K2Ca6Si4O15—structural and spectroscopical studies on a mixed tetrahedral–octahedral framework

In the course of an excursion into the system K2O–CaO–SiO2, single crystalline material of the previously unknown compound K2Ca6Si4O15 has been obtained. Single crystal X-ray diffraction experiments revealed that the new phase is monoclinic (space group P12/c1) with the following basic crystallographic data: a=7.3782(8) Å, b=5.5677(5) Å, c=17.2466(17) Å, β=90.005(8)°, Z=2. According to Liebauʹs nomenclature, the compound can be classified as a mixed anion silicate containing insular [SiO4]-groups as well as [Si2O7]-dimers in the ratio 2:1, i.e. the crystallochemical formula can be written as K2Ca6[SiO4]2[Si2O7]. The silicate anions are linked by K- and Ca-ions distributed among five different non-tetrahedral M-positions and coordinated by six to eight nearest oxygen neighbors. Alternatively, the structure can be described as a heteropolyhedral framework built up by kröhnkite-type [M(SiO4)2O2]-chains in which the MO6 octahedra are corner-linked to bridging SiO4 tetrahedra. The chains (running parallel to [0 1 0]) are located in 4.6 Å wide layers parallel to (1 0 0). Neighboring sheets are shifted relative to each other by an amount of +δ or −δ along [0 0 1]. In the derived two layer …ABABAB… stacking sequence, chains belonging to adjacent sheets are linked by corner sharing of common oxygen atoms. The resulting network contains tunnels in which the more irregularly coordinated K- and Ca-ions are incorporated for charge compensation. A comparison between the present compound and structurally related mixed tetrahedral–octahedral frameworks is given. The characterization has been completed by Raman and FTIR-spectroscopy. An allocation of the bands to certain vibrational species has been aided by density functional theory (DFT) calculations.