Structure, phase transitions and ionic conductivity of K3NdSi6O15 center dot xH(2)O. I. alpha-K3NdSi6O15 center dot 2H(2)O and its polymorphs

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.