Infrared dielectric response of ordered and disordered ferroelectric Pb(Sc1/2Ta1/2)O3 ceramics

IR reflectivity of B-site ordered and disordered Pb(Sc1/2Ta1/2)O3 ceramics was measured and evaluated between 20 and 500 K. All the IR modes were assigned and their parameters and temperature dependences determined and discussed. No softening of any phonon mode was observed near the ferroelectric transition. Therefore, the transition is of order–disorder type and Pb vibrational anharmonicity seems to be the most important cause of disorder. To our knowledge, this represents the first case of prevailingly order–disorder ferroelectric phase transition among perovskite ferroelectrics. The comparison of ordered and disordered samples shows that the strength of the weak mode near 315 cm−1 assigned to Sc–Ta vibrations can be used to estimate the degree of local order in the disordered sample. This was estimated to be 21 vol.% in good agreement with 18 vol.% estimated from X-ray diffraction.