Alkali suboxides

Clusters in alkali suboxides (a) Rb902 and (b) CS11O3. The shared faces are shaded. 

In addition to the above oxides M2O, M2O2, M4O6, MO2 and MO3 in which the alkali metal has the constant oxidation state 4-1, rubidium and caesium also form suboxides in which the formal oxidation state of the metal is considerably lower. Some of these intriguing compounds have been known since the turn of the century but only recently have their structures been elucidated by single crystal X-ray analysis. Partial oxidation of Rb at low temperatures gives RbeO which decomposes above —7.3°C to give copper-coloured metallic crystals of Rb902 ... 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

Simon, A. Structure and Bonding with Alkali Metal Suboxides. Vol. 36, pp. 81-127. 

Even the extremely electron-deficient alkali metals can form clusters when interstitial atoms contribute to their stabilization. Compounds of this kind are the alkali metal suboxides such as Rb902 it has two octahedra sharing a common face, and each is occupied by one O atom (Fig. 13.16). Flowever, the electron deficiency is so severe that metallic bonding is needed between the clusters. In a way, these compounds are metals, but not with single metal ions as in the pure metal Rb+e-, but with a constitution [Rb902]5+(e )5, essentially with ionic bonding in the cluster. 

Alkali and Alkaline Earth Metal Suboxides and Subnitrides... 

The alkali metal suboxides are well understood in terms of formation, structure, and properties, yet we still face some unsolved problems that will also be addressed in the following. In contrast, the first suboxide of barium came to light only recently [24], and one has to see whether this compound is the tip of another iceberg, as turned out to be the case with the subnitrides of alkaline earth metals. 

Table 3.1-1 summarizes the crystallographically characterized group 1 metal suboxides, and group 2 metal subnitrides, the latter containing additional alkali or other metals [25]. 

Tab. 3.1-1. Structurally characterized suboxides and subnitrides of alkali and alkaline earth metals, respectively, Z refers to the structural formula.

Fig. 3.1-3. Crystal structures of the binary alkali metal suboxides (a) [Rb902], (b) [Csnn03], (c) [Rb902]Rb3(Rb60), (d) [Csn03]Csio(Cs70), and (e) [Csn03]Cs(Cs40). O atoms drawn as small spheres, for clarity only M-M contacts in the clusters outlined.

Fig. 3.1-4. Crystal structures of the ternary alkali metal suboxides (a) [Csn03]Rb, (b) [Csn03]Rb2, (c) [Csn03]Rb7.

The structural principle of combining salt-like bonded units with stoichiometric amounts of alkali metal in the structure of Ba2ONa is equivalent to that which characterizes the alkali metal suboxides as well as the subnitrides of alkaline earth metals, such that a variation at least according to a general formula [Ba20 Na% can be expected. Indeed, there are indications of the existence of further suboxides in... 

The structural principle of alkali metal suboxides has been unique for many years. Early attempts to synthesize cluster-type subnitrides of Ba resulted in the only compound, Ba2N, that is isotypic with Ca2N. Beyond these, the Sr analogue Sr2N, and most recently Sr4N3 [34], have been characterized. 

All suboxides and subnitrides described in the preceding sections are metallic. In the case of the alkali metal suboxides this property has been demonstrated by measurements of the electrical conductivity [58], CS7O, for example, exhibits a free electron like behavior in the temperature dependence of its resistivity rather similar to the element Cs itself. The characteristic colors of the alkali metal suboxides have been mentioned before, and spectroscopic investigations to be discussed in the following provide a more quantitative access to the metallic properties and the underlying chemical bonding. 

Fig. 3.1-12. Single crystal reflectivities R of the alkali metals Rb, Cs and some of their suboxides (points) together with the Drude-Lorentz fits (lines).

---