Surface electronic structure, thermodynamic stability of Na1/2Bi1/2TiO3 (001) surfaces and their relevance to A-site cation ordering in bulk phases: A first-principles study

Karuna Kumari, P and Niranjan, Manish K (2020) Surface electronic structure, thermodynamic stability of Na1/2Bi1/2TiO3 (001) surfaces and their relevance to A-site cation ordering in bulk phases: A first-principles study. Solid State Sciences, 102. ISSN 1293-2558

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Abstract

Surface electronic structure and energetics of (001) surfaces of relaxor ferroelectric Na1/2Bi1/2TiO3 (NBT) are explored theoretically within the framework of ab-initio density functional theory. In particular, polar and nonpolar surfaces of tetragonal NBT with bulk P4mm and P4bm symmetries are explored. For P4mm symmetry, (NaO)-, (BiO)+ and (TiO2)0 terminated surfaces and for P4bm symmetry, (Na1/2Bi1/2O)0 and (TiO2)0 -terminated surfaces are considered. The surface electronic structures for different terminations are found to differ with respect to bulk and with each other due to different polarity compensation mechanisms. In case of (BiO)+ and (NaO)- terminated surfaces, the Fermi level shifts slightly above the conduction band minimum (CBM) and below the valence band minimum (VBM) respectively. The resulting localized surface gap states near the VBM and CBM provides sufficient positive and negative charge required for compensation in agreement with the polarity compensation criteria. Furthermore, localized surface states are also formed at energies (~12 eV) deep in the gap below the VBM. The relative thermodynamic stability, reconstructions and relaxations of NBT surfaces may be expected to be correlated with the formation of these deep energy localized surface states. The magnitudes of surface relaxations are found to depend strongly on the nominal charges of the surfaces. The surface energies of polar NaO, BiO and nonpolar TiO2 terminations are found to be significantly larger than that of nonpolar Na1/2Bi1/2O termination. Thus the arrangement of nonpolar-nonpolar planes forming the sequence –Na1/2Bi1/2O–TiO2–Na1/2Bi1/2O–TiO2- may be expected to be more favourable than the sequence of polar-nonpolar planes -NaO-TiO2-BiO-TiO2-NaO-. This favourable arrangement of nonpolar planes may likely influence the degree of A-site cation ordering in Na1/2Bi1/2TiO3 which in turn may influence the degree of coexistence of R3C and CC phases of NBT at the room temperature.

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