Crystallographic studies of the farringtonite-type phases γ-Zn2Co(PO4)2 and γ-(Znsub(0.50)Cosub(0.50))3(PO4)2

The two solid solutions γ-Zn2Co(PO4)2 and γ-(Znsub(0.50)Cosub(0.50))3(PO4)2 have been prepared and equilibrated at 1070 K. The crystal structures have been refined on the basis of neutron powder diffraction data (lambda approx.= 1.55 A, T = 295 K) with the Rietveld full-profile refinement technique, to Rsub(p) = 0.078 and 0.066 (Rsub(I) = 0.052 and 0.036). The metal cations are strongly ordered, with Zn2+ dominating at the five- and Co2+ at the six-coordinated sites. The MO5 polyhedra are distorted trigonal bipyramids, while the MO6 octahedra are almost regular. Some thermodynamical calculations are included. (Auth.)