Ordered-vacancy-compound semiconductors: Pseudocubic CdIn₂Se₄

Whereas substitutional adamantine compounds A_nB_4-nC₄ (e.g., ABC₂ chalcopyrites for n=2, or the AC and BC zinc-blende compounds for n=4 and 0) have four metal atoms around each nonmetal atom and vice versa, ordered-vacancy compounds OVC’s) AB₂C₄ have but three metal atoms (one A and two B’s) around each nonmetal site (C) while the fourth (unoccupied) site forms an ordered array of vacancies. An example for OVC’s is ‘‘pseudocubic’’ CdIn₂Se₄ which can be structurally derived from the layered alternate monolayer superlattice of CdSe and InSe (along the [001] direction) by removing half of the Cd atoms from each Cd plane. Such OVC’s form a natural bridge between crystal and impurity physics. Much like the metal vacancy in II-VI compounds (e.g., CdSe), the vacancy in CdIn₂Se₄ has associated with it (nonmetal) ‘‘dangling bonds’’ and ‘‘lone-pair’’ electrons, which, however, form a dispersed band in the crystal. Using all-electron mixed-basis electronic-structure techniques, we study the properties of such an ordered array of vacancies in CdIn₂Se₄ vis-a$ga—-vis the experimental data. We find vacancy-induced atomic relaxations (Se moves towards the vacant site), resonant broadening of the lone-pair dangling-bond states into a ≊3-eV band, and that the total charge density around the vacant site has little density and shows scant evidence of dangling bonds. We discuss the nature of the bonding in this system, comparing it to other covalent selenides and to the observed photoemission and optical data. A number of possible order-disorder transitions, including the disordering of cations on the vacant sites, are identified.