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

Single crystals of gamma-Na2Si2O5 have been obtained from the crystallization of a stoichiometric glass at 520 degreesC. The monoclinic crystals belong to space group C2/c (a = 33.326(2) Angstrom, b = 14.1457(7) Angstrom, c = 26.206(2) Angstrom. beta = 108.60(2)degrees, V = 11709.0(1.5) Angstrom(3), Z = 96, D-calc = 2.48 a 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 Q(3)-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 Angstrom(3). At 563 degreesC the monoclinic phase undergoes a structural phase transition into a tetragonal high temperature modification (space group I4(1)/a, a = 11.869(2) Angstrom, c = 7.176(2) Angstrom, V = 1011.0(6) Angstrom(3), 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.