Effects of Al substitution on the spontaneous polarization and lattice dynamics of the PbTi1−xAlxO3

Abstract

Al-doped PbTiO₃ solid solutions were synthesized by a solid state method. Since Al does not have bonding d-orbit or d-electrons, and the substitutions of Al³⁺ for Ti⁴⁺ in PbTiO₃ is aliovalent, the effect of Al on the structure and spontaneous polarization is quite different from that of Hf, Zr, etc. substitutions in PbTiO₃. Usually, the spontaneous polarization is weakened with decreased tetragonality in PbZr_xTi_1−xO₃ and PbHf_xTi_1−xO₃ systems; PbTi_1−xAl_xO₃ (0 ≤x≤ 0.10) solid solutions exhibit improved spontaneous polarization with decreased tetragonality (c/a). Lattice dynamics and the crystal structure of PbTi_1−xAl_xO₃ with enhanced spontaneous polarization were investigated by FT-IR, Raman scattering technique, and X-Ray Rietveld method. The Al-doping reinforced the covalence of Pb–O(II), which indicated that the Pb–O hybridization was strengthened. The three transverse optical (TO) modes of A₁-symmetry in Raman and the “stretching” and “bending” vibration modes in FTIR further verified the increase of spontaneous polarization (P_S) in the A- and B-sites.