Tetragonal-antiprismatic coordination of transition metals in intermetallic compounds: ω1-Mn6Ga29 and its structuralrelationships

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Abstract

The new phase ω1-Mn6Ga29 was synthesised in single-crystal form from the elements applying the high-temperature centrifugation-aided filtration technique. The crystal structure was determined using diffraction data collected from a twinned specimen: a new prototype, space group P1¯; a=6.3114(2) Å, b=9.9557(3) Å, c=18.920(1) Å, α=90.473(1)°, β=90.847(1)°, γ=90.396(1)°; R1=0.047, wR2=0.117 for 317 variable parameters and 7346 observed reflections; twinning matrix 0 0 13, 0 −1 0, −3 0 0; twin domains ratio 0.830(3):0.170. All manganese atoms in the crystal structure of ω1-Mn6Ga29 are coordinated exclusively by Ga forming distorted tetragonal antiprisms. The monocapped [MnGa8+1] antiprisms condense into pairs by sharing their pseudo-quadratic faces and are interconnected via common apexes and edges to form a 3D framework. The relationship between the crystal structures of ω1-Mn6Ga29 and CuAl2, α-, β-CoSn3, PtSn4, Ti4MnBi2, PdGa5, Rh3Ga16, Rh4Ga21, Al7FeCu2, Co2Al9, and RhBi4 is discussed.

Graphical abstract

ω1-Mn6Ga29 was synthesised applying the high-temperature centrifugation-aided filtration technique. The crystal structure was determined using diffraction data collected from a twinned specimen: a new prototype, space group P1¯ ; a=6.3114(2) Å, b=9.9557(3) Å, c=18.920(1) Å, α=90.473(1)°, β=90.847(1)°, γ=90.396(1)°. The basic building unit of the structure is the monocapped distorted tetragonal antiprism [MnGa8+1]. These antiprisms condense into pairs by sharing their pseudo-quadratic faces and are interconnected via common apexes and edges to form a 3D framework.

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Highlights

► The crystals of new phase ω1-Mn6Ga29 are grown from Ga flux. ► Single-crystal diffraction data is obtained from the twinned specimen. ► The ω1-Mn6Ga29 structure represents a new prototype. ► Main building unit is distorted monocapped tetragonal antiprism [MnGa8+1]. ► The structural peculiarities of ω1-Mn6Ga29 and related structures are discussed.

Introduction

While periodically reports about the preparation of new ternary intermetallic phases, which often crystallise in unique types of crystal structure, appear, the synthesis of a new binary phase is a rather seldom and an extraordinary event. Most of the binary systems are widely investigated, and the information about the composition, structure and formation conditions of the intermediate compounds is available in form of the respective phase diagrams (commonly illustrated in the corresponding composition—temperature coordinates)[1]. Nevertheless, the application of different preparation techniques, e.g., synthesis by using a metal flux rich in one of the constituent components such as Al, Ga, or Bi, allowed us recently to synthesise a series of new binary compounds and provide the clarification of their formation routes [2], [3], [4], [5].

The present work deals with the synthesis and crystal structure determination of the Ga-richest compound found up to now in the binary system Mn−Ga which is designated in the literature as “MnGa6”. This phase was reported for the first time more than forty years ago during the systematic investigation of the Mn–Ga phase diagram [6], [7]. A C-centered orthorhombic unit cell with aS=8.95 Å, bS=8.81 Å and cS=9.94 Å was suggested for the crystal structure of “MnGa6”[6]. No further crystallographic data were reported. In the subsequent study performed on this binary gallide Girgis and Schulz described the existence of superstructure reflections, which are located in rows parallel to the directions [1 1 0] and [1 1¯ 0] of the reciprocal space[8]. In addition to the C-centred lattice, the authors also noted pseudoextinction conditions for superstructure reflections and a slightly different composition of MnGa5.2 based on chemical analysis. Yet, a detailed study of the structure was not carried out, and only the similarity with the already known binary compound MnAl6 [9] was suggested.

The latest attempt to determine the structure of “MnGa6” was performed ten years ago by Boström [10], who obtained single crystals, suitable for intensity data collection, from Ga flux. The structure investigation resulted in the monoclinic space group P2 and lattice parameters aB=18.93 Å, bB=9.96 Å, cB=6.29 Å, βB=90.88°. The latter are in good agreement with the lattice dimensions reported in Ref. [6]: aB=3aS/2, bB=bS, cB=cS/2. The existence of superstructure reflections mentioned by Girgis and Schulz was interpreted in terms of single crystal twinning usually observed. However, the structural model obtained by Boström does not contain resolved Mn and Ga positions and all atoms are randomly distributed over 38 crystallographic sites. Moreover, some of these positions are split and have partial occupancies.

The observations mentioned above motivated us to reinvestigate the Ga-rich region of the system Mn–Ga in the vicinity of the composition MnGa6. Preliminary results of this work were presented on conference [11].

Section snippets

Synthesis and crystal growth

The title compound was prepared starting from compositions in the two-phase region of the phase diagram. Typically a mixture of manganese (99.9998 %, Alfa Aesar) and gallium pieces (99.999 %, Chempur) in the atomic ratio of 1:16 was placed in a glassy carbon crucible and sealed in an evacuated quartz tube. The initial reaction between the components was performed by rapid heating to 900 °C and holding at this temperature for 24 h. Subsequently, the samples were quenched in ice water. After that

Crystal structure

The crystal structure of ω1-Mn6Ga29 represents a new prototype. The characteristic building unit of the structure is a distorted tetragonal antiprism formed by Ga species around a Mn atom. One additional Ga atom caps the pseudo-quadratic face of the antiprism thereby increasing the coordination number of Mn to 9. The monocapped antiprisms [MnGa8+1] are condensed pairwise through their pseudo-quadratic faces. The so-obtained blocks are interconnected mainly via common apexes into a 3D network (

Conclusions

The crystal structure of ω1-Mn6Ga29 represents a new prototype. Within the first coordination sphere, Mn atoms are surrounded by nine Ga atoms forming distorted monocapped tetragonal antiprisms [MnGa8+1]. The latter are condensed via pseudo-quadratic faces, edges and vertices into a three-dimensional framework. The related frameworks in the compounds of transition metals with the main-group elements from groups 13–15 of the Periodic Table are described in terms of their condensation pattern and

Note added in proof

Recently, the crystal structure of the phase MnGa5−x (x∼0.15) was published in Intermetallics 29 (2012) 147–154. The crystallographic information (symmetry and lattice parameters) suggests the isotypism with the reported here ω1−Mn6Ga29.

Acknowledgments

We would like to thank the ESRF for provision of beamtime at the ID31 High Resolution Powder Diffraction beamline.

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