Renfeng Li

Local structure of liquid gallium under pressure

In situ high energy X-ray pair distribution function (PDF) measurements, microtomography and reverse Monte Carlo simulations were used to characterize the local structure of liquid gallium up to 1.9 GPa. This pressure range includes the well-known solid-solid phase transition from Ga-I to Ga-II at low temperature. In term of previous research, the local structure of liquid gallium within this domain was suggested a mixture of two local structures, Ga I and Ga II, based on fitting experimental PDF to known crystal structure, with a controversy. However, our result shows a distinctly different result that the local structure of liquid gallium resembles the atomic arrangement of both gallium phase II and III (the high pressure crystalline phase). A melting mechanism is proposed for Ga, in which the atomic structure of phase Ι breaks up at the onset of melting, providing sufficient free volume for atoms to rearrange, to form the melt.

Study of liquid gallium as a function of pressure and temperature using synchrotron x-ray microtomography and x-ray diffraction

The volume change of liquid and solid gallium has been studied as a function of pressure and temperature up to 3.02 GPa at 300 K and up to 3.63 GPa at 330 K using synchrotron x-ray microtomography combined with energy dispersive x-ray diffraction techniques. Two sets of directly measured P-V data at 300 K and 330 K were obtained from 3D tomography reconstruction data, and the corresponding isothermal bulk moduli were determined as 23.6 (0.5) GPa and 24.6 (0.4) GPa, respectively. The existence of a liquid-liquid phase transition region is proposed based on the abnormal compressibility of Ga melt at about 2.44 GPa and 330 K conditions.

Polyamorphism in Yb-based metallic glass induced by pressure

The Yb62.5Zn15Mg17.5Cu5 metallic glass is investigated using synchrotron x-ray total scattering method up to 38.4 GPa. The polyamorphic transformation from low density to high density with a transition region between 14.1 and 25.2 GPa is observed, accompanying with a volume collapse reflected by a discontinuousness of isothermal bulk modulus. This collapse is caused by that distortional icosahedron short range order precedes to perfect icosahedron, which might link to Yb 4f electron delocalization upon compression, and match the result of in situ electrical resistance measurement under high pressure conditions. This discovery in Yb-based metallic glass, combined with the previous reports on other metallic glass systems, demonstrates that pressure induced polyamorphism is the general behavior for typical lanthanide based metallic glasses.

The study of noncrystalline material under high pressure using high energy x-ray pair distribution function combined with synchrotron x-ray microtomography techniques

For non-crystalline samples, such as amorphous materials and melts, due to their lack of periodicity, scattering patterns from amorphous and liquid phases are very broad. Thus, the pair distribution function (PDF) and X-ray microtomography are important methods to investigate structural change for non-crystalline samples under high pressure conditions. Here we present experimental results on liquid gallium using microtomography and Yb-based metallic glass using PDF under high pressure.

Non-crystalline samples under high-pressure conditions

Taking advantage of synchrotron x-ray diffraction, PDF and tomographic techniques, the P-V curve of non-crystalline samples were studied under high-pressure conditions. Two element and several metallic glass cases were performed. The procedure of crystallization of amorphous Se upon compression at room temperature, which was studied in diamond anvil cell combined synchrotron x-ray PDF and 3D imaging techniques; the melting and solidification procedure of Ga in large volume press at room and high temperature; and complicated crystallization, re-rystalization, melting behavior of Ce-based metallic glass, will be presented to show the capability of revealing structure and dynamics behaviors in P-V-T-t domains using these advanced techniques.