X-ray study of deformation density and spontaneous polarization in ferroelectric NaNO2

The deformation electron density of ferroelectric sodium nitrite has been determined from X-ray diffraction data at 30 K, using Hirshfeld deformation functions. Owing to the strong correlation between odd terms of the deformation coefficients, constraints were imposed in the refinement. The net charges for Na, N and O atoms were estimated to be 0.27, 0.20 and -0.24 e, respectively. The calculated spontaneous polarization using these net charges and atomic dipole terms, 7.8 microC cm(-2), is much closer to the recently measured value, 12 microC cm-2, as compared with the value calculated from the formal point charges (74 microC cm(-2)).