Thermal Expansion in NaNO₂ Crystals

Abstract

The anisotropy and the temperature dependence of thermal expansion in NaNO₂ crystals were investigated over a temperature range from room temperature to near the melting point by a differential type dilatometer.
The anomaly of thermal expansion coefficients was recognized at 102°C, near the Curie point (163°C), and at 178°C, 189°C and 207°C respectively. The plots of the α⁻¹ vs T relation are on straight lines in the whole temperature range except for that from 102°C to 207°C in the [100] and [010] directions, though the similar linear relation exists only above 178°C in the [001] direction. These three straight lines extrapolated to the high temperature side terminate together at a point, whose temperature is higher than the melting point by a few degrees.
Obtained experimental results were discussed qualitatively in relation to the thermal motion of NO_2⁻ radicals.