Bronzes, crystal structure

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

The compound K0 3NbF3 has an average niobium valency of 2.7 and forms a crystal structure that is referred to as hexagonal tungsten bronze [239]. 

LiNbC>3 type crystals and compounds that crystallize in a tungsten bronze-type structure. 

Although the band model explains well various electronic properties of metal oxides, there are also systems where it fails, presumably because of neglecting electronic correlations within the solid. Therefore, J. B. Good-enough presented alternative criteria derived from the crystal structure, symmetry of orbitals and type of chemical bonding between metal and oxygen. This semiempirical model elucidates and predicts electrical properties of simple oxides and also of more complicated oxidic materials, such as bronzes, spinels, perowskites, etc. 

Molybdenum and Tungsten Bronzes.—The results of the majority of studies of studies of new bronze phases are summarized in Table 9. Sno.2W03 and Sno.3 WO3 have been prepared and their crystal structures determined. Whether they are bronzes is debatable since they do not involve a host lattice of WO ... 

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and... 

Andersson, S. 1965. The crystal structure of a new silver vanadium oxide bronze, Agi xV205 (x. approx. 0.32). Acta Chem. Scand. 19 1371-1375. 

Crystallization of 2-(2-thienyl)pyrroles 125 produced gold-like and bronze-like metallic crystals <02BCJ2359, 02T10233>, while structurally related derivatives produced red-violet metallic crystals <02T10225>. A crystal structure analysis of 2,5-bis(4-biphenylyl)thiophene and the effect of structure on optical properties has been reported <02AM498>. 

For the low ac-value sodium tungsten bronzes (x < 0.5), the crystal structure and x values of the crystals obtained depended strongly on the temperature of the melt. The size and homogeneity of the crystals were dependent on both temperature and electrode current. The best crystals were obtained at the lowest temperature at which they could be grown. The optimum current for best quality crystals depended upon x value and crystal structure. 

Single-crystal X-ray structures for Ag FBFfviolet) and AgFBF bronze). A structure determination was carried out on both the needle form violet crystals of AgFBF4 and the roughly octahedral-morphology... 

Disilver fluoride is a bronze-colored compound with a greenish cast when observed in bulk. It is an excellent electrical conductor. Crystal-structure determination3 shows the complete absence of elemental silver and silver(I) fluoride in the pure material and reveals the presence of successive layers of silver, silver, and fluorine in the lattice. The silver-silver distance is 2.86 A. (nearly twice the metallic radius of 1.53 A.), and the silver-fluorine distance is 2.46 A. [as in ionic silver(I) fluoride]. The compound is regarded as being intermediate in structure between a metal and a salt.4... 

Alloy systems have been known to man since the Bronze Age. It is, however, only in recent times that they have been the subject of systematic studies, and in these studies no tool has proved more powerful than the technique of crystal structure analysis. Indeed, the extension of our knowledge and understanding of the properties of intermetallic systems to which it has given rise is one of the greatest achievements of crystal chemistry. Prior to the application of X-ray methods, the investigation of the properties of alloy systems was confined principally to observations of their behaviour in the liquid state, and the behaviour of the metal as a solid could be determined only by inference from these observations. Transitions in the solid state and the effect of mechanical or heat treatment could not, of course, be observed in this way, and for information on these properties the microscope and other purely physical methods had to be invoked. Even so, these methods were all more or less indirect, and it is only since the application of X-ray analysis that it has been possible to investigate directly in the solid state, under the precise conditions which are of technical interest and without damage to the specimen, the exact positions of all the atoms in the structure, and so to refer to their ultimate cause the physical and chemical properties of the alloy. 

Some of the non-ferrous metals and alloys have already been mentioned such as copper and brass. Archaeological bronze is an alloy of copper and tin. The composition of these alloys ranges typically from 3% to 14% tin together with trace impurities such as lead and iron, depending on the chemical content of the original ores. Alloys with atin content, up to 6%, were capable of being cast and subsequently hammered into their final shape. This is due to the tin being soluble in the copper crystal structure, which allows the alloy to be deformed at room temperature. 

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