Structure, thermal stability, optoelectronic and electrophysical properties of Mg- and Na-codoped bismuth niobate pyrochlores: Experimental and theoretical study

Abstract

Novel bismuth niobates Bi1.5Mg1–xNaxNb1.5O7–δ and Bi1.5Mg0.9–xNaxNb1.5O7–δ (0 ≤ x ≤ 0.50) with the pyrochlore structure have been investigated experimentally and theoretically. The compounds were synthesized by an organic-inorganic precursors combustion method. The pyrochlore structure was formed at x = 0.10; 0.25 for cation completed and 0.25; 0.40 for cation deficient compositions, respectively. From differential scanning calorimetry (DSC) and thermogravimetry (TG), the thermal stability of the compounds up to their melting points (1250–1270 °C) was concluded. The single-crystal forms of the pyrochlores can be easily obtained from the melts. Structural characterization was performed using a Rietveld refinement against X-ray diffraction (XRD) as well as the simultaneous refinement using high resolution neutron powder diffraction (NPD) and synchrotron X-ray diffraction (SXRD) data. According to the results, the displacements of A (from 16c to 96g) and O′ (from 8a to 32e or 96g) atoms from ideal pyrochlore sites as well as the Na atoms distribution in the bismuth sub-structure, and the Mg atoms in the niobium sub-structure were determined. The same distribution of dopants in the structure was proven by DFT-PBE simulations of crystal structure and thermodynamic properties. The most preferable (Bi1.5Mg0.5)(Nb1.5Mg0.5)O7 and (Bi1.5Na0.5)(Nb1.5Mg0.5)O7 models were predicted. Both calculated/experimental direct band gap values were found to decrease from Mg-doping (3.20/3.21 eV) to Mg, Na-codoping (3.16/3.15 eV). The electrical properties were investigated by impedance spectroscopy method in air, in oxygen, and humid atmospheres in the temperature range of 25–750 °C. The typical dielectric behavior of the compounds up to 200 °C was detected. With the Na content rising the increasing of ε′ values from 86 (x = 0) to 145 (x = 0.4) was revealed at the same tanδ ≈ 0.0025 (1 MHz, 25 °C). Calculated temperature coefficients of capacitance (TCC) values varied from −594 to −724 ppm/°C (25–280 °C). Besides, contribution from electronic conductivity (T < 360 °C), proton transport below 360–400 °C as well as the ionic (oxygen) conductivity at T > 400 °C were observed.