Calorimetric investigations of NaNO3 and NaNO2 embedded into porous glasses

The results of the calorimetric investigations of porous glasses impregnated with NaNO2 and NaNO3 in a wide temperature range, including over the phase transition temperature, are presented. The specific heat anomaly connected with the melting process in NaNO2 crystals embedded into porous glasses is wider and it is shifted to lower temperature in comparison to the anomaly observed in bulk NaNO2 crystals. In crystals embedded into a matrix of porous glasses with pore diameter of 23 nm, no latent heat at melting temperatures was observed. A wider temperature range associated with a modulated phase is found to occur in NaNO2 crystals embedded into porous glasses with pore diameter of 160 nm compared to that observed in bulk crystals. It was shown that the direct phase transition from the ferroelectric phase to the paraelectric one was observed in crystals with the pore dimension comparable to the modulation period of the order parameter. We found a connection between the anomaly of the specific heat and the structural phase transition in NaNO3 embedded into porous glasses for the first time.