Quaternary rare-earth selenides with closed cavities: Cs[RE9Mn4Se18] (RE = Ho-Lu)

New quaternary selenides, Cs[RE9Mn4Se18] (RE = Ho-Lu), have been synthesized by high temperature solid state reactions of elements with a reactive CsCl flux. These compounds adopt the BaV13O18-structure type in the trigonal space group R (3) over bar (no. 148) with a = 17.4867(8)-17.263(2) angstrom, c = 9.7996(8)-9.681(2) angstrom and A= 3. The major structure motif is the 3D network that is constructed by octahedra of MSe6 (centered by disordered Mn and RE) and RESe6, in which closed cavities of cuboctahedra Cs@Se-12 are embedded. This work extends the previously established empirical A/M-structure correlation in the ternary A/RE/Q system into quaternary A/RE/TM/Q, and A/M = 0.08 (A = the number of alkali metals, M = the sum of the numbers of RE and TM metals) represents the minimum ending value. The syntheses, properties, and theoretical analyses of the title compounds are also discussed.