Low Temperature Oxygen Migration in AM2O8 Materials
In addition to their negative thermal expansion behaviour, we've shown that the AM2O8 materials show a phase transtion from an oxygen ordered to an oxygen disordered form.. For ZrW2O8 this takes the material from the acentric cubic alpha form to the centrosymmetric beta form and occurs at around 450 K. In zirconium molybdate, ZrMo2O8, evidence for the order-disorder transition is harder to obtain. However, recent experiments on HRPD have shown subtle but significant changes in the cell parameter of a quench cooled and slow cooled sample (see figure). We interpret this as being due to a static to dynamic disorder transition occuring at around 200 K - a remarkably low temperature for oxygen disorder in the solid state.
The cell parameter discrepancies of a quenched and slow cooled sample of ZrMo2O8.
In the case of ZrWMoO8 clear evidence for the order-disorder transition can be seen from the difference in cell parameter of quench-cooled and slow cooled samples.
The cell parameter discrepancies of a quenched and slow cooled sample of ZrWMoO8. By rapid cooling one can trap the disordered form of the at low temperature (red curve). On slow cooling the material orders.
The ability to trap the disordered material at low temperatures has also allowed us to study the kinetics of oxygen migration in this material using a variety of laboratory in situ diffraction studies. These have allowed us to gain information about the activation energy for oxygen migration and the growth of ordered domains in the material. This work has been selected as a "hot article" by the Royal Society of Chemistry and more details are on their website. You can read the article by clicking on the link below.
More information on this work is available in the following publications:
S. Allen; J. S. O. Evans, "Negative Thermal Expansion and Oxygen Disorder in Cubic ZrMo2O8", Physical Review B, 2003, 68, 134101.
J. S. O. Evans; P. A. Hanson; R. M. Ibberson; U. Kameswari; N. Duan; A. W. Sleight, "Low Temperature Oxygen Migration and Negative Thermal Expansion in ZrWMoO8", J. Am. Chem. Soc., 2000, 122, 8694-8699.
[Modified 10-Apr-2021 by John S.O. Evans. Pages checked for Google Chrome.]