Structure and Magnetic Properties of BiFe_0.75Mn_0.25O₃ Perovskite Prepared at Ambient and High Pressure

Solid solutions of BiFe 1– x Mn x O 3 (0.0 ≤ x ≤ 0.4) were prepared at ambient pressure and at 6 GPa. The ambient-pressure (AP) phases crystallize in space group R 3 c similarly to BiFeO 3 . The high-pressure (HP) phases crystallize in space group R 3 c for x = 0.05 and in space group Pnma for 0.15 ≤ x ≤ 0.4. The structure of HP-BiFe 0.75 Mn 0.25 O 3 was investigated using synchrotron X-ray powder diffraction, electron diffraction, and transmission electron microscopy. HP-BiFe 0.75 Mn 0.25 O 3 has a PbZrO 3 -related √2 a p × 4 a p × 2√2 a p ( a p is the parameter of the cubic perovskite subcell) superstructure with a = 5.60125(9) Å, b = 15.6610(2) Å, and c = 11.2515(2) Å similar to that of Bi 0.82 La 0.18 FeO 3 . A remarkable feature of this structure is the unconventional octahedral tilt system, with the primary a – b 0 a – tilt superimposed on pairwise clockwise and counterclockwise rotations around the b -axis according to the oioi sequence ( o stands for out-of-phase tilt, and i stands for in-phase tilt). The (FeMn)O 6 octahedra are distorted, with one longer metal–oxygen bond (2.22–2.23 Å) that can be attributed to a compensation for covalent Bi–O bonding. Such bonding results in the localization of the lone electron pair on Bi 3+ cations, as confirmed by electron localization function analysis. The relationship between HP-BiFe 0.75 Mn 0.25 O 3 and antiferroelectric structures of PbZrO 3 and NaNbO 3 is discussed. On heating in air, HP-BiFe 0.75 Mn 0.25 O 3 irreversibly transforms to AP-BiFe 0.75 Mn 0.25 O 3 starting from about 600 K. Both AP and HP phases undergo an antiferromagnetic ordering at T N ≈ 485 and 520 K, respectively, and develop a weak net magnetic moment at low temperatures. Additionally, ceramic samples of AP-BiFe 0.75 Mn 0.25 O 3 show a peculiar phenomenon of magnetization reversal.