A ⁹³Nb Solid‐State NMR and Density Functional Theory Study of Four‐ and Six‐Coordinate Niobate Systems

Abstract

A variable B(0) field static (broadline) NMR study of a large suite of niobate materials has enabled the elucidation of high-precision measurement of (93)Nb NMR interaction parameters such as the isotropic chemical shift (delta(iso)), quadrupole coupling constant and asymmetry parameter (C(Q) and eta(Q)), chemical shift span/anisotropy and skew/asymmetry (Omega/Deltadelta and kappa/eta(delta)) and Euler angles (alpha, beta, gamma) describing the relative orientation of the quadrupolar and chemical shift tensorial frames. These measurements have been augmented with ab initio DFT calculations by using WIEN2k and NMR-CASTEP codes, which corroborate these reported values. Unlike previous assertions made about the inability to detect CSA (chemical shift anisotropy) contributions from Nb(V) in most oxo environments, this study emphasises that a thorough variable B(0) approach coupled with the VOCS (variable offset cumulative spectroscopy) technique for the acquisition of undistorted broad (-1/2<-->+1/2) central transition resonances facilitates the unambiguous observation of both quadrupolar and CSA contributions within these (93)Nb broadline data. These measurements reveal that the (93)Nb electric field gradient tensor is a particularly sensitive measure of the immediate and extended environments of the Nb(V) positions, with C(Q) values in the 0 to >80 MHz range being measured; similarly, the delta(iso) (covering an approximately 250 ppm range) and Omega values (covering a 0 to approximately 800 ppm range) characteristic of these niobate systems are also sensitive to structural disposition. However, their systematic rationalisation in terms of the Nb-O bond angles and distances defining the immediate Nb(V) oxo environment is complicated by longer-range influences that usually involve other heavy elements comprising the structure. It has also been established in this study that the best computational method(s) of analysis for the (93)Nb NMR interaction parameters generated here are the all-electron WIEN2k and the gauge included projector augmented wave (GIPAW) NMR-CASTEP DFT approaches, which account for the short- and long-range symmetries, periodicities and interaction-potential characteristics for all elements (and particularly the heavy elements) in comparison with Gaussian 03 methods, which focus on terminated portions of the total structure.

Recent technique developments and applications of solid state NMR in characterising inorganic materials

“…The use of variable magnetic fields to constrain simulations of NMR spectra by comparing with experimental data collected at several magnetic fields illustrated by (a) 25 Mg in MgSO 6 .6H 2 O at three magnetic fields showing the two inequivalent magnesium sites, and (b) 93 Nb from YNbO 4 at four magnetic fields. Reproduced from [9] and [10] respectively with permission of the copyright owner. Figure 2.…”

“…An example where CSA is also present is the 93 Nb NMR spectra of YNbO 4 where although there is clearly a large second-order quadrupole interaction, there is also significant CSA. Again data collected at multiple fields (here four) strongly constrains the interactions [10]. Work on constraining the NMR interaction parameters has also been pushed forward even for much studied nuclei such as 23 Na and 27 Al.…”

“…Although 11 B is a very sensitive nucleus with a relatively high frequency and can readily distinguish BO 3 and BO 4 sites and even inequivalent sites within these groups, 10 B offers an alternative view. It is an I = 3 nucleus and spectra have been observed using a field stepping approach and spectra built up from 'slices' summed together ( Fig.…”

Structural and optical properties of ZnS/MgNb2O6 heterostructures

“…There is a growing interest in this material because this ceramic oxide presents an interesting electronic structure [3][4]. In particular, magnesium niobate MgNb 2 O 6 (MN) nanoparticles show excellent dielectric and optical properties [5][6][7][8].…”

Structure and properties of NaPO3–ZnO–Nb2O5–Al2O3 glasses

“…In fact, an isotropic chemical shift of about −700 ppm would be necessary to move the quadrupole pattern under the maxima observed in both fields. This shift could point to lower coordinated niobium sites (according to Lapina's compilations), but these structures are very unusual and only found for rare earth metals, where they might also be viewed as pseudo-octahedral [37]. Looking at the spread of isotropic chemical shifts δ, of six-fold coordinating niobates with other charge compensating metals, we find that they cannot cause such large shifts.…”