K8CaSi10O25 – Synthesis, in-situ high-temperature single-crystal diffraction and heat capacity

Abstract

Different sol-gel routes were tested to synthesize polycrystalline material of K8CaSi10O25. The best result in terms of phase purity was obtained using combustion synthesis with glycine as fuel. Single crystals were obtained from the devitrification of a glass. Cyclic single-crystal diffraction studies were carried out within the temperature range of ambient conditions to 873 K with different cooling rates. The crystal structure (space group R3c) at ambient temperature was consistent with previous investigations (a = 11.1135(6) Å, c = 21.9286(14) Å, V = 2345.5(2) Å3). A new high-temperature polymorph was discovered, showing the following crystallographic data at 873 K: space group R3‾m, a = 11.2251(8) Å, c = 11.1789(9) Å, V = 1219.9(2) Å3. On heating, an incommensurately modulated structure was observed between the two aforementioned modifications. The standard third-law entropy was determined from low-temperature Cp’s measured by relaxation calorimetry and amounts to S°(298 K) = 898.7 ± 6.3 J mol−1 K−1.