RE-O, Na-O, and O-O in the same vicinity makes it difficult to calculate these coordination numbers.

DTA measurements were used for the investigation of thermal characteristics of glasses. From these measurements, it is evident that the glass transition temperature increases with increasing the 〖RE〗_2 O_3 (RE=Pr, Er) content. FTIR was used to inspect the structural changes of the glasses. The doping of 〖RE〗_2 O_3 (RE=Pr, Er) induces depolymerization of the glasses at the Q^3 tetrahedral sites. The forming of the ionic linkages between phosphate chains is attributed to the increase in non-bridging oxygen (NBO). The cross-linkages density (CLD) increases with the 〖RE〗_2 O_3 (RE=Pr,Er) concentrations.

Absorption spectra for x=0.01 of 〖Er〗^(3+) and 0.005-0.05 for 〖Nd〗^(3+) doped glasses have been analyzed using Judd-Ofelt (JO) theory. The JO parameters have been used to predict radiative properties of luminescent levels of 〖Er〗^(3+) and 〖Nd〗^(3+) ions. Comparatively large photoluminescence lifetime 13.76 msec ( x=0.01) for 〖Er〗^(3+) and 476 μsec("x = 0.005" ) for 〖Nd〗^(3+) for the laser transition was observed. However, the quantum efficiency of the erbium doped glasses is ≈70 %. The influence of 〖RE〗^(3+) ( RE=Er, Nd) doping concentration on the emission spectra and lifetimes was investigated wit the model proposed by Auzel’s limited diffusion model, in order to study the concentration quenching effect on luminescence. In this model, the fitting of the fluorescence lifetime experimental data gives us a radiative lifetime (τ_0) and quenching concentration (N_0). For Neodymium glasses, τ_0= 467 μsec and N_0=5.98 × 〖10〗^20 ions/〖cm〗^3 〖Nd〗^(3+) ion, and in erbium glasses, τ_0=12.4 ms at N_0=1.57 × 〖10〗^20 〖ions/cm〗^3 for 〖Er〗^(3+) ion. The Inokuti-Hirayama (IH) model has been applied to the non-exponential behavior of the decay profiles to investigate the mechanism involved in the energy transfer between the donors and acceptors. Emission spectra of rare earth phosphate glasses show that their fluorescence efficiency decreases with increasing rare-earth content even at relatively at low concentrations (0.005 < x < 0.05), suggesting that concentration quenching of lasing action may be present even at these concentrations.

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