The phase diagram of NiSi under the conditions of small planetary interiors

David P Dobson,Simon A Hunt,Jabraan Ahmed,Oliver T Lord,Elizabeth T.H Wann,James Santangeli,Ian G Wood,Lidunka Vočadlo,Andrew M Walker,Andrew R Thomson,Marzena A Baron,Hans J Mueller,Christian Lathe,Matthew Whitaker,Guillaume Morard,Mohamed Mezouar

doi:10.1016/j.pepi.2016.10.005

David P Dobson, Simon A Hunt + Show 14 more

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The phase diagram of NiSi has been determined using in situ synchrotron X-ray powder diffraction multi-anvil experiments to 19GPa, with further preliminary results in the laser-heated diamond cell reported to 60GPa. The low-pressure MnP-structured phase transforms to two different high-pressure phases depending on the temperature: the ε-FeSi structure is stable at temperatures above ∼1100K and a previously reported distorted-CuTi structure (with Pmmn symmetry) is stable at lower temperature. The invariant point is located at 12.8±0.2GPa and 1100±20K. At higher pressures, ε-FeSi-structured NiSi transforms to the CsCl structure with CsCl-NiSi as the liquidus phase above 30GPa. The Clapeyron slope of this transition is −67MPa/K. The phase boundary between the ε-FeSi and Pmmn structured phases is nearly pressure independent implying there will be a second sub-solidus invariant point between CsCl, ε-FeSi and Pmmn structures at higher pressure than attained in this study. In addition to these stable phases, the MnP structure was observed to spontaneously transform at room temperature to a new orthorhombic structure (also with Pnma symmetry) which had been detailed in previous ab initio simulations. This new phase of NiSi is shown here to be metastable.

Highlights

We have known for over half a century (Birch, 1964) that the core of the Earth has a density which is somewhat lower than that of pure iron under similar conditions of pressure and temperature; this density deficit is postulated to be due to the presence of approximately 10 wt.% of a light alloying element
The sub-solidus P-T phase diagram of the NiSi system up to 19 GPa and 1773 K by in situ, multi-anvil based, synchrotron
The clear splitting of the 1 0 2 and 0 1 2 reflections at 1.81 and 1.89 Å confirms that this phase is, the orthorhombically distorted structure previously seen in samples recovered from high pressure (Wood et al, 2013) and not the tetragonal c-CuTi phase with space group P4/nmm

Summary

Introduction

We have known for over half a century (Birch, 1964) that the core of the Earth has a density which is somewhat lower than that of pure iron under similar conditions of pressure and temperature; this density deficit is postulated to be due to the presence of approximately 10 wt.% of a light alloying element (or elements). Cosmochemical arguments suggest that the core of the Earth contains 5 wt.% or more nickel and some iron meteorites contain up to 20 wt.% nickel, nickel has been largely ignored in studies of core materials. In the extensively studied FeSi system the e-FeSi structure that is stable at ambient conditions transforms to a CsCl-structured phase at $20 GPa, which remains stable to pressures greater than those in the core of the Earth (Vocadlo et al, 1999; Dobson et al, 2002, 2003; Dubrovinsky et al, 2003; Caracas and Wentzcovitch, 2004; Lord et al, 2010; Zhang and Oganov, 2010; Fischer et al, 2013; Geballe and Jeanloz, 2012).

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