Accurate charge-density distributions of cubic and tetragonal ${\mathrm{PbTiO}}_3$ and ${\mathrm{BaTiO}}_3$ have been obtained by the MEM(maximum entropy method)/Rietveld analysis using synchrotron-radiation powder data. The Pb-O bonds in tetragonal ${\mathrm{PbTiO}}_3$ show rather strong covalency, while those in cubic ${\mathrm{PbTiO}}_3$ are ionic. This is the clear evidence of the Pb-O hybridization in tetragonal ${\mathrm{PbTiO}}_3$, which has been theoretically predicted as a key factor of much larger ferroelectricity of this substance than that of ${\mathrm{BaTiO}}_3$. Tetragonal ${\mathrm{PbTiO}}_3$ forms a layered structure of a two-dimensional covalent-bonding network consisting of the $\mathrm{Ti}-O_5$ pyramid.

• Received 20 July 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.217601