Melilite-like modulation and temperature-dependent evolution in the framework structure of K2Sc[Si2O6]F

Area of Science:

• Crystallography
• Solid-state chemistry
• Materials science

Background:

• Incommensurately modulated structures (IM) exhibit complex atomic arrangements beyond simple periodic lattices.
• Understanding IM structures is crucial for predicting material properties and phase transitions.

Purpose of the Study:

• To determine and describe the incommensurately modulated crystal structure of synthetic K2Sc[Si2O6]F.
• To investigate the temperature-dependent behavior of the modulated structure.

Main Methods:

• Crystal structure solution and refinement using superspace group formalism.
• Analysis of main and satellite reflections from X-ray diffraction data.
• Investigation of temperature-dependent satellite reflection intensities.

Main Results:

• K2Sc[Si2O6]F crystallizes in a tetragonal superspace group P42/mnm(α,α,0)000s(-α,α,0)0000 with modulation wavevectors q1 and q2.
• The structure features a mixed octahedral-tetrahedral framework with variable potassium coordination.
• Satellite reflections vanish above 443 K, indicating a transition to a normal structure identical to the average room-temperature structure.

Conclusions:

• K2Sc[Si2O6]F presents a novel example of an incommensurately modulated structure with unique topological features.
• The observed temperature-dependent modulation suggests potential for tunable properties.
• The high-temperature phase represents the average structure, simplifying its description.