Structural relaxation of BaTiO3 slab with tetragonal (100) surface: Ab-initio comparison of different thickness

- Structural modifications of BaTiO3 slabs with tetragonal (100) surface are studied.
- All interlayer spacings expand except for the outmost interlayer spacing.
- The dipole moment in the TiO2 layer close to the surface is suppressed.
- In-plane displacements are insensitive to the variation in slab thickness.
- All slabs possess a non-trivial amount of spontaneous polarization.

Symmetric BaTiO3 slabs of different thickness (6, 8 and 10-unit-cell thick) with BaO (100) surface terminations in tetragonal phase were computed by first-principles. The effects of surface termination on the structural parameters were studied. In contrast to previous literature findings, all interlayer spacings were subjected to expansions except for the outmost interlayer spacing. A comparison among three slabs with varying thickness revealed that the expansion in interlayer spacings in the 8-unit-cell thick slab is smaller than the other two slabs. The surface relaxation effects penetrated into five layers from BaO (100) surface, beyond which a trailing effect in form of small expansion of interlayer spacing was noticed and there is zero rumpling. The computed in-plane ferroelectric displacements of each layer were relatively independent of the slab thickness. In agreement of previous works, symmetric tetragonal BaTiO3 slab with BaO (100) surface terminations was predicted to possess a much smaller value of in-plane spontaneous polarization than that in the bulk case. This is due to the computed polarization value using Berry phase formalism is too small with regard to ferroelectric displacements in the TiO2 layers.