Thermodynamics of fullerides [(bis-arene)2M][C60] (M = Cr, Mo and V)

V. Ruchenin1, A. Markin1 and N. Smirnova1

1Nizhni Novgorod State National Research University, Russia

Keywords: precision adiabatic calorimetry
property: heat capacity; standard thermodynamic properties
material: fullerides C60

The study of donor-acceptor complexes of fullerenes where the latter play the role of relatively strong acceptors is dictated by the search for new unique materials. Such materials exhibit interesting optical, electrical conductivity and magnetic properties. Most often, fullerene complexes with aromatic hydrocarbons as donor partners were synthesized and their properties were studied.  The temperature dependences of heat capacities of crystalline bis-(hapta6-xylene)chromium fulleride [(hapta6-xylene)2Cr]•+[C60]•- and bis-(hapta6-xylene)molybdenum fulleride [(hapta6-xylene)2Mo]•+[C60]•-, bis-(hapta6-mesitylene)chromium fulleride [(hapta6- mesitylene)2Cr]•+[C60]•- and bis-(hapta6-mesitylene)vanadium fulleride [(hapta6- mesitylene)2V]•+[C60]•- in the range from 6 to 360 K by precision adiabatic vacuum calorimetry were measured. The endothermic reversible transformations at heating was detected and its thermodynamic characteristics were estimated and analyzed. The transformation was caused by the opening of a dimeric bond between fullerene fragments and the decomposition of the tested complex on heating. The low-temperature heat capacity was analyzed on based of the heat capacity solids Debye theory and as results the conclusions about the hardness of structure for under study complex was estimated. The temperature dependence of heat capacity in the low-temperature region is well described by the limiting Debye law for the tested complex as well as for fullerite С60 and neutral dimer (С60)2. In the range 50-100 K, the heat capacity is a linear function vs. T for the tested sample and dimer (С60)2 which is typical of the solids having a structure of chain topology. The experimental data were used for calculating the thermodynamic functions, namely heat capacity, enthalpy, entropy and Gibbs function in the range from 0 to 360 K. The standard thermodynamic properties of the fulleride complex, studied earlier the neutral (C60)2 dimer and C60 fullerite were compared and discussed. On the based of our thermodynamic data, information about the structure and the composition of these and the respective data for the neutral dimer (C60)2 the some conclusion about the bound nature between fullerene fragments in the complex was made. Thus, the stability of dimeric dianions (С60-)2 depends significantly on the nature of substituent. Also, for indicated fullerides it was established that the isotherms (T = 100 K) dependences of the heat capacities and derived thermodynamic functions (enthalpy, entropy and Gibbs energy) vs. their molar weights are linear. The dependences will allow prediction of the thermodynamic properties of different fullerides of this rank. The work was performed with the financial support of the grants of RF President (Project MK-2228.2011.3).

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