Structural Principles of Semiconducting Group 14 Clathrate Frameworks

Structural Features, Chemical Diversity, and Physical Properties of Microporous Sodalite-Type Materials: A Review

This review contains data on a wide class of microporous materials with frameworks belonging to the sodalite topological type. Various methods for the synthesis of these materials, their structural and crystal chemical features, as well as physical and chemical properties are discussed. Specific properties of sodalite-related materials make it possible to consider they as thermally stable ionic conductors, catalysts and catalyst carriers, sorbents, ion exchangers for water purification, matrices for the immobilization of radionuclides and heavy metals, hydrogen and methane storage, and stabilization of chromophores and phosphors. It has been shown that the diversity of properties of sodalite-type materials is associated with the chemical diversity of their frameworks and extra-framework components, as well as with the high elasticity of the framework.

Dilute but significant: low cation concentration affects field dependent properties of Eu2Ga11Sn35

We investigate the temperature and magnetic field-dependent electrical and thermal properties of Eu2.2Ga11Sn35, revealing a change in electronic structure and an increase in magnetoresistance with increasing field, as well as the origin of magneto-suppressed thermal conductivity of this unconventional inorganic clathrate-I material with very low cation concentration.

Electronic Structure Analysis of the A10Tt2P6 System (A=Li-Cs; Tt=Si, Ge, Sn) and Synthesis of the Direct Band Gap Semiconductor K10Sn2P6

Investigating the relationship between atomic and electronic structures is a powerful tool to screen the wide variety of Zintl phases for interesting (opto-)electronic properties. To get an insight in such relations, the A10Tt2P6 system (A=Li-Cs; Tt=Si-Sn) was picked as model system to analyse the influence of structural motives, combination of elements and their properties on type and width of the band gaps. Those compounds comprise two interesting structural motives of their anions, which are either monomeric trigonal planar TtP3 5- units which are isostructural to CO3 2- or [Tt2P6]10- dimers which correspond to two edge-sharing TtP4 tetrahedra. The A10Tt2P6 compounds were structurally optimized for both polymorphs and subsequent frequency analysis, band structure as well as density of states calculations were performed. The Gibbs free energies were compared to determine temperature dependent stability, where Na10Si2P6, Na10Ge2P6 and K10Sn2P6 were found to be candidates for a high temperature phase transition between the two polymorphs. Additionally, the unknown, but predicted compound K10Sn2P6 was synthesized and characterized by single crystal and powder x-ray diffraction. It crystalizes in the monoclinic space group P 21/n and incorporates [Sn2P6]10- edge sharing double tetrahedra. It was determined to be a direct band gap semiconductor with a band gap of 2.57 eV.

Computational Screening and Stabilization of Boron-Substituted Type-I and Type-II Carbon Clathrates

Boron substitution represents a promising approach to stabilize carbon clathrate structures, but no thermodynamically stable substitution schemes have been identified for frameworks other than the type-VII (sodalite) structure type. To investigate the possibility for additional tetrahedral carbon-based clathrate networks, more than 5000 unique boron decoration schemes were investigated computationally for type-I and type-II carbon clathrates with a range of guest elements including Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba. Density functional theory calculations were performed at 10 and 50 GPa, and the stability and impact of boron substitution were evaluated. The results indicate that the boron-substituted carbon clathrates are stabilized under high-pressure conditions. Full cage occupancies of intermediate-sized guest atoms (e.g., Na, Ca, and Sr) are the most favorable energetically. Clathrate stability is maximized when the boron atoms are substituted within the hexagonal rings of the large [51262]/[51264] cages. Several structures with favorable formation enthalpies <-200 meV/atom were predicted, and type-I Ca8B16C30 is on the convex hull at 50 GPa. This structure represents the first thermodynamically stable type-I clathrate identified and suggests that boron-substituted carbon clathrates may represent a large family of diamond-like framework materials with a range of structure types and guest/framework substitutions.

Heat Transport in Clathrate Hydrates Controlled by Guest Frequency and Host-Guest Interaction

The underlying mechanism of common limited lattice thermal conductivity (κ) in energy-related host-guest crystalline compounds has been an ongoing topic in recent decades. Here, the guest-triggered intrinsic ultralow κ of the representative xenon clathrate hydrate was investigated using the time domain thermoreflectance technique and theoretical calculations. The localized guest modes were observed to hybridize with acoustic branches and severely limit the acoustic κ contribution. Besides, the strong mode coupling enables the reshaping of the overall lattice dynamics, especially for optical branches. More importantly, we identified that guest fillers prompt great phonon scattering in wide frequencies, which originates from both the guest-frequency-controlled enhancement of phase space and the host-guest-interaction-governed lattice anharmonicity. The extremely low guest frequency and strong host-guest interaction and coupling were thereby underlined to play vital but distinct roles in κ minimization. Our results unveil the dominant factors of guest reduction effects and facilitate the design of efficient thermoelectric or other thermal-related materials.

Lithium metal atoms fill vacancies in the germanium network of a type-I clathrate: synthesis and structural characterization of Ba8Li5Ge41

Clathrate phases with crystal structures exhibiting complex disorder have been the subject of many prior studies. Here we report syntheses, crystal and electronic structure, and chemical bonding analysis of a Li-substituted Ge-based clathrate phase with the refined chemical formula Ba8Li5.0(1)Ge41.0, which is a rare example of ternary clathrate-I where alkali metal atoms substitute framework Ge atoms. Two different synthesis methods to grow single crystals of the new clathrate phase are presented, in addition to the classical approach towards polycrystalline materials by combining pure elements in desired stoichiometric ratios. Structure elucidations for samples from different batches were carried out by single-crystal and powder X-ray diffraction methods. The ternary Ba8Li5.0(1)Ge41.0 phase crystallizes in the cubic type-I clathrate structure (space group Pm3̄n no. 223, a ≈ 10.80 Å), with the unit cell being substantially larger compared to the binary phase Ba8Ge43 (Ba8□3Ge43, a ≈ 10.63 Å). The expansion of the unit cell is the result of the Li atoms filling vacancies and substituting atoms in the Ge framework, with Li and Ge co-occupying one crystallographic (6c) site. As such, the Li atoms are situated in four-fold coordination environment surrounded by equidistant Ge atoms. Analysis of chemical bonding applying the electron density/electron localizability approach reveals ionic interaction of barium with the Li-Ge framework, while the lithium-germanium bonds are strongly polar covalent.

Lithium-ion Mobility in Li6 B18 (Li3 N) and Li Vacancy Tuning in the Solid Solution Li6 B18 (Li3 N)1-x (Li2 O)x

All-solid-state batteries are promising candidates for safe energy-storage systems due to non-flammable solid electrolytes and the possibility to use metallic lithium as an anode. Thus, there is a challenge to design new solid electrolytes and to understand the principles of ion conduction on an atomic scale. We report on a new concept for compounds with high lithium ion mobility based on a rigid open-framework boron structure. The host-guest structure Li₆ B₁₈ (Li₃ N) comprises large hexagonal pores filled with ∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ Li₇ N] strands that represent a perfect cutout from the structure of α-Li₃ N. Variable-temperature ⁷ Li NMR spectroscopy reveals a very high Li mobility in the template phase with a remarkably low activation energy below 19 kJ mol^-1 and thus much lower than pristine Li₃ N. The formation of the solid solution of Li₆ B₁₈ (Li₃ N) and Li₆ B₁₈ (Li₂ O) over the complete compositional range allows the tuning of lithium defects in the template structure that is not possible for pristine Li₃ N and Li₂ O.

Structural Dynamics, Phonon Spectra and Thermal Transport in the Silicon Clathrates

The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in "phonon-glass electron-crystal" materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower κlatt than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low κlatt for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system.

Chemi-Inspired Silicon Allotropes-Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets, Single-Walled Nanotubes, and Nanoparticles

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si9]4- clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si-Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si9 clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si9]4- based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.

Ternary Zinc Antimonides Unlocked Using Hydride Synthesis

Three new sodium zinc antimonides Na₁₁Zn₂Sb₅, Na₄Zn₉Sb₉, and NaZn₃Sb₃ were synthesized utilizing sodium hydride NaH as a reactive sodium source. In comparison to the synthesis using sodium metal, salt-like NaH can be ball-milled, leading to the easy and uniform mixing of precursors in the desired stoichiometric ratios. Such comprehensive compositional control enables a fast screening of the Na-Zn-Sb system and identification of new compounds, followed by their preparation in bulk with high purity. Na₁₁Zn₂Sb₅ crystallizes in the triclinic P1 space group (No. 2, Z = 2, a = 8.8739(6) Å, b = 10.6407(7) Å, c = 11.4282(8) Å, α = 103.453(2)°, β = 96.997(2)°, γ = 107.517(2)°) and features polyanionic [Zn₂Sb₅]^11- clusters with unusual 3-coordinated Zn atoms. Both Na₄Zn₉Sb₉ (Z = 4, a = 28.4794(4) Å, b = 4.47189(5) Å, c = 17.2704(2) Å, β = 98.3363(6)°) and NaZn₃Sb₃ (Z = 8, a = 32.1790(1) Å, b = 4.51549(1) Å, c = 9.64569(2) Å, β = 98.4618(1)°) crystallize in the monoclinic C2/m space group (No. 12) and have complex new structure types. For both compounds, their frameworks are built from ZnSb₄ distorted tetrahedra, which are linked via edge-, vertex-sharing, or both, while Na cations fill in the framework channels. Due to the complex structures, Na₄Zn₉Sb₉ and NaZn₃Sb₃ compounds exhibit low thermal conductivities (0.97-1.26 W·m^-1 K^-1) at room temperature, positive Seebeck coefficients (19-32 μV/K) suggestive of holes as charge carriers, and semimetallic electrical resistivities (∼1.0-2.3 × 10^-4 Ω·m). Na₄Zn₉Sb₉ and NaZn₃Sb₃ decompose into the equiatomic NaZnSb above ∼800 K, as determined by in situ synchrotron powder X-ray diffraction. The discovery of multiple ternary compounds highlights the importance of judicious choice of the synthetic method.

A Lanthanum-Filled Carbon-Boron Clathrate

We report a carbon-boron clathrate with composition 2 La@B₆ C₆ (LaB₃ C₃ ). Like recently reported SrB₃ C₃ ,^[1] single-crystal X-ray diffraction and computational modelling indicate that the isostructural La member crystallizes in the cubic bipartite sodalite structure (Type-VII clathrate) with La atoms encapsulated within truncated octahedral cages composed of alternating carbon and boron atoms. The covalent nature of the B-C bonding results in a hard, incompressible framework, and owing to the balanced electron count, La³⁺ [B₃ C₃ ]^3- exhibits markedly improved pressure stability and is a semiconductor with an indirect band gap predicted near 1.3 eV. A variety of different guest atoms may potentially be substituted within Type-VII clathrate cages, presenting opportunities for a large family of boron-stabilized, carbon-based clathrates with ranging physical properties.

Two-Dimensional Superstructures of Silica Cages

Despite extensive studies on mesoporous silica since the early 1990s, the synthesis of two-dimensional (2D) silica nanostructures remains challenging. Here, mesoporous silica is synthesized at an interface between two immiscible solvents under conditions leading to the formation of 2D superstructures of silica cages, the thinnest mesoporous silica films synthesized to date. Orientational correlations between cage units increase with increasing layer number controlled via pH, while swelling with oil and mixed surfactants increase micelle size dispersity, leading to complex clathrate type structures in multilayer superstructures. The results suggest that a three-dimensional (3D) crystallographic registry within cage-like superstructures emerges as a result of the concerted 3D co-assembly of the organic and inorganic components. Mesoporous 2D superstructures can be fabricated over macroscopic film dimensions and stacked on top of each other. The realization of previously inaccessible mesoporous silica heterostructures with separation or catalytic properties unachievable via conventional bulk syntheses is envisioned.

Si2Ge: A New VII-Type Clathrate with Ultralow Thermal Conductivity and High Thermoelectric Property

Based on global particle-swarm optimization algorithm and density functional theory methods, we predicted an alloyed Si2Ge compond with body centered tetragonal type VII clathrate (space group I4/mmm) built by a truncated octahedron fromed by six quadrangles and eight hexagons ([4668]). Si2Ge clathrate is 0.06 eV/atom lower than VII Si clathrate and thermally stable up to 1000 K. It has an indirect band gap of 0.23 eV, high p-doping Seebeck coefficient and n-doping electrical conductivity. It owns a low lattice thermal conductivity of 0.28 W/mK at 300 K because of its weak bonding and strong anharmonic interaction of longitudinal acoustic and low-lying optical phonons. The moderate electronic transport properties together with low lattice thermal conductivity results in a high optimal thermoeletric performance value of 2.54 (1.49) at 800 (1000) K in n (p)-doped Si2Ge.

Carbon-boron clathrates as a new class of sp3-bonded framework materials

Carbon-based frameworks composed of sp³ bonding represent a class of extremely lightweight strong materials, but only diamond and a handful of other compounds exist despite numerous predictions. Thus, there remains a large gap between the number of plausible structures predicted and those synthesized. We used a chemical design principle based on boron substitution to predict and synthesize a three-dimensional carbon-boron framework in a host/guest clathrate structure. The clathrate, with composition 2Sr@B₆C₆, exhibits the cubic bipartite sodalite structure (type VII clathrate) composed of sp³-bonded truncated octahedral C₁₂B₁₂ host cages that trap Sr²⁺ guest cations. The clathrate not only maintains the robust nature of diamond-like sp³ bonding but also offers potential for a broad range of compounds with tunable properties through substitution of guest atoms within the cages.

A C20-based 3D carbon allotrope with high thermal conductivity

Stimulated by the high thermal conductivity of diamond together with the light mass and rich resources of carbon, a great deal of effort has been devoted to the study of the thermal conductivity of carbon-based materials. In this work, we systematically study the thermal transport properties of a three dimensional (3D) C20 fullerene-assembled carbon allotrope, HSP3-C34, in which all carbon atoms are in sp3 hybridization. The stability of HSP3-C34 is confirmed and its thermal conductivity is obtained by using first principles calculations combined with solving the linearized phonon Boltzmann transport equation. At room temperature, the thermal conductivity of HSP3-C34 is 731 W m-1 K-1, which is larger than those of many 3D carbon allotropes, such as BCO-C16 (452 W m-1 K-1), 3D graphene (150 W m-1 K-1) and T-carbon (33 W m-1 K-1). A detailed analysis of its phonons reveals that three acoustic branches are the main heat carriers at room temperature, and the optical branches gradually become important with increasing temperature. A further study on the harmonic and anharmonic properties of HSP3-C34 uncovers that the main reasons for the high thermal conductivity are the weak anharmonicity and large group velocity resulting from the strong sp3 bonding. This study provides new insights on searching for carbon allotropes with high thermal conductivity.

Modeling of atomic-molecular structures by contiguous filling of space with Frank-Kasper atomic domains

An application of contiguous filling of space with convex polyhedra, also known as Frank-Kasper (FK) atomic domains is demonstrated here for modeling of atomic molecular structures. Both regular, when all polyhedron edges have equal length, and strained, depending on the topology of the polyhedron the length of its edges may slightly fluctuate from the common length, polyhedra are used. Polyhedra are connected to each other in agreement with Plateau's laws to form a contiguous uninterrupted space. An application of a new approach is demonstrated for a modeling of structures of graphite, graphene, graphane, diamond and two types of ice. The proposed approach allows to demonstrate a mutual arrangement of atoms in graphite layers, transitions between allotropic states of carbon atoms, to calculate the distances between layers in graphene and positions of water molecules in a square ice.

Zintl Phases as Reactive Precursors for Synthesis of Novel Silicon and Germanium-Based Materials

Recent experimental and theoretical work has demonstrated significant potential to tune the properties of silicon and germanium by adjusting the mesostructure, nanostructure, and/or crystalline structure of these group 14 elements. Despite the promise to achieve enhanced functionality with these already technologically important elements, a significant challenge lies in the identification of effective synthetic approaches that can access metastable silicon and germanium-based extended solids with a particular crystal structure or specific nano/meso-structured features. In this context, the class of intermetallic compounds known as Zintl phases has provided a platform for discovery of novel silicon and germanium-based materials. This review highlights some of the ways in which silicon and germanium-based Zintl phases have been utilized as precursors in innovative approaches to synthesize new crystalline modifications, nanoparticles, nanosheets, and mesostructured and nanoporous extended solids with properties that can be very different from the ground states of the elements.

Unconventional Clathrates with Transition Metal-Phosphorus Frameworks

In this Account, we focused on a unique class of inclusion compounds, intermetallic clathrates, which exist in a variety of structures and exhibit diverse physical properties. These compounds combine covalent tetrahedral frameworks with rattling guest atoms situated inside their framework cages. Tetrels, the group 14 elements, are the basis for conventional clathrates because they fulfill the bonding requirement of four electrons per framework atom. In analogy to the replacement of Ge with GaAs in semiconductors, we focused on unconventional tetrel-free clathrates with frameworks composed of phosphorus and late transition metals. Compared to tetrels, these elements exhibit greater flexibility in their local coordinations and bonding. Tetrel elements cannot tolerate high deviations from regular tetrahedral coordinations. Thus, they exile a number of theoretically predicted framework topologies that are composed of a single type of polyhedral cage with square faces, such as the truncated octahedron. Unconventional clathrates are capable of stabilizing both envisaged and unique, unforeseen topologies. Clathrate structures with guest atoms held inside their cages by weak electrostatic interactions are predicted to be efficient thermoelectrics due to their low thermal conductivities. Unconventional clathrates may exhibit ultralow thermal conductivities, below 1 W m^-1 K^-1, without a need for heavy elements in their frameworks. The different chemical natures of transition metals and phosphorus led to their segregation over different framework positions, fulfilling the elements' specific local coordination and bonding requirements. This resulted in the formation of short- and long-range ordered superstructures with complex phonon dispersions and ultralow thermal conductivities. Aliovalent substitutions are commonly used to tune charge carrier concentrations in materials science. They are often performed under the "doping" assumption that substitutions with neighboring elements in the periodic system should not affect the parent structure but only adjust the charge carrier concentrations. This is not the case for unconventional clathrates. We investigated the tunability of the prototype Ba₈Cu₁₆P₃₀ clathrate by the aliovalent substitution of Cu with either Zn or Ge. These substitutions resulted in significant alterations of the local chemical bonding and led to the rearrangement of the whole crystal structure. Remarkable thermoelectric properties were achieved for the substituted unconventional clathrates, exhibiting an overall order of magnitude increase in the thermoelectric performance. Aliovalent substitution allowed us to vary the charge carrier concentration in one structure type within the limits of the structure's stability. Exceeding these limits in the Ba-Cu-Zn-P system resulted in a transition from the covalent 2c-2e bonding found in conventional clathrates to the multicenter bonding common for metal-rich intermetallic compounds. This caused the complete rearrangement of the crystal structure into a new unique clathrate where a majority of the framework atoms are five- and six-coordinated, and all metal atoms are joined in Cu-Zn dumbbells. Our work shows that unconventional clathrates exhibit diverse crystal structures and unique chemical bonding, which result in tunable transport properties. While they are similar to their tetrel counterparts in some ways, they are very different in others. Specifically, the high thermal and chemical stabilities and low thermal conductivities of unconventional clathrates make them promising bases for further development of thermoelectric materials.

GenIce: Hydrogen-Disordered Ice Generator

GenIce is an efficient and user‐friendly tool to generate hydrogen‐disordered ice structures. It makes ice and clathrate hydrate structures in various file formats. More than 100 kinds of structures are preset. Users can install their own crystal structures, guest molecules, and file formats as plugins. The algorithm certifies that the generated structures are completely randomized hydrogen‐disordered networks obeying the ice rule with zero net polarization. © 2017 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Slicing Diamond for More sp3 Group 14 Allotropes Ranging from Direct Bandgaps to Poor Metals

Considerable interest in novel Si allotropes has led to intense investigation of tetrahedral framework structures during the last years. Recently, a guide to deriving sp³ -Si allotropes from atom slabs of the diamond structure enabled a systematic deduction of several low-density modifications. Some of the Si networks were recognized as experimentally known frameworks, that is, so-called "chemi-inspired" structures. Herein we present nine novel Si networks obtained by modifying three-atom-thick slabs of a cubic diamond structure after smooth distortion by applying the same construction kit. Analysis of the structure-property relationships of these frameworks by using quantum-chemical methods shows that several of them possess direct bandgaps in the range suitable for light conversion. The construction kit was also applied to higher group 14 homologues Ge and Sn, and revealed interesting differences in the band structures and relative energies of the homologues. A new modification of Sn was identified as a poor metal, which denoted significant covalent-bond characteristics.

Inorganic SnIP-Type Double Helices in Main-Group Chemistry

Inspired by the synthesis of the first atomic-scale double-helix semiconductor SnIP, this study deals with the question of whether more atomistic, inorganic double-helix compounds are accessible. With the aid of quantum chemical calculations, we have identified 31 candidates by a homoatomic substitution in MXPn, varying the Group 14 M-element from Si to Pb, the Group 17 X-element from F to I and replacing the pnictide (Pn) phosphorus by arsenic. The double-helical structure of SnIP has been used as the starting model for all candidates and the electronic structure and vibrational spectra were determined within the framework of density functional theory (DFT). Varying the outer MX or the inner Pn helix led to the conclusion that iodide- and bromide-containing MXPn compounds show similar structures to SnIP. Here, the calculations indicate interesting effects for electronic band-gap tuning. For the highly polarized fluorides, a segregation of the helices to more complex MX substructures is predicted.

Extracting Crystal Chemistry from Amorphous Carbon Structures

Carbon allotropes have been explored intensively by ab initio crystal structure prediction, but such methods are limited by the large computational cost of the underlying density functional theory (DFT). Here we show that a novel class of machine-learning-based interatomic potentials can be used for random structure searching and readily predicts several hitherto unknown carbon allotropes. Remarkably, our model draws structural information from liquid and amorphous carbon exclusively, and so does not have any prior knowledge of crystalline phases: it therefore demonstrates true transferability, which is a crucial prerequisite for applications in chemistry. The method is orders of magnitude faster than DFT and can, in principle, be coupled with any algorithm for structure prediction. Machine-learning models therefore seem promising to enable large-scale structure searches in the future.

Dispersion interactions in silicon allotropes

Periodic local-MP2 and DFT-D3 calculations show that dispersion interactions in silicon allotropes can change the energy ordering significantly.

Inorganic Double Helices in Semiconducting SnIP

SnIP is the first atomic-scale double helical semiconductor featuring a 1.86 eV bandgap, high structural and mechanical flexibility, and reasonable thermal stability up to 600 K. It is accessible on a gram scale and consists of a racemic mixture of right- and left-handed double helices composed by [SnI] and [P] helices. SnIP nanorods <20 nm in diameter can be accessed mechanically and chemically within minutes.

Si96: A New Silicon Allotrope with Interesting Physical Properties

The structural mechanical properties and electronic properties of a new silicon allotrope Si₉₆ are investigated at ambient pressure by using a first-principles calculation method with the ultrasoft pseudopotential scheme in the framework of generalized gradient approximation. The elastic constants and phonon calculations reveal that Si₉₆ is mechanically and dynamically stable at ambient pressure. The conduction band minimum and valence band maximum of Si₉₆ are at the R and G point, which indicates that Si₉₆ is an indirect band gap semiconductor. The anisotropic calculations show that Si₉₆ exhibits a smaller anisotropy than diamond Si in terms of Young’s modulus, the percentage of elastic anisotropy for bulk modulus and shear modulus, and the universal anisotropic index A^U. Interestingly, most silicon allotropes exhibit brittle behavior, in contrast to the previously proposed ductile behavior. The void framework, low density, and nanotube structure make Si₉₆ quite attractive for applications such as hydrogen storage and electronic devices that work at extreme conditions, and there are potential applications in Li-battery anode materials.

Ambient-Pressure Polymerization of Carbon Anions in the High-Pressure Phase Mg2C

Experimental and theoretical methods were employed to investigate the ambient-pressure, metastable phase transition pathways for Mg2C, which was recovered after high-pressure synthesis. We demonstrate that at temperatures above 600 K isolated C(4-) anions within the Mg2C structure polymerize into longer-chain carbon polyanions, resulting in the formation of the α-Mg2C3 (Pnnm) structure, which is another local energy minimum for the carbon-magnesium system. Access to the thermodynamic ground state (decomposition into graphite) was achieved at temperatures above ∼1000 K. These results indicate that recoverable high-pressure materials can serve as useful high-energy precursors for ambient-pressure materials synthesis, and they show a novel mechanism for the formation of carbon chains from methanide structures.

First-principles investigation of novel polymorphs of Mg2C

The calculated enthalpy curves as a function of pressure for novel Mg₂C polymorphs relative to the cubic phase.

From zeolite nets to sp3 carbon allotropes: a topology-based multiscale theoretical study

Based on the topological approach, we predicted six novel low-energy sp³-carbon allotropes that might be engineered from diamond thin films and graphene.