Exploration of AB3Si3 (A = Na/K/Rb/Cs) compounds under moderate pressure

We discovered the composition of ternary AB3Si3 (A = Na/K/Rb/Cs) compounds in the moderate pressure range of 0-100 GPa using first-principles structural prediction and systematically analyzed their structures, stability, electronic and optical properties within the framework of density functional theory. The AB3Si3 compounds exhibit a diverse phase diagram, including nine structures that are selected based on formation energies, along with a known clathrate RbB3Si3 structure with Pm3̄n symmetry. All predicted phases are thermodynamically and dynamically stable within the studied pressure range. In particular, the KB3Si3 compound with a direct band gap of 1.0 eV is identified as a promising candidate for photovoltaic materials beyond silicon-based materials, among which boron atoms form a unique regular octahedral structure; in contrast, NaB3Si3 and RbB3Si3 compounds are shown to have metallicity. Our findings enrich crystal structures of alkali-metal borosilicides and provide valuable insights into their potential applications.

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.

From Three-Dimensional Clathrates to Two-Dimensional Zintl Phases AMSb2 (A = Rb, Cs; M = Ga, In) Composed of Pentagonal M-Sb Rings

Three new antimonide Zintl phases, RbGaSb₂, CsGaSb₂, and CsInSb₂, were discovered during exploration of corresponding A-M-Sb (A = Rb, Cs; M = Ga, In) ternary systems while searching for new clathrates. The AGaSb₂ phases crystallize in the tetragonal space group P4₂/nmc (No. 137) in the LiBS₂ structure type, while CsInSb₂ crystallizes in lower symmetry in the orthorhombic space group Cmce (No. 64) in the KGaSb₂ structure type with additional disorder of one of the Cs sites. The crystal structures of all three reported AMSb₂ compounds are composed of two-dimensional [MSb₂]^- tetrahedral layers separated by Rb⁺ or Cs⁺ cations. [MSb₂]^- layers are built from fused M-Sb pentagons and hexagons, which are also the main structural units for A₈M₂₇Sb₁₉ clathrate cages. The semiconductor nature of AMSb₂ was suggested by band structure calculations and confirmed by transport property characterization. CsGaSb₂ is a rare example of an n-type pnictide Zintl phase. All reported compounds exhibit low thermal conductivity typical for complex antimonides of heavy elements.

Cage Adaption by High-Pressure Synthesis: The Clathrate-I Borosilicide Rb8B8Si38

Rb₈B₈Si₃₈ forms under high-pressure, high-temperature conditions at p = 8 GPa and T = 1273 K. The new compound (space group Pm3̅n, a = 9.9583(1) Å) is the second example for a clathrate-I borosilicide. The phase is inert against strong acids and bases and thermally stable up to 1300 K at ambient pressure. (Rb⁺)₈(B^-)₈(Si⁰)₃₈ is electronically balanced, diamagnetic, and shows semiconducting behavior with moderate Seebeck coefficient below 300 K. Chemical bonding analysis by the electron localizability approach confirms the description of Rb₈B₈Si₃₈ as Zintl phase.

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.

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.

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.