The invention relates to new films for applying to a substrate and to methods for producing them.A composition, for use in preparing a film, (preferably an epitactic film) comprising a solid solution of KNbO.sub.3 and KTaO.sub.3 forming a product of formula IK.sub.1-y M.sub.y Ta.sub.1-x Nb.sub.x O.sub.3 (I)in which M is a monovalent metal ion other than K.sup.+ (hereinafter defined as the dopant);y=0.02 to 0.20 andx=0.05 to 0.95.