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) A. The anisotropy of the tetrahedral and octahedra
l networks is lower and the displacement parameters of alkali metal atoms are smaller than for KOs2O6, so the rattling of the alkali atoms in RbOs2O6 is less pronounced. Superconducting properties of RbOs2O6 in the mixed state have been well described within the London approach and the Ginzburg-Landau parameter kappa(0) = 31 has been derived from the reversible magnetization. This parameter is field dependent and changes at low temperatures from kappa = 22 (low fields) to kappa = 31 at H_{c2}. The thermodynamic critical field H_{c}(0) = 1.3 kOe and the superconducting gap 2delta/k_{B}T_{c} = 3.2 have been estimated. These results together with slightly different H_{c2}(T) dependence obtained for crystals and polycrystalline RbOs2O6 proof evidently that this compound is a weak-coupling BCS-type superconductor close to the dirty limit.
We have synthesized, crystallized and studied the structural and electric transport properties of organic molecular crystals based on a rubrene derivative with {em t}-butyl sidegroups at the 5,11 positions. Two crystalline modifications are observed:
one (A) distinct from that of rubrene with larger spacings between the naphtacene backbones, the other (B) with a in-plane structure presumably very similar compared to rubrene. The electric transport properties reflect the different structures: in the latter phase (B) the in-plane hole mobility of 12 cm$^2$/Vs measured on single crystal FETs is just as high as in rubrene crystals, while in the A phase no field-effect could be measured. The high crystal quality, studied in detail for B, reflects itself in the density of gap states: The deep-level trap density as low as $10^{15}$ cm$^{-3}$ eV$^{-1}$ has been measured, and an exponential band tail with a characteristic energy of 22 meV is observed. The bulk mobility perpendicular to the molecular planes is estimated to be of order of $10^{-3}$ -- $10^{-1}$ cm$^2$/Vs.
