Crystal structure solution for the A₆B₂O₁₇ (A = Zr, Hf; B = Nb, Ta) superstructure

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• The A6B2O17 orthorhombic compound family exhibits complex crystal structures.
• Understanding cation ordering and structural variations is crucial for materials design.

Purpose of the Study:

• To solve and characterize the crystal structures of Zr6Ta2O17, Hf6Nb2O17, and Hf6Ta2O17.
• To classify these compounds within the A6B2O17 family and investigate structural mechanisms.
• To explore cation order-disorder phenomena and compositional variations.

Main Methods:

• Synchrotron X-ray powder diffraction
• Neutron powder diffraction
• Simulated annealing
• Charge flipping
• Rietveld refinement
• Bond valence method

Main Results:

• Crystal structure solutions were obtained for Zr6Ta2O17, Hf6Nb2O17, and Hf6Ta2O17.
• These structures are isomorphous with Zr6Nb2O17, confirming the A6B2O17 orthorhombic family (Ima2 symmetry).
• The structures comprise specific arrangements of six-, seven-, and eight-coordinated polyhedra, with mechanisms for compositional variation identified.

Conclusions:

• The A6B2O17 (A = Zr, Hf; B = Nb, Ta) orthorhombic compound family is established with Ima2 symmetry.
• Cation order/disorder potentials were analyzed using diffraction and bond valence methods.
• Compositional variations are linked to the addition/removal of specific polyhedral units and oxygen tilts.