Structures of Non-Metals

Rare gas solids can be considered as the simplest nonmetal systems. However, the high pressure polymorphism in solid helium is surprisingly complex for a simple system. Helium crystallizes only under pressure in a/cc structure and has a small range of stability of a bcc structure near 3 x 10 GPa and 1.5 K. X-ray diffraction measurement showed that He crystallizes at 11.5 GPa and 298 K in a hep structure. This modification is the only solid phase observed at room temperature to 23 GPa. Single-crystal X-ray studies of Ne and Ar showed the stability of the fee phases of these elements up to 110 and 80GPa, respectively [37, 38]. However, in Ar at P 50 GPa a hep modification appears and its concentration increases as pressure grows the fee hep transition completes at P 300 GPa. Kr starts a similar transition at 3.2 and completes it at ea. 170 GPa. Xe also starts a fee hep transformation 

At 21 GPa iodine transforms into the metallic structure of a bco type (iodine-II, a1 = 4h-8) [48], which on further compression continuously approaches bet, reaching this phase at 43 GPa (iodine-111), with the further equalization of interatomic distances. The axial ratio c/a of a ( ct-phase continuously approaches 1 and at 55 GPa there occurs a first-order phase transition into a fee structure (iodine-IV) which is stable up to 276 GPa [49, 50]. Bromine at 80 GPa transforms into a bee lattice. However, the onset of the metallic behavior takes place in the direction perpendicular to the layers via a progressive gap closure and at a pressure lower than the dissociation pressure (13 GPa for I2 and 25 GPa for Br2). In the layered direction the onset of a metallic behavior was also observed, but at a higher pressure [51]. Takemura et al. [52] discovered that between I and II phases there exists a new intermediate phase (iodine-V), using He as the pressurizing medium to obtain the pure hydrostatic compression and specify the pressure of the first transition (23.2-24.6 GPa). They characterized the new phase (a/cf type, iodine-V), and the phase II (at P = 25.6 —30.4 GPa). Reduction of volume at transitions I V is equal to 2.0 % and at V II only 0.2 %. Recently a new phase of solid bromine was revealed [53] at P 80 GPa by Raman scattering experiments. This phase was found to be the same as the iodine-V with an incommensurate structure, discovered by Takemura et al. In 

3 The crystal structure of E-oxygen at 17.6 GPa (bond distances in A). Reprinted from [56] by permission from Macmillan Publishers Ltd, Copyright 2006 

Structural changes in tellurium under pressure are similar [65], but the phase transition pressures are essentially lower than those in selenium and sulfur, viz. for the Group 16 elements (in GPa) [66]  

Since numerous attempts to realize the transition of a molecule into a monatomic metal in F2, CI2, O2, N2 have not succeeded, it makes sense to use for these transformations the ability of some solids to absorb 100- and even 1,000-fold volumes of such gases. Another approach was suggested by Ashcroft [69], who noted that CH4, SiH4, and GeH4 might become metallic at lower pressure since hydrogen is chemically pre-compressed by the Group 14 atoms. 

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Crystal data summarized first are those characteristic of structures of metallic elements, typically having highly symmetric and dense atomic arrangements. Only a few notes are reported for the close-packed structures (Mg, Cu types), since for these structures several details are presented in 3.7.6 and 3.9.2.I. Subsequently, particular structures observed for a few selected specific metals and, finally, a few typical structures of non-metallic elements are described. 

Fig. 33.12. The relation between the monoclinic CaSb2-type structure of non-metallic EuSb and the orthorhombic ZrSii-type structure of metallic YbSbi. Stippled spheres cations.

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