Real-space distribution of local WO ₄ ordering in negative thermal expansive ZrW ₂O ₈

Solids usually expand when they are heated. This is quite common behavior of solids; however, there are some exceptions. Zirconium tungstate (ZrW ₂O ₈) 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 ₄ tetrahedra ordering for the first time. We have found that (i) the WO ₄ ordering is partly inverted; (ii) WO ₄ is disordered on the nanoscale; and (iii) doping with scandium enhances the WO ₄ disordering. These findings led to construction of a microstructure model for ZrW ₂O ₈, providing a new structural perspective for better understanding of local structure and its role in phase transitions.