Unexpected stable phases of tungsten borides

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Materials Science

Background:

• Tungsten borides possess exceptional mechanical properties, rivaling superhard materials.
• Understanding their phase relations and crystal structures is crucial but challenging.

Purpose of the Study:

• To systematically investigate the tungsten-boron (W-B) system.
• To identify stable and metastable phases of tungsten borides under varying pressures.
• To characterize the mechanical properties of newly discovered phases.

Main Methods:

• Ab initio variable-composition evolutionary simulations were employed.
• Calculations were performed across a pressure range of 0 to 40 GPa.
• Structural and mechanical properties were analyzed.

Main Results:

• All known stable tungsten boride phases were successfully identified.
• Two novel stable phases (P4[combining macron]21m-WB and P21/m-W2B3) and three nearly stable phases were discovered.
• The stability of WB4 was reassessed, found to be stable above ~1 GPa, not at ambient pressure.
• P4[combining macron]21m-WB demonstrates superior hardness compared to known phases; Pmmn-WB5 exhibits the highest hardness.

Conclusions:

• The study provides critical insights into the complex phase diagram of tungsten borides.
• WB2, WB4, and WB5 are highlighted as tungsten boride compounds with significant potential for mechanical applications.
• The findings clarify previous discrepancies regarding WB4 stability and introduce new hard materials for further research.