Spin transition curves types

Fig. la-d Representation of the principal types of spin transition curves (high spin fraction (7hs) (y axis) vs temperature (T) (x axis) a gradual b abrupt c with hysteresis d two-step e incomplete... 

Despite the relative lack of predictability, the number of systems now known to display a spin transition curve of type (c) is remarkably high, and highest for iron(II) where, significantly, the change in intramolecular dimensions is the greatest for the ions for which SCO is relatively common (Fe(II), Fe(III), Co(II)). 

Thermodynamic (specific-heat) and transport (resisitivity) studies for the U(Pti xPdx)3 compounds have revealed magnetic transitions for x-values of 0.02 and higher, in contrast to the compounds below x = 0.01 that did not show anomalies in specific heat and resistivity that point to magnetic order. The sharpest transition in the specific heat studies is found for x = 0.05. For this compound, the resistivity v.v temperature curve exhibits a Cr-type of anomaly just below the magnetic ordering temperature suggesting the presence of a spin-density wave below TN. A few results from neutron diffraction studies are shown in fig. 6. 

Le Roy, et al, (2002) have reviewed all of the different types of experimental observations and theoretical calculations for HI. By an empirical analysis, they have shown that, because in HI the spin-orbit interaction is especially important, the adiabatic relativistic potential curves can explain all of the experimental data without introducing residual nonadiabatic coupling. For the lighter halogen hydrides, the J = 1/2 J = 3/2 branching ratio can be obtained from the solution of inhomogeneous coupled equations with a source term representing the initial vibrational wavefunction multiplied by the electronic transition moment (Band, et al., 1981). These calculations are based on adiabatic electronic (or diabatic relativistic) potential curves (see, for example, for HC1, Alexander, et al., 1993 and for HBr, Peoux, et al., 1997). 

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