Reversible Cation/Anion Extraction from K₂La₂Ti₃O₁₀:  Formation of New Layered Titanates, KLa₂Ti₃O_9.5and La₂Ti₃O₉

Abstract

A new soft-chemical transformation of layered perovskite oxides is described wherein K 2 O is sequentially extracted from the Ruddlesden−Popper (R−P) phase, K 2 La 2 Ti 3 O 10 ( I ), yielding novel anion-deficient KLa 2 Ti 3 O 9.5 ( II ) and La 2 Ti 3 O 9 ( III ). The transformation occurs in topochemical reactions of the R−P phase I with PPh 4 Br and PBu 4 Br (Ph = phenyl; Bu = n -butyl). The mechanism involves the elimination of KBr accompanied by decomposition of PR 4 + (R = phenyl or n -butyl) that extracts oxygen from the titanate. Analysis of the organic products of decomposition reveals formation of Ph 3 PO, Ph 3 P, and Ph−Ph for R = phenyl, and Bu 3 PO, Bu 3 P along with butane, butene, and octane for R = butyl. The inorganic oxides II and III crystallize in tetragonal structures ( II: P 4/ mmm, a = 3.8335(1) Å, c = 14.334(1) Å; III: I 4/ 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, RbSr 2 Nb 3 O 10, while III is a unique layered oxide consisting of charge-neutral La 2 Ti 3 O 9 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 KNO 3 . 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 K 2 La 2 Ti 3 O 10 ⇌ KLa 2 Ti 3 O 9.5 ⇌ La 2 Ti 3 O 9 is reported here for the first time.