Structural variations in Gd5Si4-xSnx: size vs. electronic effects

摘要

B12 icosahedra cluster-containing rare earth borosilicides REB44Si2 (RE = Tb, Dy, Ho, Er, Tm, Yb) were grown by the floating zone method and their physical properties were investigated. REB44Si2 compounds are isostructural to REB50 and they are unique among the newly discovered boron-rich phases in that it is possible to grow large crystals with dimensions exceeding 10 mm. The rare earth atoms form a coupled chain structure like a ladder along the c-axis which is also the axis along which there is a B12 icosahedral chain. Magnetic properties, resistivity, and specific heat of the compounds were investigated. The conductivity follows the 3D variable range hopping mechanism and it was determined that localization lengths ξ are extremely short, actually close to the distances of the atoms themselves. Magnetic transitions above 4 K are indicated for all the compounds except for TmB44Si2 and it is found that specific heat measurement is the expedient probe of such compounds in which the low temperature susceptibility is influenced by external conditions such as crystal growth rate. Despite dimer-like features to the magnetic transition a spin gap is not observed. The magnetic coupling in these systems is indicated to be different from the conventional mechanisms in f-electron systems of dipole-dipole coupling or RKKY interaction.