The present invention relates to a process for the preparation of a plane-parallel structure (a platelet-shaped body, or flake), comprising at least one dielectric layer consisting of oxides of one or more metal selected from groups 3 to 15 of the periodic table, which method comprises: (a) optionally applying a layer of release material on a substrate, (b) applying a composition comprising one or more precursors of one or more desired metal oxides to said release layer, or directly to a substrate with no layer of release material, (c) subjecting the one or more precursors of one or more desired metal oxides to microwave radiation to form a metal oxide layer on the substrate or on the layer of release material; and (d) separating the resulting metal oxide layer from the substrate as plane-parallel structures.