Order-disorder phase transitions in Zn2(C8H4O4)2C6H12N2 in atmospheres of noble gases

The molar heat capacity of Zn-2(C8H4O4)C6H12N2 (Zn-DMOF) was measured for the first time at 4.8-302 K in atmospheres of noble gases He-3, He-4, and Ar-40 under pressures P up to 1.52.10(5) Pa. It is shown that phase transitions reported earlier at T-c = 13, 58.9, and 131.2 K are second-order phase transitions associated with the ordering/disordering of organic ligands in Zn-DMOF. The transition entropy (Delta S-m) in the temperature range of phase transitions associated with the ordering/disordering of C6H12N2 molecules decreases at T-c = 13 K and increases at T-c = 58.9 K as P increases from 0.51.10(5) to 1.52.10(5) Pa. The values of Delta S-m do not depend on P in the region of the phase transition at T-c = 131.2 K associated with the ordering/disordering of C8H4O4 anions. (C) 2018 Elsevier Ltd.