NEGATIVE THERMAL EXPANSION AND PRESSURE INDUCED AMORPHISATION IN ZrW₂O₈ 

Figure showing a section of the a-ZrW₂O₈ crystal structure (left) and the

high-pressure amorphous ZrW₂O₈ structure (right).

Pressure-induced amorphisation (PIA) is an example of a solid state amorphisation transition and is typified by the behaviour of ZrW₂O₈ at high pressure.  Indeed, the mechanism for amorphisation in this system is particularly interesting since crystalline ZrW₂O₈ also shows isotropic negative thermal expansion (NTE) over a wide range of temperatures and NTE and PIA are believed to be theoretically linked.

The crystal structure of room temperature and pressure a-ZrW₂O₈ consists of a network of corner-linked ZrO₆ octahedra and WO₄ tetrahedra, with each of the latter having one formally non-bridging W-O bond. The structure has a low density (0.0573 atoms Å^-3) and is highly flexible. It transforms to the crystalline g-phase above 0.5GPa. This phase, albeit of lower symmetry, is topologically similar to the a-phase and possesses some shorter W-O contacts between neighbouring WO₄ tetrahedra leading to increased co-ordination. The amorphisation transition begins in polycrystalline samples around 1.5GPa and is complete above 3.5GPa when the low-pressure Bragg peaks are no longer observed. Both these high pressure phases can be recovered to ambient conditions.

In our study [1,2], which follows on from our work on the local structure of the ambient pressure crystal phase [3,4], total scattering and atomistic modelling were used to understand the local structure of the amorphous phase of ZrW₂O₈.  We show that the amorphisation process itself, the observed density change and newly measured local structure information can all be explained in terms of a structural model in which the originally non-bridging O atoms of crystalline a-ZrW₂O₈ now form W-O-W linkages.  In doing so the connectivity of oxygen-bridged W and Zr centres assumes a glass-like topology.  

References 

Home