Cage compound Sc₅Pt₂₄B₁₂: 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 Im3̄, a = 10.2042(6) Å). The refinement indicated the occupancy of icosahedral 2a and cubooctahedral 8c sites solely by Sc atoms which leads to the composition Sc₅Pt₂₄B₁₂ in contrast to the previously reported ternary stannides of Gd₃Ni₈Sn₁₆ type (RE_5-xM₁₂Sn_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 LaFe₄P₁₂vs. the Remeika phase of Yb₃Rh₄Sn₁₃-type are discussed. Analysis of chemical bonding classifies Sc₅Pt₂₄B₁₂ 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 8c 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. Sc₅Pt₂₄B₁₂ exhibits superconductivity with a transition temperature of T_C = 2.45 K.