Magnesium silicide (Mg
2
Si) undergoes a transformation from a cubic (fluorite) type lattice to a hexagonal lattice at pressures above about 25 kilobars and temperatures above 900°C. There is an increase of density of about 15 percent associated with this structure change. Both the parent and product substances are semiconductors, but of different resistivities. The high pressure phase may be recovered and is indefinitely metastable at normal ambient conditions.
Acknowledgment.-This work was supported by a (19) A . Schonberg and . 4 Mustafa, Chem R E D , 40, 181 (1947). ( 2 0 ) G. S H a m m o n d , W. P. Baker, and W. M. Moore, J . A m Chem. Soc., 83, 2795 (1961).The effects of silicon on the catalyzed diamond-forming reaction have been studied and are found to depend on the form and purity of the Si compound used to dope the reaction mix. Oxidation yielding Si02 or dissolution of Si in the catalyst nietal took place a t diamond-stable, high-pressure, high-temperature conditions. Diamond is found intergranularly in a coesite matrix, in the first case, if a SUiVdble excess of carbon is present. In the second case, some dimiond is found with S-containing S i c in a Si-Si acid-resistant matrix. Decomposable co~npounds of silicon, like CaSip, reduce diamond nucleation, slow growth, and promote the formation of rotation twins of diamond on (111). The crystals have very smooth (111) faces. and appreciable amounts of Ti arid CaO may be demonstrated to be present in some of these diamonds. The quantitative effect of Si upon nucleation and growth is not so pronounced as t h a t of either boron or aluminum, and i t is thus not possible to say t h a t Si is a cocatalyst for diamond formation when employed in conjunction with, e.g., nickel. Strong qualitative effects of these various impurities upon diamond-crystal color are observed, and these are discussed in the Appendix ( I S ) T h e existence of many of these species was in fact implied in the molecular beam experiments of S Millman.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.