Origin of Composition Variation of Ferroelectric Phase Transition Temperature in (Ba,Ca)TiO3 by Synchrotron Radiation Powder Diffraction

The ferroelectric cubic–tetragonal phase transition temperature TC in (Ba1-xCax)TiO3 (BCT) is almost unchanged and is maximum at x ~0.06 in the T–x phase diagram, in contrast to many other lead-free BaTiO3-based solid solutions that show a decrease in TC by atomic substitution. To investigate the peculiar atomic substitution effect in BCT, the crystal structure parameters and electron density distributions of BCT were analyzed precisely in the cubic phase by synchrotron radiation powder diffraction measurements and the maximum entropy method (MEM)/Rietveld method. The contraction of the lattice induced by the substitution of smaller Ca for Ba promotes the overlap of the electron cloud between the Ti and O atoms in the low x region, whereas it causes the repulsion between the Ti and O atoms in the high x region. The maximum TC can be attributed to the maximum bonding electron density between the Ti and O atoms at x ~0.06.