Stability, Chemical Bonding, and Electron Lone Pair Localization in AsN at High Pressure by Density Functional Theory Calculations.

The covalent bonding framework of crystalline single-bonded cubic AsN, recently synthesized under high pressure and high temperature conditions in a laser-heated diamond anvil cell, is here studied by means of density functional theory calculations and compared to single crystal X-ray diffraction data. The precise localization of the nonbonding electron lone pairs and the determination of their distances and orientations are related to the presence of characteristic structural motifs and space regions of the unit cell dominated by repulsive electronic interactions, with the relative orientation of the electron lone pairs playing a key role in minimizing the energy of the structure. We find that the vibrational modes associated with the expression of the lone pairs are strongly localized, an observation that may have implications for the thermal conductivity of the compound. The results indicate the thermodynamic stability of the experimentally observed structure of AsN above ∼17 GPa, provide a detailed insight into the nature of the chemical bonding network underlying the formation of this compound, and open new perspectives to the design and high pressure synthesis of new pnictogen-based advanced materials for potential applications of energetic and technological relevance.

Superconductivity in monolayer Janus titanium-sulfur hydride (TiSH) at ambient pressure.

Janus two dimensional (2D) materials are new and novel materials. As they break out-of-plane symmetry, they possess several fascinating properties which can be applied in catalytic reactions and opto-electronics. Recent synthesis of MoSH and the prediction of phonon-mediated superconductivity have opened a new way to investigate the properties of hydrogenated Janus materials (Novoselovet al2004Science306666-9; Mehtaet al2023Solid State Commun.375115347; Naiket al2023Comput. Theor. Chem.1228114278). In this work, we performed the density functional theory calculations to demonstrate that titanium sulfur hydride (TiSH) is dynamically stable and becomes phonon-mediated superconductor with the superconducting critical temperature,Tc= 9.24 K with the corresponding value of electron-phonon coupling constant,λ= 0.71, in the weak interaction limits, under ambient conditions. Eliashberg spectral functionα2F(ω)was well converged for dense grid ofq1 ×q2 ×q3 = 12 × 12 × 1 andnk1 ×nk2 ×nk3 = 140 × 140 × 1. The effect of smearing broadening was also considered for determining well converged value ofTcandλ. Figure5(b) shows that after smearing broadening of 0.02 Ry,λshows convergent values, and subsequent changes are as low as less that 5% of the peak value. Overall, our findings predicted a new member in the 2D Janus hydride family with possible applications in 2D nanomaterials and superconducting devices applications.

Electron-phonon coupling in La3In under pressure.

Conventional superconducting parameters Eliashberg spectral function, phonon DOS, average electron-phonon coupling constant & superconducting transition temperature of La3In were calculated at ambient and applied positive hydrostatic pressure. Ambient pressure electron-phonon coupling constant was found to be 1.43 and superconducting transition temperature was calculated to be 8.7 K which is close to the experimental reported value of 9.5 K. Logarithmic average phonon frequency at ambient pressure was found to be 6.21 × 10-3eV. Phonon dispersion relations for the ambient and applied positive hydrostatic pressure were calculated to confirm the dynamical stability of La3In under positive pressure. Acoustic modes of vibrations were found to mediate most of the electron-phonon-electron interaction with a very small contribution from optical modes of vibrations. Fermi surface at ambient pressure was also calculated and employed in explaining the large value of the calculated electron-phonon coupling constant. The effect of spin-orbit coupling on electronic, phonon and superconducting properties of La3In was also studied.

Presence and absence of intrinsic magnetism in graphitic carbon nitrides designed through C-N-H building blocks.

We use the first principle calculation to investigate the intrinsic magnetism of graphitic carbon nitrides (GCNs). By preserving three-fold symmetry, the GCN building blocks have been built out of different combinations between 6 components which are C atom, N atom, s-triazine, heptazine, heptazine with C atom at the center, and benzimidazole-like component. That results in 20 phases where 11 phases have been previously reported, and 9 phases are newly derived. The partial density of states and charge density have been analyzed through 20 phases to understand the origin of the presence and absence of intrinsic magnetism in GCNs. The intrinsic magnetism will be present not only because the GCNs comprising of radical components but also the [Formula: see text]-conjugated states are not the valence maximum to break the delocalization of unpaired electrons. The building blocks are also employed to study alloys between g-[Formula: see text] and g-[Formula: see text]. The magnetization of the alloys has been found to be linearly dependent on a number of C atoms in the unit cell and some magnetic alloys are energetically favorable. Moreover, the intrinsic magnetism in GCNs can be promoted or demoted by passivating with a H atom depending on the passivated positions.

Impact of the polar optical phonon and alloy scattering on the charge-carrier mobilities of FA0.83Cs0.17Pb(I1-xBrx)3 hybrid perovskites.

Lead mixed-halide perovskites are promising absorption materials that are suitable for applications in tandem solar cells using existing silicon technology. Charge-carrier mobility is an important factor that affects the performance of tandem solar cells. However, a detailed understanding of the fundamental mechanisms of lead mixed-halide perovskites remains elusive. Here, we used LO (longitudinal optical) phonons and alloy scattering to the elucidate charge-carrier mobilities in the FA0.83Cs0.17Pb(I1-xBrx)3 hybrid perovskite system. It was found that these scattering mechanisms provided very good quantitative agreement with the experimental results, between 11-40 cm2 V-1 s-1. Our findings provide new insights into charge transport scattering in lead mixed-halide hybrid perovskites and pave the way toward design of novel semiconductor alloys for solar cell applications.

Preferred oriented cation configurations in high pressure phases IV and V of methylammonium lead iodide perovskite.

A microscopic viewpoint of structure and dipolar configurations in hybrid organic-inorganic perovskites is crucial to understanding their stability and phase transitions. The necessity of incorporating dispersion interactions in the state-of-the-art density functional theory for the [Formula: see text] perovskite (MAPI) is demonstrated in this work. Some of the vdW methods were selected to evaluate the corresponding energetics properties of the cubic MAPI with various azimuthally rotated MA organic cation orientations. The highest energy barrier obtained from PBEsol reaches 18.6 meV/MA-ion, which is equivalent to 216 K, the temperature above which the MA cations randomly reorient. Energy profiles calculated by vdW incorporated functionals, on the other hand, exhibit various distinct patterns. The well-developed vdW-DF-cx functional was selected, thanks to its competence, to evaluate the total energies of different MA dipolar configurations in [Formula: see text] cubic supercell of MAPI under pressures. The centrosymmetric arrangement of the MA cations that provide zero total dipole moment configuration results in the lowest energy state profiles under pressure, while the non-centrosymmetric scheme displays a unique behaviour. Despite being overall unpolarised, the latter calculated with PBEsol leads to a rigid shift of energy from the profile obtained from the dispersive vdW-DF-cx functional. It is noteworthy that the energy profile responsible for the maximum polarised configuration nevertheless takes the second place in total energy under pressure.

Polaron transport in hybrid CH3NH3PbI3 perovskite thin films.

A comprehensive study of the transport properties of a prototypical CH3NH3PbI3 thin film is presented. The polaron-longitudinal optical (LO) phonon scattering mechanism, based on Low-Pines's polaron mobility, was studied to elucidate the charge-carrier mobility. We found that the calculated mobilities showed very good quantitative agreement with the experimental data measured in thin film samples using photoconductivity techniques. In THz mobility, the calculated results yielded room-temperature (RT) mobilities of ∼650 cm2 V-1 s-1 (single crystal) and ∼220 cm2 V-1 s-1 (disordered thin film) at a low quantum yield (φ) and 32 cm2 V-1 s-1 (high-quality thin film) at φ = 1. The dynamic disorder due to organic reorientation was included in the calculations. Its effect provided a power law mobility of µ ∝ Tm and satisfactorily supported temperature-dependent mobility over the temperature range of 80-370 K. In the orthorhombic and tetragonal phases, the charge-carrier mobilities with dynamic disorder were approximately 47% and 22% lower than those obtained from phases without dynamic disorder. The RT mobility was 26 cm2 V-1 s-1 at φ = 1. In the low-temperature orthorhombic phase, the structural phase transition was considered. The mobility followed a power law with m = -1.7. In the tetragonal and cubic phases, the mobility also followed a power law, but with m = -1.1, which is an intermediate range in optical phonon scattering. When combined with recent theoretical analysis, we also found three limitations of power law mobility with exponents between -0.46 and -1.1 for polaron-LO phonon scattering, -1.2 and -1.6 for bare carrier-LO phonon scattering, and -1.7 and -2.0 for carrier scattering off optical phonons and lattice fluctuations. This work not only provides a description of temperature-dependent mobility in CH3NH3PbI3 thin films, but also gives new insights into THz photoconductivity and the relationship between LO phonon scattering and power law mobility.

Theoretical predictions for low-temperature phases, softening of phonons and elastic stiffnesses, and electronic properties of sodium peroxide under high pressure.

High-pressure phase stabilities up to 600 K and the related properties of Na2O2 under pressures up to 300 GPa were investigated using first-principles calculations and the quasi-harmonic approximation. Two high-pressure phases of Na2O2 that are thermodynamically and dynamically stable were predicted consisting of the Amm2 (distorted P6̄2m) and the P21/c structures, which are stable at low temperature in the pressure range of 0-22 GPa and 22-28 GPa, respectively. However, the P6̄2m and Pbam structures become the most stable instead of the Amm2 and P21/c structures at the elevated temperatures, respectively. Interestingly, the softening of some phonon modes and the decreasing of some elastic stiffnesses in the Amm2 structure were also predicted in the pressure ranges of 2-3 GPa and 9-10 GPa. This leads to the decreasing of phonon free energy and the increasing of the ELF value in the same pressure ranges. The HSE06 band gaps suggest that all phases are insulators, and they increase with increasing pressure. Our findings provide the P-T phase diagram of Na2O2, which may be useful for investigating the thermodynamic properties and experimental verification.

The crucial role of density functional nonlocality and on-axis CH3NH3 rotation induced I2 formation in hybrid organic-inorganic CH3NH3PbI3 cubic perovskite.

Effects of electronic nonlocality in density functional theory study of structural and energetic properties of a pseudocubic CH3NH3PbI3 are investigated by considering coherent rotation around C-N axis of a CH3NH3 cation. A number of truly non-local and semi-local exchange correlation density functionals are examined by comparing calculated structural parameters with experimental results. The vdW-DF-cx which takes into account the non-local van der Waals correlation and consistent exchange shows the best overall performance for density functional theory study of this system. Remarkable distinctions between results from vdW-DF-cx and those from PBEsol exchange correlation functionals are observed and indicate the need of including the non-local interaction in the study of this system, especially its dynamical properties. The obtained rotational barriers are 18.56 meV/formula and 27.71 meV/formula which correspond to rotational frequencies of 3.71 THz and 2.60 THz for vdW-DF-cx and PBEsol calculations, respectively. Interestingly, the maximally localised Wannier function analysis shows the hydrogen bonding assisted covalent character of two iodide anions at a moderate rotational angle which can lead to I2 formation and then material degradation.

Theoretical aspects in structural distortion and the electronic properties of lithium peroxide under high pressure.

The structural phase transition and electronic properties of Li2O2 under pressures up to 500 GPa have been investigated using first-principles calculations. Two new structural phase transitions have been proposed at pressures around 75 GPa from the P63/mmc structure to the P21 structure and around 136 GPa from the P21 structure to the P21/c structure. The calculated phonon spectra have confirmed the dynamical stability of these structures. The pressure dependence of the lattice dynamics, O-O bond length, and band gaps in Li2O2 have also been reported. The band gaps of the P63/mmc, P21, and P21/c structures calculated by PBE and HSE06 have shown increasing trends with increasing pressure. Interestingly, the P63/mmc band gap and c/a ratio have significantly decreased with the increasing O-O bond length and ELF value around 11 and 40 GPa. At these pressures, the phonon frequency of the O-O stretching modes has softened. This finding reveals the effects of structural distortion in three phases of Li2O2. Our study provides structural understanding and the electronic properties of Li2O2 under high pressure, which might be useful for investigating the charge transport through Li2O2 in lithium-air batteries and CO2 capture.

Internal vibrations of the Li(NH3)4+ complex analyzed from ab initio, density functional theory, and the classical spring network model.

We report our theoretical findings regarding internal vibrations of the Li(NH 3) 4 (+) complex which have been studied using three different methods, namely, a classical spring network model, density functional theory, and ab initio Hartree-Fock plus Møller-Plesset correlation energy correction truncated at second-order. The equilibrium Li...N and N...N distances are found to be 2.12 and 3.47 A, respectively, in good agreement with the experimental data. The theoretically determined vibrational frequencies of the lowest modes are in good agreement with those extracted from inelastic X-ray scattering measurements. From group theory considerations, the internal vibrations of Li(NH 3) 4 (+) complexes resemble those of a tetrahedral object. Further experimental investigation is suggested.

Model of saturated lithium ammonia as a single-component liquid metal.

We use the single-component picture and the nearly-free-electron theory for describing collective excitations in the saturated Li-ammonia solution. The physical justification is discussed, and all predictions are compared with current experimental findings. The plasmon dispersion and the long-wavelength dielectric function of the solution can be explained within the homogeneous-electron-gas theory. The parameters r(s) = 7.4a(0) and epsilon(infinity) = 1.44 give a good description compared with inelastic x-ray scattering and optical data. The phonon spectrum of the solution is also examined. Within the scope of the empty core model with R(c) = 3.76a(0), the phonon dispersion at low q is reproduced. The ratio BB(free) = 1.34 is compared with 1.63 obtained from experiments.