  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.
