On the theory of structural modulation of Na2CO3

A theory is proposed for the explanation of the structural modulation of the ground state of Na2CO3. The modulation phase is seen as a generalisation of the concept of (anti) ferroelectric and ferroelastic phases out of the centre of the Brillouinzone. The instability of the structure is caused by the dipolar interactions, and is driven by the linear polarizability. It is shown that the nonlinearity in the polarizability alone, via the harmonic forces, stabilizes the system, and the Landau-free energy of the well-known form is derived.