The structure and conductivity of K(8)Nd(3)Si(12)O(32)(OH): A layered silicate with paths for possible fast-ion condution

Hydrothermally grown crystals of K(8)Nd(3)Si(12)O(32)(OH) have been examined by single crystal X-ray methods, The compound crystallizes in space group P (1) over bar and has lattice constants a = 6.9660(6) Angstrom, b = 11.4550(10) Angstrom, 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 density is 2.929 Mg m(-3). Refinement was carried out with 5667 independent and significant {[}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 factors for all atoms. The structure, closely related to that of K(8)Yb(3)Si(12)O(32)(OH), is based on unusually flat Si(12)O(32) layers that are connected by Nd octahedra to form a three-dimensional framework, The silicate layer contains two types of six-membered rings, an eight-membered ring, and a meandering twelve-membered ring. It can be generated from a condensation of wollastonite-type chains and contains structural features found in the ``ideal{''} silicate layers of alpha-K(3)NdSi(6)O(15) . 2H(2)O and beta-K(3)NdSi(6)O(15). Potassium ions are located in the interstitial sites of the neodymia-silica framework, in channels that run between silicate layers; hydroxyl groups reside within channels that run through the layers, The conductivity in the {[}0 1 0] direction is given by sigma = 1.4x 10(3) K/ Omega cm/T] x exp(-0.8 eV/k(b)T). Charge transport appears to occur by the motion of OH(-) ions. (C) 1999 Academic Press.