Thermal, structural, optical, dielectric and barocaloric properties at ferroelastic phase transition in trigonal (NH4)(2)SnF6: A new look at the old compound

Abstract

Structural phase transition P-3m1 <-> P-1 of the first order and nonferroelectric nature was found in (NH4)(2)SnF6 at about 110 K in the process of thermal, optical, dielectric and X-ray studies. Order-disorder transformation is accompanied by large changes of entropy (Delta S-0 = 16.4J/mol K) and volume (Delta V-0/V approximate to 1%). The structural model associated with three orientations of NH4 tetrahedra in P-3m1 phase and their complete ordering in P-1 phase was suggested with the entropy change (Delta S-0 = 18.3 J/mol K) close to the experimental value. A good agreement between baric coefficients measured in a direct way (dT(0)/dp = -157 K/GPa) and evaluated using entropy and volume changes at phase transition (-170 K/GPa) was found. Barocaloric effects in (NH4)(2)SnF6 are comparable with those of known solid state refrigerants and are characterised by a low-pressure need for realising their maximum values.