Contributions of electronic entropy

When high-spin Fe2+ is present in a distorted (low-symmetry) site, however, 

Electronic structure Cation Electronic configuration sa octahedral tetrahedral ASel (oct - tet) J/(deg.mole) ASel (Is—hs) J/(deg.mole) 

At elevated temperatures, the entropy difference between regular and distorted sites is reduced due to thermal population of higher energy orbitals by the sixth 3d electron of Fe2+. The probabilities, P0 and P, of this sixth electron occupying the lowest and next-lowest orbitals separated by an energy 8, at temperature Tis given by the Maxwell-Boltzmann equation, 

As the temperature rises, the probability of the sixth electron occupying orbital energy levels above the lowest level increases, and so too does the Sel of Fe2+ located in a distorted site, eq. (2.27). However, when Fe2+ is present in a regular octahedron, its sixth 3d electron remains delocalized over the three equivalent orbitals so that Sel remains constant at 9.13 J/(deg. g.ion). Therefore, 

Differences of electronic entropy also exist between high-spin and low-spin configurations (table 7.3). An alternative equation, which is regarded to be more valid than eq. (2.27) (Sherman, 1988), for evaluating the change of electronic entropy during a high-spin to low-spin transition, is 

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