Crystal structure of Ti4Ni2C

Ti4Ni2C crystallizes isotypically with Ti4Ni2O, Nb4Ni2C and Ta4Ni2C and can be considered as a partially filled Ti2Ni structure with the C atom occupying an octahedral void.


Structure description
A large number of intermetallic phases can be grouped into classes of compounds based on structural or chemical similarities.For example, Mueller & Knott (1963) Sadrnezhaad et al. (2009) and Shigeo et al. (1993) have confirmed the existence of the Ti 4 Ni 2 C phase.However, no detailed study has been performed so far with respect to the determination of its crystal structure.
In the present study, the crystal structure model of Ti 4 Ni 2 C has been refined on the basis of single-crystal X-ray diffraction data.The lattice parameter a is similar to those of previously reported isotypic phases (see above), and its chemical composition was refined to be exactly Ti 4 Ni 2 C in accordance with the EDX results (see Fig. S1 and Table S1 in the supporting information).Carbon present in the crystal structure most likely originated from the graphite crucible used during high pressure sintering (HPS).(2.mm, 48f) and six Ni1 atoms (.3m; 32e), defining the center of an icosahedron.The C1 atom occupies a position with site symmetry .3m(16d) and centers an octahedron defined by six Ti2 atoms.The shortest Ti1� � �Ti2 separation is 2.9415 (9) A ˚and the shortest Ti1� � �Ni1 separation is 2.4750 (4) A ˚; the C1-Ti2 bond length is 2.1127 (4) A ˚.

Synthesis and crystallization
The high-purity elements Ti (indicated purity 99.5%; 0.6291 g) and Ni (indicated purity 99.9%; 0.3869 g) were mixed uniformly in the stoichiometric ratio 2:1 and thoroughly ground in an agate mortar.The blended powders were then placed in a cemented carbide grinding mould of 5 mm diameter, and pressed into a tablet at about 4 MPa for 1 min.A cylindrical block was obtained without deformations or cracks.Details of the high-pressure sintering experiment using a sixanvil high-temperature high-pressure apparatus can be found elsewhere (Liu & Fan, 2018).The samples were pressurized up to 6 GPa and heated to 1573 K for 40 min, and then rapidly cooled to room temperature by turning off the furnace power.
A piece of a single-crystal (0.06�0.06�0.04mm 3 ) was selected and mounted on a glass fibre for SXRD measurements.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å
Single crystals of the intermetallic phase with composition Ti 4 Ni 2 C were serendipitously obtained by high-pressure sintering of a mixture with initial chemical composition Ti 2 Ni.The Ti 4 Ni 2 C phase crystallizes in the Fd3m space group and can be considered as a partially filled Ti 2 Ni structure with the C atom occupying an octahedral void.Ti 4 Ni 2 C is isotypic with Ti 4 Ni 2 O, Nb 4 Ni 2 C and Ta 4 Ni 2 C, all of which were studied previously by means of powder diffraction. investigated the related crystal structures of Ti 2 Cu, Ti 2 Ni, Ti 4 Ni 2 O and Ti 4 Cu 2 O by X-ray and neutron powder diffraction.They determined that the Ti 2 Ni phase crystallizes in the Fd3m space group, with cell parameter a = 11.3193(2) A ˚and with 96 atoms per unit cell; the Ti 4 Ni 2 O (Ti 4 Cu 2 O) phase also crystallizes in the Fd3m space group, with cell parameter a = 11.3279(1) A ˚[a = 11.4353(2) A ˚] and with 112 atoms per unit cell.The latter phases can be considered as partially filled Ti 2 Ni variants with the additional oxygen atom occupying an octahedral position.Holleck & Thummler (1967) studied a series of carbides, nitrides and oxides in ternary systems and reported that Nb 4 Ni 2 C (a = 11.64A ˚) and Ta 4 Ni 2 C (a = 11.61A ˚) crystallize in the same partially filled Ti 2 Ni structure.
Ti 4 Ni 2 C crystallizes isotypically with other Ti 4 Ni 2 X compounds (X = C, N, O) with a partially filled Ti 2 Ni structure in space group type Fd3m.Fig. 1 shows the distribution of the atoms in the unit cell of Ti 4 Ni 2 C. The environments of the Ti1 and C1 sites are shown in Figs. 2 and 3, respectively.The Ti1 atom is situated at a position with site symmetry .3m(multiplicity 16, Wyckoff letter c).It is surrounded by six Ti2 atoms

Figure 1 Mo
Figure 1The crystal structure of Ti 4 Ni 2 C (one unit cell), with displacement ellipsoids drawn at the 99% probability level.

Table 1
Experimental details.