User:TCO/Sandbox/Solid fluorine



The solid state of fluorine relies on Van der Waals forces to hold molecules together,(ref) which, because of the small size of the fluorine molecules, are relatively weak. Consequently, the solid state of fluorine is more similar to that of oxygen or the noble gases than to those of the heavier halogens.(ref) Solid fluorine has two phases, a lower temperature alpha structure and a higher temperature beta structure.

Fluorine solidifies at −219.6 °C (−363.3 °F) into a cubic (high symmetry) structure, called beta-fluorine. This phase is transparent and soft, with significant disorder of the molecules. At −227.5 °C fluorine undergoes a solid—solid phase transition into a monoclinic (low symmetry) structure called alpha-fluorine. This phase is opaque and hard with close-packed layers of molecules. The solid state phase change requires more energy than the melting point transition and can be violent, shattering samples and blowing out sample holder windows. In general, fluorine's solid state is more similar to that of oxygen (which has a monoclinic low-temperature structure and similar temperature alpha—beta transition) than to those of the other halogens.

Solid fluorine received significant study in the 1920s and 30s, but relatively less until the 1960s. The crystal structure of alpha fluorine, which still has some uncertainty, dates to a 1970 with a paper by Linus Pauling.



Further readings
(could perhaps use some of these as refs)

http://www.osti.gov/bridge/servlets/purl/4010212-0BbwUC/4010212.pdf (phase diagrams of the elements)

http://jcp.aip.org/resource/1/jcpsa6/v47/i2/p740_s1?isAuthorized=no (sample holder blowout)

NASA ADS: Solid Fluorine and Solid Chlorine: Crystal Structures and Intermolecular Forces by S. C. Nyburg

__NOINDEX__