Unveiling the cation ratio mediated structural modifications in TiO₂:GeO₂ mixtures for gravitational-wave detectors

Abstract Amorphous thin films of Ti doped GeO2 are of interest for coatings of the mirrors in gravitational wave detectors due to their low internal friction (Phys. Rev. Lett 127 071101). The addition of Ti to a-GeO2 enables tailoring of the optical and structural properties of the mixtures. However, the specific modifications that occur in the amorphous network with the addition of Ti are not known. In this work, x-ray photoelectron spectroscopy is used to identify modifications to the bonding of Ge and Ti atoms in mixtures of Ti doped a-GeO2 with different Ti cation content. The formation of (Ti-O-Ge) bonds is evidenced from: 1) the presence of a peak which intensity increases with Ti content and causes a shift to lower binding energy of the core level O 1s peak; 2) the shift to higher binding energy of the Ti 2p3/2 peak and a decrease in the energy split; and 3) the shift to lower binding energy of the Ge 3d5/2 peak and increase in the energy split. These changes reflect modifications to the bonding when Ge replaces Ti in Ti-O-Ti bonds and viceversa in Ge-O-Ge bonds due to their difference in electronegativity. A decrease in the O-O nearest-neighbour distance due to the incorporation of Ti atom is also observed from the broadening of the valence band spectra. The results also show the Ti doped a-GeO2 mixture with 0.44 Ti cation concentration has an optimum balance between mixed network (Ti-O-Ge) and pure (Ge-O-Ge)/(Ti-O-Ti) networks than Ti poor and Ti rich mixtures. This finding could have important consequences in the optimization of amorphous Ti doped a-GeO2 mixtures for low internal friction coatings of gravitational wave detectors.