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

Novel bismuth niobates Bi1.5Mg1-xNaxNb1.5O7-delta and Bi1.5Mg0.9-xNaxNb1.5O7-delta (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 degrees 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)O-7 and (Bi1.5Na0.5)(Nb1.5Mg0.5)O-7 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 degrees C. The typical dielectric behavior of the compounds up to 200 degrees C was detected. With the Na content rising the increasing of epsilon' values from 86 (x = 0) to 145 (x = 0.4) was revealed at the same tan delta approximate to 0.0025 (1 MHz, 25 degrees C). Calculated temperature coefficients of capacitance (TCC) values varied from -594 to -724 ppm/degrees C (25-280 degrees C). Besides, contribution from electronic conductivity (T < 360 degrees C), proton transport below 360-400 degrees C as well as the ionic (oxygen) conductivity at T> 400 degrees C were observed. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Experimental and theoretical investigations were conducted on novel bismuth niobates with pyrochlore structure, showing the effects of Na and Mg doping on their crystal structure, thermal stability, and electrical properties. The results indicated that the band gap values and electrical properties of the compounds were influenced by the concentrations of Na and Mg dopants in the structure.