Crystal Structure of BiZn2PO6. Filiation between Related Compounds

The BiZn2PO6 room-temperature crystal structure was determined from powder X-ray data using the Rietveld method. This compound crystallizes in space group Pnma, with Z=4, a=11.897(2) ?, b=5.277(1) ?, c=7.819(2) ?. The structure contains ZnO5 square pyramids associated by edge in dimers which are connected by corners to form infinite independent [ZnO4-3] zigzag double chains. The interconnection is achieved by PO3-4 tetrahedra creating six-sided tunnels parallel to the b axis containing Bi3+ cations. An intensive discussion about the structural filiation between the different members of the BiM2XO6 series (M=Mg, Ca, Cu, Cd, Pb; X=P, V, As) is given. A thermal investigation showed that BiZn2PO6 undergoes a P-to-B phase transition at about 325°C while this isostructural BiCu2PO6 remains structurally unchanged until 750°C.