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 single crystal X-ray methods. The compound crystallizes in space group P and has lattice constants a=6.9660(6) ?, b=11.4550(10) ?, c=11.6670(10) ?, alpha= 87.677(8)°, beta=87.491(9)°, gamma=79.083(8)°. 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>2.5sigma(I)] structure factors to a residual, R(F), of 0.047 (Rw(F)=0.057) using anisotropic temperature factors for all atoms. The structure, closely related to that of K8Yb3Si12O32(OH), is based on unusually flat Si12O32 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-K3NdSi6O15·2H2O and beta-K3NdSi6O15. 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=[1.4×103 K/omega cm/T]×exp(-0.8 eV/kbT). Charge transport appears to occur by the motion of OH- ions.