High-pressure phase transitions and equations of state in NiSi. II. Experimental results

Abstract

The high-pressure structures of nickel monosilicide (NiSi) have been investigated to 124 GPa by synchrotron-based X-ray powder diffraction studies of quenched samples from laser-heated diamond anvil cell experiments, and the equations of state of three of these phases have been determined at room temperature. NiSi transforms from the MnP (B31) structure (space groupPnma) to the ∊-FeSi (B20) structure (space groupP213) at 12.5 ± 4.5 GPa and 1550 ± 150 K. Upon further compression, the CsCl (B2) structure (space groupPm3m) becomes stable at 46 ± 3 GPa and 1900 ± 150 K. Thus, NiSi will be in the B2 structure throughout the majority of the Earth's mantle and its entire core, and will likely form a solid solution with FeSi, which is already known to undergo a B20 → B2 transition at high pressure. Data from the quenched (room-temperature) samples of all three phases have been fitted to the third-order Birch–Murnaghan equation of state. For the MnP (B31) structure this yieldsK0= 165 ± 3 GPa withK0′ fixed at 4 andV0fixed at 12.1499 Å3 atom−1[V0from unpublished neutron diffraction measurements on the same batch of starting material; Wood (2011), personal communication]. For the ∊-FeSi (B20) structure,K0=161± 3 GPa andK0′ = 5.6 ± 0.2 withV0fixed at 11.4289 Å3 atom−1. For the CsCl (B2) structure,K0 = 200 ± 9 GPa,K0′ = 4.6 ± 0.1 andV0= 11.09 ± 0.05 Å3 atom−1. The ambient volume of NiSi, therefore, decreases by 6% at the first phase transition and then by a further 3% at the transition to the CsCl structure. Traces of additional NiSi structures predicted by Vočadlo, Wood & Dobson [J. Appl. Cryst. (2012),45, 186–196; part I], and labelled therein asPbma-I,Pnma-II, and possibly alsoPnma-III andP4/nmm, have been detected.