Cage compound Sc 5 Pt 24 B 12 : a Pt-stuffed variant of filled skutterudite structure. Electronic and structural properties.

The title compound was obtained from elements via arc melting and its crystal structure was determined from single-crystal X-ray diffraction data (space group Im 3̄, a = 10.2042(6) Å). The refinement indicated the occupancy of icosahedral 2 a and cubooctahedral 8 c sites solely by Sc atoms which leads to the composition Sc 5 Pt 24 B 12 in contrast to the previously reported ternary stannides of Gd 3 Ni 8 Sn 16 type (RE 5- x M 12 Sn 24(+ x ) compounds). The compound is the first representative of borides crystallizing with a site exchange variant of this stannide structure type. The structural relationships of the boride structure and filled skutterudite LaFe 4 P 12 vs. the Remeika phase of Yb 3 Rh 4 Sn 13 -type are discussed. Analysis of chemical bonding classifies Sc 5 Pt 24 B 12 as a cage compound exhibiting the ionic interaction of cationic scandium species in the cages of anionic framework, formed by covalently bonded B and Pt atoms. Electronic structure calculations show that the electronic states of atoms centered around the cubooctahedral 8 c site, i.e. Sc2 3d-, Pt2 5d- and B 2p-states dominate the density of states (DOS) at the Fermi level E F . Strong effect of spin-orbit coupling on the band structure at the gamma point has been found from density functional theory calculations. Sc 5 Pt 24 B 12 exhibits superconductivity with a transition temperature of T C = 2.45 K.