Ion exchange and structural aging in the layered perovskite phases H _{1- x}Li_xLaTiO₄

Grinding together the solid acid HLaTiO₄ with stoichiometric quantities of lithium hydroxide monohydrate gives the solid solution H _1-xLi_xLaTiO₄. The structures of these crystalline phases have been refined against neutron powder diffraction data to show that all of these compounds crystallize in the centrosymmetric space group P4/nmm. The protons and lithium cations occupy sites between the perovskite layers; the former in hydroxide groups that hydrogen-bond to adjacent layers while Li⁺ is in four-coordinate sites that bridge the perovskite slabs with a geometry intermediate between square-planar and tetrahedral. The reaction proceeds rapidly, but the unit cell size continues to evolve over the course of days with a gradual compression along the interlayer direction that can be modeled using a power law dependence reminiscent of an Ostwald ripening process. On heating, these materials undergo a mass loss because of dehydration but retain the layered Ruddlesden-Popper structure up to 480 C before a substantial loss of crystallinity on further heating to 600 C. Impedance spectroscopy studies of the dehydrated materials shows that Li⁺ mobility in these materials is lower than the LiLaTiO₄ end member, possibly because of microstructural effects causing large intergrain resistance through the defective phases.