Alkali metals stoichiometry

Fig. 11. Composition dependence of the resistivity p x) for thick films of Cbo 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 stoichiometries corresponding to NaQCeo, K.iCeo and Cs,.,.Cfio[ll3].

The Bom-Haber cycle is also useful in examining the possibility of forming alkali-metal halides of stoichiometry MX2- The dominant term will clearly be the very large second-stage... 

Some of the alkali metal-group 15 element systems give compounds of stoichiometry ME. Of these, LiBi and NaBi have typical alloy stmc-tures and are superconductors below 2.47 K and 2.22 K respectively. Others, like LiAs, NaSb and KSb, have parallel infinite spirals of As or Sb atoms, and it is tempting to formulate them as M+ (E )" in which the (E )" spirals are iso-electronic with those of covalently catenated Se and Te (p. 752) however, their metallic lustre and electrical conductivity indicate at least some metallic bonding. Within the spiral chains As-As is 246 pm (cf. 252 pm in the element) and Sb-Sb is 285 pm (cf. 291 pm in the element). 

In addition to the various complex fluoro-antimonate(in) salts M SbF 4 and M SbFs mentioned above, the alkali metals form complexes of stoichiometry M ShiF , M SbyFio and M Sb4Fj3, i.e. [SbFj (SbF ),) (n = 1, 2, 3)... 

However, simple stoichiometry often conceals structural complexity as in the many alkali metal tellurides MTe (jc = 1, 1.5, 2.5, 3,... 

When the reaction is performed at relatively low temperatures that prevent strong thermal decomposition of the alkali metal carbonate, the formation of C02 will be related only to the reaction and will indicate the stoichiometry of the process. Fig 8 presents mass loss isotherms of Nb02F - M2CO3 mixtures (in which M - Li, Na, K, Rb, Cs) that were subjected to thermal treatment in air at 850°C [84, 85]. It is important to mention that parallel experiments performed without the addition of Nb02F, resulted in alkali metal carbonate mass losses that were in the same order of magnitude as the measurement errors at temperatures below 850°C. 

Compounds of the same stoichiometry type usually have the same type crystal structure within the row of alkali metals K - Rb - Cs rarely the same type structure with sodium-containing analogues and never ciystallize similarly with lithium-containing compounds. The crystal structure analysis of different fluoride and oxyfluoride compounds clearly indicates that the steric similarity between all cations and tantalum or niobium must be taken into account when calculating the X Me ratio. 

First reported by Fredenhagen in 1926 F3, F4), the graphite-alkali-metal compounds possess a relative simplicity with respect to other intercalation compounds. To the physicist, their uncomplicated structure and well defined stoichiometry permit reasonable band-structure calculations to be made S2,12) to the chemist, their identity as solid, "infinite radical-anions frequently allows their useful chemical substitution for such homogeneous, molecular-basis reductants as alkali metal-amines and aromatic radical anions N2, B5). 

A number of synthetic procedures are available (Ai2). (2) For precisely defined stoichiometries, the isobaric, two-bulb method of Herold is preferred H5, H6, H2). (2) To generate compounds suitable for organic synthesis work, graphite and alkali metal may be directly combined, and heated under inert gas (Pl, lA). (5) Electrolysis of fused melts has been reported to be effective iN2). 4) Although alkali metal -amine solutions will react with graphite, solvent molecules co-inter-calate with the alkali metal. Utilization of alkali metal-aromatic radical anion solutions suffers the same problem. 

A selection of frequent stoichiometries and common structural types found within the binary intermetallic compounds of the alkali metals is given in Table 5.6. 

Selenides, tellurides andpolonides. Se, Te and Po react easily with most metals and non-metals to form binary compounds (selenides and tellurides are common mineral forms of these elements). Non-stoichiometry is frequently observed in the compounds with the transition elements many of these compounds may be described as metallic alloys. The compounds of the metals of the first two groups may be considered the salts of the acids H2Se, H2Te, etc. The alkali metal selenides... 

The discovery of superconductivity [105] in alkali metal doped attracted the attention of a broad cross-section of the scientific community. The composition of the first alkali metal doped fullerene was determined to be [106,107]. In this compoimd, the transition to a superconducting state occurs at = 19.3 K [106]. In a short time, a large variety of alkali metal fullerides M Cgo have been synthesized and studied with respect to their solid-state properties [1, 106-114]. Thereby, the stoichiometries of as well as the alkali metal with M = Na, K, Rb, Cs have... 

This direct method is a convenient route, especially for saturation-doped fullerides MgCgo-A facile control of the alkali metal fulleride stoichiometry of M Cgg (M = K,... 

Rb, Cs) can be obtained using vapor-transport techniques [115]. Thereby, a weighed amoimt of Cjq is treated with a large excess of the alkali metal at 225 °C under vacuum. This procedure leads to the saturation-doped products MgC g. To these fullerides a specific amount of Cjq is added to give the desired stoichiometry. The process of the formation of is completed after a subsequent annealing... 

Alkali metal doping of Cjq is also possible by solution-phase techniques [1,118-121]. K CgQ and Rb CgQ containing small fractions of the superconducting M3C50 phases were prepared by allowing toluene solutions of Cjq to react with the alkali metal [118, 119]. During the reaction, the alkali metal fullerides form a black precipitate. In another example, sonication of a solution of Cjq and excess potassium in TMEDA yields K3C5q(THF)j4 with a defined stoichiometry [104],... 

The ultraviolet cutoff or the absorption edge for pure vitreous silica is 8.1 eV or 153 nm (171). This uv cutoff is influenced by the impurity level and stoichiometry of the material. Several impurities, such as the transition metals (Fe, Cu, Ti, etc) and alkali metal ions (Na, Li, K), degrade the ultraviolet performance, shifting the uv cutoff to longer wavelengths. Ferric ions (Fe3+) cause absorption or result in network defects under reducing conditions. This contaminant at only a few ppm can be detected as an absorption at 230 nm and below (176). 

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