Excited state relaxation of trivalent Cr3+ ions in different glassy
and crystalline solid-state materials was reviewed [9]. Especially,
spectral lines of Cr3+ in oxide [10–12] and non-oxide [13–15] glasses
were analyzed. The detailed study indicates that chromiumions can
occupy low-field and high-field sites [16]. It is also well noted from
the previously published results based on absorption and luminescence
spectra, that chromium at different valence states such as
Cr3+, Cr4+, Cr5+ and Cr6+ can be exist in the glass. The spectral analysis
suggests that the chromium at its low concentration exist in
Cr3+ state, which act as modifiers. When Cr2O3 is present in higher
concentrations in the glass matrix, these ions seem to exist in Cr6+
state and take part in network forming positions with CrO2−
4 structural
units in PbO–Al2O3–B2O3 glass systems [17]. It proves data
obtained from electron paramagnetic resonance (EPR) measurements.
The EPR spectra exhibit two resonance signals at g = 4.65
and 1.97 for chromium in alkali lead borotellurite glasses [18] and
g = 4.50 and 1.98 for chromium in alkaline earth alumino borate
ones [19], whichcan be attributedto Cr3+ and Cr5+ ions, respectively.
Moreover, the optical absorption spectra recorded for these systems
evidently exhibit bands characteristic of Cr3+ ions in an octahedral
symmetry. Additionally, two absorption bands between 600 and
900 nm due to 3A2–3T1 and 3A2–3T2 transitions with long tail over
1000 nm, together with broad-band luminescence centered around
1260nm are characterized for Cr4+ ions in various glasses [20]. In
spite of this fact, the lead borate glasses singly doped with Cr3+
ions were analyzed in wide spectral region in order to determine
all valence states of chromium. However, any higher valence states
than trivalent chromium Cr3+ have been observed using absorption
and luminescence spectroscopy.