Some material do not expand with temperature, some of them even shrink, like Germanium at low temperatures. Sometimes, they expand in one dimension but shrink in another direction. Negative expansion is often connected to the presence of some sub-crystal structures, moving and rotating independently.

An axample is silver-copper oxide, of the cuprite symmetry structure [2].

Ag2O    Cu2O

We can see these tiny changes of interatomic distances with an even smaller sonde (X-rays).  One of the techniques is called “Extended Fine Structure X-Ray Absorption”   It works in the following manner:

1. X-ray (synchrotron radiation) is tuned to the energy of an internal-electron level of the probe atom.

2. An absorbed X-ray quantum releases an electron from this atom.

3. This electron (i.e. the quantum wave objects) interfers with itself, getting scattered on neighborourhood atoms.

4. A fine structure, depending on the interatomic diustances   is observed in the X-ray absorption. In this mode the neighbourhood of  the X-ray absorping atom is exploited.

Two inter-penetrating networks of corner sharing M4O tetrahedra with O-M-O linear coordination

Figures are from prof. P. Fornasini, University of Trento, Physics Department,               Thanks!

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