Electronic entropy spin-state transitions

Order-disorder transitions are generally associated with (i) positional disordering, (ii) orientational disordering or (iii) disordering of electronic (or nuclear) spin states. The configurational entropy due to disordering is given by... 

Probably the most important experimental work which confirms the cooperative nature of the spin crossover transition is the heat capacity measurements on Fe(phenanthroline)2(NCS)2 and Fe(phenanthroline)2-(NCSe)2 by Sorai Seki (1974). The variation in the molar heat capacity of the latter compound with temperature is shown in Figure 3.27. Not only does the heat capacity show a sharp peak at the transition temperature (see Table 3.9), but there is also a change in the heat capacity, for the two different spin states (see Figure 3.27) at the transition. These authors propose that the total heat capacity of each spin state is made up of contributions from lattice vibrations, intramolecular vibrations, and electron thermal excitation. From this they have determined the changes in enthalpy and entropy associated with the change in spin state at the crossover. The results of these calculations are given in Table 3.9 and they... 

Ramirez et al (1970) discussed a metal-insulator transition as the temperature rises, which is first order with no crystal distortion. The essence of the model is—in our terminology—that a lower Hubbard band (or localized states) lies just below a conduction band. Then, as electrons are excited into the conduction band, their coupling with the moments lowers the Neel temperature. Thus the disordering of the spins with consequent increase of entropy is accelerated. Ramirez et al showed that a first-order transition to a degenerate gas in the conduction band, together with disordering of the moments, is possible. The entropy comes from the random direction of the moments, and the random positions of such atoms as have lost an electron. The results of Menth et al (1969) on the conductivity of SmB6 are discussed in these terms. 

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