Prediction of an Extended Ferroelectric Clathrate

Al2B12C with High Ambipolar Mobility Driven by a Unique B-C Framework

The development of materials with high ambipolar mobility is pivotal for advancing multifunctional applications, yet such materials remain scarce. Presently, cubic boron arsenide (BAs) stands out as the premier ambipolar material, demonstrating an ambipolar mobility of ∼1600 cm2 V-1 s-1 at room temperature [Science 2022, 377, 433 and Science 2022, 377, 437]. Herein, we illustrate that semiconducting Al2B12C, featuring a nonclathrate B-C framework in which a C atom bonds to the vertices of four distorted hexagonal antiprism B12 units via quasi-sp3 hybridization, is predicted to possess ambipolar carrier transport behavior. Its ambipolar mobility can reach up to ∼2095 cm2 V-1 s-1. The hole transport originates from the C pz orbitals that trap the electrons of Al atoms at the valence band maximum, forming a C-Al-C hole channel along the c-axis direction, whereas electron transport stems from the π electrons in B12 units. For Al2B12C, polar optical phonon scattering serves as the primary mechanism limiting mobility. Additionally, it displays a high absorption coefficient (105 cm-1) in the visible spectrum. These appealing properties make Al2B12C a highly promising environmentally friendly semiconductor for applications in electronics and photovoltaic devices.

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.

New High-Performance Piezoelectric: Ferroelectric Carbon-Boron Clathrate

High-performance piezoelectrics have been extensively reported with a typical perovskite structure, in which a huge breakthrough in piezoelectric constants is found to be more and more difficult. Hence, the development of materials beyond perovskite is a potential means of achieving lead-free and high piezoelectricity in next-generation piezoelectrics. Here, we demonstrate the possibility of developing high piezoelectricity in the nonperovskite carbon-boron clathrate with the composition of ScB_{3}C_{3} using first-principles calculations. The robust and highly symmetric B-C cage with mobilizable Sc atom constructs a flat potential valley to connect the ferroelectric orthorhombic and rhombohedral structures, which allows an easy, continuous, and strong polarization rotation. By manipulating the cell parameter b, the potential energy surface can be further flattened to produce an extra-high shear piezoelectric constant d_{15} of 9424  pC/N. Our calculations also confirm the effectiveness of the partial chemical replacement of Sc by Y to form a morphotropic phase boundary in the clathrate. The significance of large polarization and high symmetric polyhedron structure is demonstrated for realizing strong polarization rotation, offering the universal physical principles to aid the search for new high-performance piezoelectrics. This work takes ScB_{3}C_{3} as an example to exhibit the great potential for realizing high piezoelectricity in clathrate structure, which opens the door to developing next-generation lead-free piezoelectric applications.

Conventional High-Temperature Superconductivity in Metallic, Covalently Bonded, Binary-Guest C-B Clathrates

Inspired by the synthesis of XB₃C₃ (X = Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X = Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm3̅ symmetry XYB₆C₆ binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atm, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, T_c, can be tuned by changing the average oxidation state of the metal atoms, with T_c being highest for an average valence of +1.5. KPbB₆C₆, with an ambient-pressure Eliashberg T_c of 88 K, is predicted to possess the highest T_c among the studied Pm3̅n XB₃C₃ or Pm3̅ XYB₆C₆ phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques and then quenched to ambient conditions.

Prediction of Novel Boron-carbon Based Clathrates

Clathrates are inclusion compounds featured with host framework cages and trapped guest atoms or small molecules. Recently, the first boron-carbon (B-C) clathrate SrB3C3 was successfully synthesized at high pressures near...

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.