Synthesis, Crystal Structure, and Magnetic Properties of R₂Mg₃SiPn₆ (R = La, Ce; Pn = P, As)

Four new quaternary pnictides, R₂Mg₃SiPn₆ (R = La, Ce; Pn = P, As), were synthesized via high-temperature solid-state reactions and gas-phase transport reactions with iodine. Their crystal structures were determined by single crystal X-ray diffraction. All four compounds are isostructural and crystallize in a new structure type in the orthorhombic space group Pnma (No. 62, Z = 4), Pearson symbol oP48. The crystal structures of R₂Mg₃SiPn₆ are composed of two-dimensional puckered MgP₃ layers, which are connected in a three-dimensional framework by P–P dimers and MgSiP₄ double-tetrahedral chains. Rare-earth cations are encapsulated inside the channels of the framework running along [010]. Quantum-chemical calculations predict that La₂Mg₃SiP₆ is an indirect narrow bandgap semiconductor. The Mg–P bonding in MgP₄ tetrahedra and MgP₆ octahedra was analyzed by means of crystal orbital Hamilton population (COHP) analysis. Magnetic characterization of Ce-containing compounds confirmed the trivalent nature of cerium atoms and revealed complex ferrimagnetic ordering at low temperatures.