Polyamorphism in aluminate liquids

Abstract

Supercooled liquids in the Y 2 O 3 -Al 2 O 3 system undergo a liquid-liquid phase transition between a high-temperature, high-density amorphous polymorph (HDA form), and one with lower density that is stable at lower temperature (LDA form). The two amorphous polymorphs have the same chemical composition, but they differ in their density (∼4% density difference), and in their heat content (enthalpy) and entropy determined by calorimetry. Here we present new results of structural studies using neutron and high-energy x-ray diffraction to study the structural differences between high-density amorphous (HDA) and low-density amorphous (LDA) polyamorphs. The combined data sets show no large differences in the average nearest-neighbour Al-O or Y-O bond lengths or coordination numbers between the low-and high-density liquids. However, the data indicate that changes occur among the packing geometries and clustering of the Al-O and Y-O coordination polyhedra, i.e., within the second-nearest-neighbour shell defined by the metalmetal (i.e., Y-Y, Y-Al and Al-Al) interactions. Polarizable ion molecular dynamics simulations of Y 2 O 3 -Al 2 O 3 liquids are used to help interpret the pair-correlation functions obtained from x-ray and neutron scattering data. Unexpectedly large density fluctuations are observed to occur during the simulation, that are interpreted as due to dynamic sampling of high-and low-density configurations within the single-phase liquid at temperatures above the critical point or phase transition line. Calculated partial radial distribution functions indicate that the primary differences between HDA and LDA configurations occur among the Y-Y correlations.

Direct density determination of low- and high-density glassy polyamorphs following a liquid–liquid phase transition in Y2O3–Al2O3 supercooled liquids

“…For the AY-23/24 samples, various random points analyzed within the 'matrix' and 'inclusion' glasses gave the results reported in Table 1. As in our previous studies [11,[13][14][15][16][17][18]61], no statistically-significant difference was found to occur between 'matrix' and 'inclusion' glassy phases. Here we investigated the possible presence of concentration gradients via electron microprobe traverses across samples; none were observed (Fig.…”

“…The same result was found for glasses with a range of bulk compositions, containing between 24 and 32 mol% Y 2 O 3 . The original results have been reproduced in later studies, using different preparation methods [9,10,[12][13][14][15][16][17][18][19][20][21][22]. Aasland and McMillan proposed that the phase transition in Y 2 O 3 -Al 2 O 3 supercooled liquids was driven by density or entropy differences between the two liquid phases, occurring at constant chemical composition [11].…”

“…We later developed methods to obtain larger quantities of Y 2 O 3 -Al 2 O 3 glasses containing HDA/LDA components to carry out characterization via calorimetry, neutron/Xray diffraction, and Brillouin scattering [13][14][15][16][17][18][19][20]22,61]. In Schematic P-T relations for a polyamorphic system exhibiting a liquid-liquid phase transition at constant composition.…”

Local microstructure of amorphous Al2O3

“…Similar issues are reported for silica in [41]. The models proposed to explain the phenomena "polyamorphism" are mainly based on the assumption that two or more energetically distinct amorphous states coexist in proportions varied with temperature and pressure [6,7,31,32]. For example, a two-state model assuming the glass to be a system with high-and low-density species has successfully interpreted the polyamorphism of germania [31].…”

Local structure of liquids Al2O3 and GeO2 under densification

“…Local structure of network-forming liquid such as SiO 2 , GeO 2 and Al 2 O 3 continues to be of interest for a number of researchers in physic and material science [1][2][3][4][5][6][7][8][9][10][11][12]. It is well established that applying high pressure transforms these substances from tetrahedral to octahedral network structure.…”