Low-, medium-, and high-entropy pyrochlores in (Bi,Li,Na,La,Eu)1.9(Mg0.5Nb1.5)O7-δ compositions, their optical and dielectric properties

Abstract

The pyrochlore phase formation in the (Bi,Li,Na,La,Eu)1.9(Mg0.5Nb1.5)O7-δ compositions with different element fractions in the A sites of the A2B2O6O' structure was studied. The low- and medium-entropy pyrochlores were formed after calcination of the batch at 1000–1150 °C. The high-entropy pyrochlore composition was detected during the investigations. Its stability by ab initio calculation was proven. The main impurities (Eu0.55La0.35Bi0.04)NbO4-δ and Na0.65MxNbO3-δ were identified in the initial medium- and high-entropy compositions as more stable phases. The samples belong to wide band gap semiconductors (Egdir = 3.2–3.6 eV, Egindir = 2.9–3.0 eV). The dielectric parameters calculated from impedance spectra depend on polarizability created by different types of cations in the A2O' sublattice. The dielectric permittivity (ε' = 67–162, at 25 °C and 1 MHz) decreases with decreasing polarizability of the system, while the tangent loss remains equally low (tanδ = 0.002–0.003 up to 240 °C). The presence of impurity phases in the initial high-entropy pyrochlores adversely affected the dielectric properties. Two mechanisms of conductivity were distinguished.