Specific-heat measurements on virgin and γ-ray–irradiated single crystals of ${\mathrm{NaNO}}_2$ have been carried out to study the effects of defects on the incommensurate phase. Observation of a small specific-heat anomaly at a temperature just above the commensurate-incommensurate (CI) phase-transition point is reported for the first time. It is suggested that this new anomaly may be evidence for a defect-induced intermediate state between the commensurate and incommensurate phases. We discuss qualitatively the observed new anomaly and thermal hysteresis in the CI phase transition.

• Received 10 October 1986

DOI:https://doi.org/10.1103/PhysRevB.35.8591