Bronzes and related compounds

Structures built from ReOa blocks (a) TiNb24062. (b) high-Nb20s. Bronzes and related compounds 

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D. J.M. Bevan, P. Hagenmuller, Nonstoichiometric Compounds Tungsten Bronzes, Vanadium Bronzes and Related Compounds. Pergamon, 1975. 

Perovskites and related compounds also have a three-dimensional structure. In perovskites of formula ABOj, the octahedra of BOj lie on a cubic lattice, and are joined at the corners. Between these octahedra are large sites for the A atoms. In ReOj, the A atoms are missing, so guests can be added to the A positions. Because adjacent octahedra are joined together by only one oxygen they can rotate relative to one another, changing the shape of the A site. In LijReOj, the rotation splits the large A sites into two smaller sites more suitable for Li ions (Cava et al, 1982). Bronzes of WO3 also have a perovskite structure. 

Lithium vanadate bronzes are intercalated compounds with potential applications for lithium battery technology, since Li can be reversibly inserted into these stmctures by electrochemical reaction. Li NMR has been used to study the stmcture of 7-Lio.95V205 (Cocciantelli et al. 1992) and a series of related bronzes LixV205 (Cocciantelli et al. 1992a). The Li NMR spectrum of the -y-phase indicates the presence of a single Li site, but as the Li content is increased beyond x = 1, new lines can be resolved, corresponding... 

So characterized, extension becomes much more matter-like. To see this, consider a less remote kind of matter—the bronze of a statue. Neither the shape of the statue nor the statue itself can be said to occupy the bronze. Instead, the bronze, to use a word that is difficult to define adequately, constitutes the statue. One mark of this fact is that the statue, as well as its shape, is located wherever the bronze is, and the bronze is located wherever the statue is. Hence, if extension is the matter of a hylomorphic compound, then it, like the bronze, must constitute the composite it is the matter of. And as a result, it must go where the composite goes. Or, more accurately, the relations that hold between regions of extension must not be fixed but must instead be capable of changing over time so as to explain the differing relations between the material objects in question. 

Many of the compounds formerly regarded as derivatives of the TiOf ion have structures related to the tungsten bronzes (Figure 21-2). Typically, in the unit cell of SrTi03 there is a Ti atom at each corner, an oxygen at each face center and a Sr atom at the body center. 

Metal alloys were first used in the Bronze Age (1,400 B.C.-O B.C.), where serendipity led to the discovery that doping copper with other compounds drastically altered the physical properties of the material. Artifacts from the Middle East dating back to 3,000 B.C. are found to consist of arsenic-doped copper, due to the wide availability of lautite and domeykite ores, which are rich in both arsenic and copper. However, due to arsenic-related casualties, these alloys were quickly replaced with tin-copper alloys (bronze) that were widely used due to a lower melting point, higher hardness, and lower brittleness relative to their arsenic forerunner. 

The bronze alloys were exposed at the same time and place as the zinc during the period 1958-1978, with measurements at 2, 7 and 20 year exposures. Examination of the corrosion rates revealed two separate patterns a marine trend of high initial corrosion rate with sharp reduction in rate after the first few years, and an industrial-rural trend correspond to different chemical mechanisms at work, with only the industrial-rural corrosion being related to sulfur compounds. 

A still more extreme example shows that the perovskite structure can even occur with some of the A sites unoccupied. Sodium tungsten bronze has the ideal composition NaWOs, with the perovskite structure, but this compound shows very variable composition and colour, and is better represented by the formula Na WOg with 1 > x > 0. In the sodium-poor varieties the structure remains essentially unaltered but some of the sites normally occupied by sodium are vacant. To preserve neutrality one tungsten ion is converted from W5 to W6+ for every site so unoccupied, and this change in ionization gives rise to the characteristic alteration in colour and explains its association with the sodium content. In the extreme case, when no sodium is present, we have W03, the structure of which is closely related to that of A1F3. We have already shown how this structure, in its turn, is related to that of perovskite. 

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