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Negative Thermal Expansion

Published online by Cambridge University Press:  11 February 2011

Arthur W. Sleight*
Affiliation:
Department of Chemistry, Oregon State University, Corvallis, OR 97331–4003
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Abstract

Negative thermal expansion behavior has been found in many oxides where oxygen or a cation has a coordination number of two. The MO2, AM2O7, A2M3O12, AMO5, and AO3 families, where A is an octahedral cation, M a tetrahedral cation, and the oxygen coordination is two, have been investigated for their thermal expansion properties. Negative thermal expansion has been found in all families except the AO3 family, where very low thermal expansion was found in the case of TaO2F. Open networks are necessary to allow free transverse thermal motion of oxygen, which is the apparent cause negative thermal expansion in these families. This openness leads to two problems. One is that structure collapse transitions tend to occur as the temperature is lowered. There is little or no thermal expansion below this transition. A solution to this problem is to maintain sufficient ionic character in the bonds holding the network together. The other problem is that when the networks become sufficiently open, they tend to hydrate. This hydration destroys the negative thermal expansion of the network.

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