Formulation for the preparation of optical metal oxide layers

A formulation for preparing an optical metal oxide layer, wherein the formulation comprises at least:

• (i) an oligomer of one or more metavanadate salts, said oligomer is represented by formula I;
• wherein X represents a cation,
• m is 1, 2, or 3,
• n is an integer >2, optionally between 2 and 10 (in water);
• and
• (ii) a formulation medium.