Investigations on the polymorphism of K₄CaSi₆O₁₅ at elevated temperatures

In the present study, single crystals and polycrystalline material of K₄CaSi₆O₁₅ were prepared from solid-state reactions between stoichiometric mixtures of the corresponding oxides/carbonates. Heat capacity (C _p) measurements above room temperature using a differential scanning calorimeter indicated that two thermal effects occurred at approximately T ₁ = 462 K and T ₂ = 667 K, indicating the presence of structural phase transitions. The standard third-law entropy of K₄CaSi₆O₁₅ was determined from low-temperature C _p's measured by relaxation calorimetry using a Physical Properties Measurement System and amounts to S°(298K) = 524.3 ± 3.7 J·mol^-1·K^-1. For the 1^st transition, the enthalpy change ΔH ^tr1 = 1.48 kJ·mol and the entropy change ΔS ^tr1 = 3.25 J·mol^-1·K^-1, whereas ΔH ^tr2 = 3.33 kJ·mol^-1 and ΔS ^tr2 = 5.23 J·mol^-1·K^-1 were determined for the 2^nd transition. The compound was further characterized by in-situ single-crystal X-ray diffraction between ambient temperature and 1063 K. At 773 K, the high-temperature phase stable above T ₂ has the following basic crystallographic data: monoclinic symmetry, space group P2₁/c, a = 6.9469(4) Å, b = 9.2340(5) Å, c = 12.2954(6) Å, β = 93.639(3)°, V = 787.13(7) ų, Z = 2. It belongs to the group of interrupted framework silicates and is based on tertiary (Q³-type) [SiO₄]-tetrahedra. Together with the octahedrally coordinated Ca-cations, a three-dimensional mixed polyhedral network structure is formed, in which the remaining K-ions provide charge balance by occupying voids within the net. The intermediate temperature modification stable between T ₁ and T ₂ shows a (3+2)-dimensional incommensurately modulated structure that is characterized by the following q-vectors: q₁ = (0.057, 0.172, 0.379), q₂ = (-0.057, 0.172, -0.379). The crystal structures of the high- and the previously studied ambient temperature polymorph (space group Pc) are topologically equivalent and show a group-subgroup relationship. The index of the low- in the high-symmetry group is six and involves both, losses in translation as well as point group symmetry. The distortion is based on shifts of the different atom species and tilts of the 4- and 6-fold coordination polyhedra. Actually, for some of the oxygen atoms, the displacements exceed 0.5 Å. A more detailed analysis of the distortions relating to both structures has been performed using mode analysis, which revealed that the primary distortion mode transforms according to the Λ₁ irreducible representation of P2₁/c. However, other modes with smaller distortion amplitudes are also involved.