Theoretical investigation of surface electronic structure and thermodynamic energies of (1x1) polar and nonpolar K1/2Bi1/2TiO3 (001) surfaces

Abstract

Theoretical investigations are carried to explore surface electronic structure and surface energetics of (1 × 1) polar and nonpolar (001) surfaces of room temperature tetragonal phase of lead-free relaxor ferroelectric K1/2Bi1/2TiO3(KBT) within the framework of density functional theory. In particular, polar (KO)-, (BiO)+ and non-polar (TiO2)0 terminations with bulk P4mm symmetry and non-polar (K1/2Bi1/2O)0 and (TiO2)0 terminations with bulk P4bm symmetry are explored. The electronic structures of different terminations differ significantly with respect to bulk and with each other due to surface charge compensation. In case of BiO termination, the Fermi level shifts just above the conduction band minimum (CVM), whereas in case of KO termination, it shifts just below the valence band maximum (VBM) resulting in formation of localized surface states in the band gap with conduction and valence band character. In addition, localized surface states are also formed in the gap at ~11 eV below the VBM. These deep energy localized surface states may be expected to have important implications for surface relaxation and thermodynamic stability of surfaces and their reconstructions. For polar surfaces, surface relaxations are found to be strongly dependent on surface charge and are quite disparate as compared to that for non-polar surfaces. Surface energy of K1/2Bi1/2O termination is found to be much smaller than KO, BiO and TiO2 terminations, suggesting that stacking sequence -K1/2Bi1/2O–TiO2–K1/2Bi1/2O–TiO2- of non-polar planes is significantly favorable energetically than the sequence -KO-TiO2-BiO-TiO2- KO- of polar and non-polar planes in KBT. The preferred stacking sequence of non-polar planes may be expected to have significant influence on the degree of A-site cation ordering in K1/2Bi1/2TiO3.