Heat capacity and thermal expansion of nanosized ferroelectric (NH4)2SO4 embedded in borosilicate glasses

The thermal properties of nanocomposites obtained by incorporating a ferroelectric (NH ₄) ₂SO ₄ (AS) into porous glasses (PG) was studied for the first time. X-ray diffraction studies showed a strong increase in the ratio of the size of nanocrystallites grown in glass matrices to pore size, d _cryst/ d _pore = 0.47→1.50→5.00, with a decrease in d _pore = 320→46→7 nm. Calorimetric and dilatometric measurements showed a change from a first-order phase transition in AS single crystal to a second-order transformation in AS nanocrystallites. A restricted geometry also led to a significant decrease in the anomalous entropy in AS+PG samples compared to AS (J/mol∙K): 17.7 (AS)→12.4 (AS+PG320)→10.4 (AS+PG46)→8.1 (AS+PG7). A discussion of the nontrivial behavior of the phase transition temperature in embedded AS was carried out using the degree of PG filling, the ratio d _cryst/ d _pore, texture, and the volumetric and linear baric coefficients for the bulk AS.