Toward tetrahedral Ti3+ in oxide glasses: Spectroscopic and computational evidence for low-coordinated Ti3+ species

In oxide glasses, titanium can exist in multiple oxidation states and coordination environments, yet the spectroscopic signatures of lower-coordinated Ti3+ species remain poorly understood compared with the extensively studied Ti3+ in octahedral environments. In this work, aerodynamic levitation was employed to remelt ultra-low expansion (ULE) titanosilicate glass under reducing atmospheres at temperatures exceeding 2000 °C to promote reduction of tetrahedral Ti4+ to Ti3+ while preserving the intrinsically low coordination of titanium in the glass network. To establish coordination-dependent spectroscopic trends, highly polymerized Ti-doped sodium alumino/boro-silicate glasses were prepared under strongly reducing conditions by a conventional melt and quench process as model systems, while a sodium-titano-alumino-phosphate glass containing predominantly octahedral Ti3+ was used as a reference. All glasses were investigated using UV-Vis-NIR absorption, electron paramagnetic resonance (EPR), and Raman spectroscopy, complemented by density functional theory (DFT) and linear response time-dependent DFT (LR-TDDFT) computational predictions. The octahedral Ti3+ reference glass exhibits the characteristic absorption band near 523 nm and a nearly isotropic EPR signal around giso ≈ 1.92-1.94. In contrast, the reduced aluminosilicate and borosilicate glasses develop broad near-infrared absorption bands extending from ∼700 to 1500 nm together with EPR resonances containing high-g components approaching or exceeding g = 2.0, indicating substantial populations of lower-coordinated Ti3+ species. The reduced levitated ULE glass exhibits a dominant absorption band near 1085 nm and a rhombic Ti3+ EPR signal (gx ≈ 1.997, gy ≈ 1.942, gz ≈ 1.912), consistent with a low-symmetry and highly distorted octahedral and lower-coordinated environment. Collectively, these results provide new insight into the UV-Vis-NIR, EPR, and Raman structural characteristics of Ti3+ in different coordination environments and represent one of the first systematic efforts to distinguish lower-coordinated Ti3+ species from octahedral Ti3+ in oxide glasses.