Configuration crossover

A course now well known to secure its revelation is to closely investigate groups of materials which evidently already span the configurational crossover, like TmS-TmTe, or are likely to do so under reasonably attainable changes in applied conditions. TmSe has now proved an excellent example under ambient conditions of this unusual state and its fascinating properties. SmBg was the first material to be discovered in such a state, and SmS/Se/Te reach it, as indicated above, under relatively low pressures. The state is labelled ICF for reasons to become apparent below. 

We now return finally to take up the point with which we ended the first paragraph in section 2.1 viz. via what means is doping that bears delocalized d-electrons successful in promoting configuration crossover in a solid like SmS or TmTe Earlier we illustrated, by citing that CaS (ag = 5.68 A) doping of SmS (ag = 5.96 A) leaves the... 

Only when the d-band is sufficiently weU formed, running over all sites, host and dopant alike, is the fuU benefit of configuration crossover attained. We have seen, with the B phase of (Sm/Gd)S, that when the Sm/ d sites are excluded from the Fermi sea, the ICF conversion percentage becomes relatively low. This is so despite the continuing metallic background of the dopant d gas. With a dopant that is unable to promote metallic conditions the position will be worse. 

In the case of Sm metal, the Sm atoms on the surface are in +2 state and the Sm atoms in the interior in +3 state. This observation shows that the energy i +c which stabilizes the +3 state in the bulk and prevents it in going to +2 state, experience a reduction at the surface thereby placing the Sm atoms at the configuration crossover. A similar phenomenon has also been observed in the case of Yb ions in YbAu2. 

Fine particles of metallic Eu and Yb have been shown to exhibit valence transition with critical particle sizes of 50 A for Eu and 35 A for Yb. At these critical particle sizes, there is a reduction in the lattice constant with no change in the critical fee structure. There appears to be a change in valence from +2 to +3. This effect may be due to a shift in the position of Fermi level, E, such that E xc approaches zero resulting in the ions at the configuration crossover. ICF state can in general result in lattice constant anomaly due to the fractional occupation of the 4/ shells. Thus once ICF is identified in a system, valence fluctuation can be studied by many techniques. 

Whilst no magnetic scattering has been identified in the mixed valence phase of SmS, it has been seen in Sm 25S by Mook et al. (1978) who observed a very broad peak with a FWHM of 15 meV at 30 meV. The addition of yttrium has a similar effect to that of pressure in contracting the lattice. At this composition, the valence changes with temperature from 2.3 to 2.45 at about 200 K (Weber et al. 1989). This means that the compound is at a configuration crossover with the f level effectively pinned at the Fermi level Ep, — Ep-i = p) such that the ground state consists of a hybridised mixture of the two configurations. The valence is then determined by thermodynamic considerations in which the ionic excitation... 

CC configurational crossover FWHM full width at half maximum ... 

These dihalides have crystal structures that are also observed for the respective alkaline-earth halides (see Table 2). Neodymiiun(n) iodide, Ndl2, undergoes a high-pressure phase transition from salt-like (SrBr2 type of structure) to metallic (CuTi2 type of structure) subject to the configuration crossover 4f5d 4f 5d. The 5d electron... 

Configuration crossover Transition from one electronic configuration to another... 

Rare-earth metals R that may form clusters and encapsulate endohedral atoms (or small atom groups such as ( 2)" ) are such that they have 3d (Sc), 4d (Y), or 5d (La, Ce-Lu) states energetically available for Z—R bonding interactions. For the lanthanides, a configuration crossover from a [Xe]4f"5d°6s to a [Xe] 4f 5d 6s must be energetically favorable, which is the case with elements with low third ionization potentials, (see Fig. 6). One may translate this behavior into standard electrode potentials for the half-cells R /R ", E°... 

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]. 

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