抄録
In order to clarify complicated phase behaviors and to obtain a unified explanation in such sequential phase transitions, formation of three kinds of tetragonal phases with a fundamental structure (I4/mmm) and the superstructures (P42/mnm and Ccmm) for BaTb2Mn2O7 are mainly discussed in this study. Single crystal of this compound was also grown by Floating Zone method. The crystal structure was analyzed by four circle X-ray method. As grown single crystal showed the tetragonal phase with a superstructure with the lattice parameters of √2a x √2a x c (a and c are lattice parameters of fundamental tetragonal cell). Thermal anomalies were observed at 440 K and 740 K from DSC curve. When this as-grown specimen was annealed in purified argon atmosphere at 1373K for several days and quenched to room temperature, this phase changed into the fundamental tetragonal phase. On the one hand, When same specimen was annealed in purified argon atmosphere at 573 K for several days and slowly cooled to room temperature, phase changed into the orthorhombic space group Ccmm. As grown crystals have P42/mnm type tetragonal phase within the middle temperature, Ccmm type orthorhombic phase at room temperature and I4/mmm type tetragonal phase at high temperature.
