Antiferroelectricity and robust dielectric response owing to competing polar and antipolar instabilities in tetragonal tungsten bronze K2RNb5O15 (R: rare-earth)

Antiferroelectricity (AFE) and its robust dielectric response under a high electric field were experimentally demonstrated in tetragonal tungsten bronze ${\mathrm{K}}_{2}R{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ (R: rare-earth). Electrical resistivity and density of ceramics samples were sufficiently improved by optimizing chemical compositions and processes, and the phase transition temperatures are widely controlled by changing R ions. Typical features of AFE, i.e., a double-hysteresis loop and a dielectric peak under a DC electric field, were demonstrated at room temperature in ${\mathrm{K}}_{2}{\mathrm{Pr}}_{0.75}{\mathrm{La}}_{0.25}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$. Notably robust relative permittivity against DC fields, which might be applicable to ceramics capacitors for high-voltage usage, is realized owing to AFE. Its dielectric constant under a high DC electric field, \ensuremath{\epsilon} \ensuremath{\sim} 800 at 10 MV/m, exceeds that of conventional ${\mathrm{BaTiO}}_{3}$, \ensuremath{\epsilon} \ensuremath{\sim} 620 at 10 MV/m. First-principles calculations suggested competing polar and antipolar instabilities underlying the successive phase transitions in this system. Two different types of antipolar displacement patterns $({{\mathrm{\ensuremath{\Gamma}}}_{2}}^{\ensuremath{-}} \mathrm{and} {{M}_{1}}^{+}{{M}_{4}}^{+})$ were found and are considered to be the structural origin of AFE. These results clearly demonstrate that ${\mathrm{K}}_{2}R{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ offers a new antiferroelectric materials platform that operates around room temperature.