Pair-distribution functions of ferroelectric perovskites: Direct observation of structural ground states

Experimental information about the microscopic structure and the changes in this structure at macroscopic phase transitions for the ferroelectric perovskites provide direct tests of theoretical descriptions of ferroelectricity. Unfortunately, the local ferroelectric displacements can differ from the average periodic structure and may not be correctly described using crystallographic techniques; however, the use of real-space pair-distribution functions, which can now be obtained from scattering data with unprecedented accuracy, provides an excellent opportunity for studying the local structure. We compare crystallographic and local structures for BaTiO3, and KNbO3 above and below the low-temperature phase transitions and for PbTiO3, at low temperatures, both derived from the same powder neutron scattering data. These results elucidate the nature of the ground-state structures in each of these materials and provide insight into the mechanisms for the phase transitions.