Study On The Dielelctric Properties Of Multiferroic Rare-earth Titanates R₂Ti₂O₇

Rare-earth titanates with the general formula R2TiO7 (R=rare-earth element) attract a lot of interest of research because of its rich physical properties and application prospects. R2Ti2O7 compounds usually possess a cubic pyrochlore structure for the ions with smaller radii (e.g., R3+=Sm3+-Lu3+), or a monoclinic structure for the ions with larger radii (e.g., R3+=La3+-Nd3+). Pyrochlore materials R2Ti2O7 usually have singular magnetic states, including spin liquid and spin ice, because of its geometric frustration, coupled with the interaction between the crystal field, magnetic exchange and magnetic dipole. R2Ti2O7 compounds with monoclinic structure usually were characterized by the extremely high Curie temperatures, so many researches focus on its ferroelectricity and photocatalysis. Recently, many researchers have begun to pay attention to their dielectric and multiferroic properties. From the application point of view, dielectric properties as one of the most important issues for multiferroic materials have been extensively investigated. This paper is to research and discuss the dielectric properties on La2Ti2O7 with monoclinic structure and Dy2Ti2O7 with pyrochlore structure as the subjects investigated. The ceramic samples were prepared via conventional solid-state reaction route. The research results are as follows:(1) The dielectric properties of La2Ti2O7 were investigated as functions of temperature (113-1073 K) and frequency (100 Hz-1 MHz). Our results revealed that La2Ti2O7 ceramics exhibit intrinsic dielectric response with a dielectric constant of-57 in the temperature range below 250 K. Three thermally activated dielectric relaxations were observed when the temperature higher than 250 K. The low-temperature relaxations R1 with the activation energy of 0.38 eV is found to be a polaron relaxation results from hopping holes. The high-temperature relaxations R2 and R3 are related to the conduction progress associated with the singly and doubly ionized oxygen vacancies, respectively.(2) The dielectric properties of Dy2Ti2O7 were systematically investigated in the temperature range from 4 to 973 K and the frequency range of 100 Hz-1MHz. Both Raman and low-temperature (down to 4 K) dielectric measurements indicate incipient ferroelectricity in the Dy2Ti2O7 sample. In addition, two relaxations were observed in the temperature range above 450 K. Our results indicate that they are conductivity relaxations associated with the singly and doubly ionized oxygen vacancies, respectively.In summary, both La2Ti2O7 and Dy2Ti2O7 exhibit intrinsic dielectric response in the low temperature, and the conduction progress associated with oxygen vacancies in the high temperature range (above～450 K). Consequently, oxygen vacancies may be the dominant factor affecting the dielectric properties of R2Ti2O7 compounds.