Silicon
Framework-Based Lithium Silicides at High
Pressures
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Abstract
The bandgap and optical properties
of diamond silicon (Si) are
not suitable for many advanced applications such as thin-film photovoltaic
devices and light-emitting diodes. Thus, finding new Si allotropes
with better bandgap and optical properties is desirable. Recently,
a Si allotrope with a desirable bandgap of ∼1.3 eV was obtained
by leaching Na from NaSi<sub>6</sub> that was synthesized under high
pressure [<i>Nat. Mater.</i> <b>2015</b>, <i>14</i>, 169], paving the way to finding new Si allotropes. Li
is isoelectronic with Na, with a smaller atomic core and comparable
electronegativity. It is unknown whether Li silicides share similar
properties, but it is of considerable interest. Here, a swarm intelligence-based
structural prediction is used in combination with first-principles
calculations to investigate the chemical reactions between Si and
Li at high pressures, where seven new compositions (LiSi<sub>4</sub>, LiSi<sub>3</sub>, LiSi<sub>2</sub>, Li<sub>2</sub>Si<sub>3</sub>, Li<sub>2</sub>Si, Li<sub>3</sub>Si, and Li<sub>4</sub>Si) become
stable above 8.4 GPa. The SiSi bonding patterns in these compounds
evolve with increasing Li content sequentially from frameworks to
layers, linear chains, and eventually isolated Si ions. Nearest-neighbor
Si atoms, in <i>Cmmm</i>-structured LiSi<sub>4</sub>, form
covalent open channels hosting one-dimensional Li atom chains, which
have similar structural features to NaSi<sub>6</sub>. The analysis
of integrated crystal orbital Hamilton populations reveals that the
SiSi interactions are mainly responsible for the structural
stability. Moreover, this structure is dynamically stable even at
ambient pressure. Our results are also important for understanding
the structures and electronic properties of LiSi binary compounds
at high pressures