Luminescence characteristics of alkali borate glasses

Abstract

The results are compared with a study of the anionic structure of these glasses. It is established that the intensity of the cathodoluminescence increases with increasing fraction of alkali metals in the glasses; it is determined by the fractions of the nonbridge bonds M -O -B -in the structure of these glasses and differs in lithium and sodium glasses.Borate glasses are an important class of oxide glasses, whose anionic structure consists of diverse structural and superstructural units represented by different boro-oxygen groupings and complicated anionic complexes [1,2]. It has been determined that when the framework of alkali borate glasses undergoes polymerization bridge bonds of the type -B -O -B -as well as nonbridge bonds M -O -B -coordinated by alkali metal ions (M = Cs, Rb, K, Na, Li) can form. The fraction of the nonbridge bonds is related with the ratio of boron in ternary and quaternary coordination, depends on the alkali-metal fraction, and has a large effect on the physical properties of glass [3,4]. Increasing the alkali-metal content in the glass composition does not always increase the fraction of nonbridge bonds [5]; this is why it is important to determine the number of nonbridge bonds correctly. Vibrational spectroscopy, such as infrared spectroscopy and Raman scattering spectroscopy, cannot solve this problem. This is due to the complexity of the decomposition of the spectra, the impossibility of clearly distinguishing in them bands which are related with the vibrations of nonbridge bonds, as well as a considerable error in calculating from the intensity of these bands the nonbridge-bond fraction [6].The luminescence of glass, as other dielectric materials and substances, is determined by impurities and structural defects of different kinds as well as the formation of centers of luminescence of different nature [7]. Recently, pulsed cathodoluminescence (PCL) has been widely used to study the luminescence properties of glasses [8 -9]. The high information content and the effectiveness of this method are due to the use of pulsed excitation and to the short-time local supercooling of the sample under the action of a high-current electron beam [10]. The manifestation of the cathodoluminescence of alkali silicate glass was described in [9 -11], and the mechanism was tied to the formation of nonbridge oxygen atoms in the glass structure. This indicates the possibility of using the PCL method to study the structural features of other types of oxide glasses. The authors of the present article assumed that there should be a correlation between the intensity of the cathodoluminescence of alkali borate glasses and the fraction of nonbridge oxygen (NBO) atoms in their structure.The luminescence characteristics of alkali borate glasses were studied by means of pulsed cathodoluminescence of a series of lithium and sodium borates glasses with different alkali-metal fraction.To determine the luminescence properties of lithium and sodium borate glass, glass with the following composition was synthesized: xLi ...