A new high-pressure phase of NiSi has been synthesized in a multi-anvil press by quenching samples to room temperature from 1223-1310 K at 17.5 GPa and then recovering them to atmospheric pressure. The crystal structure of this recovered material has been determined from X-ray powder diffraction data; the resulting fractional coordinates are in good agreement with those obtained from an ab initio computer simulation. The structure, in which each atom is sixfold coordinated by atoms of the other kind, is orthorhombic (space group Pmmn) with a = 3.27, b = 3.03, c = 4.70 Å . This orthorhombic phase of NiSi may be considered as a ferroelastic distortion of the hypothetical tetragonal (space group P4/nmm) NiSi structure that was predicted to be the most stable phase (at 0 K) for pressures between 23 and 61 GPa in an earlier ab initio study by Vočadlo, Wood & Dobson [J. Appl. Cryst. (2012), 45, 186-196]. Further ab initio simulations have now shown that, with increasing pressure (at 0 K), NiSi is predicted to exist in the following polymorphs: (i) the MnP structure; (ii) the new orthorhombic structure with space group Pmmn; and (iii) the CsCl structure. Experimentally, all of these structures have now been observed and, in addition, a fourth polymorph, an "-FeSi-structured phase of NiSi (never the most thermodynamically stable phase in athermal ab initio simulations), may be readily synthesized at high pressure (P) and temperature (T). On the basis of both experiments and computer simulations it is therefore now clear that the phase diagram of NiSi at high P and T is complex. The simulated free-energy differences between different structures are often very small (<10 meV atom À1 ) and there is also the possibility of two displacive ferroelastic phase transformations, the first between structures with Pmmn and P4/nmm symmetry, and the second from P4/nmm to a different orthorhombic phase of NiSi with space group Pbma. A complete understanding of the NiSi phase diagram (which may be of relevance to both planetary cores and the use of thin films of NiSi in semiconductor technology) can, therefore, only come via in situ experiments at simultaneous high P and high T.research papers

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