ISOTROPIC MATERIALS WITH LOW OR NEGATIVE THERMAL EXPANSION. Nazy Khosrovani, Arthur W. Sleight, Department of Chemistry, Gilbert Hall, 153 Corvallis, Oregon, 97331-4003

Materials which contract on heating (negative thermal expansion materials) are of great importance since they can be mixed with other materials (positive thermal expansion materials) to make composites which have zero thermal expansion. Materials with zero or low thermal expansion have many applications in electronic devices, space telescope mirrors, stove tops, cook wares and so on.

There are two types of low or negative thermal expansion materials. The first type have anisotropic thermal expansion which means that on heating, the structure will expand in one or two dimensions and contract in the other dimension(s). Examples of anisotropic materials are [[beta]]-eucryptite, cordierite, and NZP (NaZr₂P₃O₁₂). The problem with anisotropic materials is that microcracking occurs during the heating cycle. The second type of materials have isotropic thermal expansion which means that on heating, the structure will contract (or expand) in all dimensions. Examples of isotropic materials with low or negative thermal expansion are ZrP₂O₇ and ZrV₂O₇ respectively.

We have solved the structures of ZrP₂O₇ and ZrV₂O₇, and found that ZrP₂O₇ shows normal thermal expansion up to about 290deg.C at which the phase transition occurs, and very low and positive thermal expansion after 290deg.C. However, ZrV₂O₇ shows positive thermal expansion up to about 100deg.C, and very strong negative thermal expansion after 100deg.C up to about 800deg.C at which the material decomposes. There are two phase transitions observed for ZrV₂O₇ before 100deg.C which makes the structure more challenging.

The structure of both ZrP₂O₇ and ZrV₂O₇ is cubic and the space group is Pa3. A 3x3x3 superstructure is observed for both compounds at room temperature. The 3x3x3 superstructure disappears after the phase transition is achieved in ZrP₂O₇, but it doesn't fully disappear until the second phase transition is achieved in ZrV₂O₇. We believe that the unusual thermal expansion of these compounds is a result of frustration in bending some of the P-O-P or V-O-V angles away from 180deg..

Khosrovani, N.; Sleight A.W.,Inorg. Chem. 35, 485 (1996)