Phase transition in NH₄H₂PO₄: porous glass composites

In this article, the results of the dielectric measurements after an impregnation of porous glasses (PGs) with ammonium dihydrogen phosphate NH4H2PO4 (ADP) are presented. As a result of dielectric and calorimetric studies, it was found that the phase transition in ADP crystals embedded in PG became diffused. In addition, the phase transition temperature decreased with a reduction of the pore size. We observed a shift of the phase transition temperature which is a superposition of the intrinsic size effect and the pressure effect created by the difference in thermal expansion coefficients of ADP nanoparticles and glass matrix. Accordingly, for the crystals with hydrogen bonds dTp /dp < 0 (for ADP dTp /dpp =0 = −3.4 × 10−8 K Pa−1), so in ADP-porous glass composite a small negative effective size effect occurs.