Identification of the impurity phase in high-purity CeB₆ by convergent-beam electron diffraction

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• Rare earth hexaborides exhibit high crystal perfection.
• Inert gas arc floating zone refinement produces high-purity single crystals.
• Cerium hexaboride is a prominent cathode material.

Purpose of the Study:

• To identify the impurity phase in single-crystal cerium hexaboride.
• To determine the crystallographic properties of the impurity phase.
• To confirm the identity and structure of the impurity.

Main Methods:

• Convergent-beam electron diffraction (CBED) for space group and lattice parameter determination.
• Quantitative CBED (QCBED) for atomic structure confirmation.
• Density functional theory calculations and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).

Main Results:

• An impurity phase with a tetragonal unit cell (space group P4/mbm) was identified.
• Lattice parameters of the impurity phase are a = b = 7.23 ± 0.03 Å and c = 4.09 ± 0.02 Å.
• The impurity phase was confirmed to be cerium tetraboride (CeB4).

Conclusions:

• Single-crystal cerium hexaboride grown by floating zone refinement contains significant CeB4 inclusions.
• The presence of CeB4 interrupts the expected single crystallinity of cerium hexaboride.
• Advanced electron microscopy and computational methods were crucial for phase identification and structural analysis.