Hydrogen bond network of the layered phosphates gamma-Zr(H2PO4)(PO4)centerdot 2 H2O and gamma-Hf(H2PO4(PO4)center dot 2 H2O determined by neutron powder diffraction

The hydrogen atom positions of the layered phosphates Zr(PO4)(H2PO4).2 H2O (gamma -ZrP) and Hf(PO4)(H(2)PO4).2H(2)O (gamma -HfP) have been determined from neutron powder diffraction data and the complete crystal structures ha ve been refined by the Rietveld method. The compounds crystallize in the mo noclinic space group P2(1). The final agreement factors are: R-wp = 1.47, R -exp-0.91 for gamma -ZrP and R-wp= 1.73, R-exp= 1.43 for gamma -HfP. The tw o compounds are isostructural and posses a dense hydrogen-bonding network. They are formed by layers consisting of two planes of metallic atoms joined by phosphate and dihydrogenphosphate groups. The interlayer space form cha nnels where each water molecule accepts and donates an hydrogen bond to ano ther water molecule forming a zig-zag chain. Additionally each water molecu le acts as acceptor and donor in two hydrogen bonds with oxygen atoms of th e same layer. Consecutive layers are, in this way, indirectly held together by hydrogen bonds through the water molecules.