Crystal structure of CdZn₂(PO₄)₂ and Cd₂Zn(PO₄)₂

Abstract

The crystal structures of two solid solution phases found to occur in the Zn 3 (PO 4 ) 2 –Cd 3 (PO 4 ) 2 system, one with composition near to CdZn 2 (PO 4 ) 2 and the other near to Cd 2 Zn(PO 4 ) 2 , have been determined. Although these compounds are closely related structurally, there is a miscibility gap between the two phases and an apparent phase transition in only one of these. The compounds, named Bss and Css respectively, have lattice parameters a = 9.032(4) Å, b = 11.417(5) Å, c = 5.952(6) Å, β = 98.8(2), and Z = 4 for the former compound, and a = 9.056(8) Å, b = 11.86(1) Å, c = 6.190(9) Å, β = 100.1(2) and Z = 4 for the latter. Both compounds are monoclinic and crystallize with the space group P2 1 /c with closely related, but not identical, structures to that of the mineral graftonite. The anions are slightly irregular PO 4 3− tetrahedra with average P—O bond distances of 1.538 Å and 1.545 Å for those in Bss and 1.55 Å and 1.56 Å for those in Css. The cations are found to be coordinated to 7, 4, and 5 oxygen atoms for the 3 independent sites per unit cell. In Css the second cation site has a fifth oxygen atom at 2.57 Å in comparison with a length of over 2.9 Å for this same interatomic distance in Bss. The cations appear to be ordered with Cd ++ preferentially in site 1 in Bss and Zn ++ preferentially in site 2 in Css.