Materials
Perhaps one of the most studied materials to exhibit negative thermal expansion is Cubic Zirconium Tungstate (ZrW2O8). This compound contracts continuously over a temperature range of 0.3 to 1050 K (at higher temperatures the material decomposes). Other materials that exhibit this behaviour include: other members of the AM2O8 family of materials (where A = Zr or Hf, M = Mo or W) and ZrV2O7. A2(MO4)3 also is an example of controllable negative thermal expansion.
Ordinary ice shows NTE in its hexagonal and cubic phases at very low temperatures (below -200 °C). In its liquid form, water also displays negative thermal expansivity below 3.984°C.
Quartz and a number of zeolites also show NTE over certain temperature ranges. Fairly pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 K and 120 K. Cubic Scandium trifluoride has this property which is explained by the quartic oscillation of the fluoride ions. The energy stored in the bending strain of the fluoride ion is proportional to the fourth power of the displacement angle, unlike most other materials where it is proportional to the square of the displacement. A fluorine atom is bound to two scandium atoms, and as temperature increases the fluorine oscillates more perpendicularly to its bonds. This draws the scandium atoms together throughout the material and it contracts. ScF3 exhibits this property from 10K to 1100K above which it shows the normal positive thermal expansion.
Read more about this topic: Negative Thermal Expansion
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