Cs₄Ca[Si₈O₁₉]: a new mixed tetrahedral–octahedral oxosilicate, its topological features and comparison with other interrupted framework silicates

Single crystals of a previously unknown caesium calcium silicate with the composition Ca₄Ca[Si₈O₁₉] have been obtained during a systematic study of the phase relations and compound formation in the system Cs₂O-CaO-SiO₂. Structure determination was based on a single-crystal diffraction data set recorded at 288 (2) K. The compound crystallizes in the monoclinic space group P2₁/n and has the following basic crystallographic parameters: a = 7.1670 (6) Å, b = 12.0884 (10) Å, c = 12.4019 (10) Å, β = 90.044 (8)°, V = 1074.47 (15) ų, Z = 2. The crystal structure was solved by direct methods. The sample showed twinning by pseudo-merohedry, which was accounted for in the subsequent least-squares refinements resulting in a residual of R1 = 0.036 for 1962 independent observed reflections and 149 parameters. The crystal structure of Cs₄Ca[Si₈O₁₉] belongs to the group of interrupted framework silicates, in which the [SiO₄] tetrahedra are linked in a three-dimensional network consisting of Q⁴ and Q³ groups in a 1:3 ratio. The linear backbones of the framework can be described as loop-branched dreier single chains. These ribbons are parallel to [100], and the translation period of about 7.2 Å along this axis reflects the periodicity of the chains. By sharing common corners, the condensation of these chains along the [001] direction leads to the formation of layers that are parallel to (010) and contain three- and nine-membered rings of tetrahedra. Alternatively, the crystal structure can be described as a mixed tetrahedral-octahedral framework between [SiO₄] tetrahedra and [CaO₆] octahedra containing cavities accommodating the caesium ions coordinated by seven and eight oxygen ligands, respectively. A detailed topological analysis of the mixed framework based on natural tiles is presented. Indeed, the net can be constructed from a total of only two different cages (tiles) having the following face symbols: [4³] and [3⁴.4⁶.6².7⁸]. A comparison with related silicates containing [Si₈O₁₉] anions and already classified as well as hitherto unclassified interrupted frameworks is presented. Finally, the thermal expansion tensor has been determined in the temperature interval between 193 K and 288 (2) K.