Crossover transition temperature

A series of NFS spectra of the spin-crossover complex [Fe(tpa)(NCS)2] were recorded over a wide temperature range [45]. A selection of spectra around the spin-crossover transition temperature is shown in Fig. 9.13. At 133 K, the regular quantum-beat structure reflects the quadrupole splitting from the pure high-spin (HS) phase, and the envelope of the spectrum represents the dynamical beating with a minimum around 200 ns. Below the transition, at 83 K, the QBs appear with lower frequency because of smaller AEq of the low-spin (LS) phase. Here the minima of... 

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... 

Figure 24. Plot of the particle-size distribution versus the transition temperature Tross, which describes the crossover point between an activated transport mechanism (ln(A) oc EJT and variable range hopping (VRH) (ln(R)ocT ). Note that Tdoss has a OK value at a finite (3%) particle-size distribution. (Reprinted with permission from Ref. [56], 2002, American Chemical Society.)...

A more subtle chemical influence is the variation of the anion associated with a cationic spin crossover system, or of the nature and degree of solvation of salts or neutral species. These variations can result in the displacement of the transition temperature, even to the extent that SCO is no longer observed, or may also cause a fundamental change in the nature of the transition, for example from abrupt to gradual. The influence of the anion was first noted for salts of [Co(trpy)2]2+ [142] and later for iron(II) in salts of [Fe(paptH)2]2+ [143] and of [Fe(pic)3]2+ [127]. For the [Fe(pic)3]2+ salts the degree of completion and steepness of the ST curve increases in the order io-dide[Pg.41]

Among all Fe(II) spin crossover compounds known to date, the extensively studied polymeric [Fe(4-R-l,2,4-triazole)3](anion)2 systems (R=amino, alkyl, hydroxyalkyl) appear to have the greatest potential for technological applications, for example in molecular electronics [1, 24, 25] or as temperature sensors [24, 26]. This arises because of their near-ideal spin crossover characteristics pronounced thermochromism, transition temperatures near room temperature, and large thermal hysteresis [1, 24, 27]. 

Further studies have shown that instead of TCNQ -, NCS- or NCSe- [6,7] can also occupy the trans-located axial positions, resulting in spin crossover compounds with structures comparable to those of [Fe(abpt)2(TCNQ)2] [5]. The Fe(II) spin transition is also gradual for these derivatives, however, with considerably lower transition temperatures 224 K for the NCSe- derivative and 180 K for the NCS- analogue. 

Spin transitions have also been reported for Al0.33[Fe(5-Cl-thsa)2] [110] and H[Fe(5-Cl-thsa)2] [109, 110]. For both compounds, a relatively abrupt and almost complete spin crossover occurs with Ti/2=228 K for the Al derivative, and 226 K for the H derivative. Transition temperatures determined by variable temperature heat capacity measurements are in agreement with those obtained from the magnetic susceptibility measurements. 

In summary, Fig. 6 exhibits the four characteristic temperatures of glass formation the Arrhenius temperature 7a, the crossover temperature Tj, the ideal glass transition temperature Tb, and a kinetic glass-formation temperature Tg (dehned in Section VI), to illustrate their relative locations with respect to the temperature variation of s. ... 

Figure 22. The configurational entropy Sc per lattice site as calculated from the LCT for a constant pressure, high molar mass (M = 40001) F-S polymer melt as a function of the reduced temperature ST = (T — To)/Tq, defined relative to the ideal glass transition temperature To at which Sc extrapolates to zero. The specific entropy is normalized by its maximum value i = Sc T = Ta), as in Fig. 6. Solid and dashed curves refer to pressures of F = 1 atm (0.101325 MPa) and P = 240 atm (24.3 MPa), respectively. The characteristic temperatures of glass formation, the ideal glass transition temperature To, the glass transition temperature Tg, the crossover temperature Tj, and the Arrhenius temperature Ta are indicated in the figure. The inset presents the LCT estimates for the size z = 1/of the CRR in the same system as a function of the reduced temperature 5Ta = T — TaI/Ta. Solid and dashed curves in the inset correspond to pressures of P = 1 atm (0.101325 MPa) and F = 240 atm (24.3 MPa), respectively. (Used with permission from J. Dudowicz, K. F. Freed, and J. F. Douglas, Journal of Physical Chemistry B 109, 21350 (2005). Copyright 2005, American Chemical Society.)...

Some relationship between viscosity crossover in theta solvents and polymer polarity is suggested by the results, supporting the idea of enhanced intermolecular association in poor solvents. However, from the data on hand, one could also infer a correlation with the glass transition temperature of undiluted polymer,... 

In the limit of very large viscosity, such as the one observed near the glass transition temperature, it is expected that rate of isomerization will ultimately go to zero. It is shown here that in this limit the barrier crossing dynamics itself becomes irrelevant and the Grote-Hynes theory continues to give a rate close to the transition theory result. However, there is no paradox or difficulty here. The existing theories already predict an interpolation scheme that can explain the crossover to inverse viscosity dependence of the rate... 

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