Combined theoretical and nanoscale experimental study of Pb(Ca,Ba)TiO^sub 3^, Pb(Sr,Ba)TiO^sub 3^, and Pb(Sr,Ca)TiO^sub 3^ complex perovskite structures: An investigation of the ferroelectric and electronic properties

Combined experimental and first-principles studies have been conducted to obtain a fundamental understanding of the effect of A-site chemical doping on the electronic structure and ferroelectric behavior of Pb(Ca,Ba)TiO3, Pb(Sr,Ba)TiO3, and Pb(Sr,Ca)TiO3 perovskite complex solid solutions. Rietveld refinement of powder X-ray diffraction data shows that the crystal structure of all the three compounds is distorted from the ideal cubic perovskite structure. At the nanoscale, piezoresponse force microscopy (PFM) studies show low-performance ferroelectric properties of Pb(Sr,Ca)TiO3 thin films when compared to Pb(Ca,Ba)TiO3 and Pb(Sr,Ba)TiO3 films. Theoretical analysis of the electronic band structure performed on the basis of density functional theory (DFT) allows to elucidate the origin of the different ferroelectric behaviors observed in Pb(Ca,Ba)TiO3, Pb(Sr,Ba)TiO3, and Pb(Sr,Ca)TiO3 thin films. DFT-based computational calculations reveal that there is a strong correlation between the effects of Ti 3d non-bonding orbitals (responsible for π Tisingle bondO bonding) and the ferroelectric polarization behavior of A(A'A'')BO3 complex perovskite solid solutions. In our study, very low Ti 3dxy, dxz, and dyz non-bonding electronic density state contributions were observed and the presence of mainly ionic Ca_O and Sr_O bonds. These effects are the reason for the unusually weak polarization, low tetragonality, and poor ferroelectricity of Pb(Sr,Ca)TiO3 thin films. This is in contrast to the observed behaviors of Pb(Ca,Ba)TiO3 and Pb(Sr,Ba)TiO3 thin films. However, our first-principles calculations agree well with the PFM-based experimental results obtained in the nanometer scale.