[en] The problem of isomorphism in phosphates that are structural analogues of mineral kosnarite (structural type of sodium zirconium orthophosphate NaZr₂(PO₄)₃, NZP), monazite, and others, are discussed. Data on the crystal chemistry of orthophosphates of the framework structure are systematised according to the suggested classification of their formula types. The theoretically expected solid solutions and the realised ones are viewed. Especially isomorphism of f-elements in various structures of phosphates is presented. These data form the basis on monophase ceramics that can be easily adapted to many kinds of nuclear waste compositions. (author)