First-principles calculations to investigate the properties towards lead-free tin (II) and lanthanum (III) doped lead zirconate titanate (PbZr₀.₅Ti₀.₅O₃) using density functional theory / Nur Hafiz Hussin

Lead-free elements based on Sn2+ and La3+ have been theoretically investigated as alternative materials for ferroelectric devices in PZT (PbZrTiO3). The calculations were performed using first principles calculations based on Density Functional Theory (DFT) that has been implemented in CASTEP computer code. The doping of Sn2+ and La3+ in PZT is expected to enhance the ferroelectric performance of PZT. The geometry optimization of ferroelectric phase materials were calculated using different exchange correlation functional approximations namely Local Density Approximation (LDA-CAPZ) and Generalized Gradient Approximation (GGA-PBE, GGA-WC and GGA-PBEsol). GGA-PBEsol and LDA-CAPZ show the most accurate calculation values for lattice parameter and structure volume for PZT parent material viz. tetragonal PTO (PbTiO3) and rhombohedral PZO (PbZrO3). In addition, effects of La doped tetragonal PTO and rhombohedral PZO are also comparable with experimental results. Meanwhile, GGA-PBEsol and LDA-CAPZ functionals are accurate for tetragonal and monoclinic PZT, respectively. For PLZT (PbLaZrTiO3), PLSnZT (PbLaSnZrTiO3) and SnLZT (SnLaZrTiO3) the most accurate function is GGA-PBEsol in tetragonal and monoclinic phase. The electronic band structure, density of states (DOS) and electron density indicate the existence of hybridizations among anion O 2p, cation Pb 6s/Sn 5s (special lone pair) and the Ti 3d/Zr 4d states of tetragonal PTO, SnTO (SnTiO3), and rhombohedral SnZO (SnZrO3) and PZO compounds as well as tetragonal and monoclinic PZT, PSnZT and SnZT. The hybridization of anion O 2p, cation Pb 6s/Sn 5s/ La 5d and the Ti 3d/Zr 4d also exists in PLTO (PbLaTiO3), SnLTO (SnLaTiO3), PLZT (PbLaZrTiO3) and PLSnZT (PbLaSnZrTiO3). The existence of Sn2+ and La3+ in PZT reduced the energy band gap that have consequently overcome the fatigue problem in PZT. The cohesive energy showed that the tetragonal P4mm phase of PTO and SnTO as well as rhombohedral phase PZO and SnZO are stable with the lowest energy at the ground state equilibrium structure. For tetragonal and monoclinic PZT, the introduction of Sn2+ in PZT (PSnZT and SnZT) systems increased the spontaneous polarization of PZT. Furthermore, the La doped PZT, PSnZT and SnZT have also surged the value of polarization. In this work, calculations on novel compound consist of Sn2+ and La3+ in PZT could provide new results on geometrical and electronic structure of materials. Thus, these findings will be able to assist experimentalist in reducing lead consumption by substituting or doping Sn2+ and La3+ in Pb-based system, and hence will extensively contribute to green technology.