Effect of restricted geometry on phase transitions in nanostructured sodium nitrate

The study of structure of nanocomposite material based on porous glass with 7 nm average pore diameter filled by sodium nitrate from the melt under pressure has been carried out by synchrotron radiation diffraction upon heating and cooling. The temperature evolution of the elastic peak lineshapes corresponding to the sodium nitrate structure has been analyzed, and it is shown, that in the “virgin” sample there are two types of nanoparticles with dendrite spatial organization and with fundamentally different characteristic sizes. The temperature dependences of the critical index β(T) for the structural transition in sodium nitrate have been obtained, and the crossover in this dependence for nanoparticles with a characteristic size of about 20 nm has been found at T∗ ∼ 445 K both upon heating and cooling. The temperatures of the structure and “melting – crystallization” phase transitions observed in this nanocomposite have been determined.