Optical absorption spectra and energy band gap in praseodymium borophosphate glasses

Abstract

There has been some emphasis recently on the optical properties of glassy and amorphous semiconductors doped with small amounts of rare earth ions [1–8]. These glasses are potential materials for lasing action. This arises from the fact that the electrons responsible for the spectral and magnetic properties of rare earth ions are 4f electrons, which are very effectively shielded from interaction with external forces by the overlaying 5S and 5P shells. Therefore their energies and the energies of the f–f transitions are not likely to be significantly affected by external fields. Some features of the band structure of glasses can be understood by an analysis of the optical absorption edge. The electronic states of the glasses interact with the incoming radiation and give rise to optical absorption. Doping of V2O5–P2O5 glasses by different rare earth oxides does not show any significant change in optical ‘‘band gap’’ Eopt (it varies from 2.20 eV to 2.37 eV only) [7]. This slight difference in calculated Eopt values for different dopants is a consequence of small differences in the observed position and slope of the absorption edge. A survey of the literature indicates that rare earth oxide glasses have not been studied for their optical band gap and related properties [9–11]. Ahmed et al. [4] studied the optical energy gap of praseodymium phosphate glass. They analysed their data on the basis of modern theoretical models of amorphous solids. There are no other reports on rare earth oxide glasses. Thus it was of some interest to study the nature of the optical energy gap of rare earth doped glasses and to see the effect of composition on the position of the absorption edge and the value of the so-called optical energy gap. The present communication reports such studies on Pr-doped B2O3– P2O5–BaO glasses. Borophosphate glasses of final composition B2O3–P2O5–BaO in the ratio of 40, 45 and 15 mol % were prepared from analytical reagent grades of H3BO3, NH4H2PO4 and BaCo3 in 10 g batches. The praseodymium oxide added to the glass was Pr6O11 (99.99%). The chemicals were obtained from E. Merck (India) Ltd, Glaxo India Ltd and Aldrich Chemical Company, USA, respectively. Batches containing 0.1, 0.3, and 0.5 mol % of Pr in the base glass materials were mixed in an agate pestle mortar for 30 min, and were thermally treated in an alumina crucible up to 1000 8C in steps of 100 8C starting from 300 8C. A hold time of 6 h was given at each temperature step. Homogeneity of the melt was ensured by stirring the melt with an alumina rod from time to time. The melt was quenched by pouring into a rectangular-shaped depression in a copper plate. Glasses contained no crystalline phases as revealed by an X-ray diffractogram of the specimen. The glass samples were polished using cerium oxide power. The absorption spectra were recorded using Perkin Elmer Lambda-4B UV/VIS Spectrophotometer in the wavelength range 200–900 nm. Four sharp absorption peaks at 445.5, 470.5, 482.8 and 591 nm were observed in the visible region. Fig. 1 shows the optical absorption spectra for several compositions of Pr-doped borophosphate glasses. On the basis of a comparison of the present spectra with the work already reported [1–3], the absorption peaks observed at 442.0, 469.6, 481.8 and 587.8 nm may be assigned as arising due to the transitions H4? P2, H4? P1, H4? P0 and H4? D2, respectively. This assignment of peaks similar to the ones existing in the literature for different materials containing Pr ions, is justified if we remember that f–f transitions are only weakly affected by the environment of the praseodymium ion [2, 12, 13]. This also explains the observed sharpness of the peaks. In crystalline materials, the Eopt value is directly obtained from the absorption. However, for amorphous materials in general and oxide glasses in particular, the optical absorption at the fundamental edge may be used to estimate Eopt values using a theory due to Davis and Mott [14]. The absorption in amorphous semiconductors obeys a quadratic relation for the inter-band non-direct transitions: