Modulation mechanism of growth atmosphere on composition and phase transition behavior in barium calcium titanate crystal

Barium calcium titanate (Ba1-xCaxTiO3, BCT), which can be used as a three-dimensional nonlinear photonic crystal, is an anomalous ferroelectric with tremendous electromechanical response. Its phase transition behavior depends closely on the Ca content. In particular, the rhombohedral-orthorhombic and orthorhombic-tetragonal transitions can disappear at two contents: x = 0.18 and 0.23, respectively, which is important to study a wide range of physical phenomena including the alluring quantum effects. In this work, based on the natural differences of the affinity of oxygen atoms on Ba and Ca atoms, we developed a growth atmosphere modulating mechanism to control the Ca content in BCT crystals aiming at achievement of the desired compositions (x = 0.18 and 0.23). With the same polycrystalline material (Ba0.773Ca0.227TiO3), BCT single crystals with x = 0.189 and x = 0.225 were obtained under N2 and air atmospheres, respectively. The structural and dielectric properties of the as-grown crystals were investigated and the differences were discussed. Besides, the influence of Ti3+ imperfections on the diffused phase transition was studied, and the specific heats and transition enthalpies were also measured for characterizing the different phase transition properties.