Osmium-copper alloys

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

The reason for it is not obvious since gold is not a very rare element on earth, and other metals, for example, platinum, rhodium, osmium, and rhenium, are less abundant and more expensive. Its yellow color cannot be the reason either, since other metals, such as copper, and its alloys as bronze or brass, have different colors from the bright silver of most of the metals. Probably, the reason resides in its noble character. In fact, gold does not tarnish with time, and coins and jewelry remain indefinitely unalterable even after long exposure to extremely aggressive conditions. 

Ruthenium, osmium, rhodium, iridium, palladium and platinum are the six heaviest members of Group VII1. They are rare elements platinum itself is the commonest with an abundance of about 10-6% whereas the others have abundances of the order of 10"7 % of the earth s crust. They occur in Nature as metals, often as alloys such as osmiridium, and in arsenide, sulfide and other ores. The elements are usually associated not only with one another but also with the coinage metals copper, silver and gold. The main suppliers are South Africa, Canada and the USSR. 

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