Actinide valences in xenotime and monazite

Tetravalent U, Np and Pu can be substituted by ceramic methods into the rare earth site of xenotime and monazite in air atmospheres using Ca ions as charge compensators, while no evidence of penta- or hexavalent actinide ions was found. Some Pu3+ and Np3+ can be incorporated in xenotime samples fired in a reducing atmosphere.