Structure examination of NaLaNb 2O 7 synthesized by soft chemistry

Abstract

The synthesis of NaLaNb 2O 7· xH 2O was performed by the soft chemistry using an ion-exchange reaction of KLaNb 2O 7 powder with NaNO 3 molten salt. The thermal stability of NaLaNb 2O 7· xH 2O was examined by DTA and TG analyses. The crystal structures of hydrate and anhydrate forms of this compound were solved using X-ray powder diffraction data. The cell is tetragonal (space group I4 mm, Z=2) with a=3.8981 (1) Å and c=25.7131 (9) Å for the hydrate form, and is tetragonal (space group I4/m m m, Z=2) with a=3.9022 (1) Å and c=21.1826 (8) Å for the anhydrate form. Their structures were refined using a Rietveld refinement method. Both crystal structures are built up from a double layer of perovskite units, LaNb 2O 7, and the sodium atoms located between the layers. The sodium atoms of the hydrate form are in two crystallographic independent sites, while those of the anhydrous form are in an almost perfect tetrahedral site. These structural differences are discussed based on the hydrogen bond among the water molecules inserted in the layers.