Metallic behaviour

In any group of the periodic table we have already noted that the number of electrons in the outermost shell is the same for each element and the ionisation energy falls as the group is descended. This immediately predicts two likely properties of the elements in a group (a) their general similarity and (b) the trend towards metallic behaviour as the group is descended. We shall see that these predicted properties are borne out when we study the individual groups. 

Y. Nakamura, Y. Matsushima, T. Hasegawa, and Y. Nakatami, Metal Behaviour and Suface Engineering, IITT-Intemational, Goumay-sur-Mame, France, 1989, p. 187. 

This suggests an intrinsic metallic behaviour of the single CNTs. In this respect. Fig. 12 presents the intrinsic reflectivity (a) and optical conductivity spectra (b) of a hypothetical "bulk" (i.e., / = 1) CNTs specimen, using the parameters of Table 2. The low frequency metallic behaviour is easily recognised. (The reflectivity tends to 100 % when the frequency goes to zero and... 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

In compositions with excess sodium the situation changes again The Fermi level is now shifted up into the sp-band of sodium, yielding again a metallic behaviour like in the tin-rich cases. 

The next smaller ligand-protected nanocluster that was investigated by scanning tunneling spectroscopy (STS) was the four-shell cluster Pt309phen 36O20 [20,21]. The diameter of the Pt core is 1.8 nm, about a tenth of the former example. However, even here a Coulomb blockade could only be observed at 4.2 K, i.e. at room temperature the particle still has metallic behaviour. Since... 

Scoullos, M., M. Dassenakis, and C. Zeri. 1996. Trace metal behaviour during summer in a stratified Mediterranean system the Louros estuary (Greece). Water Air Soil Pollut. 88 269-295. 

As underlined by Miller el al. (2002) in a comprehensive review about structure and bonding around the Zintl border of the Periodic Table, the Nesper criteria imply that Zintl phases (as valence compounds) are generally point compounds and may have a semi-metallic behaviour. 

Figure 5.11. Connected binary phase diagrams of the actinides. The binary phase diagrams (temperature vs. composition) for adjacent actinide metals are connected across the entire series (two-phase regions are in black, uncertain regions in grey). The transition from typical metallic behaviour at thorium to complex behaviour at plutonium and back to typical metallic behaviour past americium can be noticed (adapted from Hecker 2000).

In this section we illustrate the effect of grain boundaries on the conductivity of thin hlms of the TTF-based molecular metals TTF-TCNQ and TTF[Ni(dmit)2]2-In general the existence of grain boundaries hinders the metallic behaviour, inducing semiconducting-like activated conduction (<7rt < 10 cm ) because... 

Suppose we approach M atoms of an element having an unfilled outer shell, disposed in a lattice. The point may be made clear if we suppose to approach hydrogen atoms (outer shell 1 s ). Equations (11) and (12) would predict the broadening of the electron state in a half-filled s-band, which should therefore allow metallic behaviour. Apparently, this would happen for any inter-atomic distance a and, therefore also at infinite distance. What would change is, of course, the bandwidth, which is determined by matrix elements dependent on the interatomic distance a but the metallic behaviour, depending essentially on the fact that the electrons have available energy states within the band, should occur also at distances where the atoms may well be supposed to be isolated. 

When the cores are approached, the sub-bands split, acquiring a bandwidth, and decreasing the gap between them (Fig. 14 a). At a definite inter-core distance, the subbands cross and merge into the non-polarized narrow band. At this critical distance a, the narrow band has a metallic behaviour. At the system transits from insulator to metallic (Mott-Hubbard transition). Since some electrons may acquire the energies of the higher sub-band, in the solid there will be excessively filled cores containing two antiparallel spins and excessively depleted cores without any spins (polar states). 

The data in Table 6.7 show the transition down the group from the non-metallic behaviour of boron, through the amphoteric A1 and Ga metals, to the metallic In and Tl. 

The corresponding As species in acid solution are less stable than those of P due to the effects of the 3d contraction. Further down the group, Sb and Bi show more metallic behaviour, with the two positive ions indicated in Table 6.11, and Sb shows amphoteric behaviour the + 3 oxide reacts with sodium hydroxide solution. 

From Group 15 to Group 16. non-metallic behaviour takes over completely with no positive ions being stable. The + 6 state of sulfur is seen to have very poor oxidizing properties, and it is only in its concentrated form, and when hot, that sulfuric(VI) acid is a good oxidant. Hot concentrated sulfuric acid oxidizes metallic copper and is reduced to sulfur dioxide. The relative stabilities of the Se species with positive oxidation stales are considerably less than their S or Te counterparts, another example of the effect of the 3d contraction. 

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