Real-Space Distribution of Local WO4 Ordering in Negative Thermal Expansive ZrW2O8

Solids usually expand when they are heated. This is quite common behavior of solids; however, there are some exceptions. Zirconium tungstate (ZrW2O8) is a prototype material among them, because it has the highest degree of negative thermal expansion (NTE) over broad temperature range. Intensive investigation of NTE mechanisms has suggested the importance of metal oxygen polyhedra. However, most of the studies have been done with volume-averaged techniques, and microscopic information has been lacking. Here, our electron microscopy observations have unraveled the real-space distribution of local WO, tetrahedra ordering for the first time. We have found that (i) the WO4 ordering is partly inverted; (ii) WO4 is disordered on the nanoscale; and (iii) doping with scandium enhances the WO disordering. These findings led to construction of a microstructure model for ZrW2O8, providing a new structural perspective for better understanding of local structure and its role in phase transitions.