K4CaSi6O15—Solving a 90‐year‐old riddle

Abstract

AbstractPolycrystalline K4CaSi6O15 was prepared from (a) solid‐state reactions between stoichiometric mixtures of the corresponding oxides/carbonates and (b) combustion solution synthesis using K‐ and Ca‐nitrates, tetraethyl orthosilicate (TEOS), and glycine (fuel component) as starting reagents. The compound was characterized by powder X‐ray diffraction. Differential thermal analysis indicated that K4CaSi6O15 melts congruently at about 956°C. On cooling down from the liquid state, a distinct glass‐forming tendency was observed. Single crystals suitable for further structural investigations were obtained from sinter experiments just below the melting point. Basic crystallographic data are as follows: monoclinic symmetry, space group Pc, a = 6.9299(2) Å, b = 27.3496(9) Å, c = 12.2187(4) Å, β = 93.744(3)°, V = 2310.86(13) Å3, Z = 3. The crystal structure of K4CaSi6O15 belongs to the group of interrupted framework silicates with exclusively Q3‐units. The tetrahedral network is the first inorganic representative of the so‐called eth‐type. Charge compensation in the structure is achieved by the incorporation of potassium and calcium cations, which are coordinated by five to nine oxygen ligands. Ninety years after its first mention the present contribution proves the existence of K4CaSi6O15 as a stable phase in the ternary system K2O–CaO–SiO2.