page_title: Zirconium tungstate
section_title: Detailed crystal structure
hierarchical_section_title: Zirconium tungstate / Cubic phase / Detailed crystal structure
caption_attribution_description: An image of the crystal structure of cubic ZrW2O8. The full unit cell contents are not shown to enable structural features to be clearly shown. The image shows the positioning of the "W2O8 group" (itself consisting of two WO4 tetrahedra) along the body diagonal of the unit cell, and the corner sharing between WO4 and ZrO6 polyhedra.
context_page_description: Zirconium tungstate is a metal oxide with unusual properties. The phase formed at ambient pressure by reaction of ZrO₂ and WO₃ is a metastable cubic phase, which has negative thermal expansion characteristics, namely it shrinks over a wide range of temperatures when heated. In contrast to most other ceramics exhibiting negative CTE, the CTE of ZrW₂O₈ is isotropic and has a large negative magnitude over a wide range of temperature. A number of other phases are formed at high pressures.
context_section_description: The arrangement of the groups in the structure of cubic ZrW₂O₈ is analogous to the simple NaCl structure, with ZrO₆ octahedra at the Na sites, and W₂O₈ groups at the Cl sites. The unit cell consists of 44 atoms aligned in a primitive cubic Bravais lattice, with unit cell length 9.15462 Angstroms.
  The ZrO₆ octahedra are only slightly distorted from a regular conformation, and all oxygen sites in a given octahedron are related by symmetry. The W₂O₈ unit is made up of two crystallographically distinct WO₄ tetrahedra, which are not formally bonded to each other. These two types of tetrahedra differ with respect to the W-O bond lengths and angles. The WO₄ tetrahedra are distorted from a regular shape since one oxygen is unconstrained (an atom that is bonded only to the central tungsten (W) atom), and the three other oxygens are each bonded to a zirconium atom (i.e. the corner-sharing of polyhedra).
  The structure has P2₁3 space group symmetry at low temperatures. At higher temperatures, a centre of inversion is introduced by the disordering of the orientation of tungstate groups, and the space group above the phase transition temperature (~180C) is Pa.
  Octahedra and tetrahedra are linked together by sharing an oxygen atom. In the image, note the corner-touching between octahedra and tetrahedra; these are the location of the shared oxygen. The vertices of the tetrahedra and octahedra represent the oxygen, which are spread about the central zirconium and tungsten. Geometrically, the two shapes can "pivot" around these corner-sharing oxygens, without a distortion of the polyhedra themselves. This pivoting is what is thought to lead to the negative thermal expansion, as in certain low frequency normal modes this leads to the contracting 'RUMs' mentioned above.
caption: An image of the crystal structure of cubic ZrW2O8, showing the corner-sharing octahedral (ZrO6, in green) and tetrahedral (WO4, in red) structural units. An incomplete unit cell is shown so that the positioning of the W2O8 unit along the body diagonal of the unit cell may be seen.
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