Which materials/compounds expand in response to cold temperatures? Which materials or compounds expand in response to cold temperatures?
This material or compound should expand at a slow (but constant rate), from about 0–20 months when in the following temperature range −25 °C to −10 °C. 

Also, the compound should not react with plastic or be toxic to humans or the environment.

Please explain why the compound does this as well.
Perhaps one of the most studied materials to exhibit negative thermal expansion is cubic zirconium tungstate $(\ce{ZrW2O8}).$