93Nb and 23Na NMR in Polycrystalline Sodium Niobate

Abstract

Nuclear magnetic resonance (NMR) spectra of 23Na (I = 32) and 93Nb (I = 92) from a powdered sample of NaNbO3 have been studied. At room temperature the 23Na spectrum shows the presence of two distinct sites, one having axial symmetry with a coupling constant of 21.5 ± 0.2 MHz and the other having an asymmetry parameter lying between 0.80 and 1.0 with a coupling constant of 1.0 ± 0.1 MHz. The niobium spectrum arises from a single site with a coupling constant of 19.7 ± 0.5 MHz and an asymmetry parameter of 0.82 ± 0.02. The (12 ↔ − 12) transition in the 93Nb spectrum exhibits some unusual features which were confirmed by computer-simulated powder patterns. The temperature dependence of one 23Na site indicated a rather linear decrease in the coupling constant, from 2.6–1.6 MHz, and the asymmetry parameter was approximately zero as the temperature increased from − 170–280°C. The 93Nb quadrupole interaction showed a linear decrease not only in the coupling constant, from 19.7–10 MHz, but also in the asymmetry parameter, from 0.82–0.64, as the temperature rose from room temperature to 270°C. Electric-field-gradient calculations for 93Nb produced coupling constants lower than the measured value by about an order of magnitude while yielding values of the asymmetry parameter comparable to that observed. Similar calculations for 23Na gave relatively high values of the asymmetry parameter, and the coupling constants were of the same order of magnitude as those determined experimentally. Third-order corrections to the NMR frequencies perturbed by the quadrupole interactions for the case of the asymmetry parameter unequal to zero were calculated. These proved to be negligible except for the case of niobium at low frequencies.

Practical aspects of 51V and 93Nb solid-state NMR spectroscopy and applications to oxide materials

“…This approach was used to determine quadrupolar coupling constants in several vanadates [18,[22][23][24][25][26]. Similar 93 Nb experiments were performed on LiNbO 3 [27][28][29][30][31].…”

High-resolution solid-state NMR of quadrupolar nuclei

“…93Nb has a 100% natural abundance, and a quadrupole moment Q = -0.2 barn. We chose to investigate the system NaNbO3 because well-resolved secondorder powder patterns have been observed for the (1/2, -1/2) transition (28). Unfortunately, however, e2qQ/h is 19.5 MHz, because the NbO6 octahedra are quite distorted (29,30).…”

A simple proof that third-order quadrupole perturbations of the NMR central transition of half-integral spin nuclei are zero

“…In this paper, we show that the third-order correction to the position of the central transition is exactly zero, which partly explains this observation. This is a well-known result in the literature [10,11,18,19], but we present a new, clear, and simple derivation.This result is based on a recently published selection rule [25]. The calculation uses the principle that the NMR transitions can be calculated directly, by setting up and diagonalizing the Liouvillian [26].…”

“…In this paper, we show that the third-order correction to the position of the central transition is exactly zero, which partly explains this observation. This is a well-known result in the literature [10,11,18,19], but we present a new, clear, and simple derivation.…”

“…Higher-order corrections to the wavefunctions have been published [14][15][16][17][18][19], as have a number of exact solutions [10,[20][21][22][23][24][25]. In practice, we find that second-order perturbation theory works remarkably well.…”