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) A, b=11.4550(10) A, c=11.6670(10) A, 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.
[1]
S. Haile,et al.
Comparison of the crystal chemistry of selected MSi6O15-based silicates
,
1997
.
[2]
S. Haile,et al.
Hydrothermal synthesis of new alkali silicates I. Potassium neodymium phases
,
1993
.
[3]
F. Liebau,et al.
Structural Chemistry of Silicates: Structure, Bonding, and Classification
,
1985
.
[4]
R. D. Shannon,et al.
Ionic conductivity in sodium yttrium silicon oxide (Na5YSi4O12)-type silicates
,
1978
.
[5]
R. D. Shannon.
Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides
,
1976
.
[6]
John B. Goodenough,et al.
Fast Na+-ion transport in skeleton structures
,
1976
.
[7]
Bruno Scrosati,et al.
Fast Ion Transport in Solids
,
1993
.
[8]
G. D. Rieck,et al.
International tables for X-ray crystallography
,
1962
.