The Structure and Conductivity of K_8Nd_3Si_(12)O_(32)(OH): A Layered Silicate with Paths for Possible Fast-Ion Condution

Abstract

Hydrothermally grown crystals of K_8Nd_3Si_(12)O_(32)(OH) have been examined by single crystal X-ray methods. The compound crystallizes in space group P bar l and has lattice constants a=6.9660(6) Å, b=11.4550(10) Å, c=11.6670(10) Å, α=87.677(8)°, β=87.491(9)°, γ=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≥2.5σ(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 K_8Yb_3Si_(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 α-K_3NdSi_6O_(15)·2H_2O and β-K_3NdSi_6O_(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 σ=[1.4×10^3 K/Ωcm/T]×exp(−0.8 eV/k_bT). Charge transport appears to occur by the motion of OH^− ions.

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