The crystal and molecular structures of some sodium and potassium exchanged zirconium bis(monohydrogen orthophosphates) : the use of powder X-ray diffraction for structure solution

Abstract

Single crystal X-ray diffraction has for many become the definitive analysis technique. The procedure to collect data and to solve structures is not only readily available but is also highly automated. This is not the case for those materials which are microcrystalline or amorphous in nature. Since 1969 the Rietveld technique has been used in a limited fashion to obtain structural information from such materials. The technique was initially developed for neutron powder data and has more recently been adapted for the X-ray case. The one limitation is that a very close starting model of the structure is required before refinement can proceed. It was the purpose of this work to set up a technique which used more readily available X-ray powder intensity data as the starting point for unknown structure solution. Data were collected on a Seifert-Scintag automated powder diffractometer (PADII) with computations conducted on PDP11 family minicomputers. In cases of high crystallinity cell information was obtained by the Guinier technique. Systems which were investigated were of the inorganic layered ion exchange type. The parent compound (alpha)-Zr(HPO(,4))(,2)(.)H(,2)O, (alpha)-ZrP, can be crystallized and its crystal structure is well documented. Its ion exchanged phases, Zr(MPO(,4))(HPO(,4))(.)XH(,2)O and Zr(MPO(,4))(,2)(.)XH(,2)O, where M = Na('+), K('+) and X is an integer and varies with the phase, form only as microcrystalline powders. Due to interest in these compounds, it was decided to use them as a test of the technique developed. The structures that have been solved retain the basic layered nature of the parent with exchange ions forming hydrogen bonded networks in the monoexchanged species. Although gross structures may be solved by the developed technique, some modifications to allow better refinement from an increased number of data will be discussed.