Séminaire - Structural study of liquid nickel and cobalt under extreme conditions of pressure and temperature by means of X-ray absorption spectroscopy - Silvia Boccato

IMPMC - Sorbonne Université - 4 place Jussieu, 75005 Paris, tour 23, 4e étage, couloir 22-23, salle 401

Mercredi 25 avril 2018, 14 h 30

Silvia Boccato - ESRF (Grenoble)

Abstract

The study of the melting and compression of the liquid for the 3d metals has not only a fundamental interest but also geophysical and planetary implications. The core of rocky planets is actually mainly composed of iron alloyed with nickel and light elements. The understanding of the behavior of such materials under extreme conditions, especially in the liquid phase, can give some important insights in the understanding of the properties, such as temperature, composition, magnetic field etc. of several planets. In this framework we performed a case study of melting curve and compression of liquid nickel and cobalt. While nickel is itself present in the core of several planets, the interest of cobalt mainly lies in its electronic and structural similarity to iron, being very close in the periodic table. The structural study of highly compressed melts requires the use of a technique able to detect the solid-liquid transition at different pressures and to measure the average distance between atoms. For its short-range order sensitivity and element selectivity, X-ray absorption spectroscopy (XAS) is a suitable technique for the purpose. It was used in the work shown in this presentation to detect the melting of 3d metals such as iron, nickel and cobalt under extreme conditions of pressure and temperature. A validation of the XAS melting criterion is also provided, by means of a morphologic study of the heated samples and DFT calculations. Information about the local environment of a specific element in the material under analysis can also be extracted, and this was performed for the first time on nickel and cobalt up to 100 GPa and 4000 K. The evolution with pressure of the distance between first neighbors is compared to calculations in the literature. Discrepancies (and similarities) are discussed in the framework of different models.
The work performed on nickel and cobalt opens the way to the study of multi-element systems such as iron alloyed with nickel and light elements, with very strong implications for the study of the interior of rocky planets.