Precession electron diffraction tomography for solving complex modulated structures: the case of Bi5Nb3O15.

Area of Science:

• Crystallography
• Materials Science
• Solid-State Chemistry

Background:

• Aurivillius phases are a class of complex bismuth oxides with layered structures.
• Previous studies on Bi5Nb3O15 have yielded conflicting structural models.
• Understanding the precise crystal structure is crucial for predicting material properties.

Purpose of the Study:

• To elucidate the true crystal structure of the 1D incommensurately modulated phase Bi5Nb3O15.
• To reconcile conflicting experimental observations from different research groups.
• To characterize the unique structural features of this compound.

Main Methods:

• Solving the crystal structure using electron diffraction tomography.
• Combining electron diffraction with precession of the electron beam.
• Validating the (3+1)D structure through refinement against powder X-ray diffraction (PXRD) data.

Main Results:

• The crystal structure of Bi5Nb3O15 was determined to be a 1D incommensurately modulated phase.
• The structure contains [Bi2O2](2+) slabs and perovskite-like blocks, characteristic of Aurivillius phases.
• Aperiodic crystallographic shear planes (CSPs) along the b-direction create an original layered structure with continuous and discontinuous layers.

Conclusions:

• Bi5Nb3O15 does not possess a mixed-layer Aurivillius-type structure.
• The compound exhibits an unprecedented nonuniform stacking sequence of structural elements between CSPs.
• This study provides a coherent picture of Bi5Nb3O15, reconciling previous experimental findings.