Novel Tin Structure Motives in Superconducting BaSn5 – The Role of Lone Pairs in Intermetallic Compounds [1]

BaSn₅ is the tin richest phase in the system Ba/Sn and is obtained by stoichiometric combination of the elements. The compound peritecticly decomposes under formation of BaSn₃ and a Sn–Ba melt at 430 °C. The structure shows a novel structure motive in tin chemistry. Tin atoms are arranged in graphite‐like layers (honeycombs). Two such layers form hexagonal prisms which are centered by Sn. Consequently the central tin atom has the unusual coordination number 12. The two‐dimensional tin slabs which consist of two 3⁶ and one 6³ nets of Sn atoms are separated by 6³ nets of Ba atoms with Ba above the center of each tin hexagon. The structure of BaSn₅ can be rationalized as a variante of AlB₂ and thus also of the superconducting MgB₂. Temperature dependent magnetic susceptibility measurements show that BaSn₅ is superconducting with T_c = 4.4 K. Reinvestigation of the magnetism of the Ba richer phase BaSn₃ reveals for this compound a T_c of 2.4 K. LMTO band structure and density of states calculations verify the metallic behavior of BaSn₅. The van Hove scenario of high‐temperature cuprate superconductors is discussed for this ‘classical' intermetallic superconductor. An analysis of the electronic structure with the help of fat‐band projections and the electron localization function (ELF) shows that the van Hove singularity in the DOS originates from non‐bonding (lone) electron pairs in the intermetallic phase BaSn₅. The role of lone pairs in intermetallic phases is discussed with respect to superconducting properties.