29 research outputs found

### First-principles study of ferroelectricity induced by p-d hybridization in ferrimagnetic NiFe2O4

We investigate the ferrimagnetism and ferroelectricity of bulk NiFe$_2$O$_4$
with tetragonal $P4_122$ ~symmetry by means of density functional calculations
using generalized gradient approximation + Hubbard $U$ approach. Special
attention is paid to finding the most energetically favorable configuration on
magnetic ordering and further calculating the reliable spontaneous electric
polarization. With the fully optimized crystalline structure of the most stable
configuration, the spontaneous polarization is obtained to be 23 $\mu$C/cm$^2$
along the z direction, which originates from the hybridization between the 3d
states of the Fe$^{3+}$ cation and the 2p states of oxygen induced by
Jahn-Teller effect

### Influence of halide composition on the structural, electronic, and optical properties of mixed CH$_3$NH$_3$Pb(I$_{1-x}$Br$_x$)$_3$ perovskites calculated using the virtual crystal approximation method

We investigate the structural, electronic and optical properties of mixed
bromide-iodide lead perovskite solar cell CH$_3$NH$_3$Pb(I$_{1-x}$Br$_x$)$_3$
by means of the virtual crystal approximation (VCA) within density functional
theory (DFT). Optimizing the atomic positions and lattice parameters increasing
the bromide content $x$ from 0.0 to 1.0, we fit the calculated lattice
parameter and energy band gap to the linear and quadratic function of Br
content, respectively, which are in good agreement with the experiment,
respecting the Vegard's law. With the calculated exciton binding energy and
light absorption coefficient, we make sure that VCA gives consistent results
with the experiment, and the mixed halide perovskites are suitable for
generating the charge carriers by light absorption and conducting the carriers
easily due to their strong photon absorption coefficient, low exciton bindign
energy, and high carrier mobility at low Br contents. Furthermore analyzing the
bonding lengths between Pb and X (I$_{1-x}$Br$_x$: virtual atom) as well as C
and N, we stress that the stability of perovskite solar cell is definitely
improved at $x$=0.2

### Electronic structure and photo absorption property of pseudo-cubic perovskites CH$_3$NH$_3$PbX$_3$ (X=I, Br) including van der Waals interaction

Using density functional theory with the inclusion of van der Waals (vdW)
interaction, we have investigated electronic energy bands, density of states,
effective masses of charge carriers, and photo absorption coefficients of
pseudo-cubic CH$_3$NH$_3$PbX$_3$ (X=I, Br). Our results confirm the direct
bandgap of 1.49 (1.92) eV for X=I (Br) in the pseudo-cubic $Pm$ phase with
lattice constant of 6.324 (5.966) \AA, being agreed well with experiment and
indicating the necessity of vdW correction. The calculated photo absorption
coefficients for X=I (Br) have the onset at red (orange) color and the first
peak around violet (ultraviolet) color in overall agreement with the
experiment.Comment: 3pages, 3figures, App. Phys. Lett. 201

### Computational Prediction of Structural, Electronic, Optical Properties and Phase Stability of Double Perovskites K2SnX6 (X = I, Br, Cl)

Vacancy-ordered double perovskites K2SnX6 (X = I, Br, Cl) attract significant
research interest due to their potential application as light-absorbing
materials in perovskite solar cells. However, a deep insight into their
material properties at the atomic scale is yet scarce. Here we present a
systematic investigation on their structural, electronic, optical properties
and phase stabilities in cubic, tetragonal, and monoclinic phases based on
density functional theory calculations. Quantitatively reliable prediction of
lattice constants, band gaps, effective masses of charge carriers, exciton
binding energies is provided in comparison with the available experimental
data, revealing the increasing tendency of band gap and exciton binding energy
as lowering the crystallographic symmetry from cubic to monoclinic and going
from I to Cl. We highlight that cubic K2SnBr6 and monoclinic K2SnI6 are
suitable for the application as a light-absorber for solar cell devices due to
their proper band gaps of 1.65 and 1.16 eV and low exciton binding energies of
59.4 and 15.3 meV, respectively. The constant-volume Helmholtz free energies
are determined through phonon calculations, giving a prediction of their phase
transition temperatures as 449, 433 and 281 K for cubic-tetragonal and 345, 301
and 210 K for tetragonal-monoclinic transitions for X = I, Br and Cl. Our
calculations provide an understanding of material properties of vacancy-ordered
double perovskite K2SnX6, helping to devise a low-cost and high performance
perovskite solar cell

### First-principles study on the electronic and optical properties of inorganic perovskite Rb1-xCsxPbI3 for solar cell applications

Recently, replacing or mixing organic molecules in the hybrid halide
perovskites with the inorganic Cs or Rb cations has been reported to increase
the material stability with the comparable solar cell performance. In this
work, we systematically investigate the electronic and optical properties of
all-inorganic alkali iodide perovskites Rb1-xCsxPbI3 using the first-principles
virtual crystal approximation calculations. Our calculations show that as
increasing the Cs content x, lattice constants, band gaps, exciton binding
energies, and effective masses of charge carriers decrease following the
quadratic (linear for effective masses) functions, while static dielectric
constants increase following the quadratic function, indicating an enhancement
of solar cell performance upon the Rb addition to CsPbI3. When including the
many-body interaction within the GW approximation and incorporating the
spin-orbit coupling (SOC), we obtain more reliable band gap compared with
experiment for CsPbI3, highlighting the importance of using GW+SOC approach for
the all-inorganic as well as organic-inorganic hybrid halide perovskite
materials

### First-principles study on the chemical decomposition of inorganic perovskites \ce{CsPbI3} and \ce{RbPbI3} at finite temperature and pressure

Inorganic halide perovskite \ce{Cs(Rb)PbI3} has attracted significant
research interest in the application of light-absorbing material of perovskite
solar cells (PSCs). Although there have been extensive studies on structural
and electronic properties of inorganic halide perovskites, the investigation on
their thermodynamic stability is lack. Thus, we investigate the effect of
substituting Rb for Cs in \ce{CsPbI3} on the chemical decomposition and
thermodynamic stability using first-principles thermodynamics. By calculating
the formation energies of solid solutions \ce{Cs$_{1-x}$Rb$_x$PbI3} from their
ingredients \ce{Cs$_{1-x}$Rb$_x$I} and \ce{PbI2}, we find that the best match
between efficiency and stability can be achieved at the Rb content $x\approx$
0.7. The calculated Helmholtz free energy of solid solutions indicates that
\ce{Cs$_{1-x}$Rb$_x$PbI3} has a good thermodynamic stability at room
temperature due to a good miscibility of \ce{CsPbI3} and \ce{RbPbI3}. Through
lattice-dynamics calculations, we further highlight that \ce{RbPbI3} never
stabilize in cubic phase at any temperature and pressure due to the chemical
decomposition into its ingredients \ce{RbI} and \ce{PbI2}, while \ce{CsPbI3}
can be stabilized in the cubic phase at the temperature range of 0$-$600 K and
the pressure range of 0$-$4 GPa. Our work reasonably explains the experimental
observations, and paves the way for understanding material stability of the
inorganic halide perovskites and designing efficient inorganic halide PSCs

### First-principles study of ternary graphite compounds cointercalated with alkali atoms (Li, Na, and K) and alkylamines towards alkali ion battery applications

Using density functional theory calculations, we have investigated the
structural, energetic, and electronic properties of ternary graphite
intercalation compounds (GICs) containing alkali atoms (AM) and normal
alkylamine molecules (nC$x$), denoted as AM-nC$x$-GICs (AM=Li, Na, K, $x$=1, 2,
3, 4). The orthorhombic unit cells have been used to build the models for
crystalline stage-I AM-nC$x$-GICs. By performing the variable cell relaxations
and the analysis of results, we have found that with the increase in the atomic
number of alkali atoms the layer separations decreases in contrast to AM-GICs,
while the bond lengths of alkali atoms with graphene layer and nitrogen atom of
alkylamine decreases. The formation and interlayer binding energies of
AM-nC3-GICs have been calculated, indicating the increase in stability from Li
to K. The calculated energy barriers for migration of alkali atoms suggest that
alkali cation with larger ionic radius diffuses in graphite more smoothly,
being similar to AM-GICs. The analysis of density of states, electronic density
differences, and atomic populations illustrates a mechanism how the insertion
of especially Na among alkali atoms into graphite with first stage can be made
easy by cointercalation with alkylamine, more extent of electronic charge
transfer is occurred from more electropositive alkali atom to carbon ring of
graphene layer, while alkylamine molecules interact strongly with graphene
layer through the hybridization of valence electron orbitals.Comment: 22 pages, 9 figure

### Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites

The use of methylammonium (MA) lead halide perovskites \ce{CH3NH3PbX3} (X=I,
Br, Cl) in perovskite solar cells (PSCs) has made great progress in performance
efficiency during recent years. However, the rapid decomposition of \ce{MAPbI3}
in humid environments hinders outdoor application of PSCs, and thus, a
comprehensive understanding of the degradation mechanism is required. To do
this, we investigate the effect of water intercalation and hydration of the
decomposition and ion migration of \ce{CH3NH3PbX3} using first-principles
calculations. We find that water interacts with \ce{PbX6} and MA through
hydrogen bonding, and the former interaction enhances gradually, while the
latter hardly changes when going from X=I to Br and to Cl. Thermodynamic
calculations indicate that water exothermically intercalates into the
perovskite, while the water intercalated and monohydrated compounds are stable
with respect to decomposition. More importantly, the water intercalation
greatly reduces the activation energies for vacancy-mediated ion migration,
which become higher going from X=I to Br and to Cl. Our work indicates that
hydration of halide perovskites must be avoided to prevent the degradation of
PSCs upon moisture exposure

### Influence of M/A substitution on material properties of intermetallic compounds MSn$_2$ (M = Fe, Co; A = Li, Na): A first-principles study

Iron and cobalt distannides \ce{MSn2} (M = Fe, Co) are regarded as a
promising conversion-type anode material for lithium- and sodium-ion batteries,
but their properties are not well understood. In this work, we report a
first-principles study of alkali metal (A = Li, Na) substitutional effect on
the structural, mechanical, lattice vibrational, electronic and defect
properties of these distannides. Special attention is paid to systematic
comparison between \ce{FeSn2} and \ce{CoSn2}. Our calculations reveal that M/A
substitution induces a lattice expansion and decrease of elastic constants,
which is more announced with Na substitution than Li, and moreover changes the
elastic property of \ce{FeSn2} from ductile to brittle whereas preserves the
ductility of \ce{CoSn2}. An imaginary phonon frequency mode appears only for
\ce{FeSn2} and \ce{FeNaSn2}, and M/A substitution provokes a definite gap
between high and low frequency regions. We perform a careful analysis of
electronic density of states, band structures and Fermi surface, providing an
insight into difference of electronic structures between \ce{FeSn2} and
\ce{CoSn2}. With further calculation of defect formation energies and alkali
ion diffusion barriers, we believe this work can be useful to design
conversion-type anode materials for alkali-ion batteries

### The maximum interbubble distance in relation to the radius of spherical stable nanobubble in liquid water: A molecular dynamics study

The mechanism of superstability of nanobubbles in liquid confirmed by many
experimental studies is still in debate since the classical diffusion predicts
their lifetime on the order of a few microseconds. In this work, we study the
requirement for bulk nanobubbles to be stable by using molecular dynamics
simulations. Periodic cubic cells with different cell sizes and different
initial radii are treated to simulate the nanobubble cluster, providing the
equilibrium bubble radius and the interbubble distance. We find out that for
nanobubble with a certain radius $R$ to be stable, the interbubble distance
should be smaller than the maximum interbubble distance $L^*$ being
proportional to $R^{4/3}$