CC BY
20iSHS 2019
Moscow, Russia
AN X-RAY POWDER DIFFRACTION STUDY OF NEW CRYSTAL
Ni3.35W9.65C4
S. V. Konovalikhin*", N. Yu. Khomenko", I. I. Chuev", S. A. Gudafi, S. L. Silyakov",
and D. Yu. Kovalev"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia bSouthern Federal University, Rostov-on-Don, 344090 Russia
*e-mail: ksv17@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10068
Unknown reflections in XRD patterns of SHS products of WO3 (59%) + NiO (12%) + Al (10%) + Ca (13%) + C (6%) mixture were found. XRD analysis showed that the unknown phase is y phase (Ni3W9C4) or k phase (Ni3W10C3.4). There are no structural data in PDF-2 and ICSD databases. The aim of this paper is to establish the crystal structure of this phase. XRD study, DFT calculation, and crystal chemical simulations were carried out. Comparison of results revealed the composition and structure of the new compound.
XRD studyanalysis was performed using a DRON-3M diffractometer in Cuia radiation and in the range of 20 = 20°-80° with a step of 0.02° and a time per step of 2 s. Composition and cell parameters were determined by Rietveld method. Profile refinement of reflections was made by Pseudo-Voigt. Texture, thermal parameters, and coordinates of atoms were fixed. Occupation of 2a position by atoms W and Ni was refined in the latter stages. The weighed and profile R factors (experiment quality characteristics) calculated in the refinement process are: Rwp = 11.4%, Rp = 8.9%, Re = 26.55%, GofF = 0.43. Theoretical XRD patterns were made by Mercury 3.5.1 package.
DFT calculations were carried out using a VASP 5.0 program on supercomputer «Blokhin» of Southern Federal University. PBE pseudopotential and generalized gradient approximations (GGA) were used. K-point mesh of 3 x 3 x 3 was applied for electronic optimization. The mesh size was chosen by the trial-and-error method. We considered three meshes of 2 x 2 x 2, 3 x 3 x 3, and 4 x 4 x 4 and the one with the lowest total crystal energy turned out to be preferable.
Composition of the reaction products was the following: WC (82 %), W2C (3.5%), and Ni3WxCy (14.5%) (Fig. 1). It was shown that structures of y and k phase have different occupation of metal atoms (2a) and C (2c) [1, 2]. Comparison of XRD data for Ni3W9C4 and Ni3W10C3.4 crystals showed the difference in the intensity of 002 reflections (I002). I002 in Ni3W9C4 crystals is 10 times larger than in Ni3W10C3.4 (Fig. 2). The XRD data showed that in new crystals 002 reflections are absent (Fig. 1). Therefore, for the new phase the composition of Ni3W10C3.4 was chosen.
Analysis of XRD patterns showed that I301 (20 ~ 41.5°) is larger than I3-12 (20 ~ 42.1°) (Fig. 1), though there is an opposite ration in theoretical patterns of Ni3W10C3.4 crystal (I302 < I3-12) (Fig. 2). Crystal chemical simulation was made to get a perfect correspondence with XRD data. It was proposed a partial occupation of Ni atoms in position 2a (composition of Ni3.5W9.5C4) or in 12k (structure Ni6W?C4) Ni atoms, or in both positions simultaneously (composition Ni6.5W6.5C4).
DFT calculation showed that structures of Ni6W7C4 and Ni 6.5W6.5C4 are energetically less favorably (Table 1), furthermore, a < c for these compounds in contradiction of XRD data. The only structure with a partial occupation of Ni in position 2a is in good correspondence with experimental data (I002 ~ 0, I302 > I3-12, a > c).
XV International Symposium on Self-Propagating High-Temperature Synthesis
Fig. 1. XRD pattern of product SHS reaction.
Fig. 2. Theoretical XRD patterns y-phase crystals Ni3W9C4 and K-phase Ni3WioC3.4.
Table 1. The results of the XRD study and DFT calculation of crystals Ni3WioC3.4 and Ni3W9C4.
Phase a, Â c, Â V, Â3 N Stotai, eV/mol s eV/mol
NÍ3W9C4 7.6595 7.7102 391.5 236 -321.736 -1.363
NÍ3W10C3.4 7.8358 7.8472 417.3 208 -359.926 -1.730
Ni3.5W9.5C4 7.8247 7.7881 413.0 216 -361.227 -1.672
Ni3.5W9.5C4* 7.8344(4) 7.8048(7) 414.87(7)
NÍ6W7C4 7.6540 7.8359 397.6 236 -318.7857 -1.350
Ni5.5W5.5C4 7.6858 7.7744 397.2 240 -311.2943 -1.297
* XRD experiment.
The population of 0.65 for W atom and of 0.35 for Ni atom was found by Rietveld method. The final composition is Ni3.35W9.65C4. Figure 3 shows the crystal structure. The cell parameters are listed in Table 1.
Fig. 3. Crystal structure of Ni3.35W9.65C4. Conclusions.
The new phase of Ni-W-C system was synthesized by SHS method. Its composition (Ni3.35W9.65C4) was determined by XRD analysis, DFT calculation, and crystal chemistry simulation.
1. A. Harsta, T. Johansson, S. Rundqvist, J.O. Thomas, A neutron powder diffraction study of the K-phase in the Co-W-C system, Acta Chem. Scand. A, 1977, vol. 31, no. 4, pp. 260-264.
2. M. Schonenberg, The structure of the Co3W3C4 phase, Acta Metall., 1954, vol. 2,
pp. 837-839.
