Narrowing the gap: from semiconductor to semimetal in the homologous series of rare-earth zinc arsenides RE2−yZn4As4·n(REAs) and Mn-substituted derivatives RE2−yMnxZn4−xAs4·n(REAs) (RE = La–Nd, Sm, Gd)

Abstract

A homologous series of ternary rare-earth zinc arsenides, prepared by reactions of the elements at 750 °C, has been identified with the formula RE_2−yZn₄As₄·n(REAs) (n = 2, 3, 4) for various RE members. They adopt trigonal structures: RE_4−yZn₄As₆ (RE = La–Nd), space group Rm1, Z = 3; RE_5−yZn₄As₇ (RE = Pr, Nd, Sm, Gd), space group Pm1, Z = 1; RE_6−yZn₄As₈ (RE = La–Nd, Sm, Gd), space group Rm1, Z = 3. The Zn atoms can be partially substituted by Mn atoms, resulting in quaternary derivatives RE_2−yMn_xZn_4−xAs₄·n(REAs). Single-crystal structures were determined for nine ternary and quaternary arsenides RE_2−yM₄As₄·n(REAs) (M = Mn, Zn) as representative examples of these series. The structures are built by stacking close-packed nets of As atoms, sometimes in very long sequences, with RE atoms occupying octahedral sites and M atoms occupying tetrahedral sites, resulting in an intergrowth of [REAs] and [M₂As₂] slabs. The recurring feature of all members of the homologous series is a sandwich of [M₂As₂]–[REAs]–[M₂As₂] slabs, while rocksalt-type blocks of [REAs] increase in thickness between these sandwiches with higher n. Similar to the previously known related homologous series REM_2−xAs₂·n(REAs) which is deficient in M, this new series RE_2−yM₄As₄·n(REAs) exhibits deficiencies in RE to reduce the electron excess that would be present in the fully stoichiometric formulas. Enthalpic and entropic factors are considered to account for the differences in site deficiencies in these two homologous series. Band structure calculations indicate that the semiconducting behaviour of the parent n = 0 member (with CaAl₂Si₂-type structure) gradually evolves, through a narrowing of the gap between valence and conduction bands, to semimetallic behaviour as the number of [REAs] blocks increases, to the limit of n = ∞ for rocksalt-type REAs.