New insights into the piezoelectric, thermodynamic and thermoelectric properties of lead-free ferroelectric perovskite Na0.5Bi0.5TiO3 from Ab initio calculations

Abstract

Ab initio DFT calculations have been performed to investigate a complete set of structural, electronic, elastic, piezoelectric, thermodynamic and thermoelectric properties of lead-free perovskite Na0.5Bi0.5TiO3 (NBT) crystalizes in rhombohedral (R3c), tetragonal (P4bm) and cubic (Pm3̅m) phases. The lattice stabilities in Na0.5Bi0.5TiO3 crystals are studied from the formation energy and phonon dispersions. Electronic band profiles obtained using new KTBmBJ+so potential reveal that R3c, P4bm and Pm3̅m phases of NBT are indirect band gap semiconductors with energy values of ~ 3.29, 3.05 and 3.09 eV, respectively. Analysis of the calculated elastic constants (Cij) indicates that NBT systems are elastically stable. Ferroelectric tetragonal NBT crystal shows relatively high piezoelectric coefficients [d15 = 101.0 pC/N, d31 = 51.3 pC/N and d33 = 81.1 pC/N], compared to the rhombohedral system [d15 = 96.4 pC/N, d31 = 8.93 pC/N and d33 = 21.2 pC/N and d22 = 21.2 pC/N]. Thermodynamic quantities for rhombohedral NBT compound were calculated using the quasi-harmonic Debye model. Thermoelectricity in terms of electrical conductivity σ/τ, thermal conductivity κ/τ, Seebeck coefficient S, figure of merit ZT and thermo power for NBT crystals were examined using BoltzTraP2 code. P4bm-NBT structure showed a largest magnitude of S ~ 201.42 µV/K (at T = 500 K), following by Pm3̅m ~ 173.6 µV/K (800 K) and R3c ~ 158.6 µV/K (100 K). ZT maximized to high values ~ 2.76 (at 700 K), 2.16 (900 K) and 1.96 (400 K) for P4bm, Pm3̅m and R3c structures, respectively. Na0.5Bi0.5TiO3 (NBT) compounds could be a promising candidate for use in manufacturing high-performance piezoelectric devices and developing high-power thermoelectric generators.