A novel crystal polymorph of volborthite, Cu3V2O7(OH)2 2H2O
  A new polymorph of volborthite [tricopper(II) divanadium(V) heptaoxide dihydroxide dihydrate], Cu3V2O7(OH)2 2H2O, has been discovered in a single crystal prepared by hydrothermal synthesis. X-ray analysis reveals that the monoclinic structure has the space group C2/c at room temperature, which is different from that of the previously reported C2/m structure. Both structures have Cu3O6(OH)2 layers composed of edge-sharing CuO4(OH)2 octahedra, with V2O7 pillars and water molecules between the layers. The Cu atoms occupy two and three independent crystallographic sites in the C2/m and C2/c structures, respectively, likely giving rise to different magnetic interactions between CuII spins in the kagome lattices embedded in the Cu3O6(OH)2 layers. 
Hajime Ishikawa, Jun-ichi Yamaura, Yoshihiko Okamoto, Hiroyuki   Yoshida, G{\o}ran J. Nilsen and Zenji Hiroi
cond-mat.mtrl-sci