Thermal behaviour of natrite Na₂CO₃ in the 303–1013 K thermal range

Abstract

This study reports a structural investigation, at a fine temperature scale, of natrite Na₂CO₃ from 303 to 1013 K by laboratory parallel-beam X-ray powder diffraction. Within the investigated thermal range, Na₂CO₃ undergoes the γ C2/m(α0γ)0s → β C2/m → α P6₃/mmc phase transitions. Results indicate that all modulation amplitudes tend to decrease to zero, approaching the γ → β transition temperature. However, a high-amplitude anti-phase modulation of the two symmetry-equivalent C–O1 distances starts at ca 520 K and reaches a maximum value of ca 0.7 valence units at the γ → β transition. Therefore, it seems that O1 instability represents the driving force of the γ → β transition. The β-polymorph is characterized by a disordered structure showing multiple site splitting and a strong anisotropic microstrain, starting immediately after the γ → β transition. The structure of α-Na₂CO₃ was also refined and found to differ from reference data. Similarly, for α-K₂CO₃ and α-Rb₂CO₃ a considerable disorder at the O site was observed.

Keywords:

• natrite
• Na₂CO₃
• X-ray powder diffraction
• Rietveld method
• incommensurately modulated phase

Acknowledgements

This work was supported by Università di Roma ‘La Sapienza’.