Question: What sort of stuff srink when they are heated

Andrew Cairns answered on 13 Nov 2011:

Being scientists, we like to put things into groups, and there are three main types of stuff that shrink when heated:

1) Bridged Oxides (such as ZrW2O8, Zirconium Tungstate)
2) Bridged Cyanides (such as Zn(CN)2)
3) Molecular systems, such as Methanol at low temperatures.

The first two have a lot in common. Usually we think of bonds in materials to be like springs, so the more energy you put in (through heating) the further apart the ends of the spring go. This is just like being on a pogo stick.

In the strange materials we work with there is an atom or atoms in the middle that link two others in a line. This is like putting a weight in the middle of a piece of string and swinging it around. The faster you spin it (as in, putting energy in) the closer your hands are forced to come together. They therefore bring things closer the more energy you put in, so they shrink when heating.

The more atoms we put in, the more extreme the NTE response.

The third type are more difficult to understand – basically the molecules are held together by very weak forces so when you heat them up they move apart very quickly, and in some cases closer together. Water ice shows this type of behaviour – when ice melts (heated up) the volume shrinks. Try it!