First-principles calculation of spontaneous polarization and phase stability in NaNO2

Ferroelectricity in sodium nitrite NaNO2 has been studied from first principles. The spontaneous polarization calculated with Berry-phase theory is 0.116 C/m(2), being in excellent agreement with experimental values, 0.117 C/m(2) and 0.119 C/m(2). In order to discuss the phase transitions, we have estimated coupling parameters J(1) and J(2) in the axial next-nearest-neighbor Ising model from total-energy and force calculations. As results, J(1) and J(2) are quite sensitive to relative displacement of NO2 molecules. The transition temperature and a role of molecular displacement in the phase transitions are discussed in detail.