New insights into the structural and dynamical features of lithium hexaoxometalates Li7MO6 (M= Nb, Ta, Sb, Bi)

We present a (re)investigation of the hexaoxometalates Li8MO6 (M = Sn, Pb, Zr, Hf) and Li7MO6 (M = Nb, Ta, Sb, Bi). Lithium motion and ionic conductivity in the hexaoxometalates were studied using impedance spectroscopy (for Li7MO6, M = Sb, Bi, Ta) and Li-6 and Li-7 solid-state nuclear magnetic resonance (for Li7TaO6). The NMR data indicate a considerable exchange of Li among the tetrahedral and octahedral voids even at ambient temperature. In an investigation of the crystal structures using laboratory and synchrotron X-ray powder diffraction techniques, the structures of Li7TaO6, Li7NbO6, and Li7SbO6 could be solved and refined. All three reveal a triclinic metric (Li7SbO6, triclinic, P (1) over bar, a = 5.38503(6) Angstrom, b = 5.89164(7) Angstrom, c = 5.43074(6) Angstrom, alpha = 117.2210(6)degrees, beta = 119.6311(6)0, gamma = 63.2520(7)degrees, V = 127.454(3) Angstrom(3), Z = 1; Li7NbO6, triclinic, P (1) over bar, a = 5.37932(9) Angstrom, b = 5.91942(11) Angstrom, c = 5.37922(9) Angstrom, alpha = 117.0033(9)-, beta = 119.6023(7)degrees, gamma = 63.2570(9)degrees, V = 126.938(4) Angstrom(3), Z = 1; Li7TaO6, triclinic, P (1) over bar, a = 5.38486(2) Angstrom, b = 5.92014(3) Angstrom, c = 5.38551(2) Angstrom, alpha = 117.0108(2)degrees, beta = 119.6132(2)degrees, gamma = 63,2492(2)degrees, V = 127.208(1) Angstrom(3), Z = 1.