Reversible cation/anion extraction from K₂La₂Ti₃O₁₀: formation of new layered titanates, KLa₂Ti₃O_9.5 and La₂Ti₃O₉

Abstract

A new soft-chemical transformation of layered perovskite oxides is described wherein K₂O is sequentially extracted from the Ruddlesden-Popper (R-P) phase, K₂La₂Ti₃O₁₀ (I), yielding novel anion-deficient KLa₂Ti₃O_9.5 (II) and La₂Ti₃O₉ (III). The transformation occurs in topochemical reactions of the R-P phase I with PPh₄Br and PBu₄Br (Ph = phenyl; Bu = n-butyl). The mechanism involves the elimination of KBr accompanied by decomposition of PR₄⁺ (R = phenyl or n-butyl) that extracts oxygen from the titanate. Analysis of the organic products of decomposition reveals formation of Ph₃PO, Ph₃P, and Ph-Ph for R = phenyl, and Bu₃PO, Bu₃P along with butane, butene, and octane for R = butyl. The inorganic oxides II and III crystallize in tetragonal structures (II: P4/mmm, a = 3.8335(1) Å, c = 14.334(1) Å; III: I4/mmm, a = 3.8565(2) Å, c = 24.645(2) Å) that are related to the parent R-P phase. II is isotypic with the Dion-Jacobson phase, RbSr₂Nb₃O₁₀, while III is a unique layered oxide consisting of charge-neutral La₂Ti₃O₉ anion-deficient perovskite sheets stacked one over the other without interlayer cations. Interestingly, both II and III convert back to the parent R-P phase in a reaction with KNO₃. While transformations of the R-P phases to other related layered/three-dimensional perovskite oxides in ion-exchange/metathesis/dehydration/reduction reactions are known, the simultaneous and reversible extraction of both cations and anions in the conversions K₂La₂Ti₃O₁₀ ⇔KLa₂Ti₃O_9.5⇔ La₂Ti₃O₉ is reported here for the first time.