Pressure dependence band structure, transition metals

The assumption that the carriers are small or intermediate polarons in no way militates against discussions of ths band structure of the ground state (see e.g. Camphausen et al. 1972, Cullen and Callen 1971,1973). The absence of Jahn-Teller distortion (Goodenough 1971) also, in our view, indicates not the absence of a polaron mass-enhancement but rather a value.of V0jB not too far from the critical value. These conclusions seem to be in agreement with the considerations of Sokoloff (1972), who used a description in terms of a degenerate band of small polarons. Samara (1968) showed that pressure lowers the temperature of the Verwey transition. If this depended only on e2/ ca then the opposite should be the case. But pressure will increase B, and push the substance nearer to the critical value for the metal-insulator transition. 

Experimentally, a metal-insulator transition has been observed between 8 and 12 kbar in Rb4C60 through NMR [44]. This is illustrated in Fig. 17 by the temperature dependence of 1IT1 for different pressures. While the behavior at 1 bar is dominated by an activated component, very similar to the one of K4C60 in Fig. 14, it gradually evolves towards a linear behavior, which is characteristic of a metal. This is the so-called Korringa law where the slope is proportional to n(E )2. Remarkably, the pressure needed to close the gap is quite modest, consistent with the idea that the indirect gap in the band structure could indeed be quite small. To date, this is the only report about such a transition. 

The relation between the observed structural sequence in the lanthanides and the d-band occupancy can be examined qualitatively using the above values of the number of occupied d-electrons. The study of the pressure-induced structural transition of Lu above (Min et al. 1986c) shows that the dependence of the d-band oecupaney on the structure is small at the given lattice constant the differences obtained between struetures are only 0.02 or 0.03 electrons. Hence it is possible to relate, within a small uneertainty, the observed structures in the lanthanide metals with the absolute values of the d-band occupancy obtained in the fee structure. From fig. 6, the resulting ranges of d-band occupancy (JV ) for the stable structures are < 1.7 for hep and 1.75 [Pg.178]

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