It is not about elements you use at all, you simply need a per se transparent material (almost any dielectric with a bandgap larger than 3 eV would do) and than you have to avoid impurities and suppress scattering. if you start with reagent of sufficient purity and produce a smooth poreless solid film with stoichiometric composition, it must be transparent. Superhydrophobic films sometimes require textured surfaces, this can be an obstacle for achieving good transparency, but still it is absolutely unimportant whether you have lanthanides in your film or it is just whatever titania/alumina/silica etc. If your samples are thick, maybe you better choose a material with isortopic optical properties (for example, with a cubic crystalline lattice) and make sure you obtain only one crystallographic phase, but the main critical parameter would still be the residual porosity and, if porosity cannot be eliminated, the particle size of your material...

It is not about elements you use at all, you simply need a per se transparent material (almost any dielectric with a bandgap larger than 3 eV would do) and than you have to avoid impurities and suppress scattering. if you start with reagent of sufficient purity and produce a smooth poreless solid film with stoichiometric composition, it must be transparent. Superhydrophobic films sometimes require textured surfaces, this can be an obstacle for achieving good transparency, but still it is absolutely unimportant whether you have lanthanides in your film or it is just whatever titania/alumina/silica etc. If your samples are thick, maybe you better choose a material with isortopic optical properties (for example, with a cubic crystalline lattice) and make sure you obtain only one crystallographic phase, but the main critical parameter would still be the residual porosity and, if porosity cannot be eliminated, the particle size of your material...

If you decrease the crystallinity of the polymer membrane then you can have a transparent material.

As mentioned above, you need a fully amorphous material with no change of refractive index in the matrix. 

Ram,

As mentioned by Mr. Arun and Helmut, try to improve the amorphous state with in your material. By improving the amorphous nature of the material will technically increase your transparency.

Thank you all