Heat capacity and magnetic phase transition of two-dimensional metal-assembled complex, K[{MnIII(3-MeOsalen)}2FeIII(CN)6]

Heat capacities of the two-dimensional metal-assembled complex, K[{MnIII(3-MeOsalen)}2FeIII(CN)6] [3-MeOsalen = N,N′-ethylenebis(3-methoxysalicylideneaminato) dianion], were measured at the temperatures from 0.5 to 300 K by adiabatic calorimetry. An antiferromagnetic phase transition was observed at TN = 8.29 K. Above TN, a heat capacity tail arising from the short-range-order effect of the spins was found, which is characteristic of two-dimensional magnets. The magnetic enthalpy and entropy were evaluated to be ΔH = 373 J mol−1 and ΔS = 31.3 J K−1 mol−1, respectively. The experimental magnetic entropy is in good agreement with ΔS = R ln (5 × 5 × 2) (=32.5 J K−1 mol−1; R being the gas constant), which is expected for the metal complex with two Mn(III) ions in high spin state (spin quantum number S = 2) and one Fe(III) ion in low spin state (S = 1/2). The spin wave analysis suggests that the complex shows three-dimensional antiferromagnetic order below TN. The heat capacity tail above TN was decreased by grinding and pressurizing the crystal. This mechanochemical effect would originate in the increase of lattice defects and imperfections in the crystal lattice, leading to decrease of the magnetic heat capacity and hence the magnetic enthalpy and entropy.