Electronic and magnetic transitions

Electronic transitions like insulator-metal transitions, magnetic order-disorder transitions, spin transitions and Schottky-type transitions (due to crystal field splitting in the ground state in/element-containing compounds) profoundly influence the phase stability of compounds. A short description of the main characteristics of these transitions will be given below, together with references to more thorough treatments. 

The electronic heat capacity naturally has a pronounced effect on the energetics of insulator-metal transitions and the entropy of a first-order transition between an insulating phase with y = 0 and a metallic phase with y= ymet at Ttrs is in the first approximation Ains met5m = 7met7trs. 

Differences in the Debye temperature, or in other words the vibrational character of the two phases, will modify the transitional entropy to some extent. Still, this entropy change is normally not large for transitions where the coordination number is preserved. 

Large entropy increments accompany the spin transitions. For Co3+ in an octahedral crystal field the high-spin state has a degeneracy of 15 (the product of orbital 

A low-spin to high-spin transition relates to the crystal field splitting of the d-orbitals in an octahedral or tetrahedral crystal field. However, even in cases where the energy difference between two spin states is much larger, electronic transitions are observed. An atom with total spin quantum number S has (22 + 1) orientations. In a magnetic field the atom will have a number of discrete energy levels with 

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