Spectroscopic properties of alkali borate glasses containing Cu2+

Abstract

Binary alkali borate glasses R2O-B2O3 (R = Li, Na or K) are a model system for oxide glasses due to the major modification of their structure and properties with alkali concentration. The present study correlates optical absorption and electron paramagnetic resonance (EPR) spectroscopy data on binary alkali borate glasses containing Cu2+ in the range 10-33 mol% alkali oxides. These glasses show a color change from light cyan to sky blue with increasing alkali content. The chemical dependence of the spectroscopic properties of Cu2+ in alkali borate glasses is different from what is observed for other divalent transition metal ions (Ni2+, Co2+): (i) there is no change in the coordination of Cu2+ as a function of alkali content, confirming that tetrahedral Cu2+ is rather rare in these glasses as in crystalline oxide materials; (ii) as a consequence, the color change of Cu2+-containing glasses as a function of their chemical composition remains limited; (iii) cation field strength does not significantly influence the spectroscopic properties of Cu2+, the alkali content being the main chemical parameter governing Cu2+ speciation in these glasses; (iv) there is no clustering of Cu2+ ions in low-alkali borate glasses. By correlating the data obtained from optical absorption spectroscopy and from EPR, information is gained on the variation of Cu-site geometry as a function of the alkali content of the glass. In low-alkali glasses, the Cu2+ sites show a higher distortion than in high-alkali glasses and their coordination geometry is less distributed. This may indicate that these sites are connected with rigid superstructural borate units of these glasses. The spectroscopic parameters of Cu2+ in low-alkali glasses show also a noteworthy similarity with those of aqueous Cu2+ complexes. The smaller site distortion in high-alkali glasses is accompanied by a more covalent character of the Cu2+-O π bonds, resulting from a weakening of the B-O bonds that is caused by the progressive conversion of [3]B to [4]B with increasing alkali content. The color of the glasses changes from light cyan to sky blue due to the increase in absorption efficiency in the red portion of the visible spectrum. Indeed, in high-alkali glasses, a greater distribution of the Cu2+ sites causes a broadening of the Cu2+ absorption band. These modifications are explained by considering the influence of axial ligands on the distortion of Cu2+ sites. The intensity of this axial distortion governs the anisotropy of the Cu2+ sites, even if the contribution of axial ligands to the local charge balance of Cu2+ remains minor. The speciation of cupric ions in alkali borate glasses, as derived from this spectroscopic approach, is consistent with a bond-valence model. As these ions are used widely as a structural probe in crystals and glasses, such a combined approach may shed light on the local environment of Cu2+ in oxide glasses.