Li^＋-modified and Ca^2＋-doped PbTiO₃ humidity-sensitive nano-film (Li^＋-Ca_xPb_1－xTiO₃, x＝0.35, Li/Ti＝1.0%, mol/mol, 850 ℃/h, abbreviated as Li-CPT) was prepared by the sol-spin-coating route using Ca(OAc)₂, Pb(OAc)₂, Li₂CO₃ and Ti(O-Bu-n)₄ as starting materials. Li-CPT was studied by means of Laser Raman Spectroscopy (LRS) to characterize lattice distortion and polycrystal microstructure. In comparison with LRS of pure PbTiO₃ (abbreviated as PT), those of both Li-CPT and Ca_xPb_1－xTiO₃ (abbreviated as CPT, x＝0.08, 0.16, 0.24, 0.32, 0.40) consist of only five peaks appearing at 75, 125, 190, 273 and 585 cm^－1, while that of PT consists of ten peaks, namely E(1TO): 84, E(1LO): 105, A₁(1TO): 145, E(2TO): 203, B₁＋E: 289, A₁(2TO): 332, E(2LO): 447, E(3TO): 500, A₁(3TO): 597 and A₁(3LO): 757 cm^－1. According to "Soft Mode Theory", the five peaks of both Li-CPT and CPT, were attributed respectively to the red shift and extension of E(1TO), A₁(1TO), E(2TO), B₁＋E and A₁(3TO) vibration soft mode of PT. Other five peaks of PT, E(1LO): 105, A₁(2TO): 332, E(2LO): 447, E(3TO): 500 and A₁(3LO): 757 cm^－1, all disappeared or drowned in LRS of both Li-CPT and CPT. These results indicate that the crystal geometry and electron structure have been changed greatly after the equal-valence substitution of Pb^2＋by Ca^2＋with smaller Pauling ion radius: the substitution not only decreases the lattice distortion c/a and non-symmetry to remove film crack, but also changes the chemistry situation of M-O bond (M＝Ti/Pb), then alters the electric dipole moment and its polar rate. As a result the Raman activity has to be decreased enormously. Thus, LRS may be an effective technique for characterization of lattice distortion, and the soft mode theory may be used to explain some LRS.

Key words: Li^＋-modified and Ca^2＋-doped PbTiO₃ (Li^＋-Ca_x, Laser Raman Spectroscopy, laser microstructure, lattice distortion, soft mode theory