Theoretical investigation of local motion and establishment of displacive properties with order disorder ferroelectric properties in NaNO₂ molecule on basis of NMR data  

After discovery of ferroelectric properties of NaNO₂, it was classified as order disorder type ferroelectric material based on earlier studies. Along with this and result carried out from our recent theoretical analysis of electric field gradients due to quadrupolar nucleus ²³Na in this crystal clearly indicate the presence of another type of local motion with reorientation of NO₂ groups during polarization reversal. It is the displacement of Na atom in crystalline space with considerable amount. For this we have obtained ratios; M₁₁₁₁/M₃₃₃₃ = 0.71, M₁₁₃₃/M₃₃₃₃ = –0.74, M₂₃₂₃/M₃₃₃₃ = 0.07, M₃₁₃₁/M₃₃₃₃ = 0.17, M₁₂₁₂/M₃₃₃₃ = 0.17 of M-tensor components by using point charge model. Their values are found for flipping angle 172° w.r.t. direction specified by direction cosines l₁ = 0.996, l₂ = 0.075, l₃ = 0.0486 of NO₂ groups and displacement of Na atom in c-, a-, b-crystalline space d_c = 0.015 nm, d_a = –0.046 nm, d_b = 0.048 nm. Values of these ratios of M-tensor components are found in very good agreement with NMR experimental values of MTR’s measured by earlier researchers. For these theoretical values of M-tensor ratios, we have found minimum value of sum of square deviation S (=0.0070). This analysis confirmed the possibility of displacive nature with order disorder nature of NaNO₂ ferroelectric material. A study based on XRD analysis of NaNO₂ by M. Ichikawa et al. (2002) also confirmed that the ferroelectric transition of NaNO₂ is not of pure order-disorder type.