Skutterudites

Figure 7.6. A filled. skutterudite antimonide crystal structure. A transition niclal atom (Fc or Co) at the centre of each octahedron is bonded to antimony atoms at each corner. The rare earth atoms (small spheres) are located in cages made by eight octahedra. The large thermal motion of rattling of the rare earth atoms in their cages is believed be responsible for the strikingly low thermal conductivity of these materials (Sales 1997).

Figure 13.2 The cubic structure of skutterudite (C0AS3). (a) Relation to the ReOs structure (b) unit cell (only sufficient Co-As bonds are drawn to show that there is a square group of As atoms in only 6 of the 8 octants of the cubic unit cell, the complete 6-coordination group of Co is shown only for the atom at the body-centre of the cell) and (c) section of the unit cell showing CoAsg octahedra comer-linked to form AS4 squares.

Tesseral-kies, m. skutterudite smaltite. -system, n. (Cryst.) isometric system. 

Other binary compounds include MAs3 (M = Rh, Ir), which has the skutterudite (CoAs3) structure [33] containing As4 rectangular units and octahedrally coordinated M. The corresponding antimonides are similar. M2P (M = Rh, Ir) has the anti-fluorite structure while MP3 has the CoAs3 structure. In another compound of this stoichiometry, IrSi3, 9-coordination exists for iridium. 

The crystal skutterudite, Co43+(Asi4 )3, contains As44- groups with a... 

In crystals such as pyrite, FeS2, marcasite, FeS2, skutterudite, CoAss, etc., each metal atom is connected by covalent bonds to... 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

Fig. 26 Skutterudite-type structure in terms of a framework of M-centred octahedra and b cubic arrangement of M atoms with Pn4 rings and dodecahedral cages filled with RE atoms in the ternary variants. Reprinted with permission from [110]. Copyright the American Chemical Society...

An attractive feature of applying XPS to study these skutterudites is that the valence states of all atoms can be accessed during the same experiment. As in the study of the MnP-type compounds, these types of investigations also provide insight into bonding character and its relation to electronegativity differences. This information is obtained by analysing both core-line and valence band XPS spectra. 

Other Compounds.—Skutterudite, CoASj, has cobalt octahedrally surrounded by six arsenic atoms, the latter forming rectangles. ... 

SKS-9/PC Skutterudite Skutterudites Skyprene Skyrrn [602-06-2] Sky rockets b-sky tan thine Slag... 

Tables 3.1, 3.2 and 3.3 compiled by Povarennykh (1963) specify the initial data accepted for the calculation of hardness from formulae (3.5) and (3.6). As the ratio WJWa increases, the coefficient a decreases (Table 3.1). For compounds with ratios inverse to those given in the table, i.e., for compounds having a so-called antistructure, the coefficient a will be exactly the same, e.g., 1/2 and 2/1. In both cases, x — 80. The link attenuation coefficient / varies over a relatively narrow range, usually between 0.7 and 1.0 (Table 3.2). This coefficient requires the state of lattice linkage to be considered in each case, and like coefficient a it depends on the type of compound involved. For various types of compounds, the values of the coefficient / may be lower taking as an example minerals in the pyrite and skutterudite group, they are as follows for compounds 2/2—0.60, for 3/3—0.48 and for 4/4—0.39. The values of the coefficient y grow proportionally with coordination number (Table 3.3). The constancy of the coefficient y depends on the constancy of the coordination number which is influenced by the valence ratio of electropositive and electronegative atoms. Lattice spacings, state of chemical bonds and electron-shell structure, and for complex compounds, also the degree of action of the remain-...

Skutterudite is found in moderate-temperature veins, commonly associated with other cobalt/nickel minerals, e.g., cobaltite and nickeline. The mineral was named for its occurrence at Skutterud, Norway. Important ore sources are Norway. Bohemia. Saxony, Spain. France, and New South Wales, Australia Notable occurrences are in Ontario. Canada, mainly Sudbury, South Lorrain, and Gowganda. 

Iron usually substitutes for some nickel and cobalt in skutterudite (Klein, 2002), 369. The arsenic in the crystalline structure of skutterudite occurs as AS4 rings (Cotton et al., 1999), 387. The rings are planar and rectangular with bond lengths of 2.464 0.002 A and 2.572 0.002 A at 22 °C (Mandel and Donohue, 1971). In skutterudite, each atom of cobalt or another divalent metal is surrounded by six arsenic atoms in a roughly octahedral formation (Mandel and Donohue, 1971). Chloanthite and smaltite are arsenic-deficient forms of nickel and cobalt skutterudite, respectively (Table 2.5). 

Mandel, N. and Donohue, J. (1971) The refinement of the crystal structure of skutterudite, CoAs3. Acta Crystallo-... 

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