Flux-Assisted Growth and Characterization of Noncentrosymmetric Quaternary Rare-Earth Selenides CsRESiSe ₄ (RE = La, Pr, Nd, Sm, Gd, Tb): Crystal Structure, Magnetism, and Theoretical Studies

A series of six cesium-containing rare-earth selenosilicates, CsLaSiSe 4, CsPrSiSe 4, CsNdSiSe 4, CsSmSiSe 4, CsGdSiSe 4, and CsTbSiSe 4, with the general formula CsRESiSe 4 (RE = La, Pr, Nd, Sm, Gd, and Tb), have been successfully synthesized using a flux-assisted boron chalcogen mixture (BCM) method. Single-crystal X-ray diffraction analysis confirms that all compounds crystallize in the noncentrosymmetric orthorhombic space group P 2 1 2 1 2 1, adopting a layered structural framework with 2 ∞ [RESiSe 4 ] − two-dimensional layers. The magnetic properties of CsPrSiSe 4, CsNdSiSe 4, CsSmSiSe 4, and CsGdSiSe 4 were investigated using polycrystalline powder samples. Temperature-dependent magnetic susceptibility measurements exhibit Curie–Weiss behavior across a wide temperature range, indicating predominantly paramagnetic interactions. The effective magnetic moments are determined to be 3.46, 3.61, 1.47, and 7.1 μ B per formula unit for CsPrSiSe 4, CsNdSiSe 4, CsSmSiSe 4, and CsGdSiSe 4, respectively. Additionally, band gaps estimated from UV–vis spectroscopy are 2.7(1) eV for CsLaSiSe 4, 2.5(1) eV for CsPrSiSe 4, 2.4(1) eV for CsNdSiSe 4, 2.0(1) eV for CsSmSiSe 4, and 2.5(1) eV for CsGdSiSe 4, confirming the semiconducting nature of these materials. The electronic structures were further investigated using first-principles density functional theory (DFT) calculations to provide deeper insight into their bonding and electronic properties.