Detection of First-Order Liquid/Liquid Phase Transitions in Yttrium Oxide-Aluminum Oxide Melts

Abstract

We combine small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) with aerodynamic levitation techniques to study in situ phase transitions in the liquid state under contactless conditions. At very high temperatures, yttria-alumina melts show a first-order transition, previously inferred from phase separation in quenched glasses. We show how the transition coincides with a narrow and reversible maximum in SAXS indicative of liquid unmixing on the nanoscale, combined with an abrupt realignment in WAXS features related to reversible shifts in polyhedral packing on the atomic scale. We also observed a rotary action in the suspended supercooled drop driven by repetitive transitions (a polyamorphic rotor) from which the reversible changes in molar volume (1.2 +/- 0.2 cubic centimeters) and entropy (19 +/- 4 joules mole(-1) kelvin(-1)) can be estimated.

Polyamorphism and the universal liquid–liquid critical point in the supercooled state

“…Compositional phase separation is the best known (Poria-Koshits & Averjanov 1968), but recent experiments on refractory oxides have revealed physical phase separation promoted by critical phenomena (Greaves et al 2008). In either case, liquid coexistence results in an increase in small-angle X-ray scattering (SAXS) from which information about the length scale of compositional or physical fluctuations can be obtained (Zarzycki 1991).…”

“…17 keV X-rays were needed to penetrate the 2 mm diameter drops. By combining X-ray diffraction with SAXS and pyrometer calorimetry, the reversible changes in density and entropy that accompany this first-order transition were successfully quantified for the liquid (Al 2 O 3 ) 0.8 (Y 2 O 3 ) 0.2 or AY20 (Greaves et al 2008).…”

“…Accordingly, the SAXS experiments at ambient pressure shown in figure 1(a) follow a trajectory at ambient pressure which is almost orthogonal to the liquid-liquid phase boundary intersecting it at T LL . Approaching the critical region in this way means that the profile of density fluctuations <Δρ 2 > /ρ 0 will be weaker but also more symmetrical compared to (Fuentevilla & Anisimov 2006), the liquid-liquid critical point T C , P C , the coexistence boundary (P > P C ) and the first-order liquid-liquid transition at ambient pressure (Greaves et al 2008). …”

“…These conditions have enabled us to examine the degree to which the nearby liquid-liquid critical point T c , P c predicted by the two-liquid model (Greaves et al 2008;Ponyatovsky & Barkalov 1992;McMillan et al 2007), might display universal characteristics. In particular by employing the thermodynamic liquid mixture model (Ponyatovsky & Barkalov 1992;McMillan et al 2007), the critical liquid-liquid point T C ,P C from which the liquid coexistent phase boundary emanates has been estimated, and the gradient of the phase boundary dT/dP (Greaves et al 2008), the latter being comparatively gentler than for polyamorphic water (Debenedetti 1997;Fuentevilla & Anisimov 2006).…”

“…We have recently detected and quantified in situ the first-order liquid-liquid phase transitions (LLT) in yttria-alumina liquids (Greaves et al 2008), first proposed to explain polyamorphic phases in rapidly quenched yttria-alumina glasses differing in density and entropy (Aasland & McMillan 1994). Scattering experiments on liquid drops were performed under contactless conditions using aerodynamic levitation furnace methods (Hennet et al 2002), and in so doing reduced the danger of crystallization in the supercooled region.…”