CRYSTALLINE MATERIALS WITH VERY SMALL OR NEGATIVE THERMAL EXPANSION Arthur W. Sleight, Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003

Very low or negative thermal expansion apparently can result from three different mechanisms. In the case of anisotropic materials such as cordierite (Mg₂Al₄Si₅O₁₈), thermal expansion in certain directions is coupled with thermal contraction in other directions. The net result can be a very small volume thermal expansion. This behavior can be easily modeled as caused by the thermal expansion of Mg-O bonds. In (-eucryptite (LiAlSiO₄), the thermal expansion of Li-O bonds is very important, but the slight negative volume expansion requires that the Li distribution over tetrahedral and octahedral sites change as temperature changes. A third mechanism for very low or negative thermal expansion is based on anisotropic thermal vibration of oxygen in open network structures. This is the apparent mechanism for negative thermal expansion of certain compounds with the cubic ZrP₂O₇ structure such as ZrV₂O₇. A remarkable example of this behavior is found in cubic ZrW₂O₈, where negative thermal expansion is observed from 0.3 K to 1050 K. Materials with negative thermal expansion are finding applications in composites to lower the overal thermal expansion of the composite.