Ternary Rare-Earth Arsenides REZn3As3 (RE = La–Nd, Sm) and RECd3As3 (RE = La–Pr)

Ternary rare-earth zinc arsenides REZn3As3 (RE = La–Nd, Sm) with polymorphic modifications different from the previously known defect CaAl2Si2-type forms, and the corresponding rare-earth cadmium arsenides RECd3As3 (RE = La–Pr), have been prepared by reaction of the elements at 800 °C. LaZn3As3 adopts a new orthorhombic structure type (Pearson symbol oP28, space group Pnma, Z = 4, a = 12.5935(8) Å, b = 4.1054(3) Å, c = 11.5968(7) Å) in which ZnAs4 tetrahedra share edges to form ribbons that are fragments of other layered arsenide structures; these ribbons are then interconnected in a three-dimensional framework with large channels aligned parallel to the b direction that are occupied by La3+ cations. All remaining compounds adopt the hexagonal ScAl3C3-type structure (Pearson symbol hP14, space group P63/mmc, Z = 2; a = 4.1772(7)–4.1501(2) Å, c = 20.477(3)–20.357(1) Å for REZn3As3 (RE = Ce, Pr, Nd, Sm); a = 4.4190(3)–4.3923(2) Å, c = 21.4407(13)–21.3004(8) Å for RECd3As3 (RE = La–Pr)) in which [M3As3]3– layers (M = Zn, Cd), formed by a triple stacking of nets of close-packed As atoms with M atoms occupying tetrahedral and trigonal planar sites, are separated by La3+ cations. Electrical resistivity measurements and band structure calculations revealed that orthorhombic LaZn3As3 is a narrow band gap semiconductor.