Fabrication of Alkali Metal Boride: Honeycomb‐Like Structured NaB₄ with High Hardness and Excellent Electrical Conductivity

Abstract

The design and fabrication of novel hard materials with excellent electrical conductivity is attractive in scientific and technological application under extreme conditions. Metal borides have brought substantial interest for decades in material science because of their strong covalent BB bonding network for intrinsic incompressibility and MM bonding for electron transportation. Here the successful synthesis of a novel hard alkali metal boride as NaB4 with high thermal stability (873 K) and metallic behaviors is reported. The relatively low synthetic P/T conditions (lowest at 1.5 GPa and 1000 K) enable the easy fabrication of large bulk materials (several centimeters in diameter). The studies reveal that the Vickers hardness value of NaB4 can reach up to 26 GPa, associated with superior incompressibility along (001) direction of honeycomb‐like boron structure that exhibits the highest shear modulus up to 96 GPa for borides. The NaB4 structure undergoes an interesting metallic‐semiconducting transition under compression up to 61 GPa. This new form of hard metal boride material with pressure‐tunable electrical properties enables the development of industrial applications as future electrical devices.

Clathrate‐Like Alkali and Alkaline‐Earth Metal Borides: A New Family of Superconductors with Superior Hardness

“…[15][16][17][18] For instance, metallic calcium borides of P4/mbm CaB 4 [19] and Pm-3m CaB 6 [20] have been fabricated at 1-2 GPa and about 1300 K, which is much lower than that required P-T conditions for the synthesis of TMB systems. Very recently, we also successfully produced a novel honeycomb-like NaB 4 structure at 1.5 GPa and 1000 K, [21] predicted by our theoretical calculations as a pressure-stabilized phase at 80 GPa that can be quenchable under ambient conditions, [22] enabling fabrication of large-size productions (several centimeters in diameter). The NaB 4 sample exhibits strong BB covalent bonds with hardness up to 26 GPa, which can be comparable to previously reported TMB materials.…”

“…The ab initio MD simulations (Figure S7, Supporting Information) show these clathrate structures still keep stable at 0 GPa and 300 K. These results manifest that the predicted Cmcm CaB 7 structure can be quenchable after releasing the pressure down to ambient conditions, which provides high possibility for the experimental synthesis at relatively moderate P-T conditions (e.g., NaB 4 , DyH 3 , and uranium polyhydrides). [21,22,39,40] The length of BB bonds expand to 1.80-2.09 Å at 0 GPa from 1.60-1.76 Å at 150 GPa, which is comparable to that of the wellknown α-B (1.67−2.00 Å) [41] and β-B (1.66−1.90 Å) phases, [42] suggesting the formation of strong covalent network in the B 23 cage, an essential element of superior mechanical properties.…”

“…Such features can lead to high electronic DOS near the Fermi level, raising possibility of the formation of Cooper pairs and strengthening the electron-phonon coupling (EPC), which thus were expected to result in superconductors. [21,[44][45][46] We further calculated the superconducting transition temperature (T c ) of CaB 7 structure by the Allen-Dynes modified McMillan equation. [47] For reasonable comparison with previously reported systems, we employed a commonly accepted value of Coulomb pseudopotential parameters (µ* = 0.1).…”

“…On the basis of previous results, [6,25,26] the formation of covalent bonds between the metal and boron atoms is a disadvantage of the superconducting behaviors, while the AM atoms usually tend to form ionic bonds with boron atoms at high pressures. [21,22,27,28] MgB 2 structure containing dominated BB bonds in the company of ionic MgB bonds, exhibits strong coupling between electrons at the Fermi level and phonons from the covalent boron network, [27] thus resulting in a record T c value of 39 K in all reported borides. Similar phenomenon is also observed in NaB 6 (T c = 15.7 K) and KB 6 (T c = 14.6 K) superconductors.…”

Synthesis, crystal structure, and properties of stoichiometric hard tungsten tetraboride, WB4

“…Inexpensive hard and superhard (with a Vickers hardness of around 40 GPa and above) materials are in much demand for various industrial applications. Novel carbides, 1 nitrides, 2–8 borides, 9–23 and other compounds of transition metals, 24,25 which can potentially combine high hardness and promising optical, electronic, and other properties, are the focus of extensive investigations. 26–28 Nowadays, one of the most widely used hard materials for commercial applications is tungsten carbide, WC.…”