Sm. Haile et Bj. Wuensch, Structure, phase transitions and ionic conductivity of K3NdSi6O15 center dot xH(2)O. I. alpha-K3NdSi6O15 center dot 2H(2)O and its polymorphs, ACT CRYST B, 56, 2000, pp. 335-348
Hydrothermally grown crystals of alpha-K3NdSi6O15. 2H(2)O, potassium neodym
ium silicate, have been studied by single-crystal X-ray methods. The compou
nd crystallizes in space group Pbam, contains four formula units per unit c
ell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794
(7) Angstrom, giving a calculated density of 2.683 Mg m(-3). Refinement wa
s carried out with 2161 independent structure factors to a residual, R(F),
of 0.0528 [wR(F-2) = 0.1562] using anisotropic temperature factors for all
atoms other than those associated with water molecules. The structure is ba
sed on highly corrugated (Si2O52-)(infinity) layers which can be generated
by the condensation of xonotlite-like ribbons, which can, in turn, be gener
ated by the condensation of wollastonite-like chains. The silicate layers a
re connected by Nd octahedra to form a three-dimensional framework. Potassi
um ions and water molecules are located in interstitial sites within this f
ramework, in particular, within channels that extend along [001]. Aging of
as-grown crystals at room temperature for periods of six months or more res
ults in an ordering phenomenon that causes the length of the c axis to doub
le. In addition, two phase transitions were found to occur upon heating. Th
e high-temperature transformations, investigated by differential scanning c
alorimetry, thermal gravimetric analysis and high-temperature X-ray diffrac
tion, are reversible, suggesting displacive transformations in which the la
yers remain intact. Conductivity measurements along all three crystallograp
hic axes showed the conductivity to be greatest along [001] and further sug
gest that the channels present in the room-temperature structure are preser
ved at high temperatures so as to serve as pathways for easy ion transport.
Ion-exchange experiments revealed that silver can readily be incorporated
into the structure.