| The BiAlO3 is a novel lead free ferroelectric material along with perfect ferroelectric and piezoelectric properties which has the potential of being applied in hight temperature piezoelectric device.With the miniaturization trend in device size and dimension,the theoretical and experimental studies of ferroelectric polar surfaces have been receiving rapidly increased research attention in recently,because that the surface properties of ferroelectrics are controlled both by the well-controlled external electric field and by the growth conditions.In this paper,the dielectric and piezoelectric properties of BiAlO3 bulk phase,the surface stability and surface structure of three different stoichiometric?0001?polar surfaces and the thermodynamic stability of ferroelectric BiAlO3?0001?surfaces of different stoichiometry are studied by using first-principles calculations within density functional theory.Firstly,lattice constant,electronic structure,lattice dynamics,d ielectric and piezoelectric properties of rhombohedral BiAlO3 were computed by first-principle density functional theory simulation approach.The results showed that the O atoms give a very large contribution to the dielectric and piezoelectric responses for the system.And the dielectric and piezoelectric tensors have anisotropy for the BiAlO3with R3c symmetry.Specifically,the larger values of?3 3 and e33 are mainly fro m the contribution of A1 phonon with??=3.78THz.And the large values of?1 1,e15and e24 are basically due to the E phonons with??=1.56 and 1.96THz.The results of three different stoichiometric BiAlO3?0001?polar surfaces demonstrated that surface geometry and electronic structure are strongly dependent on the ferroelectric polar direction.The integrated Al-O3-Bi trilayer has the minimal surface energy and is the more stable construction.In addition,the favorable positive and negative surfaces have a very small surface compensation charges.The largest relaxations are found at the outmost atomic layer for positive and negative surfaces.And the significant in-plane rotations of O3 atomic layers are found at positive surface.The surface relaxations?reconstructions?make that the Bi-O and Al-O bond-lengths are reduced for the positive and negative surfaces,respectively.Thus,the hybridization of outmost metal and second O3 atoms are increased.The electronic stricture shows that the most favorable negative and positive surfaces are insulating and the energy gap of the positive surface is reduced by 0.8eV with respect to bulk.In recent years,ab initio calculations combined with thermodynamic approximate have been applied to the study of surfaces stability of nonstoichiometry which implies a contact with matter reservoirs.The calculation results indicated that the surface polarities equilibrium stoichiometry and morphologies can be strongly modified by a well-controlled the chemical potential and ferroelectric polar direction.Within the thermodynamically allowed range,the stable termination for the positive surface is the complete-Al-O3-Bi trilayer,while for the negative surface,there are three types of surface terminations and the-Bi2-O3 termination is favored under most condition.The result shows that one Bi atom of surface is relaxed beyond the top O3atomic layer.It may be considered as a mechanism that decreases the surface dipole,therefore surface is more stable.The electronic structure of negative surface indicated that the orbital hybridization between the Bi 6s and O 2p orbital are very strong in bottom of conduction and top of valence with respect to the bulk. |
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