Zirconium Tungstate

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zirconium tungstate is a highly versatile and chemically stable material with a negative thermal expansion (NTE). Its structure is composed of corner-sharing ZrO6 octahedra and WO4 tetrahedra. The octahedra and tetrahedra of this structure bind to each other by sharing one atom at their vertices, and they share one oxygen atom with every metal atom.

Tungstate exhibits a negative thermal expansion from close to absolute zero up to nearly 1000 degrees Kelvin while retaining its atomic structure. It is thought to be the result of a complicated set of vibrational modes known as Rigid Unit Modes (RUMs).

The octahedra and the tetrahedra in cubic zirconium tungstate move back and forth with the oxygen atoms moving a lot while the metal atoms move less. This guitar-string-style vibration slightly pulls the metal atoms together, which shrinks the overall size of the material.

This unusual behavior of the material is also a major reason for its strong electrical conductivity. It is not known how these tetrahedra, which are only loosely attached to each other at their vertices, can vibrate so much and in such an intricate manner.

To probe the full range of this material’s structural oscillations, scientists aimed infrared light at a sample and measured its spectrum. They found a complicated set of absorption lines well below the frequencies corresponding to stretching and bending of single bonds.

A kinetic study on the structural relaxation of amorphous zirconium tungstate shows that this is not a result of W-O bond breaking, but rather a succession of irreversible local atomic rearrangements. These results are consistent with a continuous spectrum of activation energy from 1 to 2.5 eV.