Twinning and crystal slip in black monoclinic ZnP₂

Abstract

Monoclinic βZnP₂ (black ZnP₂) single crystals, synthesized in the presence of excess P, contain regions of polysynthetic lamellar-twinned structure, with dislocations and stacking fault-like features. The twin law is a* twin axis, (100) composition plane: (100) is also the slip plane. The twin composition plane migrates across (100) lattice fringes. In the revised βZnP₂ crystal structure, Zn(1) and P(4) positions are related across a (100) twin composition plane at x = 0.84 by two twin operations, with axes through is also a possible slip plane, with three partial dislocations, ½[001], ½[001], and ½[001]. A third possible twin operation relates Zn(2) and P(1) positions across a (100) twin plane at x = 0.5, with twin axis through All twin and slip operations result in very little distortion in nearest- and nextnearest-neighbor coordination geometries. Twin and stacking fault mistakes may be facilitated by approach of the monoclinic cell parameter ratio c[α sin(β — π/2)] to 4 (which yields a pseudo-orthorhombic unit cell) and by Zn_1-xP₂ nonstoichiometry.