Li2Ca2Si2O7: Structural, spectroscopic and computational studies on a sorosilicate

•We report the crystal structure of the sorosilicate Li2Ca2Si2O7 at ambient conditions.•The thermal expansion tensor as well as the evolution of the structure between 25 and 700 °C was studied.•A topological analysis of the tetrahedral framework based on [SiO4]- and [LiO4]-units is presented.•The crystal structure of Li2Ca2Si2O7 is discussed with respect to related compounds.•Interpretation of the Raman spectra of the crystals has been aided by DFT-calculations.

Synthesis experiments in the system Li2O–CaO–SiO2 resulted in the formation of single-crystals of Li2Ca2Si2O7. Structural investigations were based on single-crystal diffraction. At ambient conditions the compound has the following basic crystallographic data: hexagonal symmetry, space group P6122, a=5.0961(2) Å, c=41.264(2) Å, V=928.07(6) Å3, Z=6. Structure solution was performed using direct methods. The final least-squares refinement calculations converged at a residual of R(|F|)=0.0260. From a structural point the lithium calcium silicate belongs to the group of pyrosilicates containing [Si2O7]-groups. Additional lithium and calcium cations are incorporated between the silicate dimers and are coordinated by four and six nearest oxygen neighbours, respectively. Each [LiO4]-tetrahedron shares two common corners with directly neighboring tetrahedra forming zweier single-chains which are running parallel to 〈1 0 0〉 in z-levels defined by the presence of the 61[0 0 1]-screw axes. From the corner-sharing [LiO4]- and [SiO4]-moieties a three dimensional framework can be constructed. An interesting feature of this framework is the presence of an O[3]-type bridging oxygen linking three tetrahedra (one [LiO4]- and two [SiO4]-units). Structural similarities with other silicates are discussed in detail. The high-temperature behavior of the Si–O, Ca–O and Li–O bond distances in Li2Ca2Si2O7 was investigated by in-situ single-crystal X-ray diffraction in the range between 65 and 700 °C. From the evolution of the lattice parameters, the thermal expansion tensor αij has been determined. The structural characterization has been supplemented by micro-Raman spectroscopy. Interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species has been aided by DFT-calculations.

Graphical abstractFramework of [SiO4]- and [LiO4]-tetrahedra in the crystal structure of Li2Ca2Si2O7Figure optionsDownload full-size imageDownload as PowerPoint slide