Determinación de los átomos de hidrógeno en intercambiadores iónicos inorgánicos policristalinos, mediante difracción de neutrones

Zusammenfassung

Metal phosphates and silicates with layered or tridimensional octahedral-tetrahedral mixed frameworks are of considerable interest due to their potential use as ionic exchangers or catalysts. Many of these phases are obtained only as microcrystalline agregates making difficult their crystal structure determination by conventional methods. Their structures may in certain cases be solved by x-ray powder diffraction methods but the determination of the hydrogen atom positions is generally out of the scope of this technique. We have used the neutron powder diffraction technique over several compounds containing hydrogen atoms without previous deuteration. Neutron diffraction patterns were collected using the powder diffractometer D2B at the Institut Laue-Langevin, Grenoble (France). The neutron diffraction study on the phases of a-Ti(HPO4)2.H2O, a-HfP and a-TiAs reveals the presence of three strong H-bond interactions, two acceptor and one donor, between the water molecule and one of the layers. A fourth weaker H-bond interaction exists with the other layer. In the phases g-Zr(H2PO4)(PO4).2H2O and g-HfP each of the two water molecules participates in four H-bonds, two with different water molecules and two with one of the layers. There is an indirect interlaminar interaction through the water molecules. The neutron diffraction study of the Ti2(OH)2OSiO4á1.5H2O phase has revealed that the exchangeable protons are located on the atom of oxygen of the cubane (TiO)4 cluster. There are six water molecules within each tunnel (two complete tunnels per unit cell) forming a hydrogen-bonded network together with the Ti-OH groups. There are no hydronium ions present. Apparently the extensive hydrogen bond network is able to stabilise the framework.