Relatore: Luca Bindi

Luogo: Sala Congressi del Plesso Q02

E-mail organizzatore: raffaella.burioni@fis.unipr.it

Abstract:

The well ordered world of solid materials was forced to reassess its rules

when an icosahedral phase of matter was first discovered in the laboratory

[1] and the concept of quasicrystals was introduced to explain it [2].

Quasicrystals are solids whose diffraction patterns are composed of Bragg

peaks, like periodic crystals, but with symmetries forbidden to crystals.

Over the last twenty-five years, more than one hundred examples have been

identified, but, until now, all have been produced in the laboratory under

controlled conditions ranging from rapid to moderately slow. The search

for a naturally-forming quasicrystal began soon after the concept of

quasicrystals was introduced. For many years, the search was informal.

However, beginning about a decade ago [3], a systematic search was

developed that led to the discovery this past year of a natural candidate

embedded in a rock found in the Koryak Mountains, northern Kamchatka [4].

It should be noted that, when the concept of quasicrystals was first

introduced, there was considerable skepticism [5] whether complex

quasiperiodic structures could ever form, even under ideal laboratory

conditions. Indeed, the first icosahedral phase, i-Al6Mn, reported by

Shechtman et al. [1] exhibited so much disorder that its identification as

a quasicrystal was challenged and alternative structural models were

proposed [5]. At the time, all known examples of icosahedral alloys were

metastable, only obtainable by rapid quenching. Then, highly perfect and

more stable quasicrystals, such as i-AlCuFe began to be discovered,

showing that quasicrystals can be formed under highly controlled

laboratory conditions. Nevertheless, one could not be sure of their

long-term stability [6] because they could not be kept in equilibrium at

low temperatures or annealed over eons. An interesting test would have

been to see if Nature beat us to the punch. If quasicrystals are on the

same footing as crystals, then it would be conceivable that quasicrystals

formed under natural conditions and simply have not been noticed until

now. The search to answer this question took more than a dozen years and

has resulted in one of the strangest scientific stories you are ever

likely to hear.

[1] Shechtman D., Blech I., Gratias D., Cahn J.W., Phys. Rev. Lett. 1984,

53, 1951. [2] Levine D., Steinhardt P.J., Phys. Rev. Lett. 1984, 53, 2477.

[3] Lu P.J., Deffeyes K., Steinhardt P.J., Yao N., Phys. Rev. Lett. 2001,

87, 275507. [4] Bindi L., Steinhardt P.J., Yao N., Lu P.J., Science 2009,

324, 1306. [5] Pauling L., Nature 1985, 317, 512. [6] Bancel P.A.,

Quasicrystals: The State of the Art, ed. By DiVincenzo D. and Steinhardt

P.J., 1991, 17.