Influence of Na2O addition on the alkali borochromate glasses: structure and ligand field

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 x Na 2 O–(30 − x ) Li 2 O–10K 2 O–10ZnO–49.7B 2 O 3 –0.3Cr 2 O 3 (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 Na 2 O 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 Na 2 O by Li 2 O content. It showed also reduction in N 4 with increasing Na 2 O. 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 ( V M ) measurements were also employed to investigate the structure. These measurements show a linear increase as Na 2 O 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 4 A 2g → 4 T 2g , 4 A 2g → 4 T 1g and 4 A 2g → 2 A 1g , 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 Cr 3+ and Cr 6+ .