[Paper Review] Ab-initio study of BaTiO3 surfaces

J. Padilla, David Vanderbilt|arXiv (Cornell University)|Feb 21, 1997

Ferroelectric and Piezoelectric Materials4 citations

TL;DR

This ab-initio study investigates the electronic and structural properties of BaTiO3 (001) surfaces in both tetragonal and cubic phases, focusing on BaO-terminated (type I) and TiO2-terminated (type II) configurations. Using first-principles total-energy calculations with full atomic relaxation, the study finds no deep-gap surface states, a reduced band gap for type-II surfaces—especially in the cubic phase—and only a modest enhancement of ferroelectricity near the surface despite substantial surface relaxation energies.

ABSTRACT

We have carried out first-principles total-energy calculations of (001) surfaces of the tetragonal and cubic phases of BaTiO3. Both BaO-terminated (type I) and TiO2-terminated (type II) surfaces are considered, and the atomic configurations have been fully relaxed. We found no deep-gap surface states for any of the surfaces, in agreement with previous theoretical studies. However, the gap is reduced for the type-II surface, especially in the cubic phase. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric order parameter is modest; we find only a small enhancement of the ferroelectricity near the surface.

Motivation & Objective

• To understand the electronic structure and stability of BaTiO3 (001) surfaces in both tetragonal and cubic phases.
• To investigate the influence of surface termination (BaO vs. TiO2) on surface electronic states and band gap formation.
• To assess the impact of surface relaxation on the ferroelectric order parameter in BaTiO3.
• To determine whether deep-gap surface states emerge, which could affect surface conductivity and electronic behavior.
• To quantify surface relaxation energies and compare them to the bulk ferroelectric well depth.

Proposed method

• Employed first-principles density functional theory (DFT) total-energy calculations within the local density approximation (LDA).
• Constructed slab models of BaTiO3 (001) surfaces with both BaO-terminated (type I) and TiO2-terminated (type II) configurations.
• Performed full atomic relaxation of surface atoms to determine equilibrium surface structures.
• Calculated electronic band structures and density of states to analyze surface states and band gap behavior.
• Computed surface relaxation energies and compared them to the bulk ferroelectric well depth.
• Analyzed the spatial variation of the ferroelectric order parameter near the surface to assess its enhancement.

Experimental results

Research questions

• RQ1 Do BaTiO3 (001) surfaces exhibit deep-gap surface states in either the tetragonal or cubic phase?
• RQ2 How does surface termination (BaO vs. TiO2) affect the band gap and electronic structure of BaTiO3 surfaces?
• RQ3 What is the magnitude of surface relaxation energy, and how does it compare to the bulk ferroelectric well depth?
• RQ4 To what extent is the ferroelectric order parameter enhanced near the surface?
• RQ5 How does the electronic structure of the surface differ between the tetragonal and cubic phases of BaTiO3?

Key findings

• No deep-gap surface states were found for any of the studied surfaces, consistent with previous theoretical findings.
• The band gap was reduced for type-II (TiO2-terminated) surfaces, particularly in the cubic phase, indicating increased electronic states near the gap edges.
• Surface relaxation energies were found to be substantial, exceeding the bulk ferroelectric well depth by several times.
• The ferroelectric order parameter showed only a small enhancement near the surface, indicating limited influence of surface effects on bulk ferroelectricity.
• The electronic structure of the TiO2-terminated surface in the cubic phase exhibited the most significant band gap reduction.
• The results suggest that surface effects in BaTiO3 are primarily structural and energetic rather than electronic in nature, with minimal impact on long-range ferroelectric order.