Ordered-vacancy-compound semiconductors: PseudocubicCdIn2Se4

Whereas substitutional adamantine compounds $A_n$ $B_{4\mathrm{-}n}$ $C_4$ (e.g., ${\mathrm{ABC}}_2$ 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) ${\mathrm{AB}}_2$ $C_4$ 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’’ ${\mathrm{CdIn}}_2$ ${\mathrm{Se}}_4$ 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 ${\mathrm{CdIn}}_2$ ${\mathrm{Se}}_4$ 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 ${\mathrm{CdIn}}_2$ ${\mathrm{Se}}_4$ 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.