Coexistence of antiferrodistortive and ferroelectric distortions at the PbTiO3(001) surface

The c(2x2) reconstruction of (001) PbTiO3 surfaces is studied by means of first principles calculations for paraelectric (nonpolar) and ferroelectric (in-plane [100] polarized) films. Analysis of the atomic displacements in the near-surface region shows how the surface modifies the antiferrodistortive (AFD) instability and its interaction with ferroelectric (FE) distortions. The effect of the surface is found to be termination dependent. The AFD instability is suppressed at the TiO2 termination while it is strongly enhanced, relative to the bulk, at the PbO termination resulting in a c(2x2) surface reconstruction which is in excellent agreement with experiments. We find that, in contrast to bulk PbTiO3, in-plane ferroelectricity at the PbO termination does not suppress the AFD instability. The AFD and the in-plane FE distortions are instead concurrently enhanced at the PbO termination. This leads to a surface phase with coexisting FE and AFD distortions which is not found in PbTiO3 bulk.