Tetragonal Allotrope of
Group 14 Elements
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Abstract
Group 14 elements (C, Si, and Ge) exist as various stable
and metastable
allotropes, some of which have been widely applied in industry. The
discovery of new allotropes of these elements has long attracted considerable
attention; however, the search is far from complete. Here we computationally
discovered a tetragonal allotrope (12 atoms/cell, named T12) commonly
found in C, Si, and Ge through a particle swarm structural search.
The T12 structure employs sp<sup>3</sup> bonding and contains extended
helical six-membered rings interconnected by pairs of five- and seven-membered
rings. This arrangement results in favorable thermodynamic conditions
compared with most other experimentally or theoretically known sp<sup>3</sup> species of group 14 elements. The T12 polymorph naturally
accounts for the experimental <i>d</i> spacings and Raman
spectra of synthesized metastable Ge and Si-XIII phases with long-puzzling
unknown structures, respectively. We rationalized an alternative experimental
route for the synthesis of the T12 phase via decompression from the
high-pressure Si- or Ge-II phase