Spectroscopy of Mn-Doped Glasses Of CaO-Ga2O3-GeO2 System

The EPR, UV and visible optical spectra of absorption, luminescence excitation and emission, as well as the luminescence kinetics of impurity Mn2+ centres in Mn-doped glasses of CaO-Ga2O3-GeO2 system have been measured and analysed. The EPR spectra of as-synthesised Mn-doped glasses consist of three broad bands with the effective g-factors geff ~ 4.3, 3.0 and 2.0. The observed spectra have been attributed to both the isolated Mn2+ (3d5, 6S5/2) ions occupying octahedral sites and revealing a broad distribution of crystal field parameters, as well as small clusters of Mn2+ ions. The optical spectroscopy has shown that the manganese impurity is incorporated into octahedral sites of CaO-Ga2O3-GeO2 glass network as both Mn2+ and Mn3+ ions. The intense broad absorption band with the maximum near 460 nm is related to the spin-allowed 5Eg --> 5T2g transition of Mn3+ ions in trigonally distorted octahedral sites of CaO-Ga2O3-GeO2 glass network. The weak spin-forbidden absorption lines of Mn2+ ions have not been observed against the background of a strong absorption band of Mn3+. The observed luminescence band with the maximum in the vicinity of 650 nm is explained by T1g --> 6A1g transition of Mn2+ ions occurring in trigonally distorted octahedral sites. The luminescence decay curve of Mn2+ could be satisfactorily described within two-exponent approximation. The two different lifetimes (t1 = 12.4 ms and t2 = 3.98 ms at lexc = 300 nm) correspond to the two types of Mn2+ centres with different local environments. A possible local structure of different Mn2+ centres in CaO-Ga2O3-GeO2 glass network is discussed.