Structure of Amorphous Tantalum Oxide and Titania-Doped Tantala: ¹⁷O NMR Results for Sol–Gel and Ion-Beam-Sputtered Materials

Amorphous thin films of high-refractive index metal oxides such as Ta 2 O 5 are critical in multilayer optical components such as the main mirrors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), but their atomic-scale structures are not well-known. Amorphous, pure, and titania-doped tantalum oxides were prepared by sol–gel synthesis and by ion-beam sputtering. The samples were successfully enriched with the nuclear magnetic resonance (NMR)-active 17 O isotope by heating them in 17 O 2 gas, and they were studied using 17 O solid-state NMR. 17 O MAS NMR spectra of pure tantala show two resonances that correspond to two- and three-coordinated oxygens, allowing the average oxygen coordination numbers to be estimated from the relative peak areas. The average coordination number for oxide ions seems to increase as tantala changes from amorphous to crystalline. Titania-doped tantalas, (Ta 1– x Ti x ) 2 O 5– x with x = 0.10, 0.25, and 0.50, were also studied. At the lower two dopant levels, Ta and Ti cations are well-mixed in the sense that TiO 2 -like domains are not observed, but Ti is preferentially associated with three-coordinated oxygens. This increases the mean oxygen coordination slightly, which may be related to previously reported improvements in thermal noise characteristics of materials for multilayer interferometer mirror coatings. 17 O MAS NMR spectra of thin-film tantala deposited by ion-beam sputtering and 17 O-enriched after synthesis are very similar in resonance positions and relative peak ratio (and, thus, average oxygen coordination number) to data for the sol–gel materials, which suggests that the amorphous structure may be similar, regardless of the preparation methods when materials are annealed at 450–500 °C.