Unusually high oxidation states of manganese in high optical basicity silicate glasses

Unusually high oxidation states of manganese were stabilized within a cesium-barium silicate (CBS) glass system of extremely high optical basicity. The highest basicity was obtained for the metasilicate glass 40Cs 2 O–10BaO–50SiO 2 (mol%) with an optical basicity of Λ = 0.81. The presence of Mn 5+ (d 2 ) as well as Mn 6+ (d 1 ) is confirmed by UV–Vis, photoluminescence, and Raman spectroscopy. The UV–Vis spectrum is dominated by the Mn 3+ (d 4 ) absorption at 526 nm for low-basicity glasses, which is replaced by a peak at 679 nm (Mn 5+ ) and, finally, a band at 603 nm (Mn 6+ ) in the glass with the highest basicity ( Λ = 0.81). In this glass, the Mn 5+ /Mn 6+ ratio varies with the melting conditions. Photoluminescence (PL) spectroscopy under 633 nm excitation confirms the presence of Mn 5+ , showing the narrow, forbidden 1 E → 3 A 2 transition located at 1191 nm with vibrational sidebands at 1245 nm and 1290 nm. The measured static fluorescence intensity due to Mn 5+ grows exponentially with increasing optical basicity. The near infrared fluorescence decay was bi-exponential, with time constants of 14 and 51 μs. The absence of Mn 4+ in CBS glasses was confirmed by PL and electron paramagnetic resonance (EPR) spectroscopy. Despite initial doping as MnO 2 , metastable Mn 4+ disproportionates into lower and higher valent manganese species, followed by reduction or oxidation of manganese to a stable species as ruled by the basicity of the glass and oxygen availability during melting. A structural study of the glasses by Raman spectroscopy revealed a resonance enhancement effect for the symmetric stretching mode of MnO 4 -tetrahedra at ∼800 cm −1 with overtones observed at higher frequencies. • Blue Mn 5+ and green Mn 6+ were stabilized in high-basicity metasilicate glasses. • Resonance Raman enhancement of MnO 4 -tetrahedra leads to a sharp band ∼800 cm −1 . • Resonance Raman spectroscopy detects Mn–O bonds even at 0.05 % Mn doping levels. • EPR results confirm that Mn 4+ is not stable in highly modified silicate glasses. • Mn 5+ gives rise to a narrow photo luminescence with an NIR emission band at 1191 nm.