A new X-ray crystallographic beamline is operational at the MAX II synchrotron in Lund. The beamline has been in regular use since August 1998 and is used both for macro-and small molecule diffraction as well as powder diffraction experiments. The radiation source is a 1.8 T multipole wiggler. The beam is focused vertically by a bendable mirror and horizontally by an asymmetrically cut Si(111) monochromator. The wavelength range is 0.8±1.55 A Ê with a measured¯ux at 1 A Ê of more than 10 11 photons s À1 in 0.3 mm  0.3 mm at the sample position. The station is currently equipped with a Mar345 imaging plate, a Bruker Smart 1000 area CCD detector and a Huber imaging-plate Guinier camera. An ADSC 210 area CCD detector is planned to be installed during 2000.

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The hydrogen atom positions in the crystalline a phase of Zr(HP04)2-H20 (a-ZRP) were determined by profile refinement of powder neutron diffraction data. The structure is built up of zirconium phosphate layers which form cavities in which the water molecules are located. The monohydrogen phosphate groups donate hydrogen bonds to the water molecule which, in turn, donates one hydrogen bond to a phosphate oxygen atom. The other water hydrogen atom does not participate in hydrogen bonding. No hydrogen bonds occur between the zirconium phosphate layers; they are held together by van der Waals forces alone. Hydroxide ions must be added to an alkali metal halide solution before the metal ions enter a-ZRP. From this fact, an assumed small Donnan potential, and the location of the hydrogen atoms, a model for the ion exchange is suggested whereby hydroxide ions create sorption sites in a-ZRP by the reaction >03POH + OH--* >03PO" + H20. This reaction should provide enough energy to move the zirconium phosphate layers apart, thus allowing hydrated metal ions to be sorbed at the phosphate groups.ion-exchange mechanism. This can be illustrated by the titration curves given in Figure 1, where the pH is shown as a function of the amount of base added per gram of a-ZRP. The uptake of Na+ and K+ occurs in two distinct steps, as if the two hydrogen phosphate groups in a-ZRP

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