Complex Structural Behavior of BiMn₇O₁₂ Quadruple Perovskite

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Quadruple perovskites are complex oxides with potential applications in electronics.
• Understanding the structural phase transitions of these materials is crucial for their technological development.
• Bismuth manganese oxide (BiMn7O12) is a specific quadruple perovskite with intriguing properties.

Purpose of the Study:

• To comprehensively investigate the structural properties and phase transitions of BiMn7O12 across a wide temperature range.
• To elucidate the crystallographic symmetries and structural changes occurring in BiMn7O12.
• To explore the influence of the Bi3+ lone electron pair on the material's structural behavior.

Main Methods:

• Laboratory and synchrotron X-ray powder diffraction (10-650 K).
• Single-crystal X-ray diffraction at room temperature.
• Differential scanning calorimetry (DSC) and second-harmonic generation.
• First-principles calculations.

Main Results:

• Identified three key structural transitions: cubic (Im3̅) above 608 K, monoclinic (I2/m(o)) between 460-608 K with orbital order, and likely Im below 460 K.
• Observed a triclinic distortion to P1 symmetry below approximately 290 K.
• First-principles calculations confirmed the stability of noncentrosymmetric structures and explained the gradual Im to P1 transition due to minimal energy difference.

Conclusions:

• BiMn7O12 displays a complex sequence of structural transitions from cubic to monoclinic, then to Im, and finally to triclinic (P1) symmetry with decreasing temperature.
• The structural behavior is significantly influenced by the Bi3+ lone electron pair, contrasting with LaMn7O12.
• The study highlights the challenges in analyzing BiMn7O12 due to twinning and scattering, yet provides crucial insights into its phase transitions.