Hafnia is the high index material of choice for UV optical coatings. However not all UV devices need the full window of transparency of hafnia and some filter and mirror designs could benefit from the availability of higher index materials. One example is the demanding requirement for bandpass filters operating in the 260-300nm region of the UV where devices are required with out of band rejection levels to greater than OD 10. Alloying hafnia with a higher index material would be expected to produce a thin film of some intermediate index, where the refractive index would primarily be determined by the relative content of the two oxides whether or not the two components of the alloy are miscible. In the former case a homogeneous material would result while in the latter case the product would be a nanocomposite material with a precipitate of one phase in a host largely comprising the other component. In the case of immiscible mixtures, the absorption edge of the composite will be governed by the high index component in substantially pure form. On the other hand, when solid solutions are formed, the bandgap will depend on composition.
Andrew M. Pitt,
"High-index oxides for UV applications (Abstract Only)", Proc. SPIE 3578, Laser-Induced Damage in Optical Materials: 1998, (7 April 1999); doi: 10.1117/12.344453; https://doi.org/10.1117/12.344453