Published: May 13, 2011 r2011 American Chemical Society 2860 dx.doi.org/10.1021/cm200241c | Chem. Mater. 2011, 23, 2860–2868 ARTICLE pubs.acs.org/cm Structural Nature of Polyamorphism in Y 2 O 3 ÀAl 2 O 3 Glasses N. K. Nasikas, †,§ S. Sen,* ,‡ and G. N. Papatheodorou † † Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH, P.O. Box 1414, GR-26504 Patras, Greece ‡ Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, United States § Department of Materials Science, University of Patras, GR-26504 Patras, Greece ’ INTRODUCTION Crystalline and amorphous rare earth aluminates constitute an important class of optical materials with a wide range of technological applications. 1À3 Particularly noteworthy in this regard is the crystalline yttrium aluminum garnet (YAG; Y 3 Al 5 O 12 ) in the Y 2 O 3 ÀAl 2 O 3 binary system that is known for its profound importance as a host material in solid state lasers. 3À7 More recently, glasses and supercooled liquids in this binary system have been extensively investigated in relation to the observation of a density-driven first-order liquidÀliquid phase transition. 8À26 The existence of this type of phase transition is a fundamental and currently greatly debated issue in the literature, i.e., whether, similar to crystalline polymorphs, two or more isocompositional glass or liquid phases with different structures and densities that are in stable or metastable equilibrium could coexist. Such a phenomenon has been termed polyamorphism in the literature. 27 In the very first report by Aasland and McMillan, 8 Y 2 O 3 ÀAl 2 O 3 glasses were prepared in the composition range of 24À32 mol % Y 2 O 3 by melting the mixture of the constituent oxides in an inert atmosphere in a modified iridium wire furnace. For every glass prepared in this compositional range, these authors observed glassy inclusions of several micrometers em- bedded in a glassy matrix using back-scattered electron (BSE) imaging. These authors verified using electron probe microana- lysis that the inclusions as well as the matrix had the same composition. Micro-Raman spectroscopy was employed to de- monstrate structural differences between these two phases in the glass samples. These results led to the conclusion that the inclusion and the matrix represented two thermodynamically distinct but compositionally identical amorphous phases. The inclusions were assigned to the low-density amorphous (LDA) phase, and the surrounding matrix phase was assigned to the high-density amorphous (HDA) phase, related via a first-order liquidÀliquid phase transition. Subsequent studies have used CO 2 laser melting and contain- erless aerodynamic levitation to prepare these glasses. 9À22 The results of these studies are generally in agreement that the observability of the coexistence of HDA and LDA phases with BSE may be a critical function of glass composition. The composi- tion range of 25À33 mol % Y 2 O 3 is more favorable for the production of single-phase glasses, whereas two-phase glasses form in the composition range of 33À40 mol % Y 2 O 3 . 13,18,21,22 These results are in contrast with the observation of two-phase glasses by Aasland and McMillan 8 in the composition range of 24À32 mol % Y 2 O 3 . Such a discrepancy may possibly arise from the difference in the synthesis techniques and hence in the thermal history of these glasses, as mentioned above. Structural studies of these glasses in the composition range of 24À40 mol % Y 2 O 3 were conducted by several laboratories using primarily 27 Al nuclear magnetic reso- nance (NMR) spectroscopy and X-ray and neutron diffraction. All Y 2 O 3 ÀAl 2 O 3 glasses in this composition range showed the presence of Al atoms in 4-, 5-, and 6-fold coordination forming AlO 4 , AlO 5 , and AlO 6 coordination polyhedra, respectively. The relative fractions of these Al species have been shown to be relatively insensitive to the glass composition. 13 The diffraction results have been interpreted to indicate that on an average the Y atoms are six- or seven-coordinated with respect to oxygen in these Received: January 24, 2011 Revised: March 29, 2011 ABSTRACT: The structural attributes of the polyamorphic high- and low-density amorphous phases in Y 2 O 3 ÀAl 2 O 3 glasses with 24À41 mol % Y 2 O 3 have been investigated using high-resolution 27 Al and 89 Y nuclear magnetic resonance spec- troscopy in combination with back-scattered electron and transmission electron microscopy imaging and differential scanning calorimetric measurements. Glasses over the entire composition range are characterized by a uniform dispersion of droplets of one phase in the matrix of the other phase resulting from a density-driven phase separation in the supercooled liquid state. Although compositionally identical, the structures of the two phases differ primarily in their Y coordination environment and in the structural order associated with the connectivity and packing of the AlÀO and YÀO coordination polyhedra. It appears that, compared to the matrix phase, the droplet phase is characterized by a significantly higher degree of short-range structural order. KEYWORDS: Y 2 O 3 ÀAl 2 O 3 , glass, supercooled liquid, polyamorphism, structure, nuclear magnetic resonance, 27 Al, 89 Y, differential scanning calorimetry, transmission electron microscopy