93Nb and 23Na NMR in Polycrystalline Sodium Niobate

Nuclear magnetic resonance (NMR) spectra of ²³Na $\mathrm{(I\hspace{0.17em}=\hspace{0.17em}}\frac{3}{2})$ and ⁹³Nb $\mathrm{(I\hspace{0.17em}=\hspace{0.17em}}\frac{9}{2})$ 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 $(\frac{1}{2}\mathrm{\hspace{0.17em}\leftrightarrow \hspace{0.17em}-\hspace{0.17em}}\frac{1}{2})$ 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.