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Single crystals of  γ -Na 2 Si 2 O 5  have been obtained from the crystallization of a stoichiometric glass at 520 °C. The monoclinic crystals belong to space group  C 2/ c  ( a  = 33.326(2) Å,  b  = 14.1457(7) Å,  c  = 26.206(2) Å,  β  = 108.60(2)°,  V  = 11709.0(1.5) Å 3 ,  Z  = 96,  D  calc  = 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  R 1 = 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 Å 3 . At 563°C the monoclinic phase undergoes a structural phase transition into a tetragonal high temperature modification (space group  I 4 1 / a ,  a  = 11.869(2) Å,  c  = 7.176(2) Å,  V  = 1011.0(6) Å 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.

Room- and high-temperature single crystal diffraction studies on γ -Na 2 Si 2 O 5 : an interrupted framework with exclusively Q 3 -units