Most materials expand when they absorb water or are exposed to heat, but some with a strange crystal structure do the opposite. Now it seems this counterintuitive behaviour is due to the formation of a tangled spaghetti structure within the crystal, a property that could lead to new anti-humidity building materials.
Zirconium tungstate, a metal oxide, shrinks by around ten per cent when water is added or it is heated to very high temperatures. 鈥淚t a remarkable material. It acts like a reverse sponge,鈥� says Andrew Goodwin of the University of Oxford.
Goodwin鈥檚 team used X-ray scattering to examine the fine structure of the crystal and discovered that it changed from an ordered crystal to a tangle of spaghetti-like strands when they added water.
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Atomic tangle
This tangle shifts the positions of some of the atoms, causing the material to contract. 鈥淭he water forces the system to make new connections which act like Velcro, pulling the structure in on itself,鈥� says Goodwin.
By looking at past studies of heated zirconium tungstate, the researchers found that the same mechanism surprisingly underlies both the heat-induced and water-induced shrinkage.
鈥淚t鈥檚 counterintuitive that it would use the same mechanism for both,鈥� says Goodwin, as heat and hydration usually result in very different effects in other materials 鈥撀� think of the difference between heating table salt and dumping it in water.
These unusual properties could one day be exploited in building materials. Since most materials expand as temperature of humidity rise, adding zirconium tungstate to the make-up of a building could help counteract this effect.
Physical Review Letters
