Investigations on the polymorphism of K₄CaSi₆O₁₅ at elevated temperatures

Area of Science:

• Solid-state chemistry and materials science.
• Crystallography and structural analysis of inorganic compounds.

Background:

• K4CaSi6O15 is an interrupted framework silicate with a mixed polyhedral network structure.
• Understanding its phase transitions is crucial for potential applications and fundamental materials science.

Purpose of the Study:

• To synthesize and characterize K4CaSi6O15, focusing on its thermal behavior and structural transformations.
• To determine the thermodynamic parameters associated with its phase transitions.
• To elucidate the crystal structures of its high-temperature polymorphs.

Main Methods:

• Solid-state reaction synthesis of single crystals and polycrystalline K4CaSi6O15.
• Differential scanning calorimetry (DSC) for heat capacity measurements and transition detection.
• Relaxation calorimetry for low-temperature heat capacity determination.
• In-situ single-crystal X-ray diffraction (XRD) across a temperature range.

Main Results:

• Two structural phase transitions were identified at T1 = 462 K and T2 = 667 K.
• Thermodynamic data: S°(298K) = 524.3 ± 3.7 J·mol⁻¹·K⁻¹, ΔHtr1 = 1.48 kJ·mol, ΔStr1 = 3.25 J·mol⁻¹·K⁻¹, ΔHtr2 = 3.33 kJ·mol⁻¹, ΔStr2 = 5.23 J·mol⁻¹·K⁻¹.
• The high-temperature phase (above T2) exhibits monoclinic symmetry (P21/c), while the intermediate phase shows a (3+2)-dimensional incommensurately modulated structure.

Conclusions:

• K4CaSi6O15 undergoes significant structural changes with increasing temperature, involving modulated structures and symmetry reduction.
• The high-temperature structure is topologically equivalent to the ambient form, with distortions arising from polyhedral tilts and shifts.
• Mode analysis identified the primary distortion mode related to the Λ1 irreducible representation of P21/c.