Syntheses, Structures, Optical and Magnetic Properties of Ba₂M<i>Ln</i>Se₅ (M = Ga, In; <i>Ln</i> = Y, Nd, Sm, Gd, Dy, Er)

The twelve quaternary rare-earth selenides Ba2MLnSe5 (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er) have been synthesized for the first time. The compounds Ba2GaLnSe5 (Ln = Y, Nd, Sm, Gd, Dy, Er) are isostructural and crystallize in a new structure type in the centrosymmetric space group P1̅ of the triclinic system while the isostructural compounds Ba2InLnSe5 (Ln = Y, Nd, Sm, Gd, Dy, Er) belong to the Ba2BiInS5 structure type and crystallize in the noncentrosymmetric space group Cmc21 of the orthorhombic system. The structures contain infinite one-dimensional anionic chains 1∞[GaLnSe5]4– and 1∞[InLnSe5]4–, and both chains are built from LnSe6 octahedra and MSe4 (M = Ga, In) tetrahedra in the corresponding selenides. As deduced from the diffuse reflectance spectra, the band gaps of most Ba2MLnSe5 (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er) compounds are around 2.2 eV. The magnetic susceptibility measurements on Ba2GaGdSe5 and Ba2InLnSe5 (Ln = Nd, Gd, Dy, Er) indicate that they are paramagnetic and obey the Curie–Weiss law, while the magnetic susceptibility of Ba2InSmSe5 deviates from the Curie–Weiss law as a result of the crystal field splitting. Furthermore, Ba2InYSe5 exhibits a strong second harmonic generation response close to that of AgGaSe2, when probed with the 2090 nm laser as fundamental wavelength.