The modulated phase of "Elongated triangles " (ELT) discovered in 1996 by P.Saint-Gregoire and I.Luk'yanchuk in quartz solves the 60-years problem of the anomalous light scattering at structural alpha-beta (a-b) transition at T=847K, which is the strongest scattering known in crystalline materials. Since its discovery in 1956, this question was the subject of many controversies and is frequently erroneously treated in classical books as the manifestation of critical opalescence, as was initially proposed by Nobel Laureate Vitaly Lazarevich Ginzburg. In fact, the scattering centers have the static nature that we associated with microscopic blocs of the novel ELT phase.
Familian already to the ancient jewelers and firstly studied by Le Chatelier in 1889 the alpha-beta transition was shown in 1975 to pass through the nonuniform incommensurate phase of "Equilateral triangles" (EQT) that exist within the interval of 1K and are clearly seen by means of electron microscopy. However, these nanoscopic triangles do not change the global symmetry of Quartz and therefore are invisible to the light. That's why the discovery of the new, less symmetrical ELT phase that lives within only 0.1K changes the situation! Having bi-axial symmetry the ferroelastic blocks of non-equilateral triangles have different optical indicatrices and therefore scatter the light.
The history of Science is a delicate issue. Discussing the problem of light scattering in quartz in the "International Light Scattering Conference in 1968, Prof. G. Benedek from MIT has made the prognosis:
"There are many old and unstylish fields of research which have again come to the center of attention as a result of the appearance of more modern experimental resources, new ideas ... Will all of these conditions be enough to make work on the scattering of light again lead to great advances in the study of crystals? We shall know the answer to this question only in the future.''
We do know the answer now !
Read more: A novel type of incommensurate phase in quartz: The elongated-triangle phase
P. Saint-Grégoire, E. Snoeck, C. Roucau, I. Luk'yanchuk, and V. Janovec; JETP Lett. 64, 410 (1996)