The temperature dependence of the intensity, frequency, and line shapes of the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ nuclear-quadrupole-resonance spectrum in potassium tetracyanoquinodimethane (K-TCNQ) showed that the Peierls transition consists of a complex set of transformations that on heating leads to a first-order transition at 130 °C with hysteresis. Above 130 °C, the high-temperature (HT) phase containing fluctuating charge-density waves (CDW) becomes incommensurate, transforms into a multisolitonic phase with narrow solitons above 141.5 °C, and near the decomposition temperature changes towards the single-soliton limit. On cooling from above 150 °C, K-TCNQ transforms directly from the HT multisolitonic to a HT commensurate phase at 141.4 °C that coexists with the low-temperature phase from 128 °C but only disappears below 90 °C. The previously undetected transition observed at 141.5 °C on heating and cooling is attributed to a change in the effective dimensionality in the ordering of the CDW’s along the TCNQ stacks. The complex set of transitions found in the fully ionic K-TCNQ showed that the Peierls transition is likely to be a two-step process even in simple salts as predicted theoretically by Zhou and Gong [J. Phys. C 21, L561 (1988)].

• Received 11 October 1988

DOI:https://doi.org/10.1103/PhysRevB.39.6765