Optical properties and site distribution of Cr ions in alkali-disilicate glasses

The optical properties of the low-field sites of Cr³⁺-doped alkali (Li, Na, K) disilicate glasses have been investigated using the single configurational coordinate model. The assumption of a Gaussian site distribution for the Cr³⁺ ions taking as parameter the zero-phonon energy has been considered. For alkali disilicate glasses the inhomogeneous contribution to the broadening of the bands, associated to the site distribution, is lower than the homogeneous one. The electron-lattice coupling S and the mean phonon energy ?ω₀ have been obtained with results around 4 and 500 cm⁻¹, respectively, similar to those obtained by other authors in oxide glasses. The site-resolved study of the emission and excitation spectra and the luminescence decay curves have been carried out as a function of temperature. On the one hand, there is evidence of a non-radiative de-excitation process that becomes important over 140 K. On the other hand, and related to the site dependence of the radiative and non-radiative probabilities, different results involving low values for the quantum efficiencies and blue shifts of the emission bands as temperature increases have been explained. Besides, the non-exponential luminescence decay curves have been fitted to an expression proposed by the authors, which takes into account non-coupled distributions for the radiative and non-radiative de-excitation probabilities for the range of temperature covering from 13 to 300 K. From the fits, the temperature dependence of the non-radiative de-excitation probability is obtained for each disilicate glass, the results are in good agreement with the expression obtained assuming the harmonic approximation in the single configurational coordinate model.