Anomalous Behavior of Heat Capacity in Ni2(bdc)2(dabco). Schottky Anomaly and Spin–Phonon Interaction

Abstract

Temperature dependence of molar heat capacity of Ni2(bdc)2(dabco) was first measured by the method of vacuum adiabatic calorimetry in the temperature region of 5–295 K. A heat capacity anomaly was discovered above 60 K. The analysis of the electronic structure of a dimeric complex Ni2(O2C-)4 testifies that the observed anomaly is the Schottky anomaly caused by thermal excitation of d electrons of Ni2+(3d8) ions. The experimental heat capacity values in the vicinity of the maximum of the Schottky anomaly exceed its calculated values by ∼10%. This fact is explained by the spin–lattice interaction between d electrons of Ni2+ ions and lattice vibrations of Ni-DMOF (Ni2(bdc)2(dabco)). The obtained experimental Cp values were used to calculate standard thermodynamic functions (heat capacity, enthalpy, and entropy) of the studied compound.