Selenium The Middle Chalcogen

Inclusion of selenium in a book on fluid metals is justified by its status as a borderline metal at high temperatures and pressures. Also, as we have discussed in chapter 2, the dramatically different molecular structure of the low-temperature liquid and the vapor means that selenium is an excellent example of a system with strongly state-dependent interactions. Structural evolution of the liquid as it is heated to the region of the critical point must inevitably lead to interesting changes in the physical properties. 

X-ray diffraction results (Tamura and Hosokawa, 1992) for the liquid structure factor 5((2) and the pair correlation function g R) are reproduced in Figs. 5.1 and 5.2, respectively. These results exhibit, at low 

Liquid structure factor S(Q) for liquid selenium measured by x-ray diffi action at various temperatures and pressures (Tamura and Hosokawa, 1992). 

The definition of the first and second peaks is preserved reasonably well with increasing temperature and pressure up to about 1300 °C. However, above this temperature, both peaks broaden dramatically. The width of the first peak at the highest temperature suggests a distribution of first neighbor distances extending over at least 0.5 A (twice the low-temperature value) and the second peak is broadened to near invisibility. 

This is not unlike the high-temperature behavior of the pair correlations in monatomic fluids including the alkali metals and mercury discussed in previous chapters. 

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