Actinide phosphates AIMIV2(PO4)3(AI = Li, Na, K, or Rb; MIV = U, Np, or Pu)

The synthesis and X-ray diffraction characterization of A I M I V 2 (PO 4 ) 3 (A I = Li, Na, K, Rb; M I V = U, Np, Pu) orthophosphates are reported. Monoclinic (a) and rhombohedral (β) polymorphs are studied. The a orthophosphate phases have an NaTh 2 (PO 4 ) 3 -type (NTP) structure, space group C2/c, Z = 4. The β ortho-phosphate phases have an NaZr 2 (PO 4 ) 3 -type (NZP) structure, space group R 3 c, Z = 6. The α → β phase transition temperature changes with the alkali metal and the actinide atom. Indexing results are used to calculate the unit cell parameters and unit cell volumes for both the monoclinic and rhombohedral phases and to elucidate tendencies in the variation of these quantities as functions of alkali and actinide cation radii. A similar and systematic variation of the unit cell dimensions is noticed for the rhombohedral orthophosphates of d (Ti, Zr, Hf) and f elements. A comparative crystal-chemical analysis of the a and β phases is carried out.