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Keywords:

  • Viologens;
  • Halometalates;
  • Charge transfer;
  • Ferroelectricity;
  • Photochromism
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As the walls of the Angers' castle (France) connect the towers, the methylviologen dications stabilise the rare halobismuthate or haloantimonate anionic chains of trans-connected octahedra. The chains are polar as highlighted by the flags at the top of the towers, which leads to a new fascinating family of ferroelectrics. In the moats, in the shadow of the castle, the clumps of crystals of halometalate viologen salts are yellow, but become black as soon as the sun appears, as a consequence of a photoinduced electron-transfer process that occurs in the presence of viologen radicals. Details are discussed in the Microreview by N. Mercier on p. 19 ff.