A systematic study on the phase diagram and superconductivity of ternary clathrate Ca-Sc-H at high pressures.

This work explores potential high-temperature superconductor materials in hydrogen-rich systems. Here, the crystal structure stabilities of ternary Ca-Sc-H systems under high-pressure (P = 100-250 GPa) and their superconductivities are investigated using the particle swarm optimization methodology combined with first-principles calculations. For the predicted candidate structures of Ca-Sc-H systems, the pressure-dependent phase diagram and thermodynamic convex hull were investigated across a wide range of compositions; the electronic properties of all the predicted phases were analyzed in detail to study the bonding behavior of these stable phases. We identified the crystal structures of four thermodynamically stable compounds: R3̄m-CaScH6, Immm-CaSc2H9,C2/m-Ca2ScH10, and R3̄m-CaScH12. Among them, R3̄m-CaScH12 was predicted to have the highest Tc value (i.e., 173 K) at 200 GPa. The discovery of this previously unreported pressure-induced decomposition of Ca-Sc-H systems will pave the way for investigations on the nature of hydrogen-metal interactions.

A two-dimensional borophene monolayer with ideal Dirac nodal-line fermions.

As a relatively new member of two-dimensional materials, borophene has gained huge interest over the past years, especially in the field of discovering new topological materials, such as Dirac nodal line semimetals. Here, based on first-principles calculations, for the first time, we find a completely flat borophene monolayer (named χ2/9) with ideal Dirac nodal line states around the Fermi level. A tight-binding model using the Slater-Koster approach is proposed to demonstrate that the unique electronic feature of χ2/9 that mainly originated from the first-nearest neighbor interactions of the pz orbitals of boron. According to our symmetry analysis, the Dirac nodal line in χ2/9 is guaranteed by the out-of-plane mirror or C2 rotational symmetry and the negligible pz orbital coupling. The chemical bonding analysis reveals the rare electronic properties of this material, which can be attributed to the multicentered π bonds.

Remarkable decrease in lattice thermal conductivity of transition metals borides TiB2by dimensional reduction.

Two-dimensional transition metals borides TixBxhave excellent magnetic and electronic properties and great potential in metal-ion batteries and energy storage. The thermal management is important for the safety and stability in these applications. We investigated the lattice dynamical and thermal transport properties of bulk-TiB2and its two-dimensional (2D) counterparts based on density functional theory combined with solving phonon Boltzmann transport equation. The Poisson's ratio of bulk-TiB2is positive while it changes to negative for monolayer TiB2. We found that dimension reduction can cause the room-temperature in-plane lattice thermal conductivity decrease, which is opposite the trend of MoS2, MoSe2, WSe2and SnSe. Additionally, the room temperature thermal conductivity of mono-TiB2is only one sixth of that for bulk-TiB2. It is attributed to the higher Debye temperature and stronger bonding stiffness in bulk-TiB2. The bulk-TiB2has higher phonon group velocity and weaker anharmonic effect comparing with its 2D counterparts. On the other hand, the room temperature lattice thermal conductivity of mono-Ti2B2is two times higher than that of mono-TiB2, which is due to three-phonon selection rule caused by the horizontal mirror symmetry.

Nanostructure engineering of two-dimensional diamonds toward high thermal conductivity and approaching zero Poisson's ratio.

Two-dimensional diamond, also called diamane, has attracted great research attention for its novel physical properties and potential applications in nanoelectronics, ultrasensitive resonators and thermal management. Compared with the hexagonal diamane, the physical properties of the rectangular diamane are less explored. In this work, using first-principles calculations, we conducted a comprehensive study on the electronic, phononic, thermal and mechanical properties of three types of rectangular diamanes. We found that rectangular diamanes possess a high Debye temperature (722-788 K) and a strong in-plane Young's modulus (405.9-575.9 N m-1). We further show close to zero Poisson's ratio in the rectangular Pmma diamane. Moreover, based on the phonon Boltzmann transport equation, high room temperature lattice thermal conductivity (910-1807 W m-1 K-1) and strong configuration and orientation dependence are demonstrated. Phonon group velocity, relaxation time and characteristic square velocity are explored and it is demonstrated that phonon harmonic behavior is responsible for the remarkable configuration dependent thermal conductivity in rectangular diamanes. The present work underscores the use of nanostructure engineering to manipulate thermal conductivity of 2D diamond, which provides opportunities for developing effective thermal channeling devices.

Pressure-induced evolution of crystal and electronic structure of neptunium hydrides.

An extensive exploration of high-pressure phase diagrams of NpHx (x = 1-10) compounds was performed by using swarm-intelligence-based CALYPSO structure searches. We propose five stable hydrogen-rich clathrate phases (P4/nmm-NpH5, Cmcm-NpH7, Fm3̄m-NpH8, P63/mmc-NpH9, and Fm3̄m-NpH10) that are composed of unusual H cages with stoichiometries H20, H24, H29, and H32 in which the H atoms are weakly covalently bonded to one another, with neptunium atoms occupying centers of the cages. The electronic structure analyses show that these predicted hydrogen-rich structures are all metallic phases, and Np-H and H-H bonds are formed by ionic and covalent bond interactions, respectively. The charge transfer from the Np atom plays an important role in the stability of the proposed structures. All hydrogen-rich clathrate structures show superconductivity behavior in their respective stability pressure range. Our work is an important step in understanding the phase stability and bonding behavior of NpHx under extreme conditions and provides a valuable reference for experimental synthesis and identification of cage-like neptunium hydrides.

Design of High-Performance Lead-Free Quaternary Antiperovskites for Photovoltaics via Ion Type Inversion and Anion Ordering.

The emergence of halide double perovskites significantly increases the compositional space for lead-free and air-stable photovoltaic absorbers compared to halide perovskites. Nevertheless, most halide double perovskites exhibit oversized band gaps (>1.9 eV) or dipole-forbidden optical transition, which are unfavorable for efficient single-junction solar cell applications. The current device performance of halide double perovskite is still inferior to that of lead-based halide perovskites, such as CH3NH3PbI3 (MAPbI3). Here, by ion type inversion and anion ordering on perovskite lattice sites, two new classes of pnictogen-based quaternary antiperovskites with the formula of X6B2AA' and X6BB'A2 are designed. Phase stability and tunable band gaps in these quaternary antiperovskites are demonstrated based on first-principles calculations. Further photovoltaic-functionality-directed screening of these materials leads to the discovery of 5 stable compounds (Ca6N2AsSb, Ca6N2PSb, Sr6N2AsSb, Sr6N2PSb, and Ca6NPSb2) with suitable direct band gaps, small carrier effective masses and low exciton binding energies, and dipole-allowed strong optical absorption, which are favorable properties for a photovoltaic absorber material. The calculated theoretical maximum solar cell efficiencies based on these five compounds are all larger than 29%, comparable to or even higher than that of the MAPbI3 based solar cell. Our work reveals the huge potential of quaternary antiperovskites in the optoelectronic field and provides a new strategy to design lead-free and air-stable perovskite-based photovoltaic absorber materials.

An ideal two-dimensional nodal-ring semimetal in tetragonal borophene oxide.

Free-standing stable two-dimensional (2D) boron monolayers, i.e., borophenes, usually settle into triangular lattices with different ratios of monoatomic vacancies. However, a stable polymorph can be drastically distinct from a free-standing one upon charge doping or on a substrate, as evidenced by the free-standing unstable hexagonal borophene that was prepared on the Al(111) substrate [Sci. Bull., 2018, 63, 282]. Moreover, 2D borophenes prefer to be oxidized to form more stable borophene oxides under ambient conditions. In this work, with the help of first-principles calculations, we propose a stable borophene oxide (t-B2O) through oxidizing the free-standing unstable T-borophene. More interestingly, t-B2O is a topological nodal-ring semimetal protected by in-plane mirror symmetry and characterized by a topological index. The energy fluctuation of the nodal ring is small and no extraneous bands are entangled with the nodal ring around the Fermi level. Two tight-binding models are developed to elucidate the orbital interactions and the formation of the nodal ring. Our work not only discovers a new ideal 2D topological nodal-ring semimetal, but the method used here also provides a fresh view in the search for 2D materials.

Broken-gap type-III band alignment in monolayer halide perovskite/antiperovskite oxide van der Waals heterojunctions.

The integration of halide perovskites with other functional materials provides a new platform for applications beyond photovoltaics, which has been realized in experiments. Here, through first-principles methods, we explore the possibility of constructing halide perovskite/antiperovskite oxide van der Waals heterostructures (vdWHs) for the first time with monolayers Rb2CdCl4 and Ba4OSb2 as representative compounds. Our calculation results reveal that the Rb2CdCl4/Ba4OSb2 vdWHs have negative binding energies and their most stable stacking possesses a rare type-III band alignment with a broken gap, which is highly promising for tunnel field-effect transistor (TFET) applications. Moreover, their electronic features can be further tuned by applying strain or an external electric field. Specifically, compressive strain can enlarge the tunneling window, while tensile strain can realize a type-III to type-II band alignment transformation. Therefore, our work provides fundamental insights into the electronic properties of Rb2CdCl4/Ba4OSb2 vdWHs and paves the way for the design and fabrication of future halide perovskite/antiperovskite-based TFETs.

Electrostatic potentials at Cu(In,Ga)Se2 grain boundaries: experiment and simulations.

In the present Letter, we report on a combined ab initio density functional theory calculation, multislice simulation, and electron holography study, performed on a Σ9 grain boundary (GB) in a CuGaSe2 bicrystal, which exhibits a lower symmetry compared with highly symmetric Σ3 GBs. We find an electrostatic potential well at the Σ9 GB of 0.8 V in depth and 1.3 nm in width, which in comparison with results from Σ3 and random GBs exhibits the trend of increasing potential-well depths with lower symmetry. The presence of this potential well at the Σ9 GB can be explained conclusively by a reduced density of atoms at the GB. Considering experimental limitations in resolution, we demonstrate quantitative agreement of experiment and theory.

Theoretical Prediction of Mixed-Valence Layered Halide Perovskites Cs4M(IV)M(II)2X12 (M = Ge, Sn; X = Cl, Br).

Charge-ordered compounds (i.e., Cu+/Cu2+, Au+/Au3+, In+/In3+, Tl+/Tl3+, Sb3+/Sb5+, and Bi3+/Bi5+) have been widely explored because of their unique physical properties. Here, a new class of ⟨111⟩-oriented mixed-valence layered halide perovskites Cs4M(IV)M(II)2X12 (M = Ge, Sn; X = Cl, Br) with C2/m, R-3m, and I41/amd space groups was predicted by first-principles calculations. Based on the decomposition enthalpy, the phonon spectrum, and the mechanical stability criteria, we found that Cs4GeGe2Cl12 (C2/m and R-3m), Cs4GeGe2Br12 (R-3m), and Cs4GeGe2Br6Cl6 (R-3m) exhibit thermodynamic, dynamical, and mechanical stability. The electronic structure calculations show that the predicted band gap of stable Cs4Ge(IV)Ge(II)2X12 varies from 1.16 to 2.25 eV. And an isolated intermediate conduction band contributed by the Ge(IV) 4s states below the Ge(II)/Ge(IV) 4p states is observed in these compounds, which is similar to previously reported Cs4CuSb2Cl12 but different from Cs4CdM(III)2Cl12 (M = Sb, Bi). In addition, the calculated static dielectric constant and optical absorption coefficient of Cs4GeGe2Br12 are close to those of typical double perovskites (e.g., Cs2AgBiBr6). We believe that our work enriches the family of mixed-valence halide perovskites and provides a new platform for potential optoelectronic semiconductor design.

Promising Thermoelectric Performance in Two-Dimensional Semiconducting Boron Monolayer.

A heavy element is a special character for high thermoelectric performance since it generally guarantees a low lattice thermal conductivity. Here, we unexpectedly found a promising thermoelectric performance in a two-dimensional semiconducting monolayer consisting of a light boron element. Using first-principles combined with the Boltzmann transport theory, we have shown that in contrast to graphene or black phosphorus, the boron monolayer has a low lattice thermal conductivity arising from its complex crystal of hexagonal vacancies. The conduction band with an intrinsic camelback shape leads to the high DOS and a high n-type Seebeck coefficient, while the highly degenerate valence band along with the small hole effective mass contributes to the high p-type power factor. As a result, we obtained the p-type thermoelectric figure of merit up to 0.96 at 300 K, indicating that the boron monolayer is a promising p-type thermoelectric material.

Structure-Composition-Property Relationships in Antiperovskite Nitrides: Guiding a Rational Alloy Design.

The alloy strategy through the A- or X-site is a common method for experimental preparation of high-performance and stable lead-based perovskite solar cells. As one of the important candidates for lead-free and stable photovoltaic absorbers, the inorganic antiperovskite family has recently been reported to exhibit excellent optoelectronic properties. However, the current reports on the design of antiperovskite alloys are rare. In this work, we investigated the previously overlooked electronic property (e.g., conduction band convergence), static dielectric constant, and exciton binding energy in inorganic antiperovskite nitrides by first-principles calculations. Then, we revealed a linear relationship between the tolerance factor and various physical quantities. Guided by the established structure-composition-property relationship in six antiperovskite nitrides X3NA (X2+ = Mg2+, Ca2+, Sr2+; A3- = P3-, As3-, Sb3-, Bi3-), for the first time, we designed a promising antiperovskite alloy Mg3NAs0.5Bi0.5 with a quasi-direct band gap of 1.402 eV. Finally, we made a comprehensive comparison between antiperovskite nitrides and conventional halide perovskites for pointing out the future direction for device applications.

From Two- to Three-Dimensional van der Waals Layered Structures of Boron Crystals: An Ab Initio Study.

A remarkable recent advancement has been the successful synthesis of two-dimensional boron monolayers on metal substrates. However, although up to 16 possible bulk allotropes of boron have been reported, none of them possess van der Waals (vdW) layered structures. In this work, starting from the experimentally synthesized monolayer boron sheet (ß12 borophene), we explored the possibility for forming vdW layered bulk boron. We found that two ß12 borophene sheets cannot form a stable vdW bilayer structure, as covalent-like B-B bonds are formed between them because of the peculiar bonding. Interestingly, when the covalently bonded bilayer borophene sheets are stacked on top of each other, three-dimensional (3D) layered structures are constructed via vdW interlayer interactions, rather than covalent. The 3D vdW layered structures were found to be dynamically stable. The interlayer binding energy is about 20 meV/Å2, which is close to the weakly bound graphene layers in graphite (∼16 meV/Å2). Furthermore, the density functional theory predicted electronic band structure testifies that these vdW bulk boron crystals can behave as good conductors. The insights obtained from this work suggest an opportunity to discover new vdW layered structures of bulk boron, which is expected to be crucial to numerous applications ranging from microelectronic devices to energy storage devices.

Engineering of charge carriers via a two-dimensional heterostructure to enhance the thermoelectric figure of merit.

High band degeneracy and glassy phonon transport are two remarkable features of highly efficient thermoelectric (TE) materials. The former promotes the power factor, while the latter aims to break the lower limit of lattice thermal conductivity through phonon scattering. Herein, we use the unique possibility offered by a two-dimensional superlattice-monolayer structure (SLM) to engineer the band degeneracy, charge density and phonon spectrum to maximize the thermoelectric figure of merit (ZT). First-principles calculations with Boltzmann transport equations reveal that the conduction bands of ZrSe2/HfSe2 SLM possess a highly degenerate level which gives a high n-type power factor; at the same time, the stair-like density of states yields a high Seebeck coefficient. These characteristics are absent in the individual monolayers. In addition, the SLM shows a suppressed lattice thermal conductivity along the superlattice period as phonons are effectively scattered by the interfaces. An intrinsic ZT of 5.3 (300 K) is achieved in n-type SLM, and it is 3.2 in the p-type counterpart. Compared with the theoretical predictions calculated with the same level of accuracy, these values are at least four-fold higher than those in the two parent materials, monolayer ZrSe2 and HfSe2. Our results provide a new strategy for the maximum thermoelectric performance, and clearly demonstrate the advantage of two-dimensional material heterostructures in the application of renewable energy.

Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study.

We explore the photovoltaic-relevant properties of the 2D MA2Pb(SCN)2I2 (where MA = CH3NH3(+)) perovskite using a combination of materials synthesis, characterization and density functional theory calculation, and determine electronic properties of MA2Pb(SCN)2I2 that are significantly different from those previously reported in literature. The layered perovskite with mixed-anions exhibits an indirect bandgap of ∼2.04 eV, with a slightly larger direct bandgap of ∼2.11 eV. The carriers (both electrons and holes) are also found to be confined within the 2D layers. Our results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2D layers aligned perpendicular to the substrates.