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This study introduces second-order mode effective charges as a new way to measure electric polarization generated by nonpolar lattice vibrations in crystalline materials. Using first-principles calculations on various oxides including hafnium dioxide and perovskites, researchers demonstrate that specific combinations of nonpolar phonons can produce polarization effects comparable in magnitude to those from conventionally polar modes. The work traces these effects to changes in chemical bonding and local electron redistribution, expanding beyond traditional first-order Born effective charge theory.
Why it matters
These findings provide new design principles for developing advanced ferroelectric and piezoelectric materials by exploiting previously underappreciated higher-order polarization effects. The results are particularly relevant for next-generation electronic devices and multifunctional materials, including hafnium oxide-based ferroelectrics already used in semiconductor applications.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
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Abstract: Born effective charge, a fundamental quantity in lattice dynamics and ferroelectrics, provides a quantitative measure of linear polarization response to ionic displacements. However, it does not account for higher-order effects, which can play a significant role in certain materials, such as fluorite HfO$_2$. In this paper, we use the second-order mode effective charges defined with the second-order atomic dynamical charges as a measure of the dipole moments generated by nonpolar lattice distortions. Using first-principles calculations, we demonstrate that specific combinations of nonpolar phonons in many oxides can induce strongly aligned second-order polarizations, reaching magnitudes comparable to those of intrinsically polar modes even in the zero frequency limit, broadening the understanding of second-order effects, which have historically been emphasized for their dynamical effects at specific frequency ranges. Through a symmetry-based analysis of the charge density, we elucidate the microscopic origin of these effects, tracing them to variations in bond covalency and local electronic rearrangements. We also demonstrate large second-order mode effective charge in well-studied perovskites, highlighting the generality of these phenomena. Our results offer insights into the design principles of next-generation ferroelectric, piezoelectric and multifunctional materials from the higher-order contribution to polarization in crystalline solids.