The authors have used spin echoes to study the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ nuclear-quadrupole-resonance (NQR) line shapes and relaxation times in Nb${\mathrm{Se}}_3$ through the charge-density-wave transition at 145 K. Of the 12 NQR lines present above 145 K, arising from the three different Nb sites each of which contributes four NQR lines, only six were observed in the frequency range studied. The authors have determined that these six lines arise from two distinct quadrupole fields with one axial or nearly axial ($\eta \simeq 0$) and the other nearly "antiaxial" ($\eta \simeq 1$). A simple argument suggests that these two sites are the ones which appear most similar in the crystal structure. Below 145 K the nearly axial line becomes nonaxial ($\eta =0.55\mathrm{}$) while the nearly antiaxial line smears slightly. Relaxation data combined with the line-shape data show that dynamical effects are present at the nearly axial site and the changes in the spectra may be due principally to a change in the dynamics of the quadrupole field at that site. Dynamics are relatively unimportant at the antiaxial site. The third site has yet to be observed.

• Received 13 June 1983

DOI:https://doi.org/10.1103/PhysRevB.29.41