The structure and conductivity of K8Nd3Si12O32(OH): A layered silicate with paths for possible fast-ion condution

Hydrothermally grown crystals of K8Nd3Si12O32(OH) have been examined by sin gle crystal X-ray methods, The compound crystallizes in space group P (1) o ver bar and has lattice constants a = 6.9660(6) Angstrom, b = 11.4550(10) A ngstrom, c = 11.6670(10) Angstrom, alpha = 87.677(8)degrees, beta = 87.491( 9)degrees, gamma = 79.083(8)degrees. There are 29 nonhydrogen atoms in the asymmetric unit. With one formula unit per unit cell, the calculated densit y is 2.929 Mg m(-3). Refinement was carried out with 5667 independent and s ignificant [I greater than or equal to 2.5 sigma(I)] structure factors to a residual, R(F), of 0.047 (R-w(F) = 0.057) using anisotropic temperature fa ctors for all atoms. The structure, closely related to that of K8Yb3Si12O32 (OH), is based on unusually flat Si12O32 layers that are connected by Nd oc tahedra to form a three-dimensional framework, The silicate layer contains two types of six-membered rings, an eight-membered ring, and a meandering t welve-membered ring. It can be generated from a condensation of wollastonit e-type chains and contains structural features found in the "ideal" silicat e layers of alpha-K3NdSi6O15 . 2H(2)O and beta-K3NdSi6O15. Potassium ions a re located in the interstitial sites of the neodymia-silica framework, in c hannels that run between silicate layers; hydroxyl groups reside within cha nnels that run through the layers, The conductivity in the [0 1 0] directio n is given by sigma = 1.4x 10(3) K/ Omega cm/T] x exp(-0.8 eV/k(b)T). Charg e transport appears to occur by the motion of OH- ions. (C) 1999 Academic P ress.