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Journal Article

Room- and high-temperature single crystal diffraction studies on γ-Na2Si2O5: an interrupted framework with exclusively Q3-units


Weidenthaler,  C.
Research Department Schüth, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Kahlenberg, V., Rakić, S., & Weidenthaler, C. (2003). Room- and high-temperature single crystal diffraction studies on γ-Na2Si2O5: an interrupted framework with exclusively Q3-units. Zeitschrift für Kristallographie-Crystalline Materials, 218(6), 421-431. doi:10.1524/zkri.218.6.421.20727.

Cite as: https://hdl.handle.net/11858/00-001M-0000-000F-98FD-D
Single crystals of ϒ-Na2Si2O5 have been obtained
from the crystallization of a stoichiometric glass at 520 ⁰C. The monoclinic crystals belong to space group C2/c (a=33.326(2)Å, b=14.1457(7)Å, c=26.206(2)Å, β=108.60(2)⁰, V=11709.0(1.5)Å3, Z=96,
Dcalc=2.48 g cm-3). The diffraction pattern showed the typical features of a superstructure. The structure was solved by direct methods including the pseudo-translational effects as a prior information for the normalization
process and subsequently refined to a residual of R1=0.058. The compound can be classified as an interrupted tetrahedral framework with exclusively Q3-units. Basic building units are spiral achter single chains running parallel [010]. Every four of these chains are linked via common corners, surrounding a central tunnel where the sodium atoms are located for charge compensation. The framework density has a value of 16.4 T-atoms/1000 Å3. At 563 ⁰C the monoclinic phase undergoes a structural phase
transition into a tetragonal high temperature modification (space group I41/a, a=11.869(2)Å, c=7.176(2)Å, V=1011.0(6)Å3, Z=4). The transition is induced by cooperative rotations of the tetrahedra and involves a bisection of the translation period along the spiral chains. The symmetries of both modifications are in a close relationship:
the space group of the RT-phase is a subgroup of index 24 of the space group of the HT-phase. Focussing on the connectivity of the tetrahedra the two phases are
identical and can be characterized as defect-cristobalites where 20% of the tetrahedra have been removed. Topologically
they can be described as three dimensional 3-connected nets of a type which has been already observed in the intermetallic compounds LiGe and LiSi, as well as in
the mineral grumantite.