Trivalent chromium is a very common ion used for coloring in the industry of glass. As glasses are melted under
high oxidizing circumstances, hexavalent chromium Cr(VI) can be fixed and found in certain advertising applications of
UV-protection receptacles. Melt quenching technique is employed to prepare oxide glasses with a general formula xNa2O–
(30 - x) Li2O–10K2O–10ZnO–49.7B2O3–0.3Cr2O3 (where x = 0, 5, 10 and 15 mol%). The amorphous nature of the
samples is confirmed using X-ray diffraction. The optical absorption spectra have been recorded, and the resulting data
were used for the ligand field theory analysis, from which the crystal-field energy (10Dq), Racah parameters (B and C) and
nephelauxetic function (h) were evaluated. The variations in the optical bandgap and the band tail with Na2O content have
been discussed in relation to the glass structure. FTIR of these glasses revealed that the borate network is affected by the
replacement of Na2O by Li2O content. It showed also reduction in N4 with increasing Na2O. This result indicates the
increase of non-bridging oxygens (NBOs) which in turn reduces the rigidity of the glass matrix and opens up the structure.
The density and the molar volume (VM) measurements were also employed to investigate the structure. These measurements show a linear increase as Na2O content increases. This can be attributed to the differences in the ionic radii of
sodium and lithium ions, being larger for sodium. UV–Visible optical absorption spectra show three characteristics bands
around * 588, 435 and 360 nm which are assigned to the transitions 4A2g ? 4T2g, 4A2g ? 4T1g and 4A2g ? 2A1g,
respectively. Racah parameters (B and C) show an opposite behavior. The obtained ratio (10Dq/B) reveals a strong ligand
field around chromium ions. Also, the nephelauxetic parameter of the ligand shows the same 10Dq behavior. The increase
of chromium ions in the trivalent state shows that it acts as modifiers, yielding the higher concentration of NBOs in the
glass matrix and confirming the FTIR and optical results. Electron spin resonance spectroscopy was measured and studied
to confirm the existence of Cr3+ and Cr6+. |
