ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2007, Vol. 71, No. 2, pp. 233–237. © Allerton Press, Inc., 2007. Original Russian Text © N.N. Ovsyuk, S.V. Goryainov, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 2, pp. 243–246. 233 INTRODUCTION Polyamorphism, i.e., the coexistence of different amorphous phases of the same composition but with different densities, has been extensively investigated since the discovery of the high-density amorphous phase of water [1]. The nature of transitions between the low-density amorphous (LDA) and high-density amorphous (HDA) phases is still not clearly understood because the interaction between the amorphous phases is rather complicated. This complexity most likely can be explained by two factors. The first factor is associ- ated with the fact that individual elementary atomic clusters are characterized by a natural dispersion of energies and the transitions between clusters of differ- ent structural types can occur in two directions. As a consequence, the energy relation between two amor- phous phases is difficult to determine reliably. More- over, the distributions of discrete energy minima in the configurational space can be smeared into energy bands. This means that, in contrast to crystals for which the phase transition point is clearly defined by the point of intersection of the curves describing the Gibbs free energy, amorphous phases can undergo a continuous transformation. The second factor, in our opinion, can be associated with the fact that the LDA and HDA phases belong to two different classes of amorphous materials. The structure of the LDA phase consists of random displacements of nuclei of the amorphous phase that do not destroy the crystal topology, whereas the HDA phase is a typical thermal glass with the atomic structure topologically nonequivalent to any crystal structure. We believe that, since the effective Hamiltonians describing the elastic strain energy in terms of atomic displacements in these phases should be constructed differently, it is necessary to use differ- ent models of amorphization. For these reasons, a gen- eral physical concept of microscopic processes respon- sible for the amorphous–amorphous phase transitions has not been proposed to date. Mutual transformations between amorphous phases can proceed continuously. This raises a natural question as to whether the volume, short-range order structure, and other properties abruptly change in amorphous phases under compres- sion. Experimental investigations have revealed that, apart from the amorphous–amorphous phase transi- tions accompanied by a continuous gradual change in the volume in SiO 2 and GeO 2 oxides [2, 3], there can occur first-order phase transitions attended by a discon- tinuous change in the volume in H 2 O and Si [4, 5]. The elucidation of the mechanisms of these transformations is the most intriguing problem. In crystals, phase tran- sitions occur either through nucleation and diffusive growth of a new phase (diffusive transformations) or through coherent displacements of atoms in the crystal lattice (martensitic transformations). However, the two aforementioned mechanisms are not appropriate for describing transformations between amorphous phases. In this respect, the question not only as to the mecha- nism but also as to the possibility of abrupt structural transformations occurring in amorphous materials under compression has until recently remained open. Brazhkin and Lyapin [6] analyzed the causes for exist- ence of different scenarios of structural transformations that lead to abrupt and smeared changes in the short- range order structure in disordered materials. These authors drew the conclusion that the type of transitions is determined by the ratio between the size of the small- est possible region with a changed short-range order and the correlation length of the medium-range order in a disordered medium. In the very recent past, amorphous zeolites have attracted the particular attention of researchers owing to the discovery of polyamorphism in these materials. Greaves and co-workers [7, 8] investigated the dynam- ics of the amorphization of crystalline zeolites with the use of X-ray diffraction and small-angle X-ray scatter- Slow Amorphization of Zeolites N. N. Ovsyuk and S. V. Goryainov Institute of Geology and Mineralogy, Siberian Division, Russian Academy of Sciences, Universitetskii pr. 3, Novosibirsk, 630090 Russia e-mail: ovsyuk@uiggm.nsc.ru Abstract—The dynamics of amorphization in two zeolites with different densities is investigated using high- pressure Raman spectroscopy. Slow amorphization of the denser zeolite under pressure leads to the formation of a low-density amorphous (LDA) phase that transforms into a more disordered high-density amorphous (HDA) phase with a further increase in the pressure. It is revealed that the LDA–HDA transformation is a first order phase transition occurring with an increase in the silicon coordination. DOI: 10.3103/S1062873807020219