Introduction

The rare earth (RE) transition metal (T) indium systems have intensively been studied with respect to phase formation, crystal structures and the broadly varying physical properties of the many RExTyInz phases [1]. Most phases have been reported with the 3d transition metals and the early (light) rare earth elements [2]. Many complete isothermal sections of the nickel and copper containing systems have been studied. Especially, the RE–Ni–In systems are rich in compounds. Typical examples are the Ce–Ni–In (12 ternary compounds) [3] and Gd–Ni–In (13 ternary compounds) [4] systems. One of the very few examples of phase analytical studies with 4d and 5d transition metals is the Ce–Pd–In system which also comprises 13 intermetallic phases [5].

The Sc–T–In systems are in contrast to the other RE–T–In systems. A general phase diagram with the so far-observed composition of ternary scandium indides in presented in Fig. 1. Hitherto only 29 ScxTyInz phases are listed in the Pearson data base [2]. From Fig. 1 it is obvious that no indium-rich phases are known, in contrast to the other RE–T–In systems which exhibit several series of indium-rich compounds, e.g., YNiAl4 type phases RETIn4 (T = Ni, Cu, Pd, Ir, Pt, Au, space group Cmcm) [2] or the series RERhIn5/REIrIn5 (HoCoGa5 type, P4/mmm) and RE2RhIn8/RE2IrIn8 (Ho2CoGa8 type, P4/mmm) [6,7,8]. More than half of the ScxTyInz phases is scandium rich, i. e., they contain ≥ 40 atom-% scandium. This peculiar crystal chemistry might be related to the small covalent radius (144 pm) [9]. All these scandium-rich phases favor Sc–T bonding.

Fig. 1
figure 1

A general phase diagram for the yet known compositions in the Sc–T–In systems. Some compositions show extended solid solutions

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The Sc50T13In3 (T = Co, Rh, Ir, Pt) [10,11,12] phases have the highest scandium content (75.6 at.%). They crystallize with a site occupancy (coloring) variant of the Ag7+xMg26–x type [13, 14]. Similar to many other series of intermetallic compounds, also these phases allow for a certain range in the valence electron count (VEC), 276 per formula unit (fu) for Sc50Co13In3 and 289 per fu for Sc50Pt13In3, underlining the electronic flexibility. Furthermore, these phases exhibit small homogeneity ranges. Precise refinements of the occupancy parameters revealed the compositions Sc50Co12.5In3.5 (VEC 273) [11], Sc50Rh13.3In2.7 (VEC 277.8), Sc50Ir13.6In2.4 (VEC 279.6) [10], and Sc50Pt13.47In2.53 (VEC 292.3) [12] for the studied single crystals.

During our recent phase, analytical studies in the scandium-rich parts of the Sc–T–In systems, besides Sc2CoIn [11], Sc14Co3In3 [15], Sc5CuIn3 [16], and Sc5Cu2In4 [17], we obtained the new members of the Sc50T13In3 series with T = Ni, Ru, and Pd, decreasing VEC to 263 for Sc50Ru13In3. The synthesis and crystal chemistry of these new scandium-rich intermetallics are reported herein.

Results and discussion

Crystal chemistry

The new Sc50T13In3 series with T = Ni, Ru, and Pd extends the family of ternary ordered Ag7+xMg26–x type compounds. Besides the prototype itself and the members with T = Co, Rh, Ir, and Pt [10,11,12] only Ga4.55Mg21.85Pd6.6 [18] has been reported. The common feature of all these phases is the formation of homogeneity ranges. All single-crystal structure refinements showed mixed occupancy at least for one of the eight crystallographically independent sites. It is obvious, that (except Sc49.59(1)Ru13In3.41(1) with a tiny degree of Sc/In mixing on 32f) the Sc50T13In3 phases preferably show the mixed occupancies on the Wyckoff sites 4a and 4b, both with the highest site symmetry m., thus preferring the cubes and icosahedra.

Herein, we exemplarily discuss the palladium member of the Sc50T13In3 series. Although the unit cells of the Sc50T13In3 phases are large (264 atoms per cell) and the structures are comprised of eight crystallographically independent sites, it is possible to describe the structure by four highly symmetrical polyhedra. Since the condensation pattern of these polyhedra is very dense, in Fig. 2, we present two different substructures, which allow an easy understanding of the complete structure.

Fig. 2
figure 2

The two different substructures of Sc50Pd13.65In2.35. The left-hand drawing emphasizes the MnCu2Al (Heusler type) arrangement of the Pd3@Sc8, Pd2/In1@Sc12, and In2@Sc12 cubes and icosahedra, while part of the condensed Pd1@Sc10 sphenocorona are presented at the right

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The left-hand drawing of Fig. 2 shows the Pd3@Sc8 cubes and Pd2/In1@Sc12 icosahedra which are arranged in a rocksalt-type pattern. The tetrahedral voids of this arrangement are filled by In2@Sc12 icosahedra and this polyhedral packing corresponds to the MnCu2Al type (Heusler phase). Many complex cubic structures can be described with such substitution patterns [19]. It is remarkable that also ScPd2In itself is a Heusler phase [20].

The large voids left by the MnCu2Al type polyhedral packing are filled by Pd1@Sc10 sphenocorona. Part of this substructure is emphasized at the right-hand part of Fig. 2. The sphenocorona are condensed via common corners and faces to a three-dimensional network which (in an alternative description) envelops the MnCu2Al-type substructure.

The main bonding interactions in the Sc50Pd13.65In2.35 structure concern Sc–Pd, Sc–In and Sc–Sc. The palladium and indium atoms are well separated within the different polyhedra. The Sc–Pd distances cover a broader range from 270 to 318 pm. Especially, the shorter ones compare well with the sum of the covalent radii [9] of 272 pm for Sc + Pd. A similar range of Sc–Pd distances occurs in binary Sc2Pd (273–287 pm) with Ti2Ni-type structure [21]. The Sc–Pd distances scale with the coordination number. The Pd3–Sc distances within the Pd3@Sc8 cubes of 282 pm are shorter than the Pd2/In–Sc2 distances of 303 pm within the Pd2/In@Sc12 icosahedra.

The In2 atoms have twelve scandium neighbors in icosahedral coordination at In2–Sc1 distances of 324 pm. This is comparable to the In@Sc12 anti-cuboctahedron with 318–321 pm In–Sc in Sc3In [22] with the hexagonal Mg3Cd-type structure. The sum of the covalent radii [9] for Sc + In of 294 pm is slightly smaller.

The dominating interactions in this scandium-rich intermetallic phase concern Sc–Sc. The latter distances range from 304 to 336 pm. Most of these distances are shorter than in hcp scandium (6 × 325 and 6 × 331 pm) [23], emphasizing the strong Sc–Sc bonding. This is also the case for other scandium-rich solids, e. g., Sc3C4 (314 pm) [24] or Sc2RuMg (304 pm) [25].

Magnetic properties

Figure 3 shows the temperature dependence of the magnetic susceptibility of the Sc50Ni13.5In2.5 and Sc50Pd13.5In2.5 samples. They show weak, overall positive susceptibilities in the whole temperature range, classifying these indides as Pauli paramagnets with similar room temperature susceptibilities of 4 × 10–3 emu mol–1. The upturns at low temperature result from tiny amounts of paramagnetic impurities. The magnetic behavior of Sc50Ni13.5In2.5 and Sc50Pd13.5In2.5 is similar to Sc50Co12.5In3.5 (12 × 10–3 emu mol–1) [11], Sc5CuIn3 (5 × 10–4 emu mol–1) [16], and Sc2Ni2In (3.8 × 10–4 emu mol–1) [26].

Fig. 3
figure 3

Temperature dependence of the magnetic susceptibility of the Sc50Ni13.5In2.5 and Sc50Pd13.5In2.5 samples measured with an applied field of 10 kOe

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Conclusion

The series of Sc50T13In3 phases with T = Ni, Ru, and Pd nicely extends the family of scandium-rich intermetallics with pronounced Sc–Sc bonding. Their complex structures are composed of four different, highly symmetric polyhedra. The three compounds show small homogeneity ranges and manifest the electronic flexibility of such structure types, since they allow for a certain range in the valence electron count. Sc50Ni13.5In2.5 and Sc50Pd13.5In2.5 are Pauli paramagnets.

Experimental

Synthesis

The three scandium-rich samples were prepared using high-purity metals as starting materials: scandium pieces (Smart Elements; 99.999%), nickel wire (Goodfellow GmbH, 99.99%), ruthenium powder (Agosi; 99.99%), palladium sheets (Agosi; 99.99%), and indium ingots (Smart Elements; 99.995%). The metals were weighed in the atomic ratio of 75.7Sc:20.5 T:3.8In with a total weight of ∼ 0.5 g, and sealed in tantalum crucibles under argon atmosphere (purified using titanium sponge (900 K), silica gel, and molecular sieves) by arc-welding [27]. The synthesis was conducted in a high-frequency furnace (Hüttinger Elektronik, Freiburg, Typ TIG 1.5/300), equipped with a water-cooled chamber [28]. The heating procedure consisted of three subsequent steps: (i) the ampoule was heated up to 1720 K and cooled down to 1170 K within 2 h; (ii) annealing at 1170 K for one hour; (iii) gradual cooling to 870 K and keeping this temperature for another 3 h. The gray products showed no reaction with the tantalum crucibles. The samples contained prismatic crystals, which exhibited metallic luster. All three compounds were stable in air over months.

X-ray diffraction on powders and single crystals

The polycrystalline Sc50T13In3 samples were mortared to fine powders and examined by X-ray powder diffraction (Enraf–Nonius FR 552 Guinier camera, CuKα1 radiation, α-quartz (a = 491.30 and c = 540.46 pm) as internal standard). The diffraction patterns were collected with an imaging plate technique (Fuji film, BAS-READER 1800). The pattern of the Sc50Pd13In3 sample is presented as an example in Fig. 4. The cubic lattice parameters were deduced from least-squares refinements. Intensity calculations (Lazy-Pulverix routine [29]) facilitated the correct indexing. The powder lattice parameters (a = 1763.8(2) pm for Sc50Ni13In3, a = 1772.6(2) pm for Sc50Ru13In3, and a = 1785.26(9) pm for Sc50Pd13In3) match with the single-crystal data listed in Table 1. Small differences account for the homogeneity ranges.

Fig. 4
figure 4

Experimental (bottom) and calculated (top) X-ray powder pattern (CuKα1 radiation) of Sc50Pd13In3

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Table 1 Crystallographic data and structure refinement details for Sc50T13In3; Ag7+xMg26–x type, space group Fm\( \bar{3} \), Z = 4
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Small prismatic crystal splinters were selected from the crushed Sc50T13In3 samples and glued to glass fibers with bees wax. The Sc49.59Ru13In3.41 crystal is shown as an example in Fig. 5. For a first quality check, Laue patterns were collected on a Buerger precession camera (white Mo radiation, Fuji-film imaging plate). Complete data sets were recorded at room temperature using a Stoe IPDS-II image plate system (graphite monochromatized Mo radiation; λ = 71.073 pm) in oscillation mode. Numerical absorption corrections were applied to the data sets. Details on the crystallographic data are given in Table 1.

Fig. 5
figure 5

Scanning electron micrograph of the Sc49.6Ru13In3.4 single crystal (ca. 70 × 70 × 165 µm3) mounted on a quartz fiber

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EDX data

The three single crystals were semiquantitatively analyzed by EDX in a Zeiss EVO® MA10 scanning electron microscope (variable pressure mode (60 Pa) and W cathode) using Sc, Ni, Ru, Pd, and InAs as standards. The measurements (76 ± 1 at.% Sc: 19 ± 1 at.% Ni: 5 ± 1 at.% In, for the Sc50Ni13.16In2.84, 75 ± 1 at.% Sc: 20 ± 1 at.% Ru: 5 ± 1 at.% In for the Sc49.59Ru13In3.41 and 75 ± 1 at.% Sc: 21 ± 1 at.% Pd: 4 ± 1 at.% In for the Sc50Pd13.65In2.35 crystal) confirmed the compositions refined from the single crystal X-ray data. The standard uncertainties account for the irregular crystal surface. No impurity elements (especially with respect to the tantalum containers) were detected.

Structure refinements

The three data sets showed face-centered cubic lattices with low Laue symmetry and space group Fm\( \bar{3} \) was found to be correct in agreement with previous work on the isotypic compounds with T = Co, Rh, Ir, and Pt [10,11,12]. The atomic parameters of the platinum compound were taken as starting values and the structures were refined on F2 with the Jana 2006 [30] software package using anisotropic displacement parameters for all atoms. Since all representatives of this structure type show small homogeneity ranges, the occupancy parameters of all sites were refined in separate series of least-squares cycles to check the correct compositions. Each crystal showed one mixed occupied site. These site occupancies were refined as least-squares variables (the enhanced standard deviation for the palladium-based crystal is a consequence of the close scattering power of palladium and indium, which differ by three electrons only). The final difference Fourier syntheses were flat. The final atomic coordinates, displacement parameters, and interatomic distances (exemplarily for the palladium compound) are listed in Tables 2 and 3.

Table 2 Atomic coordinates and displacement parameters [pm2] for Sc50T13In3 (Ag7+xMg26–x type, space group Fm\( \bar{3} \), Z = 4)
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Table 3 Interatomic distances (pm) for Sc50Pd13.65In2.35
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CCDC–2015860 (Sc50Ni13.16In2.84), CCDC–2015859 (Sc49.59Ru13In3.41), and CCDC–2015828 (Sc50Pd13.65In2.35) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Magnetic characterization

The Sc50Ni13.5In2.5 and Sc50Pd13.5In2.5 samples were investigated using the Vibrating Sample Magnetometer (VSM) option of a Physical Property Measurement System (PPMS) by Quantum Design. The samples were ground to a fine powder and each was filled into a polypropylene capsule, which was then inserted into a brass sample holder, which was attached to the sample holder rod of the VSM. The measurements were conducted from 2.5 to 300 K and a field of 10 kOe.