K2Ca3Si3O10, a novel trisilicate: high-pressure synthesis, structural, spectroscopic and computational studies

Single crystals of K2Ca3Si3O10 have been obtained in a multi-anvil high-pressure synthesis experiment performed at 10 GPa and 1000 degrees C. The compound adopts the monoclinic space group C2/c with four formula units per cell and the following lattice parameters (at 25 degrees C): a = 10.3539(19) angstrom, b = 10.6013(16) angstrom, c = 9.8221(19) angstrom, beta = 118.079(13)degrees, V = 951.2(3) angstrom(3). The crystal structure was determined from single-crystal X-ray diffraction data using direct methods (Mo-K alpha radiation, 2 theta(max) 59.53 degrees, R-int = 5.90 %) and refined to R(vertical bar F vertical bar) = 5.64 % using 1042 observed reflections with I > 2 sigma(I). The structure belongs to the group of oligosilicates consisting of [Si3O10] groups. The trimers of the anion complex are located in layers parallel to (001) at about z approximate to 1/4 and 3/4, respectively. Ca(1)-octahedra provide linkage between (1) the Si3O10 groups of a single layer by corner sharing of the equatorial oxygen atoms of the terminal tetrahedra and (2) the trimers belonging to adjacent sheets by corner sharing of the apical O-atoms. To an upper limit of 3.3 angstrom, the remaining two crystallographically independent non-tetrahedral cation sites Ca(2) and K(1) are coordinated by 8 and 10 oxygen atoms, respectively. From a topological point of view the crystal structure of K2Ca3Si3O10 can be classified as a new type of mixed tetrahedral-octahedral framework. The thermal expansion of K2Ca3Si3O10 has been determined in the temperature range between 25 and 750 degrees C. The temperature dependence of the cell volume can be described with a second-order polynomial: V(T) = 0.00002(3) T-2 + 0.026(2)T + 949.36(34). Structural investigations were completed by Raman spectroscopic studies. The assignment of the bands to certain vibrational species was aided by density functional theory (DFT) calculations.