Local structure in ZrW(2)O(8) from neutron total scattering
Matthew G Tucker1, David A Keen, John S O Evans
1ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK.
Summary
Investigating the negative thermal expansion (NTE) material ZrW(2)O(8) using neutron scattering reveals that rigid unit mode (RUM) motions of its polyhedra drive the material
Area of Science:
- Materials Science
- Solid-State Chemistry
- Crystallography
Background:
- Negative thermal expansion (NTE) materials exhibit unique properties where they contract upon heating.
- Zirconium tungstate (ZrW(2)O(8)) is a well-known NTE material exhibiting complex structural behavior.
- Understanding the local atomic structure is crucial for explaining NTE mechanisms.
Purpose of the Study:
- To investigate the local atomic structure of the low-temperature ordered phase of ZrW(2)O(8).
- To elucidate the relationship between local structure, atomic motion, and negative thermal expansion.
- To validate the rigid unit mode (RUM) model for NTE in ZrW(2)O(8).
Main Methods:
- Neutron total scattering experiments were performed on ZrW(2)O(8).
- Reverse Monte Carlo (RMC) modeling was employed to analyze the scattering data.
- Instantaneous distributions of bond lengths and angles were derived from RMC models.
Main Results:
- RMC models successfully reproduced both local and average crystal structures.
- The majority of atomic mean-squared displacements are attributed to rigid unit mode (RUM) motions.
- Specific RUM motions involve ZrO(6) octahedra and WO(4) tetrahedra.
Conclusions:
- The local structure of ZrW(2)O(8) is consistent with rigid polyhedra (ZrO(6) and WO(4)) linked by flexible Zr-O-W bonds.
- Rigid unit mode (RUM) motions are the dominant mechanism responsible for negative thermal expansion in ZrW(2)O(8).
- This study provides detailed structural evidence supporting the RUM interpretation of NTE in this material.
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