Structural and superconducting properties of RbOs₂O₆ single crystals

Single crystals of RbOs₂O₆ have been grown from Rb₂O and Os in sealed quartz ampoules. The crystal structure has been identified at room temperature as cubic with the lattice constant a=10.1242(12) Å. The anisotropy of the tetrahedral and octahedral networks is lower and the displacement parameters of alkali metal atoms are smaller than those for KOs₂O₆, so the “rattling” of the alkali atoms in RbOs₂O₆ is less pronounced. The superconducting properties of RbOs₂O₆ in the mixed state have been described well within the London approach and the Ginzburg–Landau parameter κ(0)≅31 has been derived from the reversible magnetization. This parameter is field dependent and changes at low temperatures from κ≅22 (low fields) to κ≅31 at H_c2. The thermodynamic critical field H_c(0)=1.3 kOe and the superconducting gap 2Δ_o∕k_BT_c≅3.2 have been estimated. These results, together with the slightly different H_c2(T) dependence obtained for crystals and polycrystalline RbOs₂O₆, evidently prove that this compound is a weak-coupling BCS-type superconductor close to the dirty limit.