Na₄Co₃(PO₄)₂P₂O₇, A NEW SODIUM COBALT PHOSPHATE CONTAINING A THREE-DIMENSIONAL SYSTEM OF LARGE INTERSECTING TUNNELS. C. Ruiz-Valero¹, E. Gutierrez-Puebla¹, A. Monge¹, U. Amador², C. Parada² and F. Sanz², ¹Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049, Madrid, Spain, ²Facultad de Ciencias Químicas. UCM. 28040 Madrid, Spain

The phosphates of transition metals form a huge family of compounds. The association of metallic or semi-metallic rows or layers of MO₆ octahedra with insulating PO₄ rows or layers gives rise to original physical properties. Moreover, it is well known that many phosphates are good ionic conductors. Due to these reasons, a great effort, during the last years, has been devoted to the study of mixed transition-metal phosphates.

Single crystals of the new Co(II) phosphate Na₄Co₃(PO₄)₂P₂O₇ have been isolated and their structure has been determined by X-ray diffraction techniques. The structure of this compound is built up from corner- and edge-sharing between CoO₆ octahedra and PO₄ and P₂O₇ groups giving rise to a polyhedral connectivity which produces large tunnels running along the three main crystallographic directions The structure consists of infinite layers with composition (Co₃P₂O₁₃)_[[infinity]] parallel to the bc plane. Interlayer linkages are made via P-O-P bridges of the pyrophosphate groups in such a way that large tunnels extending along the [010] and [001] directions occur between two neighbouring sheets. Besides, there also exist channels along the [100] direction crossing the (Co₃P₂O₁₃)_[[infinity]] layers. A complex scheme of tunnel intersections gives rise to the formation of a three-dimensional channel network which hosts the sodium cations.

This compound crystallizes in the orthorhombic noncentrosymmetric space group Pn2₁a with a=18.046(5)Å, b=6.533(2)Å, c=10.536(2)Å, V=1242.1(5)ų and Z=4. The structure was refined from 1791 reflections with I>2[[sigma]](I) were used for structure solution and refinemt (R=0.039).