The incommensurate phase of sodium carbonate: an infrared absorption study

The alpha - beta , beta - gamma and gamma - delta transitions in Na 2 CO 3 are observed through frequency changes in the carbonate ion internal modes. The incommensurate phase shows particularly interesting behaviour, as the nu 4 mode splits into two components at the beta - gamma transition which represent edge singularities. As the temperature is lowered five other, weaker components appear as subsidiaries to the edge singularities. These are interpreted as representing commensurate domains in a multisoliton lattice. It is shown that by solving the sine-Gordon equation numerically it is possible to calculate theoretical lineshapes in good agreement with the experimental spectra, although at the moment this must necessarily be done using a physically unrealistic model.