Li- and Na-doped bismuth titanate pyrochlores: From the point of view ab initio calculation and experiment

Abstract

A detailed combined density functional theory (DFT) and experimental study of the substitution Li and Na in Bi2Ti2O7 pyrochlore (BTO) has been performed. The DFT simulation predicted preference of A-site doping Bi1.5M0.5Ti2O7 rather than B-site doping Bi2Ti1.5M0.5O7 (M – Li, Na) and explained the phase formation process in the Bi-Na-Ti-O system. The computational results are strongly confirmed during the experiment. Na-doped BTO pyrochlores could be obtained only at the low sodium content along with the location of Na atoms at the partially occupied A-sites of pyrochlore, e.g., the Bi1.5Na0.125Ti2O7-δ pyrochlore has been synthesized for the first time. In contrast, Li-doped BTOs could be synthesized at various Li content. The electronic properties of Li-, Na-doped BTOs, which were modeled with range separated hybrid functionals, are found to be in good agreement with the experimental optical properties. These doped pyrochlore compositions were stated to be wide-band indirect semiconductor materials with close band gaps ca. 2.7–2.8 eV. The comprehensive comparative impedance study of the Bi1.6Li0.1Ti2O7-δ and Bi1.5Na0.125Ti2O7-δ pyrochlores have been performed in dry and wet conditions. Below 200 °C, the compounds have high specific resistance (ca. 108–109 Ω·m). With rising temperature from 200 to 750 °C, the bulk conductivity increased up to 10−3 S·cm−1. The conductivity value of Bi1.6Li0.1Ti2O7-δ is lower than that of Bi1.5Na0.125Ti2O7-δ at the same temperatures. The increase in the electrical conductivity of both compounds in a wet atmosphere compared to a dry one was detected. New Li-, Na-doped BTO pyrochlores might be recommended as mixed electronic-ionic conductors.