Miki Ueda

First-principles investigation of phenomenological properties in Ba1−xSrxTiO3

First-principles calculations were used to evaluate the coefficients of the phenomenological equation for perovskite compounds of Ba1−xSrxTiO3 up to the eighth order. The second- and fourth-order coefficients for the cubic structures phenomenological equation agreed with the previously reported values [King-Smith and Vanderbilt, Phys. Rev. B 49, 5828 (1994), Nishimatsu et al., Phys. Rev. B 82, 134106 (2010)]. The coefficients of the phenomenological equation, up to the eighth order, were used to estimate the polarization P3 and the piezoelectric constant d33. While the P3 values estimated in this study and those evaluated by first-principles calculation matched, the d33 values differed significantly depending on the estimation method. The difference is likely due to the estimated relative dielectric susceptibility coefficient η33. When η33 is estimated accurately, the properties of perovskite oxides can probably be predicted easily.

Growth of (1-x)NaNbO3–xBaTiO3 Single Crystals by Slow-Cooling and Flux Methods

(1-x)NaNbO3–xBaTiO3 single crystals were grown by slow-cooling and flux methods. In the slow-cooling method, 0.88NaNbO3–0.12BaTiO3 powder was melted in a platinum crucible at 1500 °C and cooled down to 1000 °C. Slow cooling at less than 21 °C/h led to a bulky single crystal, and the BaTiO3/NaNbO3 ratio was graded inside the single crystal as expected from the phase diagram. Further slow cooling at 5 °C/h with the aim of improving the compositional uniformity increased the level of contamination of platinum in the single crystals from the crucible. Cuboidal single crystals with a (100) facet were grown in Na2B4O7 flux through a cooling process from 1200 to 1000 °C at 1 °C/h. There was no compositional gradient inside the cuboidal crystals, and the BaTiO3 content of the single crystals remained within 2–3% irrespective of the raw powder/flux ratio and the BaTiO3 composition in the raw powder. The obtained single crystals were orthorhombic ferroelectrics with a phase transition temperature of 310–330 °C.