Nd₃₉Ir_10.98In_36.02 – A complex intergrowth structure with CsCl- and AlB₂-related slabs

1 Introduction

Iridium reacts with indium forming the binary compounds IrIn₂ [1] and IrIn₃ [2, 3] of which IrIn₃ is dimorphic with a cementite-type low-temperature modification. The three phases show pronounced covalent Ir–In besides In–In bonding. IrIn₂ and IrIn₃ are metallic conductors. Especially IrIn₃ has thoroughly been investigated with respect to its thermoelectric properties [4].

Much larger structural diversity occurs if a third element is introduced. The systematic study of ternary systems with alkaline earth (AE) and rare earth (RE) elements revealed a manifold of interesting structures with polyanionic [Ir_xIn_y]^δ− networks. The following series of compounds have been reported: (i) the equiatomic indides REIrIn (RE = Ce, Sm, Gd–Ho) with hexagonal ZrNiAl type structure [5], (ii) AEIrIn₂ (AE = Ca, Sr, Ba) [6–9] and REIrIn₂ (RE = La–Nd, Sm) [5] with MgCuAl₂ type structure, (iii) the indium-rich compounds AEIrIn₄ (AE = Ca, Sr, Ba) [10–12] with distorted bcc indium cubes as building units, (iv) RE₂IrIn₈ (RE = La, Ce, Yb) [5, 13] and REIrIn₅ (RE = La, Ce, Pr, Yb) [14–17] which are intergrowth variants of Cu₃Au and CsCl slabs, (v) Ba₂Ir₄In₁₃ [12] with its own structure type, (vi) the rare earth-rich phases RE₄IrIn (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) [18–20], (vii) Y₁₄Ir₃In₃ [19], (viii) Sc₅₀Rh_13.3In_2.7 and Sc₅₀Ir_13.6In_2.4 [21] with a complex cubic structure that is derived from the Mg₁₉(Mg_0.9Ag_0.1)₇ type, and the terbium phases Tb₃Ir_1.62In_0.33 and Tb₃Ir_1.52In_0.44 [22] with ordering variants (including defect formation) of the tetragonal Y₃Rh₂ structure. Detailed property studies were performed for CeIrIn₅ and Ce₂IrIn₈ in the context of superconductivity and heavy fermion materials [23, 24].

During our systematic phase analytical studies of new quaternary indides related to the RE₇Ni₅Ge₃In₆ (RE = La, Ce, Pr, Nd, Sm) phases [25, 26] we investigated also the iridium based systems and got hints for a new ternary neodymium–iridium–indide with approximate composition of Nd₂IrIn₂. Herein we report on the targeted synthesis and the structure of Nd₃₉Ir_10.98In_36.02.

2 Experimental

3 Discussion

The ternary indide Nd₃₉Ir_10.98In_36.02 crystallizes with a new structure type, Pearson code oP172, and the Wyckoff sequence h¹⁹g²³da. This complex structure type is a further tile in the overall mosaic of the large family of rare earth-based indides [34].

Although the Nd₃₉Ir_10.98In_36.02 unit cell is very large, the structure can easily be described by the intergrowth concept of small simple slabs [35, 36]. A projection of the Nd₃₉Ir_10.98In_36.02 structure onto the xy plane is presented in Fig. 1. The neodymium atoms lie all on mirror planes and are stacked in AA sequence. These neodymium layers can be considered as a tessellation of distorted squares and triangles of different size. This structural motif has repeatedly been observed for many intermetallic phases, and different crystal structures with this building principle have been reviewed [30–32]. So far, in these examples the square prismatic voids were filled with the p element (CsCl-related slab) and the trigonal prismatic voids (AlB₂-related slab) by the smaller transition metal. The complex tessellation in Nd₃₉Ir_10.98In_36.02, however, leaves triangles and squares with different distortions and different size. Thus, one observes indium occupancy in two quite large trigonal-prismatic voids, and one medium-sized void shows iridium-indium mixing. In order to emphasize these differences in the prism size, the Nd–Nd distances are also listed in Fig. 1. This transition metal-indium ordering readily reminds of the structures of Lu₃Co_1.87In₄ [37], Gd₃Rh_1.94In₄ [38], and Sc₃Rh_1.594In₄ [39]. These three indides crystallize with ordered superstructures of the ZrNiAl type with indium-transition metal ordering within the trigonal prisms of rare earth elements. The ordering leads to distinctly different sizes of the trigonal prisms, e.g., 447 vs. 380 pm edge length for the triangle of the indium-, respectively, rhodium-filled trigonal prisms as in Gd₃Rh_1.94In₄.

Fig. 1:

Projection of the Nd₃₉Ir_10.98In_36.02 structure onto the crystallographic xy plane. Neodymium, iridium, and indium atoms are drawn as light grey, blue, and magenta circles, respectively. The mixed occupied site is drawn by checkered hatching. Atom designations and relevant interatomic distances are given. The tessellation with CsCl (distorted cubes) and AlB₂ (distorted trigonal prisms) related slabs is emphasized.

Four of the distorted trigonal prisms are labeled with interatomic distances in Fig. 1. The smallest of these prisms is filled by the Ir5 atoms. The neodymium triangle shows Nd–Nd distances of 376, 387, and 406 pm with an average of 390 pm. The largest prism is filled by the In18 atoms: 2 × 477 and 498 pm Nd–Nd with an average of 484 pm. The arrangement is similar for the strongly distorted In17 prism: 402, 489, and 491 pm Nd–Nd with an average of 460 pm. The mixed occupied site shows a situation in between: 399, 402, and 411 pm Nd–Nd with an average of 404 pm. Thus, the occupancy of the trigonal prisms nicely correlates with their size. Occupancy of the larger prisms with a smaller iridium atom would lead to non-optimized bonding.

All trigonal prisms are capped on the rectangular faces by three additional indium atoms. Such tri-capped trigonal prisms (coordination number 9) are a frequent building unit in many intermetallic structure types [36, 40]. The distorted square prisms are also capped on all six rectangular faces, leading to the typical 8 + 6 coordination of the bcc arrangements. However, one has to keep in mind that not all of these 14 neighbors have bonding contacts with the central indium atoms, and the cubes should be considered as a geometrical building unit for easier description of the structure.

The shortest distances in the Nd₃₉Ir_10.98In_36.02 structure occur for Ir–In. They range from 283 to 311 pm and are all longer than the sum of the covalent radii of 276 pm [41]. This is indicative of weak Ir–In bonding. In IrIn₃ (265 pm) [2] and CaIrIn₂ (271–280 pm) [7] with strongly bonding Ir–In interactions, the Ir–In distances are distinctly shorter. Strong bonding interactions occur between the neodymium and iridium atoms. This is already expected from the course of the electronegativities. The shortest Nd–Ir distances of 286–301 pm compare well with the sum of the covalent radii of 290 pm. Such short Nd–Ir distances (291–296 pm) occur also in binary Nd₅Ir₃ with iridium in square-antiprismatic neodymium coordination [42].

Finally we turn to the indium substructure. The short In–In contacts occur (i) between pairs connecting adjacent cubes: In2–In4 (306 pm), In1–In15 (318 pm), In3–In14 (313 pm), In10–In12 (312 pm), (ii) between one trigonal prism and two cubes: In5–In17 (290 pm), In7–In17 (293 pm), and (iii) between one trigonal prism and three cubes: In8–In18 (302 pm), In6–In18 (296 pm), In13–In18 (315 pm). All these distances are indicative of substantial covalent In–In bonding. Most In–In distances in Nd₃₉Ir_10.98In_36.02 are even shorter than in tetragonal body-centered indium (4 × 325 and 8 × 338 pm) [43].

The indium subunits observed in Nd₃₉Ir_10.98In_36.02 are already known from related structures. In₂ units (305 and 313 pm) also occur between neighboring In@Nd₈ cubes in Nd₁₁Pd₄In₉ [32], and a dumb-bell with 309 pm In–In in a stuffed square anti-prismatic yttrium coordination has been observed in Y₁₄Ir₃In₃ [19]. The trigonal InIn₃ unit is comparable to that in the Gd₃Rh_1.94In₄ structure (297 pm In–In) [38].

Summing up, Nd₃₉Ir_10.98In_36.02 is a further example of a complex structure type with simple basic building units. The mixed Ir/In occupancy within one trigonal prism is indicative of a small homogeneity range. Further synthetic work is in progress in order to examine the existence range of this structure type with other rare earth elements.