Intermetallic phases in the Sc–Ir–In system – synthesis and structure of Sc_1.024Ir₂In_0.976 and Sc₃Ir_1.467In₄

1 Introduction

Scandium as the smallest element in the series of rare earth (RE) metals plays its own peculiar crystal chemical role [1], [2], [3], [4]. In many cases it extends the series of rare earth compounds and forms a compound that is isotypic with the lutetium one; however, due to its small size superstructure or defect formation may also occur. Two recent striking examples are Sc₅Cu₂In₄ [5] which adopts a superstructure of the Lu₅Ni₂In₄ type or the series of ScPt_1–xIn compounds [6] which show modulated structures, driven by ordering of filled and vacant platinum sites.

Table 1 gives an overview on the known Sc_xT_yIn_z phases. Especially the Sc–Co–In and Sc–Ni–In systems have thoroughly been studied and numerous indide structures were refined from X-ray diffraction data [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. In comparison with the many RE_xT_yIn_z phases with the larger rare earth elements it is obvious that most Sc_xT_yIn_z phases are either scandium- or transition metal-rich. The few examples with ‘higher’ indium content are Sc₃Co_1.64In₄ (46.3 at.%) [20] and Sc₁₀Ni₉In_19.44 (50.6 at.%) [19], in contrast to compounds like RENiIn₂ (50 at.%) [32], RENiIn₄ (66.6 at.%) [33], RE₂IrIn₈ (72.7 at.%) or REIrIn₅ (71.4 at.%) [34]. The latter all exhibit pronounced indium substructures [35], while the scandium-rich phases show the typical condensation patterns of metal-rich structures with Frank–Kasper-related polyhedra. Representative examples are the structures of Sc₅₀Pt_13.47In_2.53 [10] or Sc₁₄Co_3.10In_2.59 [7].

The Sc-T-In systems with the 4d and 5d metals are still not completely studied, especially those with the heavy 5d elements. In continuation of our phase-analytical work in these systems we have now obtained the new Heusler phase Sc_1.024Ir₂In_0.976 and the compound Sc₃Ir_1.467In₄ which crystallizes with a new superstructure variant of the ZrNiAl family. The synthesis and crystal chemical details are reported herein.

2 Experimental

2.2 X-ray diffraction

The polycrystalline Sc₃Ir_1.5In₄ and ScIr₂In samples were studied by powder X-ray diffraction (Guinier technique, Enraf-Nonius FR552 camera, CuKα₁ radiation, imaging plate detector, Fujifilm BAS-1800). α-Quartz (a = 491.30, c = 540.46 pm) was taken as an internal standard. The lattice parameters (Table 2) were refined from the experimental 2θ values. Correct indexing of the complex pattern was ensured with intensity calculations using the Lazy Pulverix routine [38]. The Guinier pattern of the Sc₃Ir_1.5In₄ sample is shown in Figure 1. The main product is the hexagonal phase besides the by-product ScIr₂In and residual indium. The powder and single-crystal lattice parameters of both phases show good agreement (Table 2).

Table 2:

Crystallographic data and structure refinement for Sc_1.024Ir₂In_0.976; MnCu₂Al type, space group Fm3‾m, Z = 4, and for Sc₃Ir_1.467In₄; space group P6‾, Z = 2.

Empirical formula	Sc1.024Ir2In0.976	Sc3Ir1.467In4
Formula weight, g mol−1	542.5	876.1
Lattice parameters (single-crystal data)
a, pm	639.97(19)	769.99(5)
c, pm	a	684.71(4)
V, nm3	0.2621	0.3516
Lattice parameters (Guinier powder data)
a, pm	640.3(2)	769.7(2)
c, pm	a	685.0(2)
V, nm3	0.2625	0.3515
Calculated density, g cm−3	13.75	8.28
Crystal size, µm3	25 × 35 × 40	15 × 35 × 35
Diffractometer type	IPDS-II (Stoe)	IPDS-II (Stoe)
Detector distance, mm	70	70
Exposure time, min	15	30
ω Range/step width, °	0–180/1.0	0–180/1.0
Integr. Param. A/B/EMS	14.0/−1.0/0.030	14.0/−1.0/0.030
Abs. coefficient, mm−1	111.9	43.1
F(000), e	893	744
θ range, °	5.52–32.96	2.97–33.43
hkl range	±9, ±9, ±9	±11, ±11, ±10
Total no. reflections	348	4288
Independent reflections/Rint	41/0.0248	967/0.0267
Refl. with I ≥ 3σ(I)/Rσ	41/0.0044	906/0.0173
Data/ref. Parameters	41/7	967/33
Goodness-of-fit on F2	1.60	1.13
R1/wR2 for I ≥ 3σ(I)	0.0162/0.0376	0.0176/0.0360
R1/wR2 for all data	0.0162/0.0376	0.0207/0.0371
Extinction coefficient	170(30)	79(9)
Twin ratio	–	0.465(7): 0.535(7)
Largest diff. Peak/hole, e Å−3	1.18/−0.91	1.11/−1.21

Figure 1:

Experimental Guinier powder pattern (CuKα₁ radiation) of the Sc₃Ir₂In₄ sample (middle) compared to calculated patterns of Sc₃Ir_1.467In₄ (top) and the Heusler phase ScIr₂In (bottom). The blue bars mark the 2θ ranges that are characteristic for the Heusler phase. The strongest reflection for residual indium is marked by an arrow.

Small single crystals were isolated from both carefully crushed samples and glued to quartz fibres using beeswax. Their quality was tested on a Buerger camera (white Mo radiation). Complete intensity data sets were collected on a Stoe IPDS-II diffractometer (graphite monochromatized MoKα radiation; oscillation mode). Numerical absorption corrections were applied to the data sets. Details of the data collections, the crystallographic parameters and the refinements are summarized in Table 2.

3 Crystal chemistry

We start our crystal chemical discussion with the Heusler phase Sc_1.024Ir₂In_0.976. Although the refined composition is close to the ideal one, we observe substantial disorder, i.e., Sc/In and In/Sc mixing on the 4b and 4a sites, indicating the existence of a small homogeneity range. The unit cell of Sc_1.024Ir₂In_0.976 is presented in Figure 2. The mixed occupied sites are emphasized by segments. Together, the scandium and indium atoms build up a rocksalt-like atomic arrangement in which all tetrahedral sites are filled with iridium atoms. The Heusler phase structure (the prototype is MnCu₂Al) is one of the simple, basic structure types of intermetallic compounds and the crystal chemical facets are summarized in standard textbooks [42], [43], [44].

Figure 2:

The crystal structure of the Heusler phase Sc_1.024Ir₂In_0.976. Scandium, iridium and indium atoms are drawn as medium grey, blue and magenta circles, respectively. The mixed occupied sites are emphasized by segments.

The structure of Sc₃Ir_1.467In₄ belongs to the family of ZrNiAl derived superstructures [45, 46]. It is closely related to the superstructure variant Sc₃Rh_1.594In₄ [12]. The differences in the site occupancies and in the atomic parameters are compared in Figure 3. The main difference concerns the vacant 1b site in the iridium compound. The latter is occupied by 77.5% rhodium in Sc₃Rh_1.594In₄. We can thus clearly designate Sc₃Ir_1.467In₄ as a new ZrNiAl superstructure. Sc₃Rh_1.594In₄ is derived from the aristotype ZrNiAl via two symmetry reductions (t2 and i3). The corresponding group-subgroup scheme is discussed in detail in [12].

Figure 3:

Refined atomic coordinates and site occupancies in the structures of Sc₃Rh_1.594In₄ [12] and Sc₃Ir_1.467In₄.

The vacant 1b site in the Sc₃Ir_1.467In₄ structure substantially influences the z coordinates of both indium sites and the x and y coordinates of the scandium atoms (Figure 3). These scandium displacements are readily visible in the projection of the structure along the c axis. The top drawing of Figure 4 shows an extended unit cell and emphasizes the trigonal prisms formed by the scandium and indium atoms. Adjacent triangular faces of the Ir3@Sc₆ prisms are slightly twisted with respect to each other. The middle and bottom drawings of Figure 4 show cutouts within distinct z ranges. These partial projections also show that only every other of the In₆ prisms along the z axis is occupied by iridium.

Figure 4:

Projection of the Sc₃Ir_1.467In₄ structure onto the ab plane. Scandium, iridium and indium atoms are drawn as medium grey, blue and magenta circles, respectively. Displacement ellipsoids are drawn at a 99% level. The upper drawing shows the complete unit cell, while sections from z = 0.4–0.8 and 0–0.4 are shown in the middle and at the bottom. The trigonal prismatic building units are emphasized. The green circle highlights the cutout presented in Figure 5.

For better illustration of the occupancies and the resulting structural distortions we present a cutout (emphasized in Figure 4 by green shading) of a row of condensed In₆ prisms in Figure 5. The triangular faces of the prisms are congruent; however, the c/a ratios of the prisms change drastically: 1.11 for the filled prisms and 0.93 for the empty one. The In–In distances in the row of prisms range from 313 to 372 pm. At least the shorter ones are comparable to those in tetragonal body-centered indium metal (4 × 325 and 8 × 338 pm In–In) [47]. Such In–In distance ranges are typically observed in indium-rich intermetallic phases. An overview is given in [48].

Figure 5:

A row of trigonal prisms formed by the In2 atoms in Sc₃Ir_1.467In₄. Scandium, iridium and indium atoms are drawn as medium grey, blue and magenta circles, respectively. Displacement ellipsoids are drawn at a 99% level. Relevant interatomic distances are given in pm.

The Ir1–In2 distances within the filled trigonal prisms are 269 pm, slightly shorter than the sum of the covalent radii [49] of 276 pm for Ir + In. We can therefore assume substantial covalent Ir–In bonding. This is comparable to the characteristics of other ternary iridium indides [50] with pronounced [Ir_xIn_y]^δ– polyanionic networks. If iridium atoms would occupy the centers of the compressed empty prisms, the Ir–In distance would be too short (249 pm).

As a consequence of the prism elongation, the scandium atoms of the adjacent prisms shift towards the rectangular faces of the filled In₆ prisms, while in the case of a compressed prism they shift in opposite direction. This leads to weak Sc1–Ir1 bonding with Sc1–Ir1 distances of 294 pm, even slightly shorter than in the cubic Laves phase ScIr₂ (305 pm) [51]. The empty In₆ prisms in the Sc₃Ir_1.467In₄ structure remind of the structure of Ti₃Rh₂In₃ [52], where the small titanium atoms lead to a shrinking of the unit cell leaving the complete column of condensed In₆ prisms unoccupied. With the slightly larger zirconium atoms, a statistical occupancy with 12% rhodium in the subcell has been observed for ZrRh_0.71In [53]. This is similar to the results for recently observed scandium phases Sc₃Pt_2.072(3)In₃ and Sc₃Pt_2.095(4)In₃ [6].

The present structure refinement on Sc₃Ir_1.467In₄ clearly shows that precise single-crystal data are essential for characterizing the crystal chemical peculiarities. The family of compounds that derives from the aristotype ZrNiAl is large, with more than 1700 entries in the Pearson database [50]. These many compounds show different structural facets: coloring with different elements on the four crystallographically independent Wyckoff positions, isosymmetric phase transitions [54], [55], [56], occupancy modulations [6] or other superstructure variants [45, 46]. Such structural subtleties cannot reliably be determined with powder diffraction data.