First-Principles Study of High-Pressure Phase Stability and Electron Properties of Be-P Compounds

Abstract

New, stable stoichiometries in Be-P systems are investigated up to 100 GPa by the CALYPSO structure prediction method. Along with the BeP₂-I4₁/amd structure, we identify two novel compounds of Be₃P₂-P-42₁m and Be₃P₂-C2/m. It should be noted that the Be-P compounds are predicted to be energetically unfavorable above 40 GPa. As can be seen, interesting structures may be experimentally synthesizable at modest pressure. Our results indicate that at 33.2 GPa, the most stable ambient-pressure tetragonal Be₃P₂-P-42₁m transitions to the monoclinic Be₃P₂-C2/m structure. Moreover, the predicted Be₃P₂-P-42₁m and Be₃P₂-C2/m phases are energetically favored compared with the Be₃P₂-Ia-3 structure synthesized experimentally. Electronic structure calculations reveal that BeP₂-I4₁/amd, Be₃P₂-P-42₁m, and Be₃P₂-C2/m are all semiconductors with a narrow band gap. The present findings offer insight and guidance for exploration toward further fundamental understanding and potential applications in the semiconductor field.

Keywords: first-principles; high pressure; phase transition; structural prediction.

Figure 1

(color online). In relation to elemental beryllium and phosphorus solids, relative enthalpies of creation of the Be−P phase. Solid lines connect the stable phases to form convex shells (solid stars). Open stars indicate the unstable/meta stable phases.

Figure 2

(color online). Be−P compounds composition-pressure phase graph. The metallic and insulating phases are represented by blue and red, respectively. Stable (metastable) phases are represented by solid (dashed) lines.

Figure 3

(color online). Crystal structures of the predicted Be−P system for (a) Be₃P₂−P−42₁m, (b) Be₃P₂−C2/m, (c) Be₃P₂−Ibam, (d) Be₃P₂−Cmcm, (e) Be₃P₂−C2/m, (f) BeP₂−I4₁/amd, (g) BeP₂−P4₃2₁2, and (h) BeP₂−Imma. Be atoms and P atoms are represented by the purple and blue spheres, respectively. Inside the red circular region, the structures are stable.

Figure 4

(color online). Phonon−dispersion contours and PHDOS projected on Be and P atoms for (a) Be₃P₂−P−42₁m at 0 GPa, (b) Be₃P₂−C2/m at 33.2 GPa, and (c) BeP₂−I4₁/amd at 0 GPa.

Figure 5

(color online). Electronic band structures and PDOS for (a) Be₃P₂−P−42₁m at 0 GPa, (b) Be₃P₂−C2/m at 33.2 GPa, and (c) BeP₂−I4₁/amd at 0 GPa. Note that zero energy is on the Fermi level. The dotted (red) and solid (black) lines represent results obtained using the PBE and HSE functional, respectively.

Figure 6

(color online). The ELF graphs for the structures of (a) Be₃P₂−P−42₁m, (b) Be₃P₂−C2/m, and (c) BeP₂−I4₁/amd with isosurface of 0.8.