Second phase transition

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... 

S-shaped current potential curves emerge when a surface phase transition of an organic adsorbate is coupled with a faradaic reaction of some electroactive species. As a representative of such a system, the periodate reduction on Au(lll) single crystal electrodes in the presence of camphor was studied [160], Camphor adsorbed on Au(lll) electrodes exhibits two first-order phase transitions upon variation of the electrode potential [161]. In a cyclic voltammogram, the phase transition manifests itself in a pair of needle-like peaks (Fig. 28 (A)). Between the peak pairs, a condensed, well-ordered camphor film exists. At more negative potentials, the camphor coverage is low, while the state of the adsorbate at positive potentials beyond the second phase transition is not yet known. The small hystereses between the respective anodic and cathodic peaks are caused by the finite nucleation rate of the respective thermodynamically stable phase. 

Figure 6.8-21 Raman spectra of hexachloroethane in the region of the second phase transition at 71 °C.

Around 1.8 % Th,TQ reaches a minimum and increases again with still increasing x. Concomitantly, a second phase transition in the superconducting state, which does not destroy superconductivity, is observed. The critical temperature of this transition is almost independent of X and both transitions merge into one for x > 0.05 (25). 

A coexistence region in SQA was also evidenced by 13C 2D temperature jump correlation NMR.172 Proton 1H spin lattice and spin-spin relaxation measurement on SQA was performed within 320 460 K.138 In addition to a typical critical behavior manifested at the known temperature of the anti-ferroelectric phase transition near Tc 373 K, both relaxation data also show a second critical behavior around 420 K, which opens the question of a second phase transition for SQA. 

Equation 2.110, we obtain the analytical expression for the pseudo-second-phase-transition temperature (Wang, 1995a)... 

For simplicity, assume that vab is the geometrical mean of va and vb, i.e., the Berthelot law, and extend the free energy derived from Equation 2.f22 in the power series of Sa and, S),. The zero determinant of the quadratic coefficients gives the pseudo-second-phase-transition temperature as follows... 

In the second-order approximation of the free energy, above T, Sa and Sb are both zero. Below T, the order parameters are non-zero. At T, the quadratic terms vanish. This formula illustrates the pseudo-second-phase-transition temperature vs. the molecular parameters that shows the same shapes as those of the numerical calculations of the N-I transition temperature Tc, mentioned above. 

All f 2Cu2ln indides order ferromagnetically with Curie temperatures between 26.7 and 85.5 K. Strong crystal field anisotropies result in Tc values that do not scale with the de Getmes frmction of the rare earth elements (Fisher et al., 1999). Dy2Cu2ln shows a second phase transition to an antiferromagnetic ground state at 7n = 22 K. 

Sakurai et al. (1995) have established that of SmPdSn is equal to 12.2 K. At 4.2 K there is a second phase transition which is apparently connected with a change of the SmPdSn magnetic structure. 

Fig. 5 Examples of self-made models of the students Second phase — transition into the micro-world ...

On the basis of resistivity measurements on a series of Ui jThjBei3 compounds as a function of pressure, such as shown in fig. 49, Borges et al. (1988a,b) have suggested a correspondence between normal-state properties and the appearance of a minimum in 7 o(x,F), i.e., the onset of a second phase transition. With increasing Th content the low-... 

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