Abstract
In situ x-ray diffraction measurements and inverse Monte Carlo simulations of pair distribution functions were used to characterize the local structure of molten AuGa2 up to 16 GPa and 940 K. Our results document systematic changes in liquid structure due to a combination of bond compression and coordination increase. Empirical potential structure refinement shows the first-neighbor coordination of Ga around Au and of Au around Ga to increase from about 8 to 10 and 4 to 5, respectively between 0 and 16 GPa, and the inferred changes in liquid structure can explain the observed melting-point depression of AuGa2 up to 5 GPa. As intermetallic AuGa2 is an analogue for metallic SiO2 at much higher pressures, our results imply that structural changes documented for non-metallic silicate melts below 100 GPa are followed by additional coordination changes in the metallic state at pressures in the 0.2–1 TPa range achieved inside large planets.
Highlights
The study of densified liquid structures provides fundamental information for understanding thermodynamic phase diagrams, including the effects of distinct crystalline phases on melting at various pressures and temperatures
There is only limited understanding of the local structure of solid amorphous AuGa2, with the available information amounting to the total structure factor obtained from electron diffraction[7], which does not provide such detail as pair distribution functions from experiment or modelling (e.g., Monte-Carlo modelling)
In spite of our limited Q range, which constrains the simulation of measurements to low magnitudes of the scattering vector Q, we were able to observe distinct features in the diffraction patterns for liquid AuGa2 at pressures at which different sub-solidus crystal structures are stable (Table 2, Fig. 1a)
Summary
Data were obtained by heating each of the crystalline phases of AuGa2 to temperatures slightly above melting (Table 1), the two-dimensional diffraction patterns recorded by image plate confirming the absence of any crystalline phase after melting[3]. The most prominent peak (Peak 3, Fig. 1c) in the structure factor S(Q) at temperatures above the melting temperatures of cubic and orthorhombic phases (P ≤ 11 GPa) has a clear shoulder on the low-Q side (Peak 2, Fig. 1c). This is distinct from the result for the monoclinic-to-melt pattern (P = 16 GPa, Fig. 1a), which shows a broad peak at Q = 2.62(5) Å−1 attributable to overlapping of the prominent peak and shoulder. Peak 3 for the liquid S(Q) is in the range of 2.87–2.95 Å−1 at 11 GPa and below, matching the highest peaks in the diffraction patterns of the
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