On the ambient pressure polymorph of K2Ca3Si3O10—An unusual mixed-anion silicate and its structural and spectroscopic characterization

• We report the crystal structure of the ambient pressure polymorph of K2Ca3Si3O10.
• The compound is the first example of a silicate where isolated [SiO4]-tetrahedra and rings occur concomitantly.
• The crystal structure of K2Ca3Si3O10 is discussed with respect to related compounds.
• Interpretation of the Raman spectra of the crystals has been aided by DFT-calculations.
• DFT-calculations have been also used to investigate the stability ranges of the ambient and the high-pressure form.

An ambient pressure polymorph of K2Ca3Si3O10 has been synthesized via solid state reactions. Single-crystal X-ray diffraction experiments show, that this new modification crystallizes in the triclinic space group P  1¯ with the following lattice parameters: a=5.6699(6) Å, b=7.3754(12) Å, c=11.8310(13) Å, α=86.199(11)°, β=80.625(9)°, γ=88.700(11)°. The structure was solved by direct methods and subsequently refined to a residual of R1=0.0261 for 1761 independent observed reflections (I>2σ(I)) and 163 parameters. A special feature of the crystal structure is the coexistence of two different types of silicate anions. Isolated [SiO4]- tetrahedra as well as [Si4O12]- vierer single rings occur in the ratio 2:1, resulting in the crystallochemical formula K4Ca6[SiO4]2[Si4O12]. To the best of our knowledge, this is the first example of an oxo-silicate where insular and cyclic silicate anions appear concomitantly. Charge compensation is provided by Ca and K cations. All calcium atoms are coordinated by 6 oxygen atoms, forming distorted octahedra. By sharing common corners, edges and faces, these [CaO6]-polyhedra build up octahedral layer-like motifs parallel to (010). Potassium ions are located in voids between the silicate anions and [CaO6]-octahedra and are coordinated by 8–9 oxygen atoms. Further characterization of this new compound was carried out by electron microprobe analysis and Raman spectroscopy. DFT calculations were employed (i) to assign Raman bands to certain vibrational modes and (ii) to determine the relative stabilities of the monoclinic high-pressure and the triclinic ambient pressure polymorph of K2Ca3Si3O10.

An ambient pressure polymorph of K2Ca3Si3O10 was synthesized via solid state reactions. Structural analysis showed that this new compound is built up of isolated [SiO4]-tetrahedra, as well as [Si4O12]- vierer single rings – a combination of silicate anions, which has not been reported so far.Figure optionsDownload as PowerPoint slide