Comprehensive thermodynamic study of three Co(II)- and Fe(II)-based octacyanoniobates

A comprehensive study of thermodynamic properties of three samples of bimetallic molecular magnets [Co-II(pyrazole)(4)](2x)[Fe-II(pyrazole)(4)](2(1-x))[Nb-IV(CN)(8)]center dot 4H(2)O with x = 1 (Co2Nb), 0.5 (CoFeNb), and 0 (Fe2Nb) is reported. The three samples display the same crystallographic structure crystallizing in the tetragonal system with space group I4(1/a). Their heat capacities are measured in the temperature range 0.36-100 K without applied field as well as in the field of mu H-0 = 0.1, 0.2, 0.5, 1, 2, 5, and 9 T. The results imply the presence of the second-order phase transitions to magnetically ordered phases at 4.87(8), 7.1(2), and 8.44(3) K for x = 1, 0.5, and 0, respectively. The corresponding thermodynamic functions are analyzed to discuss the stability of the mixed compound and the magnetocaloric effect (MCE). The Gibbs energy of mixing is found to be positive but smaller in magnitude than the energy of thermal fluctuations indicating that the mixed sample is most probably metastable in the full detected temperature range. The enthalpy of mixing is negative, which points to favoring a direct neighborhood of the Co(II) and Fe(II) ions in the solid solution CoFeNb. The negative values of the entropy of mixing are explained by considering the enhanced rigidity of the crystal lattice of the solid solution sample. To extract the magnetic contribution to the heat capacity, an approach based on a reasonable frequency spectrum is adopted Taking advantage of the in-field heat capacity measurements, MCE was described in terms of the isothermal entropy change AS M and the adiabatic temperature change Delta T-ad. The magnitudes of these quantities are typical for the class of molecular magnets. The values of vertical bar Delta S-M vertical bar(max) detected for mu(0)Delta H = 5 T amount to 7.04, 5.26, and 4.93 J K-1 mol(-1) for Co2Nb, CoFeNb, and Fe2Nb, respectively, and are on the order of those obtained for the same field change in the isostructural compounds. The values of Delta T-ad detected for mu(0)Delta H = 5 T amount to 4.16, 2.47, and 2.01 K for Co2Nb, CoFeNb, and Fe2Nb, respectively, and are larger or comparable with those observed for the isostructural compounds. Temperature dependences of exponent n, quantifying the field dependence of Delta S-M, display minima close to the transition temperatures, implying through their values that the studied compounds belong to the universality class of the three-dimensional Heisenberg model. The regeneration Ericsson cycles employing the studied compounds as the working substance were considered. Most surprisingly, the Ericsson cycle operating between the temperatures corresponding to the full width at half maximum of the vertical bar Delta S-M vertical bar signal (T-C, T-H) turns out to be totally ineffective. Through shifting the temperature of the hot reservoir T-H down to the temperature T-max corresponding to vertical bar Delta S-M vertical bar(max), the coefficient of performance is rendered positive and comparable with that of the Carnot cycle. A detailed analysis indicates that the regeneration Ericsson cycle operating between T-C and T-max should be most efficient for the maximal studied value of the applied field (=9 T) with irrelevant differences between the studied compounds.