Fig. 1. (a) XRD patterns of recovered samples synthesized under different high P–T conditions. (b) High P–T forming region of WP. No reaction is observed below black dashed line. Phase-pure WP occurs above black line. Between the two lines formed is WP with by-products of WP₂ and WP₄.

Fig. 2. Refined XRD pattern of polycrystalline WP synthesized at 5 GPa and 1600 °C taken at ambient conditions by using a copper radiation. Inset shows a polyhedral view of its crystal structure.

Fig. 3. SEM and optical observations and single-crystal diffraction measurements for WP, showing SEM images of WP prepared at 5 GPa and different temperatures with different holding durations at (a) 5 GPa and 1600 °C for 30 min, (b) 5 GPa and 1600 °C for 60 min and followed by quenching, (c) 5.0 GPa and 1600 °C for 60 min with a cooling rate of 40 °C/min, (d) 5.0 GPa and 2400 °C for 5 min, (e) 5.0 GPa and 3000 °C for 0.5 min with a cooling rate of 600 °C/min, (f) 5.0 GPa and 3200 °C for 0.5 min with a cooling rate of 800 °C/min, (g) optical image of WP synthesized at 3200 °C. Size of the background grid is 1 mm × 1 mm, (h) Laue diffraction pattern, and secondary symmetry of the (100) plane is identified, (i) single-crystal diffraction patterns. In panels (h) and (i), diffraction data are taken at room temperature.

Fig. 4. Element mapping for WP crystals based on EDS measurement.

Fig. 5. (a) Determined TG-DTA data for polycrystalline WP. (b) XRD patterns of the sample treated at 1400 °C to look into decomposition process of panel (a).