0360-5043/01/2701- $25.00 © 2001 MAIK “Nauka /Interperiodica” 0037 Glass Physics and Chemistry, Vol. 27, No. 1, 2001, pp. 37–47. Original English Text Copyright © 2001 by Fizika i Khimiya Stekla, Pontuschka, Kanashiro, Courrol. 1 INTRODUCTION The profusion of electron–hole radiative and nonra- diative recombination processes in glasses previously exposed to ionizing radiation is rich of interesting phe- nomena that involve charge and energy transfers among populations of specific impurity and defect states found in specific structural units having their characteristic local symmetry. As is well known, in the absence of impurities, the photoelectrons released by borate glasses under irradi- ation are trapped at metastable states located at oxygen vacancies of the glass matrix with an average depth of about 0.2 eV [1] below the bottom of the conduction band, thus producing the boron electron centers (BEC) [2]. The corresponding holes left in the valence band are self-trapped at the site of a bridging oxygen between a threefold-coordinated and a fourfold-coordi- nated boron of the glass network, thus producing the boron–oxygen hole center (BOHC) [3] with an energy of about 1.0 eV [1] above the top of the valence band. 1 In accordance with the decision of the Editorial Board of Fizika i Khimiya Stekla (Glass Physics and Chemistry), this article is pub- lished without review. The article was submitted by the authors in English. When impurity ions are present in the glass composi- tion, specific fractions of electrons and holes are scav- enged under conditions governed by chemical affinity and oxidation state of the system, so that the BEC and BOHC concentrations immediately after the cessation of the sample irradiation are no more equal. The com- plete recombination of the totality of the e – –h + pairs produced by the ionizing radiation is accomplished by a number of reactions which are not always feasible to follow in detail. A procedure generally adopted in order to take into account the several recombinations, which occur in such a complex system as a multicomponent glass con- taining impurities, is to write down the differential equations of the possible reactions involving the elec- trons and holes. As the exact integration of the system of equations is in general impracticable, numerical integration procedures are preferred, and the adjusted coefficients are used to obtain useful information, such as the activation energies and cross sections [4–6]. The time derivatives of the BEC and BOHC concentrations can be determined from the plots of the ESR line inten- sities obtained during thermal decay measurements or from thermoluminescence (TL) studies. The corre- lated ESR and TL kinetic studies are very useful in Luminescence Mechanisms for Borate Glasses: The Role of Local Structural Units 1 W. M. Pontuschka*, L. S. Kanashiro*, and L. C. Courrol** * Instituto de Física da Universidade de São Paulo, Caixa Postal 66318, 05315-970 São Paulo, Brazil ** Divisão de Óptica Aplicada, IPEN/CNEN, Caixa Postal 11049, 05508-900, São Paulo, Brazil Abstract—The increasing knowledge of local order and detailed structural information on the neighboring atoms of impurity ions or radiation-induced defect states in borate glasses is a powerful argument for more detailed models leading to a better understanding of luminescence mechanisms, recombination kinetics, and other related phenomena. Examples of some previous models for the thermoluminescence quenching mecha- nisms for gamma-irradiated aluminoborate glasses doped with Fe are briefly revised. The Racah parameters and the ligand field intensity for a transition element ion such as Cr 3+ are found to be useful parameters, sensitive to the nature of local symmetry and distortions, which can be controlled by an adequate glass composition design. The Fano antiresonances observed in the optical absorption spectra appear at the positions expected from the Tanabe–Sugano diagram. The optical absorption spectrum of barium aluminoborate glasses doped with Cr 3+ and Nd 3+ is merely a superposition of the respective independent absorptions. However, the fluores- cence spectrum appears to be shifted, thus indicating the occurrence of an energy transfer process from Cr 3+ to Nd 3+ excited states, which is observed even at room temperature. Both the fluorescence and excitation spectra of glasses doped with chromium and neodymium show the Fano antiresonance effect but exhibit a Lamb shift of the valleys associated with neodymium over the emission bands of Cr 3+ . PROCEEDINGS OF THE INTERNATIONAL CONFERENCE “THERMODYNAMICS AND CHEMICAL STRUCTURE OF MELTS AND GLASSES” (St. Petersburg, Russia, September 7–9, 1999)