An Investigation of Distortions of the Dion−Jacobson Phase RbSr2Nb3O10 and Its Acid-Exchanged Form with 93Nb Solid State NMR and DFT Calculations

Abstract

The different niobium sites in a Dion−Jacobson triple-layered perovskite, RbSr2Nb3O10, and its acid-exchanged version, HSr2Nb3O10, were investigated by using solid state NMR and DFT methods. 93Nb electric field gradient (EFG) and chemical shift anisotropy (CSA) tensor values were extracted and site assignments made by using DFS, VOCS, MQMAS, and {1H}-93Nb CP NMR techniques. The exterior niobium site exhibited a quadrupolar coupling (CQ) of 45 MHz and a CSA span of 820 ppm while the interior site exhibited a reverse trend with a CQ of 93 MHz and a CSA span of 530 ppm. Both EFG and CSA tensors for the sites exhibited near axial symmetry although the CSA tensors had a greater deviation. The symmetry of the tensors is in conflict with the previously proposed structure for RbSr2Nb3O10 and implies a lower symmetry space group and a possible tilting of the octahedra. Acid exchange altered the EFG tensors for both sites reducing their CQ values to 39 and 86 MHz, respectively, while not substantially changing the tensor symmetry. The effect on the CSA tensor occurred only for the surface site with the span reduced to 560 ppm. The effect of the tilting of the octahedra and alteration of bond angles and bond lengths on the EFG tensor was investigated through periodic DFT calculations. Interior and exterior sites of the rubidium form exhibited different tilt along the c-axis of 14° and −3°, respectively, based on the 93Nb EFG values. Calculations on the acid-exchanged composition implied that structural alterations must occur with both sites. The exterior octahedra experience changes in both bond length and bond angles, while only a subtle elongation of the interior site is found.