Competing antiferroelectric and ferroelectric interactions inNaNbO3: Neutron diffraction and theoretical studies

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Abstract

Neutron diffraction studies using powder samples have been used to understand the complex sequence of low temperature phase transitions of $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ in the temperature range from $12\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Detailed Rietveld analysis of the diffraction data reveals that the antiferroelectric to ferroelectric phase transition occurs on cooling around $73\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, while the reverse ferroelectric to antiferroelectric transition occurs on heating at $245\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. However, the former transformation is not complete until it reaches $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and there is unambiguous evidence for the presence of the ferroelectric $R3c$ phase coexisting with an antiferroelectic phase $(Pbcm)$ over a wide range of temperatures. The coexisting phases and reported anomalous smearing of the dielectric response akin to dipole glasses and relaxors observed in the same temperature range are consistent with competing ferroelectric and antiferroelectric interactions in $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$. We have carried out theoretical lattice dynamical calculations which reveal that the free energies of the antiferroelectric $Pbcm$ and ferroelectric $R3c$ phases are nearly identical over a wide range of temperature. The small energy difference between the two phases is of interest as it explains the observed coexistence of these phases over a wide range of temperature. The computed double well depths and energy barriers from paraelectric $Pm\overline{3}m$ to antiferroelectric $Pbcm$ and ferroelectric $R3c$ phases in $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ are also quite similar, although the ferroelectric $R3c$ phase has a slightly lower energy.

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