Structural transition and temperature dependencies of thermal expansion coefficients of NaNO$_3$ embedded into the nanoporous glass

The temperature evolution of the crystal structure of a nanocomposite material obtained by introducing sodium nitrate NaNO$_3$ from a melt under pressure into a nanoporous alkali borosilicate glass with an average pore diameter of 7 nm has been studied by the method of diffraction of synchrotron radiation in a wide temperature range upon heating. Analysis of the experimental diffraction patterns revealed a significant decrease in the temperature of the structural (orientational) transition by more than 50 K (up to 496 K) compared to bulk sodium nitrate. From the temperature dependence of the intensity of the superstructure peak (113), the dependence of the critical exponent $\beta(T)$ for this transition was obtained and a significant difference from the critical exponent for a bulk material was found in the temperature range from 455 K to the transition temperature. Analysis of the broadening of Bragg reflections made it possible to estimate the average size ($\sim$ 40 nm) of sodium nitrate nanoparticles into the pores. An increase in the linear coefficient of thermal expansion in the [001] direction was found in NaNO$_3$ nanoparticles in comparison with bulk material at temperatures above 450 K.