Site-occupancy scheme in disordered Ca₃RE₂(BO₃)₄: a dependence on rare-earth (RE) ionic radius

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Polycrystalline calcium rare-earth orthoborates (Ca3RE2(BO3)4) are an important class of materials.
• Understanding their structural variations with rare-earth element substitution is crucial for materials design.

Purpose of the Study:

• To determine the crystal structures of Ca3RE2(BO3)4 compounds.
• To investigate trends in structural properties as a function of rare-earth ionic radius.
• To establish predictive models for related A3RE2(BO3)4 systems.

Main Methods:

• Powder X-ray diffraction was employed to determine the structures.
• Analysis focused on unit-cell dimensions, interatomic distances, and fractional site occupancies.
• Data was correlated with rare-earth ionic radii.

Main Results:

• Unit-cell volume and the 'a' parameter show linear dependence on RE ionic radius.
• Unit-cell parameters 'b' and 'c' exhibit nonlinear behavior.
• Rare-earth atoms occupy all three cationic sites (M1, M2, M3) with occupancies varying based on RE ionic radius.
• Smaller RE ions favor the M3 site, while larger RE ions show increased occupancy at M1 and M2 sites.

Conclusions:

• The study establishes clear trends in structural properties of Ca3RE2(BO3)4 orthoborates.
• Predictive insights into cationic site occupancy are provided for unexplored A3RE2(BO3)4 compounds (A = Ca, Ba, Sr).
• This work aids in the targeted synthesis and application of rare-earth orthoborate materials.