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

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 C2/m(0)0s C2/m P63/mmc phase transitions. Results indicate that all modulation amplitudes tend to decrease to zero, approaching the 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 transition. Therefore, it seems that O1 instability represents the driving force of the transition. The -polymorph is characterized by a disordered structure showing multiple site splitting and a strong anisotropic microstrain, starting immediately after the transition. The structure of -Na2CO3 was also refined and found to differ from reference data. Similarly, for -K2CO3 and -Rb2CO3 a considerable disorder at the O site was observed.