Low-energy tetrahedral polymorphs of carbon, silicon, and germanium

Abstract

Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching (AIRSS) approach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of P bam symmetry with 24 atoms in the unit cell was found to be a low energy, low-density metastable polymorph in carbon, silicon, and germanium. In silicon, Pbam is found to have a direct band gap at the zone center with an estimated value of 1.4 eV, which suggests applications as a photovoltaic material. We have also found a low-energy chiral framework structure of P 41212 symmetry with 20 atoms per cell containing fivefold spirals of atoms, whose projected topology is that of the so-called Cairo-type twodimensional pentagonal tiling. We suggest that P41212 is a likely candidate for the structure of the unknown phase XIII of silicon. We discuss Pbam and P41212 in detail, contrasting their energetics and structures with those of other group 14 elements, particularly the recently proposed P42/ncm structure, for which we also provide a detailed interpretation as a network of tilted diamond-like tetrahedra.

Phonons inSi24at simultaneously elevated temperature and pressure

“…3b) is nearly the same as for the phonons in diamond cubic Si, measured by inelastic neutron scattering. At ambient temperature, the pressure shift of the A (6) g mode of Si 24 is approximately the same as the pressure shift of the single Raman mode of diamond cubic Si 12 , although these authors also report some negative Grüneisen parameters for two-phonon Raman features.…”

“…The vertex interactions that give the strengths of phonon-phonon interactions may also change with ∆P . 3g vibrational mode involves more bending of interatomic bonds, whereas bond stretching dominates most other modes such as A (6) g . It is typical for bond stretching in solid crystals to be associated with positive Grüneisen parameters and positive thermal expansion 27 .…”

“…Long-standing questions for silicon remain relevant, however, particularly with regard to overcoming intrinsic limitations of the fundamental indirect band gap. The integration of optical and electronic function on a single silicon wafer has been of enduring interest to the semiconductor industry [1][2][3][4] , for example, and many recent calculations suggest metastable, low-energy forms of silicon with properties that could potentially accommodate this possibility (see for example [5][6][7][8][9] ). Recently, a new allotropic modification of silicon was produced using a high-pressure precursor method 10 .…”

Novel silicon phases and nanostructures for solar energy conversion

“…13 Recent ab initio random structure searches identified solids with P4 2 =ncm 14 and P4 1 2 1 2 (Ref. 15) symmetries as possible candidates for the elusive Si-XIII phase, and a new low energy structure (Pbam) with an estimated direct gap of 1.4 eV. 15 Another recent study identified a dynamically and thermally stable hexagonal phase, named h-Si 6 , with a direct band gap of 0.61 eV.…”

Ab initio prediction of superdense tetragonal and monoclinic polymorphs of carbon

“…Carbon crystals share many structural similarities with silicon and germanium because of their common s 2 p 2 valence electron configuration 23,24 , thus it is instructive to compare the structural form and relation among various polymorphs of these elements. Silicon and germanium crystallize in cubic diamond structure at ambient conditions; they transform to the β-Sn phase at high pressure and turn into the BC8 and R8 Si and ST12 Ge phase upon decompression [25][26][27] .…”