Thermal and physical properties of sodium niobate ceramics over a wide temperature range

The temperature dependences of the heat capacity $C_p(T)$ and thermal expansion coefficient $\alpha(T)$ of NaNbO$_3$ ceramic samples have been investigated in the temperature range from 2 to 800 K. In addition to the anomalies associated with the known phase transitions at temperatures $T_6\approx$ 265 K, $T_5\approx$ 638 K, $T_4\approx$ 760 K, and $T_3\approx$ 793 K, anomalies in the behavior of $C_p(T)$ and $\alpha(T)$ have been observed near $T_{5''}\approx$ 500 K and $T_{5'}\approx$ 600 K. It has been found that all the observed structural transformations, according to the values of the entropy change, are not related to the ordering of structural elements. It has been shown that, with an increase in the temperature, the unit cell volume during the phase transitions near 265, 515, 604, and 638 K decreases. The specific features of the transition to the phase R3c have been examined. Two possible scenarios of the sequence of phase transformations in the temperature range between $T_5$ и $T_6$ have been analyzed.