Synthesis and characterization of the rare-earth Dion-Jacobson layered perovskites, APrNb2O7 (A = Rb, Cs and CuCl).

The new double-layered perovskites, APrNb(2)O(7) (A = Rb, Cs) have been prepared by a high temperature ceramic method. Rietveld refinement of X-ray powder diffraction data confirmed the orthorhombic and tetragonal structures, respectively; RbPrNb(2)O(7) was refined in space group Imma (a = 5.4534(7) Å, b = 22.012(1) Å, c = 5.4549(7) Å) and CsPrNb(2)O(7) in P4/mmm (a = 3.8668(2) Å, c = 11.163(1) Å). (CuCl)PrNb(2)O(7), topochemically prepared by replacement of Rb(+)/Cs(+) with CuCl(+), contains a 2D Cu-Cl network between the PrNb(2)O(7) slabs (orthorhombic space group Pbam, a = 7.7328(6) Å, b = 7.7113(4) Å and c = 11.6706(3) Å). The parent compounds both show paramagnetic behavior µ(eff) (Rb) = 3.34(1)µ(B) and µ(eff) (Cs) = 3.60(2)µ(B) while the (CuCl)PrNb(2)O(7), paramagnetic (µ(eff) = 4.020(8)µ(B)) down to 20 K, exhibits antiferromagnet-like behavior below 20 K.

Topochemical synthesis of alkali-metal hydroxide layers within double- and triple-layered perovskites.

The formation of alkali-metal hydroxide layers within lamellar perovskites has been accomplished by a two-step topochemical reaction strategy. Reductive intercalation of ALaNb2O7 with alkali metal (A = K, Rb) and RbCa2Nb3O10 with Rb leads to A2LaNb2O7 and Rb2Ca2Nb3O10, respectively. Oxidative intercalation with stoichiometric amounts of water vapor, produced by the decomposition of calcium oxalate monohydrate in a sealed ampule, allows the insertion hydroxide species. Compounds of the form (A2OH)LaNb2O7 (A = K, Rb) and (Rb2OH)Ca2Nb3O10 are accessible. X-ray diffraction data indicates a clear layer expansion of almost 3 Å on the insertion of hydroxide relative to that of the parent. Rietveld refinement of neutron diffraction data collected on deuterated samples of (Rb2OD)LaNb2O7 (P4/mmm space group, a = 3.9348(1) Å, c = 14.7950(7) Å) finds that both rubidium and oxygen species reside in cubic sites forming a CsCl-like interlayer structure between niobate perovskite blocks. Hydrogens, attached to the interlayer oxygens, are disordered over a 4-fold site in the x-y plane and have O-H bond distances (0.98 Å) consistent with known hydroxide species. This synthetic approach expands the library of available topochemical reactions, providing a facile method for the construction of alkali-metal hydroxide layers within receptive perovskite hosts.