Origin of diffuse superstructure reflections in labuntsovite-group minerals

The average crystal structures of two natural porous titanosilicates of the labuntsovite group, lemmleinite-Ba and lemmleinite-K, ideally [Na.sub.4][K.sub.4][Ba.sub.2][Ti.sub.8][([Si.sub.4][O.sub.12]).sub.4] [O.sub.4][(OH).sub.4]*8[H.sub.2]O and [Na.sub.4][K.sub.4][K.sub.2] [Ti.sub.8][([Si.sub.4][O.sub.12]).sub.4][O.sub.2][(OH).sub.6]*8 [H.sub.2]O, respectively, have been refined from single crystal X-ray diffraction data. Both samples represent an extensive solid solution with labuntsovite sensu strictu [Na.sub.4][K.sub.4][D.sub.2][Ti.sub.8] [([Si.sub.4][O.sub.2]).sub.4][O.sub.4][(OH).sub.4]*10[H.sub.2]O where D = Mn, Fe, and Mg. In addition to the sharp Bragg reflections both crystals, space group C2/m, a = 14.3, b =13.8, c = 7.75 [Angstrom], [beta] = 117[degrees], exhibit diffuse layers at c*/2 intervals indicating faulty superstructures with c = 15.7 [Angstrom]. The diffuse layers consist of two types of reflections. The dominant type is strongly diffuse and smeared along a* indicating an I-centered Bravais lattice. The other type is very sharp but also weak and is in agreement with a C-centered lattice. Models for both superstructures have been developed on the basis of crystal-chemical principles and their theoretical diffraction patterns have been calculated and compared with the observed diffuse layers yielding excellent qualitative agreement. X-ray structure refinements of the average structure at -160 and 22 [degrees]C indicate temperature independent (static) disorder of Ti within rather rigid Ti[O.sub.6] octahedra connected to chains that extend along a. This Ti disorder is interpreted in terms of long-range order of OH and O in the superstructures where these anions occupy the corner-connecting octahedral apices in an ordered fashion. An additional effect of OH, O order is an ordered arrangement of extraframework Ba (K) that only bonds to O but not to OH sites exposed on the channel walls. Temperature dependent cell dimensions between -160 and +200 [degrees]C suggest a phase transition at ca. -80 [degrees]C. However, the structural data obtained from the average structures, refined at -160 and 22 [degrees]C, did not allow us to draw crystal-chemical conclusions about the nature of the phase transition. Dehydration of the investigated lemmleinite-Ba starts at ca. 150 [degrees]C leading to increasing extraframework disorder and decreasing crystal quality as evidenced by strong smearing of the originally sharp Bragg reflections.