IF YOU squeeze tellurium hard enough, it forms an atomic structure so complex that it never repeats itself. The revelation has surprised researchers who have spent years trying to deduce what happens to this mysterious element at high pressures.
One way to study a material at high pressure is to compress a sample between the flattened tips of diamond crystals, fire X-rays at it, and study the diffraction pattern formed by the X-rays bouncing off the atoms inside. But when researchers tried this with tellurium, the resulting pattern included a series of faint spots that nobody could explain.
One problem is that the tellurium crystal tends to shatter at intense pressures, blurring the resulting X-ray pattern. So Malcolm McMahon and his team at the Centre for Science at Extreme Conditions at the University of Edinburgh increased the pressure extremely slowly, in an attempt to keep the crystal whole. Eventually they managed to apply more than 5 gigapascals to the crystal 鈥� 50,000 times atmospheric pressure 鈥� without it shattering. 鈥淲e tried 29 times before we got one that worked,鈥� says McMahon. The researchers finally had a much clearer X-ray pattern to look at, but when they did, they were in for a surprise.
Advertisement
The pressure forced the atoms into an entirely new structure, McMahon told the Condensed Matter and Materials Physics meeting in Belfast last week. The underlying structure is quite regular 鈥� as though boxes with atoms at the centre and at each of the corners have been stacked. But this structure is distorted by a Slinky-like wave that pushes each atom a little out of place. Because the length of the wave does not match the length of the boxes, no two sections of crystal are alike.
Although such structures are known in some compounds, researchers are only just starting to realise that they can also occur in simple elements, and at high pressures. McMahon鈥檚 group announced similar results for selenium. The findings could reveal why elements such as tellurium and selenium superconduct more easily at high pressures.
