New layered rubidium rare-earth selenides: Syntheses, structures, physical properties, and electronic structures for RbLnSe₂

The compounds RbLnSe₂ (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Lu) have been synthesized by means of the reactive flux method at 1173 K. These isostructural compounds, which have the α-NaFe0₂ structure type, crystallize with three formula units in space group D_3d⁵-R̄3m of the trigonal system in cells at T = 153 K of dimensions (a, c in Å) La, 4.4313(4), 23.710(3); Ce, 4.3873(3), 23,656(3); Pr, 4.3524(11), 23.655(7); Nd, 4.3231(5), 23.670(4); Sm, 4.2799(4), 23,647(3); Gd, 4,2473(7), 23,689(5); Tb, 4.2197(4), 23.631(3); Ho, 4.1869(6), 23.652(5); Er, 4,1541(8), 23.576(7); Lu, 4.1294(6), 23,614(5). The structure consists of close-packed Se layers in a pseudocubic structure distorted along [111]. The Rb and Ln atoms occupy distorted octahedral sites in alternating layers, The Rb-centered octahedra share edges with the Ln-centered octahedra between layers. Within a given layer, both the Rb-centered and Ln-centered octahedra share edges with themselves. RbTbSe₂ and RbErSe₂ exhibit Curie-Weiss paramagnetism between 5 and 300 K, and RbCeSe₂ exhibits Curie-Weiss paramagnetism between 100 and 300 K. The optical transitions for RbCeSe₂, RbTbSe₂, and RbErSe₂ are in the 2.0-2.2 eV region of the spectrum, both from diffuse reflectance spectra and from first-principles calculations. These calculations also provide insight into the electronic structures and chemical bonding in RbLnSe₂. A quadratic fit for the lanthanide contraction of the Ln-Se distance is superior to the linear one only if the closed-shell atoms La and Lu are included.