Cubic tungsten nitrides with high elastic stiffness are promising replacements for metal carbides used in tool applications to achieve enhanced working efficiency. However, due to the difficulties in preparing these nitrides at ambient pressure, their crystal structures and mechanical properties remain largely elusive, which have limited the functionality of these materials. Here, we report a comprehensive study of cubic tungsten nitrides synthesized by a high-pressure method, leading to definitive structural identifications of rocksalt cF8-WN and NbO-type cP6-WN involving atomic deficiencies. Combined with calculations, we find that the structural stabilities of both nitrides are closely related to the atomic deficiency that prevents the filling of unfavorable W: 5d-t2g bands. The disordered N vacancies are decisive for stabilizing cF8-WN, while the ordering of W and N vacancies occurs at 3 GPa and relatively low temperatures and leads to the formation of nearly stoichiometric cP6-WN, rather than previously misassigned cP7-W3N4. Both nitrides exhibit similar excellent mechanical and thermal properties, rivaling and even exceeding WC. Besides, their formation mechanisms are also explored to be associated with atomic vacancies, shedding light on the rational design of functional nitrides by defect chemistry.

We report two reversible pressure-induced isosymmetric phase transitions in α-Ag 2 S that are accompanied by two compressive anomalies at 7.5 and 16 GPa, respectively. The first transition arises from a sudden and drastic puckering of the wrinkled Ag-S layers, which leads to an anomalous structural softening at high pressure and gives rise to the ultrahigh compressive ductility in α-Ag 2 S. The second transition stems from a pressure-driven electronic state crossover from a conventional semiconductor to a topological metal. The band-crossing points near the Fermi energy form a nodal-line structure due to the preservation of the time-reversal and space-inversion symmetries under pressure. Our findings not only reveal the underlying mechanism responsible for the ultrahigh ductility in this class of inorganic semiconductors, but also provide a distinctive member to the growing family of topological metals and semimetals.

As one of important members of refractory materials, tungsten phosphide (WP) holds great potential for fundamental study and industrial applications in many fields of science and technology, due to its excellent properties such as superconductivity and as-predicted topological band structure. However, synthesis of high-quality WP crystals is still a challenge by using tradition synthetic methods, because the synthesis temperature for growing its large crystals is very stringently required to be as high as 3000 °C, which is far beyond the temperature capability of most laboratory-based devices for crystal growth. In addition, high temperature often induces the decomposition of metal phosphides, leading to off-stoichiometric samples based on which the materials’ intrinsic properties cannot be explored. In this work, we report a high-pressure synthesis of single-crystal WP through a direct crystallization from cooling the congruent W–P melts at 5 GPa and ∼ 3200 °C. In combination of x-ray diffraction, electron microscope, and thermal analysis, the crystal structure, morphology, and stability of recovered sample are well investigated. The final product is phase-pure and nearly stoichiometric WP in a single-crystal form with a large grain size, in excess of one millimeter, thus making it feasible to implement most experimental measurements, especially, for the case where a large crystal is required. Success in synthesis of high-quality WP crystals at high pressure can offer great opportunities for determining their intrinsic properties and also making more efforts to study the family of transition-metal phosphides.

Transition metal nitrides (TMNs) exhibit fascinating physical properties that show great potential in future device applications. Stacking two-dimensional TMNs with other functional materials with different orientations and symmetries requires separating epitaxial TMNs from the growth substrates. However, the lattice constants of TMNs are incompatible with those of most sacrificial layers, resulting to a great challenge in fabricating high-quality single-crystalline TMN membranes. In this study, we report the application of a water-soluble BaO sacrificial layer as a general method for creating freestanding TMN membranes. Using CrN as an example, the relatively small lattice mismatch and identical cubic structure between BaO and CrN ensure the formation of heterostructures. We directly observe the planar atomic structure and correlate its electronic state with its intrinsic transport properties using millimeter-size CrN membrane. Our research enables the fabrication of freestanding TMN membranes and transfers them to arbitrary substrates. By integrating TMN membranes with other materials will stimulate further studies in the emergent phenomena at heterointerfaces.

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