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Based on the structural data of phases α (hexagonal; 756–972 K), β (monoclinic; 605–751 K), γ (incommensurate, monoclinic; 295 K) and δ (lock-in, monoclinic; 110 K) of sodium carbonate, Na2CO3, we could draw a parallel between the phase transitions and the evolution of the second coordination sphere of the C atoms. The temperature-dependent structures observed in the β phase are reproduced in the incommensurate γ phase as a modulation wave, which relates to the content of the symmetrically equivalent {110} lattice planes in the α phase. By decreasing the temperature, the phase transitions are associated with a stepwise increase in the number of Na ions participating in the second coordination sphere of the C atoms. Over the full temperature range, this number increases from 3 to 7. The C—O distances and the mobility of the O atoms depends on the number of Na ions in the vicinity of the C atoms.

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