Decreasing giant splitting of longitudinal and transverse optical phonons in PbxSr1−xTiO3 due to Pb covalency

In contrast to the increase of giant splitting of longitudinal optical (LO) and transverse optical (TO) phonons in polycrystalline BaxSr1−xTiO3 [Phys. Rev. B. 64, 224103-1 (2001)], a decreasing splitting of the softest A1(1TO) and the hardest A1(3LO) was observed in PbxSr1−xTiO3. A similar structure but anomalous discrepancies in lattice dynamics of ABO3 perovskite are related to the diverse electronic states of Pb2+ and Ba2+ ions. In this article, Pb–O interaction is proposed to be more hybridized than Ba–O, and thus the change of effective charge to the LO–TO splitting may be the dominant mechanism in PbxSr1−xTiO3, while unit-cell volume change is in the BaxSr1−xTiO3 systems.