Lattice dynamics of perovskite Pb_xCa_1-xTiO ₃

Investigation of the structural and vibrational properties of the Pb _xCa_1-xTiO₃ (PCT) system exhibits two phase transitions in the range of x=0-1 by using x-ray powder diffraction and Raman spectroscopy. The first transition occurring at x∼0.65 corresponds to the tetragonal-to-cubic phase and the second one occurs near x=0.35 is attributed to the orthorhombic-to-cubic phase. The absence of the intermediate tetragonal phase between orthorhombic and cubic phases (0<x<0.35) may be mostly attributed to the very restricted region in the PCT system. In addition, a decreasing giant LO-TO splitting behavior similar to that exhibited in Pb _xSr_1-xTiO₃ system for lower Pb concentration was also observed, which has been attributed to the highly Pb-O covalent bonding to reduce the long-range Coulomb interaction. Comparison of the change of line shapes of A₁ (1TO) mode among three PbTiO₃-based perovskites indicates that the anharmonicity will become more conspicuous due to the larger high-order potential terms of Ba²⁺ and Ca²⁺ substitution than Sr²⁺ substitution for Pb²⁺.