Calorimetric investigation of phase transitions in the layered antiferromagnetic molecule-based material {N(n-C5H11)4[MnIIFeIII(ox)3]}∞(ox=oxalato)

Heat capacity studies of the layered molecule-based magnetic material {N(n-C5H11)4[MnIIFeIII(ox)3]}∞ (ox=oxalato) have been made in the 2–280 K range. Two distinct heat capacity anomalies were detected at 27.1 K (TN) and 226 K corresponding to magnetic and structural phase transitions, respectively, along with a hump around 23 K. The magnetic heat capacities above TN can be well represented by <f>S=5/2</f> two-dimensional antiferromagnetic Heisenberg model of a honeycomb lattice with intralayer exchange interaction J/kB=-3.3 K. Application of spin-wave theory indicated the existence of a very weak interlayer magnetic interaction below TN. The anomaly at 27.1 K is due to the antiferromagnetic transition, in conformity with the magnetic results reported earlier. The hump around 23 K might be associated with the existing uncompensated magnetic moments. The estimated magnetic entropy (33.22 J K-1 mol-1) is close to the expected magnetic entropy (R ln (6×6)=29.80 J K-1 mol-1) for the spin multiplicity of high spin MnII and FeIII ions. The heat capacity anomaly at 226 K may be assigned to a structural phase transition of order–disorder type due to increasing conformational change of the n-C5H11 chains in the organic cation. [Copyright &y& Elsevier]