Disorder in Ho₂Ti_2- Zr O₇: pyrochlore to defect fluorite solid solution series.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Pyrochlore (A2B2O7) is a versatile isometric structure type with diverse compositions and derivatives.
• Structural disorder significantly influences properties like magnetism and ionic conduction.
• Understanding disordering mechanisms is key to tailoring material functionalities.

Purpose of the Study:

• To investigate the structural disorder in Ho2Ti2-xZrxO7 solid solutions.
• To correlate structural changes with Zr-content using advanced scattering techniques.
• To elucidate the atomic processes governing the pyrochlore to defect fluorite transformation.

Main Methods:

• Neutron and X-ray total scattering experiments.
• Simultaneous Rietveld refinement for long-range structure.
• Small-box refinement PDF analysis for short-range structure.
• High-temperature oxide melt solution calorimetry for energetics.

Main Results:

• A rapid structural transformation from pyrochlore to defect fluorite occurs at x ≈ 1.2.
• A local weberite-type phase (C2221) increases linearly with Zr-content.
• Negative enthalpies of mixing suggest thermodynamic stability of the solid solution.
• Short-range ordering indicates deviations from ideal random mixing models.

Conclusions:

• The solid solution is best described as a mixture of pyrochlore and weberite-type nanodomains.
• Oxygen Frenkel defects drive local ordering and the formation of weberite-type nanodomains.
• Accumulation of weberite-type nanodomains triggers the long-range pyrochlore to defect fluorite transformation.