Study of Thermal Expansion of NaNO2with Special Emphasis on the Vicinity of the Transition Points

By use of a capacitance method, the thermal expansions of NaNO 2 ( T C =163.9°C, T N =165.2°C) along the a -, b - and c -axes were measured from -180°C to 240°C with the accuracy of 1×10 -6 cm in Δ l . Anomalies associated with the phase transitions were observed over the wide temperature range 100 to 200°C. The thermal expansion showed a distinct jump at T C , and the thermal expansion coefficient exhibited a divergent character near T N . The critical exponents were found to be 0.34∼0.36 for T > T N and 0.09∼0.20 for T < T N . These agree very well with those of the specific heat. It was revealed that the Néel temperature determined by the analysis of the thermal expansion data lies 0.1∼0.2 deg below the peak position of the dielectric constant. The pressure dependences of the transition points deduced from the Clausius-Clapeyron equation and the Pippard equation were found to agree with the result of the hydrostatic pressure experiment.