Polaronic phase transitions and complex permittivity of solid polar insulators with gigantic dielectric response

Abstract

Gigantic dielectric response (GDR) in polar insulators attracts significant scientific interest due its remarkable physics and the bright future of GDR materials in energy storage and memory devices. So far, the physical mechanism of extremely high complex dielectric permittivity (with the real part up to 106) is not established convincingly. Moreover, the application area of GDR materials is currently restricted due to unacceptable losses. In this paper, the polaronic approach for elucidation the GDR in solid polar insulators is developed based on a classical Fröhlich polaron model and polaronic phase transition criteria, pioneered by Fratini and Quémerais [Eur. Phys. J. B 14, 99 (2000)]. The dielectric functions of isolated Fröhlich polarons, a polaronic Wigner crystal, a “polaronic liquid” and an “electron liquid” exhibit surprising diversity. In particular, sufficient for GDR “dipolar” permittivity and a moderate level of dielectric loss are expected for a “polaronic liquid,” while extremely high metal‐like negative real part of the complex permittivity and its excessive imaginary part are customary for the “electron liquid.” Natural explanations for so‐called “low‐temperature anomalies” in the GDR response observed in codoped nickel oxide, CaCu3Ti4O12 and other quaternary compounds as well as a realistic model for the spectra of optical conductivity and dielectric permittivity obtained from those materials in the far‐ and midinfrared regions at different temperatures are provided based on the proposed polaronic approach and the Fano resonance model. Furthermore, this polaronic approach provides physically transparent guidelines for the design of new GDR materials, i.e., selection of appropriate host insulators and dopants (codopants) to them, estimations of their critical concentrations and temperature ranges corresponding to a GDR with an acceptable loss level. Simulated effect of polaron concentration n on static dielectric permittivity of CaCu3Ti4O12 (CCTO) and NiO, codoped with Li, Ti (LTNO).