Hydrothermal Synthesis and Crystal Structure of a New BariumVanadium Bronze Ba1+xV8O21 with a Tunnel Structure

A barium vanadium bronze Ba1+xV8O21 has been hydrothermally synthesized and structurally characterized. Hydrothermal treatment of a suspension of VO2 powders in Ba(NO3)2 solution at 350°C yielded a fibrous brown compound having nonstoichiometric composition Ba1 +xV8O21 (x=0.13). Single-crystal X-ray diffractometry revealed the monoclinic system C2/m with a=15.144(6), b=3.596(4), c=14.972(3) ?, beta=90.08(3)°, and Z=2, with the refinement based on 2021 reflections with I>3sigma(I) converged to R=0.054 and Rw=0.046. A new tunnel-type bronze structure was disclosed in which VO6 octahedra and VO5 trigonal bipyramids form a V-O framework with a tunnel cavity running along [010]. The Ba atom partially occupies the tunnel site with more than half occupancy of 56.6%, which causes displacement of the Ba atom and further displacement of V and O atoms along the tunnel axis. Ba1+xV8O21 is the first tunnel-type barium vanadium bronze whose structure has been fully determined.