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

Abstract

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 40Cs₂O–10BaO–50SiO₂ (mol%) with an optical basicity of $\Lambda$ = 0.81. The presence of Mn⁵⁺ (d²) as well as Mn⁶⁺ (d¹) is confirmed by UV–Vis, photoluminescence, and Raman spectroscopy. The UV–Vis spectrum is dominated by the Mn³⁺ (d⁴) absorption at 526 nm for low-basicity glasses, which is replaced by a peak at 679 nm (Mn⁵⁺) and, finally, a band at 603 nm (Mn⁶⁺) in the glass with the highest basicity ($\Lambda$ = 0.81). In this glass, the Mn⁵⁺/Mn⁶⁺ ratio varies with the melting conditions. Photoluminescence (PL) spectroscopy under 633 nm excitation confirms the presence of Mn⁵⁺, showing the narrow, forbidden ¹E $\to$³A₂ transition located at 1191 nm with vibrational sidebands at 1245 nm and 1290 nm. The measured static fluorescence intensity due to Mn⁵⁺ 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 Mn⁴⁺ in CBS glasses was confirmed by PL and electron paramagnetic resonance (EPR) spectroscopy. Despite initial doping as MnO₂, metastable Mn⁴⁺ 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 MnO₄-tetrahedra at ∼800 cm⁻¹ with overtones observed at higher frequencies.