Topochemical Reduction of Rynersonite-type Niobium and Tantalum Oxides

SrM2O6 (M = Nb, Ta) phases adopt rynersonite-type structures, in which pairs of MO6 octahedra share edges to form ‘dimers’. Reaction of SrTa2O6 with LiH reductively inserts lithium into the framework, resulting in a change in crystal symmetry (Pnma to P21/n) yielding a phase in which there are two crystallographically distinct types of M-M dimer: one with a short Ta-Ta distance (2.71Å) and one with a long Ta-Ta distance (3.15Å). Thermogravimetric data are consistent with a formula of LiSrTa2O6, suggesting that a mixed valent Ta4+/Ta5+ phase has been prepared. We believe the short Ta-Ta contact is the first example of a Ta-Ta bond in an extended oxide. Lithiation of SrNb2O6, SrNbTaO6 and CaTa2O6 leads to qualitatively similar materials. We propose to collect neutron powder diffraction data from the SrM2O6 and lithiated LiSrM2O6 phases to establish their crystal structures so we can better understand the electronic states of the materials.