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Structural changes in crystal phase transition discovered

Electron density distribution of the frontier orbital of a TMTTF molecule.

Structural changes accompanying phase transition in an organic crystal tetramethyltetrathiafulvalene (TMTTF)2PF6 are said to have been unveiled by a research team at Nagoya University.

Changing temperature or pressure causes phase transitions in the crystal structure of molecular conductors and their related conduction properties. Scientists can usually determine the crystal structure using X-ray diffraction. However, structural change accompanying phase transition in TMTTF has defied examination for almost 40 years.

"Researchers have questioned that the TMTTF salt shows a charge disproportionation transition at 67K but no relevant changes in its crystal structure. This transition is a long-standing mystery known as a 'structure-less transition'," explains lead author Shunsuke Kitou.

Just above the temperature that liquid nitrogen freezes, the organic crystal is said to behave as an insulator. But as the temperature is lowered it goes through electronic and magnetic changes.

According to the researchers, until now these structural changes were too small to measure directly. Using an X-ray source, the team could precisely determine the crystal structure at each stage. The transition is said to involve the formation of a 2D Wigner crystal, based on a change in the distribution pattern of electrons in the structure.

"We have precisely characterised the subtle structural changes across this transition and finally provided a complete physical explanation for the apparent unchanging structure of this organic conductor," says project leader Hiroshi Sawa.

"Accurate crystallographic data is still lacking for many organic conductors. A better understanding of their complex behaviour could pave the way to a range of new functional electronic materials."

Peggy Lee

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