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

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 â 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.