24 research outputs found
A Stable, Magnetic, and Metallic Li<sub>3</sub>O<sub>4</sub> Compound as a Discharge Product in a Li–Air Battery
The
Li–air battery with the specific energy exceeding that of a
Li ion battery has been aimed as the next-generation battery. The
improvement of the performance of the Li–air battery needs
a full resolution of the actual discharge products. Li<sub>2</sub>O<sub>2</sub> has been long recognized as the main discharge product,
with which, however, there are obvious failures on the understanding
of various experimental observations (e.g., magnetism, oxygen K-edge
spectrum, etc.) on discharge products. There is a possibility of the
existence of other Li–O compounds unknown thus far. Here, a
hitherto unknown Li<sub>3</sub>O<sub>4</sub> compound as a discharge
product of the Li–air battery was predicted through first-principles
swarm structure searching calculations. The new compound has a unique
structure featuring the mixture of superoxide O<sub>2</sub><sup>–</sup> and peroxide O<sub>2</sub><sup>2–</sup>, the first such example
in the Li–O system. The existence of superoxide O<sub>2</sub><sup>–</sup> creates magnetism and hole-doped metallicity.
Findings of Li<sub>3</sub>O<sub>4</sub> gave rise to direct explanations
of the unresolved experimental magnetism, triple peaks of oxygen K-edge
spectra, and the Raman peak at 1125 cm<sup>–1</sup> of the
discharge products. Our work enables an opportunity for the performance
of capacity, charge overpotential, and round-trip efficiency of the
Li–air battery
Robust Diffusive Proton Motions in Phase IV of Solid Hydrogen
Systematic
first-principles molecular dynamics (MD) simulations
with long simulation times (7–13 ps) for phase IV of solid
hydrogen using different supercell sizes of 96, 288, 576, and 768
atoms established that the diffusive proton motions process in the
graphene-like layer is an intrinsic property and independent of the
simulation cell sizes. The present study highlights an often overlook
issue in first-principles calculations that long time MD is essential
to achieve ergodicity, which is mandatory for a proper description
of dynamics of a system. It is inappropriate to make arguments on
the analysis of MD results, which are far from ergodic. Furthermore,
we have simulated the vibrational density of states of phase IV based
on our proton diffuse model at a pressure range of 225–300
GPa, which is qualitatively in agreement with experimental data
Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides
The
oxidation state of an element influences its chemical behavior
of reactivity and bonding. Finding unusual oxidation state of elements
is a theme of eternal pursuit. As labeled by an alkali-earth metal,
barium (Ba) invariably exhibits an oxidation state of +2 by a loss
of two 6s valence electrons while its inner 5p closed shell is known
to remain intact. Here, we show through the reaction with fluorine
(F) at high pressure that Ba exhibits a hitherto unexpected high oxidation
state greater than +2 in three pressure-stabilized F-rich compounds
BaF<sub>3</sub>, BaF<sub>4</sub>, and BaF<sub>5</sub>, where Ba takes
on the role of a 5p element by opening up its inert 5p shell. Interestingly
enough, these pressure-stabilized Ba fluorides share common structural
features of Ba-centered polyhedrons but exhibit a diverse variety
of electronic properties showing semiconducting, metallic, and even
magnetic behaviors. Our work modifies the traditional knowledge on
the chemistry of alkali-earth Ba element established at ambient pressure
and highlights the major role of pressure played in tuning the oxidation
state of elements
Silicon Framework-Based Lithium Silicides at High Pressures
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 Siî—¸Si 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
Siî—¸Si 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 Liî—¸Si binary compounds
at high pressures
Gold as a 6p-Element in Dense Lithium Aurides
The
negative oxidation state of gold (Au) has drawn a great attention
due to its unusual valence state that induces exotic properties in
its compounds, including ferroelectricity and electronic polarization.
Although monatomic anionic gold (Au<sup>–</sup>) has been reported,
a higher negative oxidation state of Au has not been observed yet.
Here we propose that high pressure becomes a controllable method for
preparing high negative oxidation state of Au through its reaction
with lithium. First-principles calculations in combination with swarm
structural searches disclosed chemical reactions between Au and Li
at high pressure, where stable Li-rich aurides with unexpected stoichiometries
(e.g., Li<sub>4</sub>Au and Li<sub>5</sub>Au) emerge. These compounds
exhibit intriguing structural features like Au-centered polyhedrons
and a graphene-like Li sublattice, where each Au gains more than one
electron donated by Li and acts as a 6p-element. The high negative
oxidation state of Au has also been achieved through its reactions
with other alkali metals (e.g., Cs) under pressures. Our work provides
a useful strategy for achieving diverse Au anions
Gold with +4 and +6 Oxidation States in AuF<sub>4</sub> and AuF<sub>6</sub>
An important goal
in chemistry is to prepare compounds with unusual
oxidation states showing exciting properties. For gold (Au), the relativistic
expansion of its 5d orbitals makes it form high oxidation state compounds.
Thus far, the highest oxidation state of Au known is +5. Here, we
propose high pressure as a controllable method for preparing +4 and
+6 oxidation states in Au via its reaction with fluorine. First-principles
swarm-intelligence structure search identifies two hitherto unknown
stoichiometric compounds, AuF<sub>4</sub> and AuF<sub>6</sub>, exhibiting
typical molecular crystal character. The high-pressure phase diagram
of Au fluorides is rather different from Cu or Ag fluorides, which
is indicated by stable chemical compositions and the pressures needed
for the synthesis of these compounds. This difference can be associated
with the stronger relativistic effects in Au relative to Cu or Ag.
Our work represents a significant step forward in a more complete
understanding of the oxidation states of Au
Unexpected Trend in Stability of Xe–F Compounds under Pressure Driven by Xe–Xe Covalent Bonds
Xenon difluoride
is the first and the most stable of hundreds of
noble-gas (Ng) compounds. These compounds reveal the rich chemistry
of Ng’s. No stable compound that contains a Ng–Ng bond
has been reported previously. Recent experiments have shown intriguing
behaviors of this exemplar compound under high pressure, including
increased coordination numbers and an insulator-to-metal transition.
None of the behaviors can be explained by electronic-structure calculations
with fixed stoichiometry. We therefore conducted a structure search
of xenon–fluorine compounds with various stoichiometries and
studied their stabilities under pressure using first-principles calculations.
Our results revealed, unexpectedly, that pressure stabilizes xenon–fluorine
compounds selectively, including xenon tetrafluoride, xenon hexafluoride,
and the xenon-rich compound Xe<sub>2</sub>F. Xenon difluoride becomes
unstable above 81 GPa and yields metallic products. These compounds
contain xenon–xenon covalent bonds and may form intercalated
graphitic xenon lattices, which stabilize xenon-rich compounds and
promote the decomposition of xenon difluoride
Predicted Lithium–Boron Compounds under High Pressure
High pressure can fundamentally alter the bonding patterns
of light
elements and their compounds, leading to the unexpected formation
of materials with unusual chemical and physical properties. Using
an unbiased structure search method based on particle-swarm optimization
algorithms in combination with density functional theory calculations,
we investigate the phase stabilities and structural changes of various
Li–B systems on the Li-rich regime under high pressures. We
identify the formation of four stoichiometric lithium borides (Li<sub>3</sub>B<sub>2</sub>, Li<sub>2</sub>B, Li<sub>4</sub>B, and Li<sub>6</sub>B) having unforeseen structural features that might be experimentally
synthesizable over a wide range of pressures. Strikingly, it is found
that the B–B bonding patterns of these lithium borides evolve
from graphite-like sheets in turn to zigzag chains, dimers, and eventually
isolated B ions with increasing Li content. These intriguing B–B
bonding features are chemically rationalized by the elevated B anionic
charges as a result of Li→B charge transfer
Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth-Metal Subnitrides
High pressure is able to modify profoundly
the chemical bonding
and generate new phase structures of materials with chemical and physical
properties not accessible at ambient conditions. We here report an
unprecedented phenomenon on the pressure-induced formation of semiconducting
electrides via compression of layered alkaline-earth subnitrides Ca<sub>2</sub>N, Sr<sub>2</sub>N, and Ba<sub>2</sub>N that are conducting
electrides with loosely confined electrons in the interlayer voids
at ambient pressure. Our extensive first-principles swarm structure
searches identified the high-pressure semiconducting electride phases
of a tetragonal <i>I</i>4Ì…2<i>d</i> structure
for Ca<sub>2</sub>N and a monoclinic <i>Cc</i> structure
shared by Sr<sub>2</sub>N and Ba<sub>2</sub>N, both of which contain
atomic-size cavities with paring electrons distributed within. These
electride structures are validated by the excellent agreement between
the simulated X-ray diffraction patterns and the experimental data
available. We attribute the emergence of the semiconducting electride
phases to the p<i>–</i>d hybridization on alkaline-earth-metal
atoms under compression as well as the filling of the p<i>–</i>d hybridized band due to the interaction between Ca and N. Our work
provides a unique example of pressure-induced metal-to-semiconductor
transition in compound materials and reveals unambiguously the electron-confinement
topology change between different types of electrides
Nonmetallic FeH<sub>6</sub> under High Pressure
High pressure induces
unexpected chemical and physical properties
in materials. For example, hydrogen-rich compounds under pressure
have recently gained much attention as potential room-temperature
superconductors, and iron hydrides have also gained significant interest
as potential candidates for being the main constituents of the Earth’s
core. It is well-known that pressure induces insulator-to-metal transitions,
whereas pressure-induced metal-to-insulator transitions are rare,
especially for transition metal hydrides. In this article, we have
extensively explored the structural phase diagram of iron hydrides
by using ab initio particle swarm optimization. We have found a new
stable stoichiometry, FeH<sub>6</sub>, above 213.7 GPa with <i>C</i>2/<i>c</i> symmetry. Interestingly, <i>C</i>2/<i>c</i> FeH<sub>6</sub> presents an unexpected nonmetallicity,
and its band gap becomes larger with increasing pressure. This is
in sharp contrast with <i>P</i>2<sub>1</sub>/<i>m</i> FeH<sub>4</sub>. The nonmetallicity of <i>C</i>2/<i>c</i> FeH<sub>6</sub> mainly originates from the pressure-induced
hybridization between the Fe and H orbitals. This new compound shows
a unique structure with a mixture of nonbonded hydrogen atoms in a
helical iron framework. The strong Fe–Fe interaction and ionic
Fe–H bonds are responsible for its structural stability. In
addition, we have also found a more stable tetragonal FeH<sub>2</sub> structure with the same <i>I</i>4/<i>mmm</i> symmetry as the previously proposed one, the X-ray diffraction pattern
of which perfectly agrees with that of the experiment