Structural phase transitions in EuNbO₃ perovskite

The crystal structures of europium niobate, EuNbO₃, have been examined over a wide temperature range between 20 and 500 K using synchrotron X-ray diffraction. We have observed two successive structural phase transitions at 360 and 460 K. Below 350 K, EuNbO₃ adopts an orthorhombic perovskite structure (space group Imma), which is characterized by NbO₆ octahedral tilting about the pseudocubic two-fold axis. The result differs from previous reports in which EuNbO₃ was assigned to a cubic aristotype (space group Pm3m) of perovskite at room temperature. At around 360 K, EuNbO₃ undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm) with the NbO₆ octahedral tilting about the pseudocubic four-fold axis. As the temperature is further increased, the I4/mcm tetragonal phase changes into the Pm3m cubic aristotype at 460 K. The tetragonal-to-cubic transformation is characterized as a continuous phase transition.