Transitions Jahn-Teller

Both spin-crossover transitions (HS < LS, FO LS) are first order accompanied by definite jumps of populations, while the cooperative Jahn-Teller transition (HS FO) is weak first-order (very close to a second-order transition). It suggests a possibility of observation of hidden cooperative Jahn-Teller transition (the broken line in Fig. 7) between the metastable HS and FO phases, if the HS phase could be supercooled enough below the spin-crossover transition temperature Tc by a rapid cooling. 

The three adjustable parameters are determined, A/kB = 90 K, Jo/kB = -36 K, and J /kB = 125 K, so as to reproduce the spin-crossover transition temperature Tc = 48 K, the virtual Jahn-Teller transition temperature rJT = 6 = 26 K, and the effective LS-HS gap in the LS phase Acff/kB = 340 K. (Note Aeff is approximated by A + 2Jx in this mean-field model.) This choice of model parameters gives a phase sequence from the LS to HS with increasing temperature, corresponding to the arrow path in Fig. 7. Temperature dependence of thermodynamic quantities (Fig. 8) is calculated along the path indicated by the arrow in Fig. 7, where the discontinuities arising from the first-order spin-crossover transition are recognized Ap0 = 0.99, AH = 0.64 kJ mol-1, and AS = 13.3 J K-1 mol-1 These theoretical... 

Jahn-Teller effects are rarely adopted in theoretical treatments of spin-crossover phenomena, except Kambara s model for Fe(II) complexes [24] and Bersuker s for Fe(III) complexes [19,25]. In the Kambara theory, abrupt spin-crossover transitions are given an FO => LS character, while gradual transitions are of the HS LS type. However, experimental evidences of cooperative Jahn-Teller transitions (FO HS) have not yet been reported for Fe(II) spin-crossover complexes. Kambara s theory ignores the spin-orbit interaction and appears to overestimate the Jahn-Teller coupling, producing an unphysical (for Fe(II) complexes) FO => HS transition. 

Thomas, H. Theory of Jahn-Teller Transitions. In Riste, T. (ed.) Electron-Phonon Interactions and Phase Transitions, p. 245. New Yoik-London Plenum Press 1977... 

Fig. 18. p vs. T curves of PrCu2, NdCu2, and SmCu2. The inset shows the resistivity behaviour in the vicinity of the Jahn-Teller transition temperature in PrCu2 (Sassik and Gratz, 1981). 

If the crystal is strained homogeneously by external stresses or by undergoing a cooperative Jahn-Teller transition, the total strain ej-(i) = -I- Sp(i) is the sum... 

DyV04 undergoes a co-operative Jahn-Teller transition (see section 6) at around... 

Fig. 10. Temperature variation of the lattice parameters of TbV04, measured by X-ray diffraction, showing the distortion below the Jahn-Teller transition temperature To = 33 K. (Will et al. 1972.)...

Fig. 11. Heat capacity of TmVO from 0.6 to 3K, excluding the lattice contribution. The anomaly at the Jahn-Teller transition at 2.1S6K [experimental points (o)] is fitted to molecular-field theory (full curve). (Cooke et al. 1972.)...

Fig. 17.41. Lowest CEF-energy levels for three vanadates below (left) and above (right) the Jahn-Teller transition temperature (from Gehring and Gehring, 1975).

A divergent central peak was found by quasi-elastic neutron scattering at the Jahn-Teller transition temperature of TbV04 (Hutchings et al. 1975). 

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