A Kinetic Product, Thermodynamic Stability, and Phase Boundaries on the c(2x2) and c(4x4) reconstructions of BaTiO3(001)
The step orientations on four BaTiO3 (001) surfaces were quantified from Scanning Tunneling Microscopy images in order to determine the relationship between surface reconstructions and step stability. The (1x1), c(2x2), (√5x√5) R26.6°, and (√13x√13) R33.7° surface reconstructions are compared. A well-known procedure that renders non-polar and low energy surface energies is applied to consider model step face structures, taking into account for the first time the adatoms of the surface reconstruction. A comparison of the number of steps with two crystallographically equivalent step face structures but different reconstruction termination shows that the surface adatoms affect the stability of steps. Except for the (1x1) surface Ti adatoms influence the choice of crystallographic direction of the step edge in all the surfaces mentioned above. The observed step edge directions are in accordance with general energetic principles where the facets exposed are most favorable surface planes in the cubic system.
A combined experimental and theoretical study elucidates the c(2x2) and c(4x4) surfaces of BaTiO3 (001). Scanning Tunneling Microscopy (STM) proves the coexistence of these two phases where Density Functional Theory calculations asserts that the c(4x4) is a thermodynamically stable phase with the coexistence of the c(2x2) due to a more kinetic favorable path of formation. The boundaries between the two phases were successfully calculated and match STM atomically resolved images. The stoichiometry of the c(2x2) and c(4x4) surfaces are TiO2-(TiO)1/2 and TiO2-(Ti3O3)1/8 (TiO)1/8 , respectively. Other than the size of the surface unit cell we find that the main difference between the c(2x2) and c(4x4) is that the latter introduces a Ti-O cluster for its stabilization.