Synthesis of a series of rare-earth-based multi-anion chalcogenide iodides RE3Si2SexS8−xI (RE = La, Ce, Pr, and Nd) using the flux-assisted boron-chalcogen mixture method

Abstract

Single crystals and polycrystalline powders of rare earth mixed chalcogenide iodides La3Si2Se1.21S6.79I, Ce3Si2Se1.39S6.61I, Pr3Si2Se1.22S6.78I, and Nd3Si2Se1.18S6.82I were prepared using the reactive flux-assisted boron-chalcogen mixture (BCM) method at 850 °C. All compounds crystallized in the monoclinic crystal system, space group C2/c (space group number 15). The series adopts the La3Si2O8Cl structure type, containing isolated SiQ4 tetrahedra connected by REQ8 (RE = La, Ce, Pr and Nd) polyhedra; this arrangement creates tunnels that are filled by I atoms. The partial substitution of S by Se was carried out to modulate the optical properties. Phase pure samples and uniform solid solutions were obtained for all compositions as determined using powder X-ray diffraction patterns. Polycrystalline powders were used for physical property measurements, including magnetic susceptibility and UV-Vis diffuse reflectance. The solid-state UV-Vis data for the polycrystalline La3Si2Se1.21S6.79I, Ce3Si2Se1.39S6.61I, and Pr3Si2Se1.22S6.78I samples revealed band gaps of Eg = 2.5(1), 2.2(1), and 2.3(1) eV, typical of semiconductors. Magnetic measurements indicated that Ce3Si2Se1.39S6.61I and Nd3Si2Se1.18S6.82I exhibit paramagnetic behavior with slightly negative Weiss constants θ = −25 and −38. The photoluminescence spectrum of Ce3Si2Se1.39S6.61I exhibits a broad emission band around ∼493 nm.