Crystal Structure and Orbital Ordering in BiMnO 3 + δ (0 < δ ≤ 0.14) Ceramics

The crystal structure of BiMnO 3 + δ is studied as a function of the nominal oxygen content using X-ray synchrotron diffraction. It is found that an increase in the nominal oxygen-ion concentration leads to a series of phase transitions from the monoclinic ( C 2/ c ) to another monoclinic phase ( P 2 1 / c ) and then to the orthorhombic structure Pnma through the formation of a two-phase structural state. The indicated sequence of the phase transitions is accompanied by gradual destruction of the orbital ordering formed by the orbitals of Mn 3+ ions, which is caused by the nonuniform distribution of vacancies of manganese ions in the B positions of the perovskite lattice.