Unconventional Polarization Response in Titanite-Type Oxides due to Hashed Antiferroelectric Domains

Domains in a crystal, which have crystallographic uniformity and are geometrically segmented, typically arise from various phase transitions. The physical properties within individual domains are inherently the same as those in the homogeneous bulk. As a result, sufficiently large domains have little influence on the bulk properties. However, as the domains decrease in size to the nanoscale, for instance, due to multiple phase instabilities or spatial inhomogeneities, then the materials often acquire exceptional functionalities that are unattainable without these domains. This effect is exemplified by the ultrahigh dielectric and piezoelectric responses observed in ferroelectric oxides with nanoscale polar domains as well as in ferroelectric relaxors with polar nanoclusters. Here, we demonstrate that hashed nanoscale domains in an antiferroelectric material are also capable of boosting dielectric permittivity in an unconventional way. This discovery has been made in an antiferroelectric titanite-type oxide, CaTi(Si_1-Ge)O₅, in which the permittivity significantly increases when the antiferroelectric order becomes short-range. Our transmission electron microscopy observations have clarified that polar regions simultaneously appear around antiphase boundaries in the antiferroelectric phase of CaTi(Si_1-Ge)O₅. As the concentration of the antiphase boundary increases, the polar regions become denser and play a crucial role in boosting the permittivity. At the composition of x = 0.5, the value of the permittivity finally reaches double that in the bulk and shows excellent linearity, at least until an electric field of 500 kV/cm is applied. The present findings highlight the promise of domain engineering for boosting the permittivity in antiferroelectrics as a way to develop materials with excellent dielectric properties.