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Exposure time (minutes)

Fig. 14.1. Dependence of the resistivity of a K^O, film on exposure time to a K molecular beam in ultrahigh vacuum (UHV) at ambient temperature near 74°C. For this particular sample only the end point stoichiometry was determined explicitly; the stoichiometry at the minimum was determined from other similarly prepared samples. The arrows indicate changes in the resistivity of a similar sample, as it was heated from 60°C to 134°C. The dashed curve is a model fit to the data points [14.5], at x = 4 and a pronounced maximum at x = 6. Overdoping beyond x = 6 leads to a metallic overcoat on the sample but no further uptake of K or Rb into the crystal lattice.

Somewhat similar behavior is shown in Fig. 14.2 for several alkali metal dopants where the resistivity is plotted against alkali metal concentration x in MxC60 for M = Na, K, Rb, and Cs as determined by photoemission spectroscopy [14.3]. Furthermore, even at the minimum resistivity in M^Qo, the value of p found in Fig. 14.2 for KjC«, (2.5 x 10~3 fl-cm) is high, typical of a high-resistivity metal [14.5]. Conduction is believed to occur by charge transfer from one C60 molecular ion to another along (110) directions through the weak overlap of the wave functions on adjacent C60 anions, perhaps enhanced by a Jahn-Teller distortion of the molecule to lower the degeneracy of the ground state (see §14.2.4).

Fig. 14.2. p(x) for thick films of CM doped with Na, K, Rb, and Cs. Points indicate where exposure to the alkali-metal source was stopped and x-ray and ultraviolet photoemission spectra were acquired to determine the concentration x. The labels indicate the known fulleride phases at 300 K. The minima in p(x) occur for stoichiometrics corresponding to Na2C60, K3CMI, RbjQo, and Csj^C^. Structure in p(x) can be associated with the development of different stable phases [14.3].

Concentration (x)

Fig. 14.2. p(x) for thick films of CM doped with Na, K, Rb, and Cs. Points indicate where exposure to the alkali-metal source was stopped and x-ray and ultraviolet photoemission spectra were acquired to determine the concentration x. The labels indicate the known fulleride phases at 300 K. The minima in p(x) occur for stoichiometrics corresponding to Na2C60, K3CMI, RbjQo, and Csj^C^. Structure in p(x) can be associated with the development of different stable phases [14.3].

Consistent with band structure studies [14.9], nuclear magnetic resonance (NMR) Knight shift studies of 39K [14.10] indicate that the charge transfer from the alkali metal dopants of the CWI is complete, so that the alkali metal ions do not participate in electrical conduction. From the charge transfer and the stoichiometry of the compound, the carrier density can be inferred. Then assuming three electrons per C^q anion in K3C50, the carrier density is expected to be ~ 4 x 1021/cm3, which is quite low for conducting systems. Very weak structure is observed (Fig. 14.2) in the p(x) data for K^Qq at x = 4 [14.3], suggesting that I^Q,, might form as a minority phase in this experiment. Whereas the dependence of the room temperature resistivity on alkali metal concentration is similar for potassium and rubidium (including the magnitudes of pmin in Fig. 14.2, which are 3.1 x 10"3 il-cm for K3C60 and 2.8 x 10~3 il-cm for Rb3C60), the behavior of p(x) is qualitatively different in three respects for sodium and cesium doping: the magnitude of pmin, the value of x where pmin occurs, and the shape of the p vs. x curves. The stable phases for the M^Qq compounds are indicated on the figure. At all other x values, measurements are made on multiphase samples [14.3].

Of the various stoichiometrics and alkali metal species for the bulk compounds indicated on Fig. 14.2, only the compounds close to the KjQq and 60 stoichiometry (see Table 14.1) exhibit metallic temperature coefficients of the resistivity (see §14.1.2). It is significant that metallic conductivity occurs over a very narrow stoichiometry range near x — 3 in KXC60

Table 14.1

Table 14.1

Quantity

K^C^o

RbjQo

Reference

Space group

Fm33

Fm33

[14.10]

CM—Qo distance (Â)

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