Abnormal positive thermal expansion in Mo substituted ZrW 2O 8

Abstract

ZrW 2O 8 displays the unusual property of an isotropic bulk contraction in volume as a function of temperature. We report here on the positive thermal expansion (PTE) property caused by substituting Mo for W sites in ZrW 2O 8 at room temperature. The room temperature crystal structure of ZrW 2− x Mo x O 8 compounds, which were synthesized using a low temperature route, could be divided into ordered phase with α-ZrW 2O 8 structure (0≤ x≤0.5) and disordered phase with β-ZrW 2O 8 (0.5< x≤1.5) and c-ZrMo 2O 8 structure (1.5< x≤2). ZrW 2− x Mo x O 8 adopting β-ZrW 2O 8 structure shows abnormal PTE property at room temperature due to the existence of water molecules, while ZrW 2− x Mo x O 8 adopting the other two structures (α-ZrW 2O 8 and c-ZrMo 2O 8) shows negative thermal expansion (NTE) property from room temperature until decomposition for α-ZrW 2O 8 structure or trigonal phase transition for c-ZrMo 2O 8 structure. The reason lies in the fact that the oxygen migration caused by the Mo substitution leads to the re-arrangement of W(Mo)O 4 tetrahedra lying along the 3-fold axis, only particular W/Mo ratio (0.5< x≤1.5) produces a special crystal structure, which allows the entrance of water molecules.