  Single crystals of KOs2O6 have been grown in a sealed quartz ampoule.
Detailed single crystal X-ray diffraction studies at room temperature show
Bragg peaks that violate Fd-3m symmetry. With a comparative structure
refinement the structure is identified as non-centrosymmetric (F-43m). Compared
to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O
octahedral network exhibit breathing mode like volume changes accompanied by
strong anisotropic character of the K channels. The crystals show metallic
conductivity and a sharp transition to the superconducting state at Tc = 9.65
K. Superconducting properties have been investigated by magnetization
measurements performed in a temperature range from 2 to 12 K and in magnetic
fields from 0 to 60 kOe. The temperature dependence of the upper critical field
Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has
been obtained near Tc. The upper critical field at zero temperature was
estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli
paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL)
coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter
alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6
might behave unconventionally at low temperatures and high magnetic fields.
