Li+ ion exchange in H2SrTa2O7 via low temperature acid/base reactions

Layered materials form a wide range of technologically important ceramics. Within the class of layered materials, ion-exchangeable tantalates have shown great promise. However, the majority of ion exchange methods require large molar excesses of reagents, high temperatures, and by-products or undesired second phases that have to be removed in subsequent reaction steps. Here we show how direct reaction of H2SrTa2O7 with varying quantities of LiOH.H2O at room temperature can exchange protons for Li+. This process does not form a continuous series such as that observed in H2-xLixLa2Ti3O10, instead forming LiHSrTa2O7 balanced with either unreacted H2SrTa2O7 or LiOH.H2O depending on the initial ratio of H2SrTa2O7 to LiOH.H2O. When H2SrTa2O7 is mixed at room temperature with less than one equivalent of LiOH.H2O it reacts to produce a mixture of H2SrTa2O7 and LiHSrTa2O7. Mixing equimolar amounts of H2SrTa2O7 and LiOH.H2O produces a single phase of LiHSrTa2O7. Room temperature mixing with x LiOH.H2O (1 < x < 2) followed by heating to 120 °C gives a two-phase product mixture of LiHSrTa2O7 and Li2SrTa2O7 and a single phase Li2SrTa2O7 from a reaction with x = 2.

LiHSrTa2O7 can be further dehydrated at 360 °C to form the defect layered perovskite, ☐LiSrTa2O6.5. Materials with Li+ and vacancies on the same crystallographic site often show fast ion conduction but controllable synthesis has previously proved challenging. The synthesis and degree of exchange is investigated using X-ray powder diffraction data, thermogravimetric analysis and the ionic mobility assessed via a.c. impedance spectroscopy. As use of LiOH.H2O has now been shown to directly control ion exchange in both n = 1 and n = 3 (ideal) and n = 2 (distorted) Ruddlesden Popper phases the application of this methodology generally to layered materials is discussed.