Metal-semiconductor transition

The pressure-induced semiconductor-metal transitions of SmSei xSx solid solutions are discontinuous for x 0.2 and continuous forx 0.2 as detected by electrical resistivity q versus pressure measurements. The strength of the first-order phase transition (expressed by Ag/g p where Ag is the resistivity jump and Qtr s the resistivity at the transition pressure Ptr) decreases smoothly to zero at x = 0.2. This composition is characterized by a sharp break in slope at 34.6 kbar when q versus p is plotted but no hysteresis is noticed as pressure is released. Transition pressure p r and Ag/ptr in comparison with the theoretical phase transition strength (calculated with a modified Falicov-Kimball model) as a function of composition are shown in Fig. 81, p. 170, Bucher, Maines [2], also see Bucher et al. [3]. The change from continuous to discontinuous transition is interpolated to be at x = 0.28 the experimental value is 0.25 (determination technique not given in the paper), Narayan, Ramaseshan [4]. The course of the configuration crossover f d°- f d for Sm(Se,S) under pressure is illustrated by Wilson [5]. 

The concentration dependence of the Se NMR shift at 77 and 302 K is shown in Fig. 82, p. 170, for SmSei xSx with x = 0 to 0.5. The slopes dK/dx are negative and amount to -1.114 and -0.411% per x at 77 and 302 K, respectively. The non-4f contribution K is estimated to change by dKo/dx = (0.38 0.10)% per x. The observed hyperfine field at the Se site is a consequence of some type of interaction which induces a net spin polarization of the anion p shell and varies roughly as the inverse 7th power of the lattice spacing, Brog, Kenan [6]. 

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Phase transitions are involved in critical temperature thermistors. Vanadium, VO2, and vanadium trioxide [1314-34-7] V2O3, have semiconductors—metal transitions in which the conductivity decreases by several orders of magnitude on cooling. Electronic phase transitions are also observed in superconducting ceramics like YBa2Cu30y but here the conductivity increases sharply on cooling through the phase transition. 

Bipolarons are mobile electron pairs. They have been identified in a small temperature interval above the semiconductor-metal transition in Ti407 In this case, they represent mobile electron-pair bonds in a mixed-valence compound, the electrons condensing into metal-metal homopolar bonds that are ordered at low temperatures, but become disordered and mobile at somewhat higher temperatures. 

Jurczek, E., Model Study of the Semiconductor-Metal Transition in BaBij xPbxOs by Use of a Spatially Inhomogeneous Order-Parameter Approximation. Phys. Rev. B 35(13) 6997 (1987). 

Figure 6,23 Electrical resistivity of NiS2 (Se showing semiconductor-metal transition. (After Bouchard et al, 1973.)...

The transition from LDA to HDA Si was observed in the successive experiment by McMillan et al. [264]. In situ Raman spectra and electronic resistance measurements were performed with optical observation. After compression, the LDA form prepared by solid-state metathesis synthesis [10] was found to be transformed to the HDA form at 14 GPa. The electronic resistance exhibited a sharp decrease at 10-14 GPa (Fig. 15), which is consistent with the early experimental findings by Shimomura et al. [260], Optical micrographs show that HDA Si is highly reflective, whereas LDA Si is dark colored and nonreflective. This finding again supports that the LDA-HDA transition of Si is accompanied by a semiconductor-metal transition. Reverse transitions with large hysteresis were also observed LDA Si began to form from HDA Si at 4-6 GPa after decompression from --20 GPa. 

Rn Semiconductor metal transition occurs at 150°K (cooling), at 180°K (heating). The resistivity changes through the transition by a factor of 10. The low-temperature phase is monoclinic. A noncooperative, high temperature transition occurs over the temperature range 110°C < T < 260°C. 

Fig. 11. Lattice constants of monochalcogenides of the RE. We have indicated the value (5.71 A) for metallic SmS at 6.5 kbar, after the first order semiconductor-metal transition. Fully trivalent SmS should have an interpolated lattice constant of 5.61 A. The case of the Tm monochalcogenides is discussed in the text.

Fig. 20. Concentration dependence of the lattice constant of SmS for substituents having different valences. Divalent substituents (e.g. Ca or Yb) do not induce the semiconductor-metal transition.

In some sense this is similar to a uniaxial stress applied to the bulk crystal. For the semiconducting SmS (100) surface such surface relaxation, if large enough, is expected to have drastic influence on the electronic structure of the Sm ions, because the semiconductor-metal transition occurs when the bulk lattice constant has been... 

Although indicative of the semiconductor-metal transition, the conductivity data alone cannot be accepted as rigid proof of the formation of Si-II during indentation. Unfortunately, due to obvious complications with the experimental setup, no in situ indentation diffraction data are available as of today. On the other hand, a number of papers provide indirect evidence of the Si-I -> Si-II transformation during indentation based on the post-indentation characterization techniques. We will start with a discussion of the electron microscopy results. 

TmSe is another interesting example for anomalous elastic properties. TmSe with the two magnetic configurations 4f (Tm ) and 4f (Tm ) exhibits an antiferromagnetic ordering at Tjf 3K. As seen from table 6, is small and V < 0. Very illustrative is the semiconductor-metal transition in the TmSe-TmTe system (Boppart 1985). In fig. 57 the phase diagram p-V-x exhibits close similarities to the common liquid-gas transition (TmSei Te ). The critical point is givey by — 0.45, — 0.8 GPa. Because of the same symmetry between the... 

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