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, RbSr 2 Nb 3 O 10 , and its acid-exchanged version, HSr 2 Nb 3 O 10 , were investigated by using solid state NMR and DFT methods. 93 Nb electric field gradient (EFG) and chemical shift anisotropy (CSA) tensor values were extracted and site assignments made by using DFS, VOCS, MQMAS, and { 1 H}-93 Nb CP NMR techniques. The exterior niobium site exhibited a quadrupolar coupling (C Q ) of 45 MHz and a CSA span of 820 ppm while the interior site exhibited a reverse trend with a C Q 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 RbSr 2 Nb 3 O 10 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 C Q 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 93 Nb 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.

Combined solid-state NMR, FT-IR and computational studies on layered and porous materials

“…219 The crystal structure of some layered ceramics based on mixed oxides, La 2 Sn 2-x Ti x O 7 and RbSr 2 Nb 3 O 10 , was determined by a combination of XRD and 119 Sn and 87 Rb NMR. In both cases, the resonance spectra were interpreted with the aid of DFT calculations of chemical shifts and EFG tensors: in one example 220 the theoretical analysis also allowed to predict the relative stability of two layered phases undistinguishable from the NMR results, while in another case 221 the geometrical deformation of the layers in a Dion-Jacobson phase was described by comparing the theoretical and the recorded resonance spectra.…”

Density-functional computation of 93Nb NMR chemical shifts

“…On the other hand, solid-state 93 Nb NMR spectroscopy has gained some importance in the characterization of metallic and oxidic materials, [7,8] high-temperature superconductors being particularly attractive targets. With the advent of ultrahigh-field spectrometers, it is now also becoming possible to record transition-metal NMR spectra of molecular solids when the corresponding acquisition in solution is not feasible.…”

Tuning the Photocatalytic Activity of Layered Perovskite Niobates by Controlled Ion Exchange and Hydration

“…93 Nb NMR studies have shown that upon proton exchange, the local environments of the niobium sites at the interface of the layers change when compared with the parent compounds, according to Wang et al. [17,18] The proton exchange leads to a change in the Nb-O bonds length and also bond angles. Based on our observations, such changes could affect the water intercalation in between the perovskite layers of the protonexchanged compounds.…”

“…93 Nb NMR studies on the bulk and proton exchanged materials have shown that the distortion of the NbO6 octahedra in KCa2Nb3O10 is relaxed when the potassium ions are exchanged with protons due to the change in the local environments of the niobium sites at the interface of the layers. [17] In case of Sr containing compounds, the NbO6 octahedra is distorted following proton exchange. [18] The distortion of the octahedra can generate internal electric fields due to dipole moments and such electric fields can mitigate the effect of the recombination of charge carriers thus improving the photocatalytic activity.…”