Effects associated with confined geometry in nanocomposites based on mesoporous 2D-SBA-15 and 3D-SBA-15 matrices containing sodium nitrite nanoparticles

Temperature dependences of the ferroelectric order parameter for nanostructured sodium nitrite under heating and cooling were obtained by analyzing the temperature evolution of diffraction spectra of neutron scattering by composites obtained by introducing sodium nitrite into pores of mesoporous 2D-SBA-15 (the average pore diameter is 69(4) $\mathring{\mathrm{A}}$) and 3D-SBA-15 (94(5) $\mathring{\mathrm{A}}$) matrices. It was demonstrated that the phase transition to the paraelectric phase in the process of heating occurs at $T_C$ = 433 $\pm$ 1 K in both nanocomposite materials. It was found that these sizes decrease at approaching to the ferroelectric phase transition point on heating. Temperature hysteresis ($\sim$15–20 K) in the temperature dependence of the order parameter between the heating and cooling regimes was revealed.