Single-crystal structure determination of two new ternary bis­muthides: Rh₆Mn₅Bi₁₈ and RhMnBi₃

A study of the ternary Rh–Mn–Bi phase diagram revealed the existence of two new ternary bis­muthides, viz. hexa­rhodium penta­manganese octadecabismuthide (Rh₆Mn₅Bi₁₈) and rhodium manganese tribismuthide (RhMnBi₃). Their crystal structures represent new structure types.

Abstract

A study of the ternary Rh–Mn–Bi phase diagram revealed the existence of two new ternary bis­muthides, viz. hexa­rhodium penta­manganese octa­deca­bis­muthide (Rh₆Mn₅Bi₁₈) and rhodium manganese tribismuthide (RhMnBi₃). Their crystal structures represent new structure types. Rh₆Mn₅Bi₁₈, with a Wyckoff sequence a f2 g2 i5, crystallizes in the tetra­gonal system (space group P4₂/mnm; Pearson symbol tP58), and RhMnBi₃, with a Wyckoff sequence a c g i q, crystallizes in the ortho­rhom­bic system (Cmmm; oS20). In the Rh₆Mn₅Bi₁₈ structure, the transition metal atoms are linked into ribbon-like structural units aligned along the [001] direction, whereas planar sheets are formed in RhMnBi₃. In both crystal structures, the units formed by the transition metal atoms are enveloped by Bi atoms, which themselves form a loosely bound network. The linkage results in a layer structure for RhMnBi₃, while in the case of Rh₆Mn₅Bi₁₈, a three-dimensional network is formed; the latter, however, contains several areas where Bi⋯Bi distances suggest van der Waals inter­actions. Both phases under discussion have analogous structural motifs.

Introduction  

For decades, there has been an ongoing search for ferromagnetic materials free of rare earth elements. One promising candidate is the inter­metallic phase α-BiMn; unfortunately, it has not been possible to synthesize this phase as a single-phase bulk material in spite of intensive research (e.g. Liu et al., 2004 ▸; Rama Rao et al., 2013 ▸; Cui et al., 2014 ▸; Chen et al., 2015 ▸; Marker et al., 2018 ▸). A possible approach to circumvent these problems was considered to be the addition of a third com­ponent, e.g. Rh, which forms an inter­metallic phase with Bi that is isotypic with α-BiMn (Ross & Hume-Rothery, 1962 ▸; Kainzbauer et al., 2018 ▸).

Street et al. (1974 ▸) identified a ferromagnetic compound, i.e. Mn₅Rh₂Bi₄ (cubic, Fm m), with a Curie temperature of 266 K. A similar observation was made by Taufour et al. (2015 ▸), who described the ferromagnetic compound Mn_1.05Rh_0.02Bi, with a Curie temperature below 416 K. Furthermore, Suits (1975 ▸) discovered ferromagnetism in Bi-substituted RhMn with the composition RhMn_0.8Bi_0.2. Based on these observations, a systematic study of the ternary Rh–Mn–Bi system at different temperatures was considered of inter­est, with the focus on finding additional inter­metallic phases which might possibly exhibit ferromagnetism. The synthesized samples were checked by powder X-ray diffraction (PXRD) investigations. As a result of this ongoing research, the phases hexa­rhodium penta­manganese octa­deca­bis­muthide (Rh₆Mn₅Bi₁₈) and rhodium manganese tribismuthide (RhMnBi₃) were detected; admittedly, they are not ferromagnetic.

A literature survey of the ternary Rh–M–Bi systems (M = 3d transition metal) shows that they are relatively unexplored. Except for the aforementioned phases, only a handful of compounds are known. Examples are RhNiBi₂ (Zhuravlev et al., 1962 ▸) and RhNiBi₆ (Fjellvåg & Furuseth, 1987 ▸). It may be of particular inter­est that Rh₆Mn₅Bi₁₈ is probably one of the first reported ternary manganese pnictide phases, with a network formed by Bi atoms where alkaline or rare earth metal elements are absent. Further examples are known to crystallize in the cubic structure type Cu₄Mn₃Bi₄ (Street et al., 1974 ▸; Szytula et al., 1981 ▸).

Experimental  

Synthesis and crystallization  

Bulk samples were prepared from pure element pieces of Bi (99.999%, ASARCO, New Jersey, USA) and Mn (99.95%, Alfa Aesar, Johnson Matthey Chemicals, Karlsruhe, Germany), and from Rh powder (99.95% ÖGUSSA, Austria). Except for Rh, the metals were pulverized manually and sieved (grain size <0.09 mm). For the Rh₆Mn₅Bi₁₈ phase, 76.40 mg Rh, 33.81 mg Mn and 389.26 mg Bi in powder form were mixed, and for RhMnBi₃, the amounts were 95.14 mg Rh, 67.50 mg Mn and 838.14 mg Bi; in both cases, the powder mixtures were pressed into pellets in a 5 mm pressing cylinder under a load of 20–25 kN. The bulk samples for the Rh₆Mn₅Bi₁₈ phase were sealed in an evacuated silica-glass tube and melted over an oxyhydrogen flame under shaking, with optical control of the melting process. For the alloying process, the samples were heated quickly to 1373 K, cooled over a period of 5 d to 613 K and annealed at this temperature for two weeks. The bulk samples for the RhMnBi₃ phase were prepared as sinter pellets. The pellet was sealed in an evacuated silica-glass tube with a small alumina plate at the bottom and covered with an inverted closed silica-glass tube to reduce the gas volume (annealing time of four months). After the annealing process at 613 K in a muffle furnace (Nabertherm, Germany, temperature accuracy ±5 K), all samples were quenched in cold water.

Small single crystals of Rh₆Mn₅Bi₁₈ and RhMnBi₃ were obtained in several inhomogeneous bulk samples. The target compounds had a metallic luster and were selected manually using an optical stereomicroscope. Adherent bis­muth was removed with a scalpel. The entire preparation process was performed in an Ar-filled glove-box (Labmaster SP MBraun, H₂O and O₂ levels below 0.1 ppm). Differential thermal analysis was performed on a DSC 404F1 Pegasus (Netzsch, Selb, Germany) and showed that the Rh₆Mn₅Bi₁₈ compound is stable up to 730 K. Phase identification was performed under ambient conditions by PXRD on a Bruker D8 Advance diffractometer in Bragg–Brentano pseudo-focusing geometry, using Cu Kα radiation and a LynxEye^® one-dimensional silicon strip detector. Energy-dispersive X-ray spectroscopy analyses on a scanning electron microscope (Zeiss Supra 55 VP) confirmed that the elemental compositions corresponded to those from the single-crystal X-ray structure determination. Morphologically, both new bis­muthides are acicular and flaky.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. A number of crystal chips were checked for their scattering behaviour and, in particular, to exclude admixtures as adherent bis­muth. Crystals of sufficient quality were used for collection of the intensity data in the full reciprocal sphere. To minimize absorption effects, the crystals were mounted approximately parallel to the φ axes with their longest extension. As the crystal structures are composed of structural units only bonded by weak Bi—Bi bonds, extensive cleavage of the crystals is evident. As a consequence of this behaviour, only a crystal of limited quality could be found for Rh₆Mn₅Bi₁₈, even though a large number of crystals was checked by single-crystal X-ray diffraction; thus, the R _int value and, consequently, the structure refinements remained poor. Nevertheless, the structure type could be clearly established.

Table 1. Experimental details.

	Rh6Mn5Bi18	RhMnBi3
Crystal data
Chemical formula	Rh6Mn5Bi18	RhMnBi3
M r	4653.80	784.79
Crystal system, space group	Tetragonal, P42/m n m	Orthorhombic, C m m m
Temperature (K)	293	293
a, b, c (Å)	18.526 (3), 18.526 (3), 4.1722 (11)	8.885 (3), 13.696 (6), 4.1310 (12)
α, β, γ (°)	90, 90, 90	90, 90, 90
V (Å3)	1432.0 (6)	502.7 (3)
Z	2	4
Radiation type	Mo Kα	Mo Kα
μ (mm−1)	115.57	110.13
Crystal size (mm)	0.16 × 0.03 × 0.02	0.10 × 0.05 × 0.03
Data collection
Diffractometer	Nonius KappaCCD	Nonius KappaCCD
Absorption correction	Multi-scan (SCALEPACK; Otwinowski & Minor, 1997 ▸)	Multi-scan (SCALEPACK; Otwinowski & Minor, 1997 ▸)
T min, T max	0.009, 0.011	0.003, 0.005
No. of measured, independent and observed [I > 2σ(I)] reflections	21380, 1784, 1297	3699, 667, 522
R int	0.169	0.141
(sin θ/λ)max (Å−1)	0.803	0.806
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.044, 0.088, 1.05	0.094, 0.257, 1.19
No. of reflections	1784	667
No. of parameters	50	21
Δρmax, Δρmin (e Å−3)	4.24, −3.66	11.68, −8.87

Computer programs: COLLECT (Hooft, 1999 ▸), DENZO and SCALEPACK (Otwinowski & Minor, 1997 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸) and CrystalMaker (CrystalMaker, 2009 ▸).

A careful inspection of the reciprocal space gave no evidence for any superstructure reflections; twinning was not recognized. As mixed occupation of individual atom positions was not evident and the anisotropic displacement parameters were not conspicuous, a violation of centrosymmetry can be excluded within the accuracy of the structure refinements. Due to the high mosaicity of both samples, their extinction is neg­ligible. Complex neutral atomic scattering functions were applied (Prince, 2006 ▸). The program STRUCTURE TIDY (Gelato & Parthé, 1987 ▸) was used to standardize all atomic coordinates.

Results and discussion  

As mentioned above, only a few pnictides are known with Rh and a second 3d transition metal as constituents (Street et al., 1974 ▸; Szytula et al., 1981 ▸; Huang et al., 2015 ▸). The title phases are probably also the only reported ternary bis­muthides containing a platinum group element and Mn, which adopt new structure types.

Rh₆Mn₅Bi₁₈ crystallizes in the tetra­gonal space group P4₂/mnm (Pearson symbol tP58). The asymmetric unit contains ten atoms, which are listed together with their Wyckoff letters and site symmetries in Table 2 ▸. Fig. 1 ▸ shows the whole crystal structure and the main structural element of Rh₆Mn₅Bi₁₈ formed by extensive linkage of the Mn and Rh atoms. It is characterized by double chains running parallel to [001], each with the formal composition Rh₃Mn₂. They are linked by an additional Mn1 atom to form ribbons with a linear Rh1—Mn1—Rh1 configuration. The central part of the chains consists of the atoms Rh2, Mn2 and Mn3, the Rh1 atom points towards the linking atom Mn1, and the Mn1 atom itself is surrounded in a bicapped square-prismatic coordination (CN = 10, position 2a) (see Fig. 2 ▸ a and Table 3 ▸). The ribbons are surrounded by Bi atoms, with Rh/Mn—Bi bond distances > 2.814 Å. All Bi atoms are exclusively bonded to one Rh₃Mn₂—Mn1—Rh₃Mn₂ ribbon. The Bi atoms themselves form an extended three-dimensional anionic network. The Bi—Bi bonds are longer than 3.316 Å; although Bi—Bi distances in the network were found up to 3.5808 (12) Å, which is slightly longer than the inter­layer Bi—Bi distance in native Bi under ambient conditions (3.529 Å; Donohue, 1974 ▸), bonding inter­actions are still implicated. In addition to the inter­atomic bonds, weak van der Waals Bi4⋯Bi4 [3.920 (2) Å] and Bi2⋯Bi5 [3.848 (1) Å] inter­actions contribute to the cohesion of the network. These longer distances are not shown in Fig. 1 ▸(a). The coordination spheres around all the transition-metal positions are depicted in Fig. 2 ▸.

Table 2. Fractional atomic coordinates, Wyckoff letter and site symmetry of Rh₆Mn₅Bi₁₈ .

	Wyckoff letter	Site symmetry	x	y	z
Bi1	4g	m.2m	0.17112 (3)	0.82888 (3)	0
Bi2	8i	m..	0.08340 (3)	0.25924 (3)	0
Bi3	8i	m..	0.37210 (3)	0.50115 (3)	0
Bi4	8i	m..	0.08560 (4)	0.56218 (4)	0
Bi5	8i	m..	0.20740 (4)	0.41210 (4)	0
Rh1	4f	m.2m	0.10090 (7)	0.10090 (7)	0
Rh2	8i	m..	0.18578 (7)	0.67667 (7)	0
Mn1	2a	m.mm	0	0	0
Mn2	4f	m.2m	0.24406 (14)	0.24406 (14)	0
Mn3	4g	m.2m	0.33193 (14)	0.66807 (14)	0

Figure 1.

(a) The crystal structure of Rh₆Mn₅Bi₁₈; the view is slightly inclined to the [001] direction. (b) The Rh₆Mn₅ ribbons (distances and angles are listed in Table 3 ▸). (c) A clinographic projection of the central parts of the ribbons (atoms Bi2, Bi3, Rh1, Mn1 and Mn3). Colour code: green represents Bi, blue Mn and red Rh atoms.

Figure 2.

A schematic representation of the distinct atomic coordination spheres in the Rh₆Mn₅Bi₁₈ structure. All neighbours within 3.2 Å are shown. (a) The bicapped square prism around the Mn1 atom [CN (coordination number) = 10]. (b) The distorted cubocta­hedron around the Mn2 atom (CN = 12). (c) The distorted cubocta­hedron around the Mn3 atom (CN = 12). (d) The tricapped trigonal prism (alternatively monocapped tetra­gonal anti­prism) around the Rh1 atom (CN = 9). (e) The capped square anti­prism (alternatively tricapped trigonal prism) around the Rh2 atom (CN = 9). The colour coding is as in Fig. 1 ▸.

Table 3. Selected geometric parameters (Å, °) for Rh₆Mn₅Bi₁₈ .

Rh1—Mn1	2.6435 (19)	Rh2—Mn2ii	2.7568 (10)
Rh1—Mn3i	2.729 (3)	Mn2—Mn3iii	2.884 (4)
Rh2—Mn3	2.712 (3)
Mn1—Rh1—Mn3iii	130.15 (6)	Rh2v—Mn3—Mn2ii	58.93 (7)
Mn3i—Rh1—Mn3iii	99.69 (13)	Rh1ii—Mn3—Mn2ii	83.82 (7)
Rh1—Mn1—Rh1iv	180.00 (8)	Rh1vi—Mn3—Mn2ii	176.49 (13)
Rh2v—Mn3—Rh2	83.27 (12)	Rh1vi—Mn3—Mn2vi	83.82 (7)
Rh2v—Mn3—Rh1ii	118.81 (3)	Mn2ii—Mn3—Mn2vi	92.67 (15)
Rh1ii—Mn3—Rh1vi	99.69 (13)

Symmetry codes: (i) ; (ii) ; (iii) , ; (iv) ; (v) ; (vi) .

A characteristic feature of the ribbons are eight-membered rings formed by two Mn1 and two Mn3 atoms, as well as four Rh1 atoms. In addition, four-membered rings are built by two Mn3, one Mn2 and one Rh1 atom. These two kinds of rings are planar by space-group symmetry. Only the Rh2 atoms are, respectively, above and below the layers; see Fig. 1 ▸(b). These structural units are the common structural motif of the two title compounds. However, tetra­gonal symmetry causes a herring-bone pattern of these one-dimensional structural units along [001] in Rh₆Mn₅Bi₁₈, whereas they are linked into a two-dimensional arrangement in RhMnBi₃ (see below).

RhMnBi₃ crystallizes in the ortho­rhom­bic space group Cmmm (Pearson symbol oS20). Like Rh₆Mn₅Bi₁₈, RhMnBi₃ represents a new structure type and exhibits a layer structure consisting of planar Mn–Rh sheets parallel to (010) surrounded by Bi atoms, as presented in detail in Fig. 3 ▸. Fig. 3 ▸(a) shows the crystal structure along c, clearly indicating the layering. Bi—Bi bond distances between the layers are mainly in the range of van der Waals inter­actions, except for the Bi2⋯Bi2 distances of 3.590 (3) Å, which are slightly longer than the inter­layer distance in native Bi (3.529 Å), but are still assumed to exhibit weak bonding inter­actions. The planar nets formed by the transition metals shown in Fig. 3 ▸(b) consist of eight-membered rings of alternating Mn and Rh atoms, similar to the motif shown in Fig. 1 ▸(b). The coordination spheres around all the transition-metal positions are depicted in Fig. 4 ▸.

Figure 3.

(a) The crystal structure of RhMnBi₃, viewed approximately parallel to [001]. (b) The exactly planar Mn–Rh nets; the view is slightly inclined to the b direction (distances and angles are listed in Table 5 ▸). (c) A clinographic projection of the main structural elements parallel to [001]. The colour coding is as in Fig. 1 ▸.

Figure 4.

A schematic representation of different atomic coordination spheres in the RhMnBi₃ structure, showing all neighbouring atoms up to a distance of 3.3 Å. (a) The monocapped square anti­prism around the Rh1 atom (CN = 9). (b) The distorted cubocta­hedron around the Mn1 atom (CN = 12). (c) The bicapped square prism around the Mn2 atom (CN = 10). The colour coding is as in Fig. 1 ▸.

The asymmetric unit of the structure of RhMnBi₃ itself contains five atoms, which are listed together with their Wyckoff letters and site symmetries in Table 4 ▸.

Table 4. Fractional atomic coordinates, Wyckoff letter and site symmetry of RhMnBi₃ .

	Wyckoff letter	Site symmetry	x	y	z
Bi1	4i	m2m	0	0.33689 (14)	0
Bi2	8q	..m	0.19449 (14)	0.12399 (10)	½
Mn1	2c	mmm	½	0	½
Mn2	2a	mmm	0	0	0
Rh1	4g	2mm	0.3016 (4)	0	0

Finally, Fig. 5 ▸ illustrates clearly the relationship between the structures of Rh₆Mn₅Bi₁₈ and RhMnBi₃.

Figure 5.

Comparison of the similar structural motif in the two new bis­muthides under discussion. (a) In Rh₆Mn₅Bi₁₈, the motif consists of Bi2, Bi3, Mn2, Mn3 and Rh1 atoms. (b) In RhMnBi₃, all distinct atom positions listed in Table 4 ▸ are included, and the motif is repeated infinitely forming two-dimensional layers (cf. Fig. 3 ▸). The colour coding is as in Fig. 1 ▸.

Supplementary Material

CCDC references: 1850893, 1850892

Table 5. Selected geometric parameters (Å, °) for RhMnBi₃ .

Mn1—Rh1	2.715 (2)	Mn2—Rh1	2.680 (4)
Rh1i—Mn1—Rh1ii	99.05 (12)	Rh1ii—Mn1—Rh1	80.95 (12)
Rh1i—Mn1—Rh1iii	80.95 (12)	Mn2—Rh1—Mn1	130.48 (6)
Rh1i—Mn1—Rh1	180.0

Symmetry codes: (i) ; (ii) ; (iii) .

Funding Statement

This work was funded by Austrian Science Fund grant P 26023. University of Vienna grant open-access funding.