High-pressure phase transitions and equations of state in NiSi. III. A new high-pressure phase of NiSi

Abstract

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

High-temperature ab initio calculations on FeSi and NiSi at conditions relevant to small planetary cores

“…The sequence of stable NiSi structures originally proposed was as follows: MnP → P4/nmm (or the CuTi phase) → Pbma-I → Pnma-III (FeB) → CsCl, with transitions occurring at 23, 61, 168 and 247 GPa, respectively . Following this work, Wood et al (2013) found a new stable phase of NiSi in experiments carried out in a MAP. This new phase of NiSi, with Pmmn symmetry (an orthorhombic distortion of the tetragonal CuTi structure), had not originally been considered by Vočadlo et al (2012), but further ab initio calculations revealed that this Pmmn phase was indeed more stable than either of the P4/nmm, Pbma-I or Pnma-III (FeB) phases, resulting in a much simpler phase diagram at 0 K. The new phase stability sequence in NiSi, therefore, became MnP → Pmmn → CsCl, with transitions occurring at 21 and 264 GPa (Wood et al 2013).…”

“…Following this work, Wood et al (2013) found a new stable phase of NiSi in experiments carried out in a MAP. This new phase of NiSi, with Pmmn symmetry (an orthorhombic distortion of the tetragonal CuTi structure), had not originally been considered by Vočadlo et al (2012), but further ab initio calculations revealed that this Pmmn phase was indeed more stable than either of the P4/nmm, Pbma-I or Pnma-III (FeB) phases, resulting in a much simpler phase diagram at 0 K. The new phase stability sequence in NiSi, therefore, became MnP → Pmmn → CsCl, with transitions occurring at 21 and 264 GPa (Wood et al 2013). More recent static computer simulations by Gavryushkin et al (2015) confirmed the stability of the Pmmn phase but also suggested that a tetragonally distorted (a/c ~ 0.8) CsCltype structure formed above 213 GPa, becoming fully cubic above 522 GPa.…”

“…In addition, the structures of other phases investigated for NiSi Wood et al 2013) were also used as starting points (see section "Stability of NiSi-structured phases in FeSi at 0 K"), with convergence criteria to ensure errors of less than 0.001 eV per atom. Geometry optimisation was performed, generating internal energy values for a set of specified volumes.…”

The NiSi melting curve to 70GPa

“…Up to 1700 K the assemblage consists of B20-NiSi plus the orthorhombic Pmmn 289 phase ( Fig. 6a and 7b; Wood et al, 2013) which are known to share a broad two-phase region. The 290…”

“…Both studies also reported equations of state (EoS) for these 49 structures as well as for the room pressure phase, at 0 K (for the ab initio study), and 300 K (for the 50 LH-DAC study). A third installment (Wood et al, 2013) reported the discovery of another high-51 pressure phase, with space group Pmmn, in multi-anvil press (MAP) experiments together with ab 52 initio simulations of this new structure, including a 0 K EoS. The present study, in which the 53 melting curve of NiSi is reported, forms a further part of this ongoing effort; it is accompanied by 54 Dobson et al (submitted) in which the sub-solidus phase relations of NiSi are described using off-55 line and in situ MAP experiments, in combination with the in situ LH-DAC experiments described 56…”