Abstract: The invention relates to the use of a compound of formula KMgF3 to trap metals present in the form of fluorides and/or of oxyfluorides in a gaseous or liquid phase. It also relates to a compound of formula KMgF3 which has a surface specific area at least equal to 30 m2/g and at most equal to 150 m2/g and also to its methods of preparation. The invention notably finds application in the nuclear industry, in which it can advantageously be used to purify uranium hexafluoride (UF6) present in a gaseous or liquid stream, with regard to metal impurities which are also present in this stream.