Superconductivity in Rare Earth Carbide Halides

The Structures of the layered carbide halides RE2X2C2 are closely related to those of the metallic hydride halides discussed in the previous subsection, except for the fact that Q units occupy the centers of the metal atom octahedra and the halogen atoms take different positions above and below the metal atom bilayers. Regardless, the strictly two dimensional character is preserved. 

The structures of the alkali metal suboxides have been discussed in terms of bare metal clusters stabilized by interstitial oxygen atoms. Due to the direct M-M bonding between the clusters, the suboxides are metallic and show electrical properties [220] which are similar to the free alkali metals. 

Thin composite layers of CS2O and Cs on Ag play an important role in the IR sensitive SI photocathodes which have been manufactured for approximately sixty years. [245, 246] The characteristic spectral response of such cathodes can be explained by allowing for the presence of alkali metal suboxides, CsnOj or CS3O , since these have the appropriate electronic properties. Thus, these materials have a low work function of approximately 1 eV (compared to 2 eV for elemental Cs) and low energies of the surface plasmons (1.5 eV for (IIS11O3). The enhancement of the photoelectric yield is due to surface plasmon decay. [247] 

The small value for the plasmon energy is simply related to the low carrier concentration and intimately linked with the electron poor nature of the alkali metals. The low work function has a more interesting explanation. Although it was shown earlier that the electrical properties of the alk i metals and their suboxides are very similar, this is not quite true. In elemental Cs, the conduction electrons are allowed to take up all the space between the atoms. In contrast, in a crystal of Cs,03 the conduction electrons meet a regular array of accumulated negative charge (6-) in each cluster center which is highly repulave for the con- 

5 Discrete and Condensed Transition Metal Chtsters in Solids 

---