Crystal structure of the new silicide LaNi_11.8–11.4Si_1.2–1.6

1 Introduction

The most efficient method to discover new ternary materials with various characteristics is the investigation of the complicated interactions of the components and the construction of the isothermal sections. From this point of view, the RE–Ni–Si systems have attracted particular interest (RE is lanthanoid). These ternary systems are characterized by a variety of interactions of the components. A fairly high number of compounds with various crystal structures and interesting physical properties have been found in many of the RE–Ni–Si systems [1], [2], [3], [4]. Twenty ternary compounds were found to exist in the La–Ni–Si system [5]. However, despite the fact that the isothermal section of this system has been constructed, the crystal structures of some compounds have not yet been determined. A good example is the series of ternary compounds with the approximate composition La(Ni, Si)₁₃, which exists within the section of 7.14 wt% La [6]. According to references [7], [8], LaNi₉Si₄ is isotypic to CeNi_8.5Si_4.5. The crystal structure of the other two compounds, namely LaNi_11.6–9.5Si_1.4–3.5 and LaNi_8.8–8.4Si_4.2–4.6, are known to be derived from the NaZn₁₃ type [6], [9], though only the cell parameters for these compounds have been determined. Therefore, we decided to investigate their crystal structures in more detail. The results of this investigation are reported herein.

2 Experimental details

Starting materials for the preparation of the samples were ingots of lanthanum, nickel, and silicon with nominal purities La ≥ 99.85 wt%, Ni ≥ 99.99 wt% and Si ≥ 99.999 wt%. The samples were synthesized by arc-melting under a purified argon atmosphere, using Ti as a getter and a tungsten electrode. To achieve complete reactions, the samples were melted twice. The ingots were annealed at 600 °C in evacuated quartz ampoules for 720 h and subsequently quenched in cold water. The weight loss during the preparations was less than 1% of the total mass, which was 2 g.

Initially, the samples were studied by X-ray powder diffraction (DRON-2.0 powder diffractometer) using FeKα radiation. The indexing of the obtained diffraction data for the ternary samples was performed by comparison with calculated data using the program Powder Cell [10]. The compounds were further investigated using X-ray powder data: diffractometer STOE STADI P with a linear PSD detector (CuKα₁ radiation, curved germanium [1 1 1] monochromator; 2θ range 6.0 ≤ 2θ ≤ 111.060° with a step width of 0.015° in 2θ; PSD step 0.480° in 2θ, scan time 250 s per step). The crystal structure refinement was performed using the Dbws software [11].

Single crystals were selected by mechanical fragmentation from the sample with the composition La_7.1Ni_78.6Si_14.3. Laue and rotation diffraction patterns of selected single crystals showed cubic symmetry. Integrated intensities were measured at room temperature with graphite-monochromatized МоKα radiation (λ = 0.71073 Å) on an Oxford Diffraction Xcalibur four-circle diffractometer equipped with an Atlas CCD camera. Data collection and reduction were carried out using the programs CrysAlis CCD and CrysAlisRed [12] taking into account a numerical absorption correction. The structure was solved by Direct Methods and refined using the Shelxl-2014 program package [13].

Further details on the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49 7247 808-666; crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-2062187.

3 Results and discussion

The powder X-ray diffraction patterns of La_7.1Ni_85.9Si_7.0, La_7.1Ni_82.9Si_10.0 and La_7.1Ni_78.6Si_14.3 showed reflections of a new compound with cubic structure and some additional phases: binary Ni₂Si (Co₂Si type), LaNi₉Si₂ (CeNi_8.6Si_2.4 type) and LaNi₂Si₂ (CeAl₂Ga₂ type) or pure Ni (Cu type) [14]. First we assumed that the compounds are isostructural to the NaZn₁₃ type. However, the analysis of the powder X-ray diffraction patterns of the La_7.1Ni_82.9Si_10.0 sample revealed that the structure of this compound is more complex and may be related to the CaCu_6.5Al_6.5 structure type (Pearson code cF112, space group Fm3‾c, а = 11.935 Å [15], [16]), which is a distortion variant of NaZn₁₃ [17]. All further calculations have confirmed this supposition. Crystal data and details of structure refinement are listed in Table 1. Figure 1 illustrates the X-ray diffraction pattern of the respective sample.

Figure 1:

Observed (circles), calculated (solid line) and difference powder X-ray diffraction patterns of the sample La_7.1Ni_82.9Si_10.0 (1: LaNi_11.8Si_1.2; 2: Ni).

Because a single crystal suitable for intensity data collection was available, the following part of the investigation was performed using single-crystal X-ray diffraction. The crystallographic data and details on the data collection for a single crystal obtained from the La_7.1Ni_78.6Si_14.3 sample are listed in Table 2. An analysis of systematic absences in the data set of the single crystal has pointed towards the possible space group Fm3‾c (no. 226), and the structure refinement confirmed that conclusion. The initial atomic parameters were deduced from an automatic interpretation of Direct Methods, and the structure was successfully refined with the CaCu_6.5Al_6.5 structure type with anisotropic atomic displacement parameters for all the atoms. The final electron density difference map was smooth and did not reveal any significant residual peaks. Therefore, from the reasonable values of the residual factors of the structure reliability, it may be concluded that the investigated compound adopts the CaCu_6.5Al_6.5 type. The final atomic positional and displacement parameters for the silicide are presented in Table 3. All the crystallographic positions are fully occupied, but the Wyckoff site (96j) is occupied by a mixture of nickel and silicon atoms. Based on this data, the refined composition LaNi_11.4Si_1.6 was deduced for the investigated crystal.

When the cell parameters of the refined phases together with the occupancy of the positions (a = 11.25486(8) Å and G(96j) = 0.897Ni/0.103Si for LaNi_11.8Si_1.2; a = 11.256(4) Å and G(96j) = 0.869Ni/0.131Si for LaNi_11.4Si_1.6) are compared with each other, the existence of a small homogeneity range with the composition LaNi_11.8–11.4Si_1.2–1.6 may be assumed. Herewith, the increase in the unit cell parameters is in good agreement with the increase in the silicon content of the compound (r(Ni) = 1.24 Å < r(Si) = 1.32 Å [18]). It should be noted, that the coordination of atoms in positions with mixed occupation is also slightly different for both phases (see Table 1 and 3).

The main interatomic distances and their reduced values from the sum of the atomic radii (Δ = 100(δ−∑r)/∑r, where ∑r is the sum of the respective atomic radii), and the coordination numbers of the atoms for LaNi_11.4Si_1.6 are listed in Table 4 (values of the atomic radii are taken from [18]: r(La) = 1.87, r(Ni) = 1.24, r(Si) = 1.32, and calculated r(M) = 1.25 Å). Deviations in the interatomic distances deduced from the sums of the relevant atomic radii are very small (no more than 6%). Some M–M and M–Ni interatomic distances are slightly shorter when compared with the sum of the respective atomic radii. Hence, the feature of our compound is that within its structure, the lanthanum atoms are not in contact with each other.

Projections of the unit cell and the coordination polyhedra of the atoms are shown in Figure 2. The lanthanum atoms are confined within 24-vertex snub cubes [La(M₂₄)]. The nickel atoms are located inside the icosahedra [Ni(M₁₂)] with coordination number 12. The environment of the Ni/Si atoms on sites with mixed occupancy consists of atoms of all three kinds with the coordination polyhedron [M(La₂Ni₁M₉)], that has 12 vertices.

Figure 2:

Projection of the unit cell of LaNi_11.4Si_1.6 onto the crystallographic ab plane and coordination polyhedra of the atoms.

The LaNi_11.4Si_1.6 structure can be considered as a packing of different polyhedra (Figure 3). To the first type of the polyhedra one can attribute the Ni-centered icosahedra [Ni(M₁₂)], which are isolated and mutually rotated by 90°. The other type of polyhedra includes the coordination polyhedra around the lanthanum atoms [La(M₂₄)], which are connected with each other through common faces. These basic polyhedral building units almost completely fill the space in the structure of the compound. Moreover, the Ni-centered icosahedra and the La-centered snub cubes are packed in a CsCl manner (Figure 3).

Figure 3:

The packing of the icosahedra [Ni(M₁₂)] and the snub cubes [La(M₂₄)] in the crystal structure of LaNi_11.4Si_1.6.

According to previous reports [6], [9], the compound with the approximate composition LaNi_11.6–9.5Si_1.4–3.5 and an unknown crystal structure exists partly in the range of the compound LaNi_11.8–11.4Si_1.2–1.6. Taking into account that the area of existence of LaNi_11.6–9.5Si_1.4–3.5 has been determined approximately by using Debye-Scherrer techniques, it can be argued that the aforementioned silicide investigated by us is a completely new compound. In view of our results and the inaccuracy of literature data, new precise investigations of the existence region and the crystal structure of the ternary compound which can form across the section with 7.14 wt% La need to be performed in future work.

4 Conclusions

The crystal structure of LaNi_11.8–11.4Si_1.2–1.6 has been investigated using powder and single crystal X-ray diffraction data. The investigated silicide belongs to a larger class of compounds which derives from the NaZn₁₃ family. The LaNi_11.8–11.4Si_1.2–1.6 compound forms a cubic CaCu_6.5Al_6.5-type unit cell (space group Fm3‾c, a = 11.25486(8)–11.256(4) Å). A mixture of Ni/Si atoms occupies one crystallographic position and the remaining two positions are occupied by La and Ni atoms.