Antiferroelectricity and robust dielectric response owing to competing polar and antipolar instabilities in tetragonal tungsten bronze K2RNb5 O15 (R: rare-earth)

Antiferroelectricity (AFE) and its robust dielectric response under a high electric field were experimentally demonstrated in tetragonal tungsten bronze K2RNb5O15 (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 K2Pr0.75La0.25Nb5O15. 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, ϵ ∼800 at 10 MV/m, exceeds that of conventional BaTiO3, ϵ ∼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 (Γ2- and M1+M4+) were found and are considered to be the structural origin of AFE. These results clearly demonstrate that K2RNb5O15 offers a new antiferroelectric materials platform that operates around room temperature.