We have determined the crystal structure of the title compound, which has a triclinic cell with cell parameters of a=6.844 Å, b=6.933 Å, c=9.339 Å, α =76.617 °, β =84.188 °, γ =74.510 ° and space group Pī. The crystal structure suggests the chemical formula CoMoO 4. 3/4H 2 O. The structure consists of MoO 4 tetrahedra and CoO 6 octahedra, confirming the earlier XANES investigation on the hydrate. The comparison of the crystal structures of the hydrate and the α-, β-, and hp-phases shows that the hydrate exhibits metal cation coordinations similar to those of the β-phase, but had arrangements of CoO 6 and MoO n polyhedra similar to those of the hp-phase.

We have explored several structure-inheriting solid-state reactions (SISSRs) under hydrothermal conditions for syntheses in the Co-Mo-O system. And we found an interesting hydrothermal SISSR from CoMoO 4. 3/4H 2 O to high-pressure (hp-) phase of CoMoO 4 , which enabled us to considerably reduce the severe conditions for the synthesis of hp-CoMoO 4 . As similar hydrothermal SISSRs are expected to be useful tools for material syntheses, we also briefly discuss them as a means of developing novel material syntheses and designs.

a b s t r a c tHydrothermal synthesis in the M/Mo/O (M ¼ Co,Ni) system was investigated. Novel transition metal tetramolybdate dihydrates MMo 4 O 13 Á 2H 2 O (M ¼ Co,Ni), having an interesting pillared layer structure, were found. The molybdates crystallize in the triclinic system with space group PÀ1, Z ¼ 1 with unit cell (6)1 for NiMo 4 O 13 Á 2H 2 O The structure is composed of two-dimensional molybdenum-oxide (2D Mo-O) sheets pillared with CoO 6 octahedra. The 2D Mo-O sheet is made up of infinite straight ribbons built up by corner-sharing of four molybdenum octahedra (two MoO 6 and two MoO 5 OH 2 ) sharing edges. These infinite ribbons are similar to the straight ones in triclinic-K 2 Mo 4 O 13 having 1D chain structure, but are linked one after another by corner-sharing to form a 2D sheet structure, like the twisted ribbons in BaMo 4 O 13 Á 2H 2 O (or in orthorhombic-K 2 Mo 4 O 13 ) are.

Cobalt I 7200 Transition Metal Tetramolybdate Dihydrates MMo4O13·2H2O (M: Co, Ni) Having a Novel Pillared Layer Structure. -The title compounds (III) crystallize in the triclinic space group P1 with Z = 1 (single crystal XRD). Tetranuclear units of four edge-sharing MoO6 octahedra are connected via corners to ribbons, which are further linked by corner sharing to form molybdenum oxide sheets. These Mo-O sheets are pillared by CoO6 octahedra. -(EDA*, K.; OHSHIRO, Y.; NAGAI, N.; SOTANI, N.; WHITTINGHAM, M. S.; J. Solid State Chem. 182 (2009) 1, 55-59; Dep. Chem., Grad. Sch. Sci., Kobe Univ., Nada, Kobe 657, Japan; Eng.) -Schramke 13-021