Cation Induced Structural Variation in Topochemically Modified Dion-Jacobson Perovskite Solid Solutions

The continuous solid solution series based on the ion exchangeable Dion-Jacobson layered perovskites, A_1-xA'_xLaNb₂O₇ (A/A' = Li, Na, K, Rb, Cs; 0 ≤ x ≤ 1), has been investigated to illuminate the relationship between composition and structure. Topochemical synthesis of the solid solutions from combinations of various alkali metal cations has been achieved by reacting pure end members (ALaNb₂O₇) at appropriate ratios and temperatures. All adjacent sets of alkali metals (Li/Na, Na/K, K/Rb, and Rb/Cs) readily formed solid solutions, while only the one non-adjacent solid solution, K_1-xCs_xLaNb₂O₇ (K/Cs), could be obtained. Local cation coordination and the corresponding layer alignments vary as a function of composition where the relative concentration of the larger cation dictates structure. Thermal analysis of the solid solutions, A_1-xA'_xLaNb₂O₇ (A/A' = Li, Na, K) showed that the lithium- and sodium-containing compositions were thermally unstable. This study demonstrates that the systematic variation in average cation sizes in the solid solution series allows for structural control in these important perovskite hosts.