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.


Introduction
For decades, there has been an ongoing search for ferromagnetic materials free of rare earth elements. One promising candidate is the intermetallic phase -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 -BiMn (Ross & Hume-Rothery, 1962;Kainzbauer et al., 2018). Street et al. (1974) identified a ferromagnetic compound, i.e. Mn 5 Rh 2 Bi 4 (cubic, Fm3m), with a Curie temperature of 266 K. A similar observation was made by Taufour et al. (2015), who described the ferromagnetic compound Mn 1.05 Rh 0.02 Bi, with a Curie temperature below 416 K. Furthermore, Suits (1975) discovered ferromagnetism in Bi-substituted RhMn with the composition RhMn 0.8 Bi 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 (Rh 6 Mn 5 Bi 18 ) and rhodium manganese tribismuthide (RhMnBi 3 ) were detected; admittedly, they are not ferromagnetic. 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 RhMnBi 3 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 AE5 K), all samples were quenched in cold water.
Small single crystals of Rh 6 Mn 5 Bi 18 and RhMnBi 3 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, H 2 O and O 2 levels below 0.1 ppm). Differential thermal analysis was performed on a DSC 404F1 Pegasus (Netzsch, Selb, Germany) and showed that the Rh 6 Mn 5 Bi 18 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 K radiation and a LynxEye 1 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 ' 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 Rh 6 Mn 5 Bi 18 , 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.
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   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.
are probably also the only reported ternary bismuthides containing a platinum group element and Mn, which adopt new structure types.
Rh 6 Mn 5 Bi 18 crystallizes in the tetragonal space group P4 2 /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 Rh 6 Mn 5 Bi 18 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 Rh 3 Mn 2 . 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. 2a 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 Rh 3 Mn 2 -Mn1-Rh 3 Mn 2 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 Bi4Á Á ÁBi4 [3.920 (2) Å ] and Bi2Á Á ÁBi5 [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.
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    . RhMnBi 3 crystallizes in the orthorhombic space group Cmmm (Pearson symbol oS20). Like Rh 6 Mn 5 Bi 18 , RhMnBi 3 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 Bi2Á Á ÁBi2 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.
The asymmetric unit of the structure of RhMnBi 3 itself contains five atoms, which are listed together with their Wyckoff letters and site symmetries in Table 4.

Hexarhodium pentamanganese octadecabismuthide (Rh6Mn5Bi18)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 11.68 e Å −3 Δρ min = −8.87 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.