Pressure-induced valence transitions

We can describe the process of pressure induced valence transitions (as in Fig. 1 of Ref. ( )) as a three step phenomenon. The first, in the pressure range P P] only qualitative changes in the wave functions (as described by Equation 1 ) are induced, for a given ground state configuration, 1.0, because... 

In conclusion, the valence classification of Yb compounds, based on the SIC-LSD total energies, maps very well onto the physical properties observed experimentally. In particular, this allows to identify the third group of compounds as the heavy-fermion and mixed-valent systems on the trivalent side, and, on the divalent side, those systems that are likely to imdergo pressure-induced valence transitions. 

Svane A, Strange P, Temmerman WM et al (2001) Pressure-induced valence transitions in... 

Lattice constant variation of 4f levels pressure-induced valence transitions... 

We realize of course, that with 4f-5d energy gaps between 1 and 2eV and with standard deformation potentials of the order of 5 eV (Jayaraman 1979) the pressure-induced valence transition will be between 100 and 200kbar, and in fact, the pressure-... 

The values of Bq and Bq for typical Ce Kondo compoimds are summarized in Table 1. It is seen that the values of Bq are strongly dependent on the nature of the compound. However, the values of Bq show a systematic behavior. The Bq is between 2 and 3 for the Ce Kondo compounds showing pressure-induced continuous valence transition (Kagayama and Oomi, 1996). In this sense, CeAu2Si2 is classified as a Kondo compound showing pressure-induced valence transition, which may occur very slowly. 

A pressure-induced transition from an NaCl to a CsCl structure is observed for EuSe at about 145 kbar, but there was none for SmSe up to 200 kbar and for YbSe up to 250 kbar, Jayaraman et al. [3] also see the individual sections pp. 140,185, and 399. A pressure-induced valence transition is observed for SmSe and a compositionally induced valence transition in the case of TmSe (see p. 324), both retaining the cubic NaCl structure. 

At room temperature the pressure-induced phase transition in SmS differs from those in SmSe and SmTe in that it is discontinuous (55). It is not known whether it remains discontinuous at T=0. It was pointed out by Davis (56) that the two modes of behavior could be explained by assuming different mechanisms for the transition, i.e. an f6 - f5d delocalization in SmS and a simple Se(4p)—Sm(55) band-gap closing in the selenide and telluride. This suggestion was supported by APW calculations of the band structure of the Sm monochalcogenides. In the case of SmS the 4/states were found to lie in the band gap, but in SmSe and SmTe they were located below the p-valence bands,... 

Cerium metal is discussed in ch. 4 and only a brief mention of its high pressure behavior will be made here (for references see the list in ch. 4). Cerium can exist at atmospheric pressure in the fee (y) or dhep (iS) form and undergoes an isostructural transition near 100 K to another fcc-form referred to as o-Ce. The y-a Ce transition occurs at 7 kbar at room temperature and this transition is accompanied by about 8% volume decrease. This is one of the most widely studied transitions as a function of pressure and temperature and is believed to involve a valence change from 3 towards a higher valence state (3.7 ). The y to a transition line terminates at a critical point the very first example in which a solid - solid transition was shown to exhibit a liquid-vapor-like critical point. A pressure-induced phase transition near 50 kbar, initially reported to be yet another isostructural transition has been shown to be from fee (a-Ce) to an orthorhombic phase with the a-U structure. Stager and Drickamer (1964) have reported a pronounced resistance anomaly near 120 kbar indicative of a phase transition, but the nature of this transition is unknown. The fusion behavior of Ce is again unique in that it exhibits a minimum. 

SmSe exists, like SmS (see Rare Earth Elements C 7, 1983, p. 221), in a semiconducting form with divalent Sm and a metallic form with Sm in the intermediate valence state between two and three. Both forms have a cubic NaCl structure, but the metallic SmSe has smaller lattice constants. In contrast to SmS, the pressure-induced phase transition of SmSe is continuous. Only Singh et al. [1, 2] report the existence of hexagonal SmSe films. 

An interesting aspect of Au oxidation states is provided by the investigation of the pressure-induced transition from the mixed-valence state of Au(l)/Au(lll) to the single valence state of Au(ll) as described for M2[Au(l)X2][Au(lll)X4] (M = Rb, Cs X = Cl, Br, 1) [385, 386]. The valence states of Au(l) and Au(lll) at ambient pressure were clearly distinguishable. With increasing pressure, the doublets gradually increase their overlap. Finally, the Au Mbssbauer spectrum of CS2AU2I6 shows, at 12.5 GPa of applied pressure, only one doublet which was associated with Au(II). 

Other Papers.—Various iron species prepared by the vacuum pyrrolysis of acetyl-ferrocene-furfural resins at 400°C have been studied by Mossbauer spectroscopy. These consist of an amorphous glass-like carbon matrix containing free iron atoms, Fe+ ions, iron clusters, superparamagnetic iron, and ferromagnetic iron.333 The effect of pressure of up to 50kbar on the absorption spectra of five iron(m), two iron(n) and one mixed valence compound has been studied. In six of the compounds, but not in basic ferric acetate or soluble Prussian Blue, the observed pressure-induced bands were assigned to d-d transitions of converted iron(n) for the ferric compounds and to spin-forbidden d-d bands for the ferrous compounds. The charge-transfer band from iron(n) to iron(m) in soluble Prussian Blue showed a blue shift at pressures up to 7.2 kbar.334... 

In Table 5, the calculated and available experimental data are collected for the isostructural pressure transitions in Yb chalcogenides. The general trends are reproduced by the calculations. Only for the case of YbO, the calculated transition pressure seems to be significantly too high. In the intermetallic YbAls compound, pressure induces a continuous increase of valence (Kumar et al., 2008), which is in good agreement with the calculated rate of depletion of the 14th f-band in the SIC-LSD calculations. 

High pressure has proven to be an extremely powerful variable for the study of electronic phenomena in the solid state. Modern solid state models can be tested by their ability to account for the phenomena seen as function of pressure. Since the pioneering high-pressure studies in the optical region, the term pressure tuning (Drickamer 1982) describes the experimentally controlled volume dependence of electronic levels. Pressure in the lanthanides induces mixed valence or valence transitions, induces heavy fermion supraconductivity in other words high pressure... 

Independent of the conditions inducing the valence transition (pressure, temperature or chemical alloying) the valence numbers observed just after the first-order transition, converge to u = 2.6 0.05. The theoretical answer to why just the ratio 3/2 of the two valence state populations stabilizes SmS at low volumes might be obtained in the framework of the thermodynamics of valence fluctuations (see e.g. Wohlleben (1984a,b)). 

---