Short- and medium-range order in Zr${}_{80}$Pt${}_{20}$ liquids

The atomic structures in equilibrium and supercooled liquids of Zr${}_{80}$Pt${}_{20}$ were determined as a function of temperature by in situ high-energy synchrotron diffraction studies of the levitated liquids (containerless processing) using the beamline electrostatic levitation (BESL) technique. The presence of a pronounced pre-peak at q ∼ 1.7 Å${}^{-1}$ in the static structure factor indicates medium-range order (MRO) in the liquid. The position and intensity of the pre-peak remain constant with cooling, indicating that the MRO is already present in the liquid above its melting temperature. An analysis of the liquid atomic structures obtained using the Reverse Monte Carlo method utilizing both the structure factor $S$($q$) from x-ray diffraction experiments and the partial pair-correlation functions from ab initio molecular dynamics simulations show that the pre-peak arises from a Pt-Pt correlation that can be identified with icosahedral short-range order around the Pt atoms. The local atomic ordering is dominated by icosahedral-like structures, raising the nucleation barrier between the liquid and these phases, thus assisting glass formation.

Figure 1

Static structure factor $S$($q$) for (a) liquid Zr${}_{80}$Pt${}_{20}$ at 1607 °C, 1474 °C, 1345 °C, 1217 °C, 1090 °C, and 978 °C. The insets show the evolution of $S$($q$) with decreasing temperature for (b) the primary peak, (c) the second peak, and (d) the pre-peak (indicated with an arrow in the total structure factor).

Figure 2

Experimental $S$($q$) (solid lines) and the corresponding CRMC fit (dashed lines) for liquid Zr${}_{80}$Pt${}_{20}$ at 1345 °C, 1217 °C, 1090 °C, and 978 °C.

Figure 3

(a) Structure factor $S$($q$) of Zr${}_{80}$Pt${}_{20}$ liquid at 1345 °C with the constrained RMC fit. Partial pair-correlation functions $g_{i-j}(r)$ from MD and CRMC simulations for (b) Zr-Pt, (c) Zr-Zr, and (d) Pt-Pt.

Figure 4

$S$($q$) produced from CRMC simulations at all temperatures. (a) Total structure factor; (b) Zr-Pt partial structure factor, showing an asymmetric first peak and no developing shoulder in the second peak; (c) Zr-Zr partial structure factor showing a developing should in the second peak; and (d) Pt-Pt partial structure factor displaying a prominent pre-peak at all temperatures.

Figure 5

The prominent HA indices for the RMC liquid structure as a function of temperature.

Figure 6

(a) Dominant Zr-centered Voronoi polyhedra as a function of temperature. (b) Dominant Pt-centered Voronoi polyhedra as a function of temperature.