First-Principles Calculation of Spontaneous Polarization and Phase Stability in NaNO2

Ferroelectricity in sodium nitrite NaNO 2 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 NO 2 molecules. The transition temperature and a role of molecular displacement in the phase transitions are discussed in detail.