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

The molar heat capacity of Zn₂(C₈H₄O₄)₂ ^.C₆H₁₂N₂ (Zn-DMOF) was measured for the first time at 4.8–302 K in atmospheres of noble gases ³He, ⁴He, and ⁴⁰Ar under pressures P up to 1.52·10⁵ 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 (ΔS_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 0.51·10⁵ to 1.52·10⁵ Pa. The values of ΔS_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 ²⁻ anions.