The Ternary Alkaline-Earth Metal Manganese Bismuthides Sr2MnBi2

Oct 2, 2017 - First-principle calculations and magnetization measurements show that this electron-balanced compound is a frustrated magnetic semicondu...
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The Ternary Alkaline-Earth Metal Manganese Bismuthides Sr2MnBi2 and Ba2Mn1−xBi2 (x ≈ 0.15) Alexander Ovchinnikov,† Bayrammurad Saparov,†,‡ Sheng-Qing Xia,†,§ and Svilen Bobev*,† †

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States § State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China ‡

S Supporting Information *

ABSTRACT: Two new ternary manganese bismuthides have been synthesized and their structures established based on single-crystal X-ray diffraction methods. Sr2MnBi2 crystallizes in the orthorhombic space group Pnma (a = 16.200(9) Å, b = 14.767(8) Å, c = 8.438(5) Å, V = 2018(2) Å3; Z = 12; Pearson index oP60) and is isostructural to the antimonide Sr2MnSb2. The crystal structure contains corrugated layers of corner- and edge-shared [MnBi4] tetrahedra and Sr atoms enclosed between these layers. Electronic structure calculations suggest that Sr2MnBi2 is a magnetic semiconductor possessing Mn2+ (high-spin d5) ions, and its structure can be rationalized within the Zintl concept as [Sr2+]2[Mn2+][Bi3−]2. The temperature dependence of the resistivity shows behavior consistent with a degenerate semiconductor/poor metal, and magnetic susceptibility measurements reveal a high degree of frustration resulting from the two-dimensional nature of the structure. The compositionally similar Ba2Mn1−xBi2 (x ≈ 0.15) crystallizes in a very different structure (space group Imma, a = 25.597(8) Å, b = 25.667(4) Å, c = 17.128(3) Å, V = 11253(4) Å3; Z = 64; Pearson index oI316) with its own structure type. The complex structure boasts Mn atoms in a variety of coordination environments and can be viewed as consisting of two interpenetrating 3D frameworks, linked by Bi−Bi bonds. Ba2Mn1−xBi2 can be regarded as a highly reduced compound with anticipated metallic behavior.

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INTRODUCTION Zintl phases have gained much attention as promising materials for thermoelectric applications owing to their nominally semiconducting properties and complex crystal structures that contribute to their low thermal conductivity. Although the textbook examples of Zintl phases are typically composed of main-group elements, a similar electron counting can be applied to some semiconducting transition-metal (TM) compounds to rationalize them within the framework of the Zintl formalism. Since the discovery of Yb14MnSb11,1 one of the bestperforming thermoelectric materials at temperatures around 1000 K, active exploratory work has been done to study other manganese pnictides. These efforts uncovered exciting compositional and structural complexity in the AE−Mn−Sb and AE−Mn−Bi systems (AE stands for alkaline-earth metal or the nominally divalent Eu and Yb).2−9 The crystal structures of such compounds typically feature [MnSb4] and [MnBi4] tetrahedra forming polyanionic sublattices that can contain isolated units or units linked by corner- or edge-sharing. Whereas the electronic properties of these compounds can be tuned by optimizing the carrier concentration, the thermal conductivity can be lowered by going to heavier pnictogens, e.g., from antimonides to bismuthides. In addition, by increasing the Mn/AE ratio, extended covalent frameworks can be built, giving rise to rich structural variability. The latter © XXXX American Chemical Society

can also lead to interesting magnetic topologies as new Mn− Mn exchange interactions develop. Several years ago, we began systematic studies in the AE− Mn−Sb and AE−Mn−Bi systems,2−4 where a number of ternary compounds had been previously discovered.5,6 Most of the results from the earlier exploratory work have already been published.2−4,7−9 Here, we report on two new bismuthides with complex structures, Sr2MnBi2 and Ba2Mn1−xBi2 (x ≈ 0.15), which complement (and extend) the previous work on similar compounds by our group.

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EXPERIMENTAL SECTION

Synthesis of Sr2MnBi2. All manipulations were performed in an argon-filled glovebox. At first, Sr2MnBi2 was serendipitously discovered while trying to synthesize Sr11Mn8Bi14, an unknown analogue of Ba11Cd8Bi14.10 After a series of optimizations, the best procedure for obtaining suitable single crystals of Sr2MnBi2 was found to involve Pb flux and Sr, Mn, and Bi (all with purity >99.9 wt %, Alfa Aesar). A mixture of Sr, Mn, Bi, and Pb metals with the atomic ratio 2:1.5:2:12 was placed in an alumina crucible and sealed in an evacuated fused silica tube. After that, the tube was heated up to 1273 K with a rate of 100 K/h, kept at this temperature for 24 h, and cooled down to 823 K with a rate of 5 K/h. At this temperature, the sample was taken out from the furnace, and the Pb flux was rapidly removed by Received: July 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b01851 Inorg. Chem. XXXX, XXX, XXX−XXX

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Single-Crystal X-ray Diffraction (SCXRD). Suitable single crystals were selected in the glovebox and scooped from a droplet of dry Paratone N oil using low-background plastic loops. The crystals were protected from the ambient atmosphere in a cold nitrogen stream. Data collection was done on a Bruker SMART CCD diffractometer equipped with monochromated Mo Kα radiation (λ = 0.71073 Å). The raw data were integrated using the program SAINT.12 Semiempirical absorption corrections were applied with the SADABS software.13 Crystal structures were solved by direct methods and refined by full matrix least-squares methods on F2 using SHELXL.14 Atomic coordinates were standardized using STRUCTURE TIDY.15 Details of the data collection, crystallographic parameters, and selected interatomic distances are summarized in Tables 1−5.

centrifugation. The sealed tube was brought back into the glovebox, and after it had been opened, small black crystals of Sr2MnBi2 were found in the crucible. They could be easily distinguished (and picked out with tweezers) under an optical microscope from the larger irregularly shaped crystals of Sr11Bi10.11 Attempts to grow crystals of Sr2MnBi2 out of Sn or Bi fluxes were also made, but failed. Single crystals of Sr2MnBi2 were found to be modestly air-sensitive, showing some tarnishing of the crystal surfaces after an exposure to the ambient atmosphere for 1 day. Synthesis of Ba2Mn1−xBi2 (x ≈ 0.15). This phase was initially obtained as one of the products of a reaction of Ba, Mn, and Bi, aimed at Ba2MnBi2. Ba, Mn, and Bi in the ratio 2:1:2 (total mass ca. 500 mg) were loaded in a Nb tube, which was arc-welded at both ends and enclosed in an evacuated fused silica tube. The assembly was heated in a tube furnace to 1273 K with a rate of 200 K/h, kept at this temperature for 10 h, and cooled down to 573 K with a rate of 5 K/h, and then it was taken out from the furnace and air-quenched. After that, the Nb tube was brought back into the glovebox and opened. Powder X-ray diffraction analysis on the as-synthesized product showed that the main crystalline phases were Bi and Ba11Bi10.11 In addition to them, reflections indicative of a previously unidentified phase (minor component) were also present in the PXRD pattern. Single crystals of Ba2Mn1−xBi2 were found upon inspection of the product under an optical microscope. Neither additional annealing of the samples nor variations of the experimental conditions yielded phase-pure products. This, the small crystal sizes, and the high airsensitivity precluded any physical property measurements. Structure solutions and refinements from single-crystal X-ray diffraction data showed a very complicated structure with more than 30 independent positions in the asymmetric unit, and a positional disorder concerning Mn atoms on two sites (vide inf ra). Following the refinements, the idealized composition of the compound would be Ba32Mn15Bi32 (=Ba2Mn0.9375Bi2). Additional occupational disorder on the two Mn sites is possible, and when taken into consideration yields a refined formula Ba32Mn∼13.6Bi32 (=Ba2Mn0.85(1)Bi2). This small compositional variation could not be ascertained by means of elemental analysis; therefore, to gain some insight into this problem, several experiments were set up with varied stoichiometric amount of Mn. Further single-crystal X-ray diffraction data confirmed a small phase width, reflected in deviations in all lattice constants for two crystals from different batches (designated as sample I and sample II hereafter) as follows: a = 25.619(3)/25.598(5) Å; b = 25.691(4)/ 25.668(5) Å; c = 17.156(2)/17.128(3) Å; V = 11292(3)/11254(3) Å3) for samples I/II, respectively. Due to the apparent small phase width, the new barium manganese bismuthide is referred to as Ba2Mn1−xBi2 throughout this paper. Despite repeated attempts, so far, Ba2Mn1−xBi2 is the only known compound with this structure, except perhaps for Ba2Zn1−xSb2 (same body-centered orthorhombic symmetry with unit cell parameters a = 25.089(4) Å; b = 25.204(4) Å; c = 17.003(2) Å; V = 10752(3) Å3), which is not fully characterized as of yet. Ba2Zn1−xSb2 was prepared employing the same procedure as for Ba2Mn1−xBi2 using elemental Ba, Zn, and Sb as starting materials. We present preliminary data on Ba2Zn1−xSb2 for the sake of comparison with the Mn phase and to show the versatility of the new structure type reported in the present paper. Details of the data collection, and structural parameters for the second crystal of Ba2Mn1−xBi2 (sample II), and for Ba2Zn1−xSb2 are given in Tables S1−S5. Powder X-ray Diffraction (PXRD). PXRD measurements were performed on a Rigaku Miniflex diffractometer (Cu Kα radiation, λ = 1.5418 Å) operating inside a nitrogen-filled glovebox to prevent oxidation of the samples. Data were collected in a θ−θ scan mode between 5° and 60° with a step size of 0.05° and 2 s/step counting time. Scanning Electron Microscopy (SEM). The morphology and composition of the Sr2MnBi2 and Ba2Mn1−xBi2 single crystals were studied on a JEOL JSM-6335F scanning electron microscope (accelerating voltage 15 kV) equipped with an energy-dispersive Xray (EDX) detector.

Table 1. Selected Single-Crystal Data Collection and Structure Refinement Parameters for Sr2MnBi2 and Ba2Mn1−xBi2 (x ≈ 0.15) [Sample I] refined composition −1

fw/g·mol T/K λ/Å space group Z a/Å b/Å c/Å V/Å3 ρcalc/g·cm−3 μMoKα/cm−1 R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]a R1 [all data]a wR2 [all data]a Δρmax,min/e·Å−3

Sr2MnBi2

Ba2Mn0.85(1)Bi2

648.14

738.39 120 0.71073

Pnma (No. 62) 12 16.200(9) 14.767(8) 8.438(5) 2018(2) 6.40 695.6 0.027 0.057 0.034 0.060 1.93, −1.94

Imma (No. 74) 64 25.597(8) 25.667(4) 17.128(3) 11253(4) 6.98 622.6 0.044 0.091 0.058 0.096 4.46, −5.13

a R1 = ∑||Fo|−|Fc||/∑|Fo|; wR2 = [∑[w(Fo2−Fc2)2]/∑[w(F02)2]]1/2, where w = 1/[σ2Fo2+(A·P)2+(B·P)], and P = (Fo2 + 2Fc2)/3; A and B are the respective weight coefficients (please see CIF in the Supporting Information). CIFs have depository numbers CCDC 1563384 for Sr2MnBi2 and CCDC 1563382 for Ba2Mn0.85(1)Bi2, respectively.

Physical Property Measurements. Four-contact electrical resistivity measurements for Sr2MnBi2 were conducted on a Quantum Design Physical Property Measurement System (PPMS) in the temperature range 3−300 K. Two single crystals with approximate dimensions of 1.2 × 1.0 × 0.2 and 1.5 × 1.2 × 0.5 mm3 were selected from the same batch, and their surface was mechanically cleaned with a scalpel. After that, metal wires were attached using a conductive silver paint. The cleaning step was aimed at the removal of the residual lead flux on the surface; however, the measurements on both crystals still indicated a presence of lead (vide inf ra). The temperature dependence of the magnetization for Sr2MnBi2 sample was measured on a Quantum Design MPMS SQUID magnetometer in an external field of 500 Oe within the temperature range 5−300 K. The measured sample (ca. 30 mg) consisted of several hand-picked single crystals, enclosed in low background gel-cap holders and secured with cotton tips. High-temperature magnetization measurements were performed on a Quantum Design Vibrating Sample Magnetometer (VSM) in an external field of 500 Oe within the temperature range 300−700 K. The material was affixed to the holder using an alumina cement. After the measurements, the crystals were ground in the glovebox and checked with PXRD. No side phases were observed in the X-ray powder patterns, indicating that no decomposition had occurred during the measurements. B

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of 720 K.24 It is reasonable to assume that ternary manganese pnictides follow similar trends. As a result, the single crystal growth of ternary manganese bismuthides appears to be more challenging since the formation of flux-specific competing phases (like MnSn2 in the present case) has to be considered. It is worthwhile to note that no binary phases have been reported in the Mn−Pb system.23 Crystal growth attempts from Bi flux were also made, but the main product, instead of the targeted Sr2MnBi2, was the known compound SrMnBi2.25 Synthesis of Ba2Mn1−xBi2. Always produced as a minor side product, as described above, Ba2Mn1−xBi2 may be a metastable phase. Attempts to vary the temperature, annealing duration, and cooling rate of the synthetic protocol did not yield purer samples. Different compositions were also tested in a series of Pb-flux reactions, but these attempts did not produce Ba2Mn1−xBi2. The commonly observed ternary phase in these cases was BaMn2Bi2.26 Scanning Electron Microscopy. Typical single crystals of Sr2MnBi2 and Ba2Mn1−xBi2 are shown in Figure 1. Semi-

Since the Ba2Mn1−xBi2 crystals from the sealed Nb tubes were very small in size and highly air-sensitive, no physical property measurements could be made. Thermal Analysis. Simultaneous thermogravimetric differential scanning calorimetry (TG/DSC) analysis was carried out using a SDT Q600 analyzer, supplied by TA Instruments. A single crystal of Sr2MnBi2 was loaded in a small alumina pan, which was subsequently capped by an alumina cover. After equilibration at 323 K, the temperature was ramped up to 1273 K with a rate of 10 K/min under a constant flow (100 mL/min) of high-purity argon to prevent oxidation of the sample. No discernible thermal events were observed, suggesting that the material does not melt/decompose under these conditions (Figure S1). Computational Details. Spin-polarized scalar-relativistic electronic structure calculations were performed on Sr2MnBi2 using the Siesta16 and TB-LMTO-ASA codes.17 At first, several possible magnetic arrangements were generated following the broken symmetry approach. To limit the number of magnetic structures, all symmetrically equivalent Mn−Mn pairs were assumed to have the same type of coupling (ferro- or antiferromagnetic). This resulted in eight different magnetic structures (Figure S2). Total energy calculations were performed for all of them with the Siesta code using the Perdew−Burke−Ernzerhof parametrization of the GGA functional.18 The Brillouin zone was sampled with a 4 × 4 × 8 k-point grid, and the plane-wave kinetic energy cutoff was set to 1000 Ry after checking for convergence of the total energy and magnetic moments. For the most energetically favorable arrangement, additional calculations were done with the TB-LMTO-ASA package. The von Barth−Hedin LDA functional19 was employed, and the same k-point grid was used as in the Siesta calculations. To satisfy the atomic sphere approximation (ASA), an introduction of empty spheres was necessary. Crystal orbital Hamilton population (COHP) curves were generated by the procedure implemented in the LMTO code. In the case of Ba2Mn1−xBi2, a non-spin-polarized calculation was performed for an idealized Ba32Mn15Bi32 composition within the LMTO code on a 4 × 4 × 4 k-point grid. In the model, the disordered Mn1a and Mn1b sites were averaged, and the position was taken as fully occupied. In a similar manner, we calculated the electronic structure of the presumed to be structurally related and nonmagnetic Ba2Zn1−xSb2 (Tables S1, S4, and S5). The calculated densities of states are given in Figures S3 and S4, respectively.

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RESULTS AND DISCUSSION Synthesis of Sr2MnBi2. Several different compositions of the reaction mixture and varying temperature profiles were employed to optimize the yield and quality of the Sr2MnBi2 single crystals. In all cases, a presence of the Sr11Bi10 side phase could not be avoided. The dull black crystals of Sr2MnBi2 could be easily distinguished from the silver Sr11Bi10 crystals. In addition to the different appearance, the crystals of the binary strontium bismuthide are much more air-sensitive than Sr2MnBi2 and demonstrate visible deterioration of the crystal surfaces when exposed to the ambient atmosphere. It was already mentioned that the best way to make single crystals of Sr2MnBi2 was from Pb flux. Interestingly, when Sn was used instead, Sr2MnBi2 could not be obtained. In these experiments, Sr11Bi10 was always the main product of the reaction, with needle- or rod-like MnSn2 crystals being the secondary phase. In contrast, the isotypic Sr2MnSb2 was successfully grown from Sn flux.20 An apparently lower stability of the manganese bismuthides is a likely explanation of this difference. This tendency can be illustrated by comparing the decomposition or melting temperatures of the formally equimolar binary manganese pnictides. Thus, MnP and MnAs melt congruently at 1420 and 1210 K, respectively,21,22 and MnSb decomposes peritectically at 1130 K.23 MnBi undergoes a peritectic decomposition at a significantly lower temperature

Figure 1. SEM photographs of as-synthesized crystals of Sr2MnBi2 and Ba2Mn1−xBi2. The cracks and rough surface of the latter are likely signs of decomposition due to a brief exposure to air.

quantitative EDX analysis confirmed the presence and approximate ratios of the expected elements. For the fluxgrown Sr2MnBi2 crystals, small regions containing Pb were also present on the crystal surfaces, but other regions show no signal for Pb, ruling out flux as an inadvertent dopant of the sample. Physical property measurements also provide clues for the presence of elemental Pb (vide inf ra). Crystal Structure of Sr2MnBi2. Sr2MnBi2 is isotypic to the antimonide Sr2MnSb2 (space group Pnma (No. 62), Z = 12).20 This is only the second compound known to form with this structure. There are ten independent sites in the asymmetric C

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Inorganic Chemistry unit (Table 2), and based on the refinements, the structure appears to be devoid of any disorder. Table 2. Atomic Coordinates and Equivalent Displacement Parameters (Å2) of Sr2MnBi2 atom

site

x

y

z

Ueqa

Sr1 Sr2 Sr3 Sr4 Mn1 Mn2 Bi1 Bi2 Bi3 Bi4

8d 8d 4c 4c 8d 4c 8d 8d 4c 4c

0.11002(5) 0.11373(5) 0.12263(7) 0.37273(7) 0.20372(9) 0.0784(1) 0.02212(2) 0.24847(2) 0.25134(3) 0.49964(3)

0.06020(6) 0.59689(6) 1/4 1/4 0.08911(9) 1/4 0.09493(2) 0.57266(2) 1/4 1/4

0.6243(1) 0.0883(1) 0.0040(1) 0.0926(1) 0.2589(2) 0.4031(2) 0.23944(4) 0.42933(4) 0.42639(6) 0.79142(6)

0.0163(2) 0.0162(2) 0.0159(3) 0.0165(3) 0.0162(3) 0.0159(4) 0.0155(1) 0.0152(1) 0.0155(1) 0.0153(1)

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

The structure is best viewed as corrugated layers of fused [MnBi4] tetrahedra and Sr atoms sandwiched between them (Figure 2). The [MnBi2] layers feature trimeric repeating units, composed of edge-shared [MnBi4] tetrahedra that are cornerlinked to the adjacent ones.

Figure 3. Structural details concerning Sr2MnBi2. (a) Part of the [MnBi2] layer drawn along [100], and (b) coordination environment of the Sr atoms in. Sr, Mn, and Bi atoms are shown in orange, blue, and red, respectively. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2. Crystal structure of Sr2MnBi2. Sr, Mn, and Bi atoms are shown in orange, blue, and red, respectively, and the unit cell is outlined. Displacement ellipsoids are drawn at the 50% probability level.

Table 3. Selected Interatomic Distances (Å) in Sr2MnBi2 The two symmetrically independent Mn positions in the structure have different site symmetries. Mn1 is at a general position (Wyckoff site 8d), and Mn2 is at a mirror plane (Wyckoff site 4c). The Mn1 atoms are located in the vertices of a distorted hexagonal net, which explains the pseudohexagonal metrics of the unit cell with b/c = 1.75 ≈√3. The resultant [(Mn1)Bi4] tetrahedra connect to each other by corner-linking exclusively, forming a phyllosilicate-type [Mn2Bi5] sublayer (Figure 3a). Mn2 atoms reside above and below this net in alternating fashion. [(Mn2)Bi4] and [(Mn1)Bi4] tetrahedra are joined by edge-sharing so that the resulting [MnBi2] layers can be broken down into trimers of edge-sharing [MnBi4] tetrahedra that are corner-linked to the adjacent trimers. The [(Mn1)2(Mn2)Bi6]12− layers depicted in Figure 3a can thus be represented by the notation 2∞[{(Mn1)Bi3/2Bi1/3}2{(Mn2)Bi1/1Bi2/2Bi1/3}]12−. The Mn−Bi distances in both [MnBi4] tetrahedra range from 2.81 to 2.95 Å (Table 3). These values are in good agreement with the typical Mn−Bi bonding contacts in other Mn(II)

atoms Sr1

Sr2

Sr3

D

−Bi1 −Bi4 −Bi2 −Bi2 −Bi1 −Bi3 −Bi4 −Bi3 −Bi1 −Bi1 −Bi2 −Bi2 −Bi4 −Bi2 × 2 −Bi1 × 2 −Bi3

distance 3.340(2) 3.400(2) 3.404(1) 3.452(2) 3.583(2) 3.986(2) 3.380(1) 3.429(2) 3.443(2) 3.534(2) 3.613(2) 3.630(2) 3.191(2) 3.408(2) 3.441(2) 4.129(2)

atoms

distance

Sr4

−Bi4 −Bi3 −Bi2 × 2 −Bi1 × 2

3.269(2) 3.435(2) 3.551(2) 3.621(2)

Mn1

−Bi3 −Bi2 −Bi2 −Bi1 −Mn2

2.870(2) 2.881(2) 2.897(2) 2.948(2) 3.354(2)

Mn2

−Bi3 −Bi1 × 2 −Bi4

2.808(2) 2.826(2) 2.876(2)

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Inorganic Chemistry Table 4. Atomic Coordinates and Equivalent Displacement Parameters (Å2) of Ba2Mn1−xBi2 (x ≈ 0.15) atom

site

x

y

z

Ueqa

Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Ba9 Ba10 Ba11 Ba12 Mn1ab Mn1bb Mn2 Mn3 Mn4 Mn5 Bi1 Bi2 Bi3 Bi4 Bi5 Bi6 Bi7 Bi8 Bi9 Bi10 Bi11 Bi12 Bi13 Bi14

16j 16j 16j 16j 8i 8i 8h 8h 8g 8g 8f 8f 16j 16j 16j 16j 8f 4e 16j 16j 16j 16j 8i 8i 8i 8h 8h 8h 4e 4e 4e 4e

0.08154(5) 0.09023(4) 0.25146(4) 0.40706(4) 0.12582(7) 0.13152(7) 0 0 1/4 1/4 0.10861(7) 0.37680(6) 0.0801(4) 0.1135(4) 0.2075(1) 0.2924(1) 0.2405(2) 0 0.13544(3) 0.13564(3) 0.18598(3) 0.31448(3) 0.13133(5) 0.29423(4) 0.30337(4) 0 0 0 0 0 0 0

0.01505(5) 0.65735(5) 0.13239(4) 0.15929(5) 1/4 1/4 0.11936(8) 0.12211(7) 0.17418(7) 0.66103(6) 0 0 0.1745(3) 0.1440(4) 0.0497(1) 0.5347(1) 0 1/4 0.11567(3) 0.11210(3) 0.55597(3) 0.07218(3) 1/4 1/4 1/4 0.04640(4) 0.12810(5) 0.55277(4) 1/4 1/4 1/4 1/4

0.24950(7) 0.15187(7) 0.51087(6) 0.07348(7) 0.6282(1) 0.0677(1) 0.6246(1) 0.0680(1) 1/4 1/4 0 0 0.2134(5) 0.2373(6) 0.1757(2) 0.1842(2) 0 0.3129(5) 0.09950(5) 0.63135(4) 0.15201(4) 0.15305(4) 0.28335(7) 0.07264(6) 0.61962(6) 0.42186(6) 0.28287(7) 0.11976(6) 0.1466(1) 0.5074(1) 0.74756(9) 0.9464(1)

0.0272(3) 0.0240(2) 0.0208(2) 0.0232(2) 0.0279(4) 0.0292(4) 0.0300(4) 0.0277(4) 0.0375(5) 0.0212(3) 0.0223(3) 0.0192(3) 0.030c 0.030c 0.0248(6) 0.0207(6) 0.047(2) 0.043(2) 0.0290(2) 0.0208(2) 0.0207(2) 0.0239(2) 0.0365(3) 0.0243(2) 0.0239(2) 0.0214(2) 0.0327(3) 0.0228(2) 0.0365(4) 0.0307(4) 0.0237(3) 0.0258(3)

a c

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. bOccupancies of Mn1a and Mn1b are 0.361(8) and 0.293(8), respectively. Isotropic, and the value constrained to the average of all Mn sites.

bismuthides, e.g., Ca21Mn4Bi18 (dMn−Mn = 2.80−2.98 Å),2 and on a par with the sum of the respective Pauling single-bonded radii (rMn = 1.17 Å, rBi = 1.52 Å).27 The shortest distance between the Mn atoms within the trimers is 3.35 Å, which is significantly longer than the Mn−Mn contacts in metallic Mn (dMn−Mn = 2.24−2.91 Å). The Mn−Mn distance for the cornersharing tetrahedra is 5.19 Å, and the shortest Mn−Mn distance between the adjacent layers is 6.66 Å. Among the four crystallographically nonequivalent Sr atoms, Sr2 and Sr4 are octahedrally coordinated by the Bi atoms with the Sr−Bi distances ranging from 3.27 to 3.63 Å. Sr1 and Sr3 adopt a square-pyramidal coordination with dSr−Bi = 3.19−3.58 Å (Figure 3b). The sixth closest Bi atom, which could complete an octahedral coordination, is located at a distance of 3.99 Å from Sr1 and 4.12 Å from Sr3, respectively. These distances are out of the typical range for Sr−Bi bonds, and the corresponding bonding interactions were revealed by our first-principle calculations to be negligible. In the absence of any homoatomic Bi−Bi bondsthe shortest contact between the Bi atoms in Sr2MnBi2 is 4.56 Å the valence electrons can be partitioned as follows: [Sr2+]2[Mn2+][Bi3−]2. This charge-balanced composition puts the discussed compound in the broad class of transition-metalbearing Zintl phases.

Crystal Structure of Ba2Mn1−xBi2. Ba2Mn1−xBi2 crystallizes in a complex structure that bears little similarity to the crystal structures of other bismuthides in this compositional field, e.g., Sr2MnBi2, Ca9Mn4Bi9,7,28 and AE2ZnBi2 (AE = Sr, Ba),29,30 in spite of their stoichiometry being 2−1−2 or close to it. There are 32 independent sites in the asymmetric unit (Table 4) of this large orthorhombic structure.31 Up until now, there are no precedents for such atomic arrangement, which should be classified with the Pearson index oI316 in its own structure type (assuming that the two disordered Mn atoms account for one fully occupied position, this would equate to a chemical formula Ba2Mn0.9375Bi2, i.e., Ba32Mn15Bi32). To facilitate the description of such a complex crystal structure, which as already mentioned is not devoid of disorder, it is convenient to view it as consisting of two types of interpenetrating 3D frameworks, as shown in Figure 4. The first framework is built up of corner- and edge-sharing Bi-centered [BiBa7] monocapped trigonal prisms. For the four symmetrically nonequivalent Bi atoms in these prisms, the Bi− Ba distances lie in the range 3.54−4.00 Å, 3.47−3.79 Å, 3.63− 3.95 Å, and 3.51−3.71 Å for Bi6, Bi7, Bi8, and Bi10, respectively (Table 5). The second framework incorporates several kinds of Mn-centered polyhedra (Figure 4b). Mn5 atoms are coordinated by one Bi atom, four Bi2 dumbbells, and E

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Figure 4. (a) Crystal structure of Ba2Mn1−xBi2 viewed approximately along [001]. Bi−Bi bonds are omitted for clarity. Ba, Mn, and Bi atoms are shown in orange, blue, and red, respectively. (b) A fragment of the Mn−Bi framework viewed approximately along [001] (upper image) and along [010] (lower image), with the homoatomic Bi−Bi bonds displayed. Displacement ellipsoids are drawn at the 50% probability level.

one linear Bi3 unit. The resulting distorted octahedra are packed into columns along [001], with the Bi3 units pointing in the same direction within a column and in the opposite direction for adjacent columns. The four Bi2 dumbbells around Mn5 (2 × Bi5−Bi6 and 2 × Bi9−Bi8) link the Mn−Bi substructure with the Bi−Ba framework. The Bi−Bi distances in the dumbbells are 3.12 Å for Bi5−Bi6 and 3.17 Å for Bi9− Bi8. These distances are comparable with those found in other bismuthides with similar moieties, e.g., Ba11Bi10 (dBi−Bi = 3.15 Å).11 The slightly asymmetric Bi3 units display Bi−Bi contacts of 3.41 and 3.43 Å. These values also fall in the typical range expected for such fragments, e.g., dBi−Bi = 3.43 Å for the Bi3 fragments in Sr14MnBi11.32 Mn2 and Mn3 atoms are 4-fold coordinated by Bi atoms. Mn2- and Mn3-centered tetrahedra share common edges forming tetranuclear Mn4 clusters with Bi atoms capping all faces and terminating all vertices. The [Mn4Bi8] units (stellae quadrangulae) are stacked along [001] and are linked via the intermediate Mn4 atoms. The latter ones adopt an unusual 2fold coordination environment, typically observed for highly reduced Mn pnictides, e.g., Mn3Pn (Pn = P, As).33,34 Acknowledging that such coordination is atypical for Mn, we attempted to refine this site as partially occupied by Ba or Bi, but the resulting models showed some unreasonably short interatomic distances or senseless environment around the discussed position. Finally, the partially occupied Mn1a and Mn1b positions provide a linkage between the Mn5- and Mn2-centered polyhedra. Mn1a (with the occupancy 0.36) is accommodated in a tetrahedron built up of two Bi2 dumbbells and one Bi3 unit from the Mn1 domain and one Bi atom from the Mn2 coordination sphere, resulting in face-sharing with the Mn1centered octahedron and corner-sharing with the Mn2-centered tetrahedron. The 4-fold coordination environment of Mn1b

Table 5. Selected Interatomic Distances (Å) in Ba2Mn1−xBi2 (x ≈ 0.15) atoms Mn1a

Mn1b

distance

−Mn1b −Bi5 −Bi9 −Bi1 −Bi11 −Mn5 −Bi1 −Bi5 −Bi9 −Bi4 −Mn2

1.23(2) 2.628(9) 2.653(9) 2.845(9) 3.044(9) 3.30(1) 2.53(1) 2.87(1) 3.04(1) 3.21(1) 3.57(1)

−Bi3 −Bi1 −Bi4 −Bi4 −Mn3 −Mn3 −Mn2 −Mn4 −Bi3 −Bi4 −Bi2 −Bi3 −Mn3 −Mn4

2.798(3) 2.824(3) 2.824(3) 3.042(3) 3.072(4) 3.232(4) 3.347(6) 3.376(3) 2.833(3) 2.850(3) 2.856(3) 2.911(3) 3.130(6) 3.537(3)

a

atoms Mn4 Mn5

Bi6

Bi7 Mn2

Mn3

Bi8

Bi10

Bi11 Bi13

−Bi3 × 2 −Bi4 × 2 −Bi11 −Bi9 × 2 −Bi12 −Bi5 × 2 −Bi5 −Ba3 × 2 −Ba4 × 2 −Ba9 × 2 −Ba5 −Ba10 × 2 −Ba6 −Ba2 × 2 −Ba3 × 2 −Bi9 −Ba12 × 2 −Ba1 × 2 −Ba4 × 2 −Ba7 −Ba1 × 2 −Ba2 × 2 −Ba8 −Ba11 × 2 −Bi14 −Bi14

distance 3.285(2) 3.727(3) 2.850(8) 3.171(2) 3.330(8) 3.400(2) 3.117(2) 3.539(1) 3.710(2) 3.781(1) 4.002(2) 3.474(1) 3.616(2) 3.658(2) 3.788(1) 3.173(2) 3.627(2) 3.704(2) 3.750(2) 3.946(2) 3.511(2) 3.584(2) 3.676(2) 3.711(2) 3.428(2) 3.405(2)

a

This unphysical distance is an artifact of the lower occupancies of Mn1a and Mn1b, 0.36 and 0.29, respectively.

F

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value is comparable with an energy difference that can result from some significant structural changes. This emphasizes the conjecture that the effect of magnetism on the net stabilization of the crystal structures must not be underestimated. The two lowest lying Sr2MnBi2 magnetic structures with respect to the total energy (AFM1 and AFM2, Figure S2) differ only in how the adjacent two-dimensional Mn layers couple to each other. The energy difference between these two structures amounts to 17.8 meV/(unit cell), with AFM2 being less stable than AFM1. The third most energetically favorable magnetic arrangement, with a different coupling within the layers, is by 136.6 meV/(unit cell) less stable than AFM1. These numbers show that the main stabilization comes from a specific pattern of magnetic interactions in the two-dimensional Mn sheets. The coupling between the sheets plays a marginal role, leading to low-dimensional magnetism, reflected in magnetic frustration as observed in the magnetization data (vide inf ra). The AFM1 and AFM2 arrangements, identified as the two most energetically favorable candidates, display almost identical electronic structures and magnetic moments. In both cases, an electronic bandgap of about 0.4 eV is observed at the Fermi level. The small bandgap and the presence of the heavy element Bi make Sr2MnBi2 a suitable material for testing of thermoelectric properties and for rational property optimizations. To get a deeper insight into the electronic structure and the chemical bonding in Sr2MnBi2, we switched from the pseudopotential Siesta code to the full-electron TB-LMTOASA approach, this time focusing on the AFM1 structure only. The electronic density of states (DOS) is shown in Figure 5a. Due to the spin polarization, just above the Fermi level EF, the DOS is mainly composed of the empty Mn(3d) states (“spinup” for the “spin-down” Mn atoms and vice versa). Well above EF, the empty Sr(5d) states are predominant. Due to a partial electron transfer from the less electronegative metal atoms onto the Bi atoms, the Bi states are mainly located below EF. However, a fully ionic picture is not applicable here even for the Sr−Bi interactions, as a significant hybridization of the Sr(5s), Sr(5p), and Sr(5d) states with the Bi(6p) states is observed below EF, indicating a pronounced degree of covalency between the Sr and Bi atoms. Such a situation is not uncommon for bismuthides, due to the metallic character of Bi, and was also observed for Ba2Cd3Bi4,37 as well as Ba2Mn1−xBi2 (Figure S3). The covalent Mn−Bi interactions in Sr2MnBi2 are indicated by the hybridization of Mn(3d) and Bi(6p) states. The cumulative COHP curve for the symmetrically independent Sr−Bi interactions (3.19 Å ≤ d ≤ 3.63 Å) shows exclusively bonding interactions below EF (Figure 5b). These interactions are only slightly underoptimized due to the presence of unpopulated bonding states just above EF. This underoptimization is compensated by the fully optimized Mn− Bi interactions (Figure 5c, 2.80 Å ≤ d ≤ 2.95 Å). Interestingly, even the relatively long Mn−Mn contact of 3.35 Å shows a sizable bonding interaction, with a −ICOHP value of 0.48 eV/bond (Figure 5d). Similarly, a non-negligible −ICOHP of 0.38 eV/bond was calculated for a Mn−Mn contact of 3.2 Å in La3MnBi5.38 These results show that interactions between transition metal (TM) atoms may play an important role in stabilizing crystal structures of the TMbearing multinary pnictides. The TM−TM interactions are of course mediated by the available d- or f-electrons, and are naturally coupled to the magnetism in such compounds. These peculiarities, specific for transition metals only, provide an

(with the occupancy 0.29, located at a distance of 1.24 Å from Mn5a) includes two Bi2 dumbbells from Mn1 and two Bi atoms from Mn2, which prompts an edge-sharing connection of the tetrahedron around Mn1b with the adjacent Mn1- and Mn2centered polyhedra. The partial occupation of Mn1a and Mn1b hints toward a possible homogeneity range in Ba2Mn1−xBi2. Indeed, the refined occupation factors differed slightly between the two single crystals, and the total Mn1a + Mn1b was always less than unity. In light of the crystal structure description above, the idealized formula of the discussed compound can be written as Ba32Mn15Bi21[Bi2]4[Bi3], where the Bi2 dimers and Bi3 trimers are differentiated from the Bi atoms that do not participate in homoatomic bonding. Assuming the formal charges Ba2+, Bi3−, [Bi2]4−, and [Bi3]7−, the net oxidation state of Mn would be +1.47. Since Mn is typically found as Mn2+ (d5 configuration) in most pnictides,35,36 the compound under consideration should be considered as an example of an electron-excessive (highly reduced) system. Although no physical property measurements are available, metallic behavior can be anticipated for this phase. We also would like to note that Ba32Mn15Bi32 represents the limiting case with no Mn vacancies, while in reality, ca. 10% of the Mn positions are empty and the actual chemical formula is Ba32Mn15−xBi32 (x ≈ 1.4, or Ba2Mn1−xBi2 with x ≈ 0.15). This increases the charge on the Mn to +1.62, but in order to bring the valence electron balance to the realm of the Zintl concept, the formula should be Ba32Mn11Bi32 (or Ba2Mn1−xBi2 (x = 0.3125)), which is not supported experimentally. Electronic Structure and Chemical Bonding in Sr2MnBi2. Spin-polarized electronic structure calculations for Sr2MnBi2 were performed with the Siesta code for the eight magnetic arrangements listed in Table 6. Two Mn−Mn Table 6. Calculated Total Energies (Siesta) of Different Magnetic Structures for Sr2MnBi2 Mn−Mn pair (Å) structure

3.35 (intralayer)

5.19 (intralayer)

6.66 (interlayer)

E with respect to AFM1 (meV/unit cell)

AFM1 AFM2 AFM3 AFM4 AFM5 AFM6 FiM FM

AFM AFM AFM FM FM FM AFM FM

AFM AFM FM FM AFM AFM FM FM

AFM FM FM AFM AFM FM AFM FM

0 17.8 136.6 895.3 769.6 780.7 148.2 954.1

couplings within the two-dimensional Mn layers and one coupling between the layers were chosen to set up these magnetic structures (Figure S2). Six of them are antiferromagnetic (AFM), one is ferrimagnetic (FiM), and one is ferromagnetic (FM). For all these test cases, the converged ordered magnetic moments on Mn atoms were in the range 5.0−5.1 μB, in perfect agreement with the expected value of 5.0 μB for the high-spin Mn2+. In general, lower total energies were achieved for the structures with an antiferromagnetic coupling between the Mn atoms located at the distance 3.35 Å. When a ferromagnetic coupling was assumed for this pair, the calculations yielded a strong destabilization of up to 954.1 meV/(unit cell). The latter G

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Figure 5. Calculated total and partial densities of states for Sr2MnBi2 in the model AFM1 structure (a); (b, c, d) cumulative COHP curves for the Sr−Bi, Mn−Bi, and Mn−Mn contacts, respectively.

additional tool for designing new materials with potentially magnetic and transport interesting properties. Physical Properties of Sr2MnBi2. The electrical resistivity of the Sr2MnBi2 single crystals is weakly temperaturedependent in the range 3 K < T < 300 K (Figure 6). The small anomaly upon cooling (near 260 K) is likely associated with the contacts. The overall behavior of the ρ(T) curve indicates poorly metallic or degenerate semiconducting behavior. It is important to note that the resistivity data are affected by small amounts of remaining Pb flux. Its presence is also reflected in the drop of the resistivity below T ≈ 7.5 K, which corresponds well to the superconducting transition temperature of Pb (Tc = 7.2 K). The incorporation of the metal flux during the crystal growth is a well-known problem,39 which can be resolved sometimes by selective etching of the flux,39 or by conducting centrifugation at a higher temperature.26 The high chemical activity of Sr2MnBi2 precludes the use of etching agents, and the attempts to remove the molten Pb at higher temperatures did not produce large enough crystals. Nevertheless, the intrinsic behavior of Sr2MnBi2 seems to have the prevailing contribution to the measured data, as it can be deduced from the resistivity at room temperature: its value is orders of magnitude higher than that of metallic Pb.40 In addition, the signature of a possible phase transition can also be seen in the plot as a kink of the resistivity curve at ca. 50 K. This transition, associated with an antiferromagnetic (AFM)

Figure 6. Temperature dependence of the electrical resistivity (ρ) for a single crystal of Sr2MnBi2. Data gathered upon cooling are indicated by the blue arrow, and the data upon heating are indicated by the red arrow.

ordering, is also visible in the magnetization data (Figure 7). Upon thermal cycling, a sizable hysteresis of the electrical resistivity above the transition temperature is observed, pointing toward the first-order nature of this transition. H

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A strong magnetic frustration can be also inferred from the fact that Sr2MnSb2 does not order magnetically down to 5 K.18

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CONCLUSIONS We synthesized two new ternary alkaline-earth metal manganese bismuthides and studied their crystal structures. Whereas Sr2MnBi2 adopts a known structure type (Sr2MnSb2) and conforms to the Zintl notation, Ba2Mn1−xBi2 (x ≈ 0.15) crystallizes in its own structure, possessing a complex atomic arrangement and partial disorder. First-principle calculations on Sr2MnBi2 confirm its classification as a Zintl phase. The calculated bandgap of about 0.4 eV and the presence of a heavy element (Bi) make Sr2MnBi2 an interesting material for thermoelectric applications. A strongly two-dimensional nature of the exchange interactions in Sr2MnBi2 found in our calculations leads to a pronounced magnetic frustration, observed in the magnetization data. In addition, the importance of magnetic interactions for the stabilization of crystal structures was emphasized in our study. Rational synthesis of transition-metal-bearing pnictides can open new ways of designing interesting magnetic topologies and bonding patterns with possible implications for valuable properties.

Figure 7. Temperature dependence of magnetic susceptibility for Sr2MnBi2 in a field of 500 Oe. Inset: high-temperature part (300 K < T < 700 K) of χ(T). Vertical bars denote standard deviations.

■

Temperature dependence of the magnetic susceptibility is displayed in Figure 7. An increase of the magnetic susceptibility below the transition temperature is likely due to some paramagnetic impurities or intrinsic defects. The sharp drop of χ(T) upon cooling below T ≈ 7.5 K is associated with the superconducting transition of the remaining elemental Pb. Above the AFM transition, the magnetic susceptibility decreases monotonically with increasing the temperature. Although a linear fit of the inverse susceptibility data (Figure S5) yields a reasonable magnetic moment of 6.02 μB, close to the expected spin-only value of 5.92 μB for a high-spin Mn2+ ion, the appearance of the χ(T) curve and the strongly negative Weiss constant of −569 K suggest that Sr2MnBi2 does not enter the paramagnetic regime in this temperature range. Our high temperature magnetization data (Figure 7, inset) show that, within the margins of the experimental error, the almost linear decrease of χ(T) persists at least up to 700 K, i.e., well above the presumed AFM ordering. All of the above indicates that the energy scale of the leading exchange interaction in Sr2MnBi2 exceeds the thermal energy. At the same time, the magnetic interactions must be highly anisotropic, judging from the comparably low ordering temperature. This conclusion is corroborated by our first principle calculations, which reveal a strongly two-dimensional nature of magnetism in Sr2MnBi2. As a result of the anisotropic exchange, Sr2MnBi2 represents a frustrated magnetic system. To reliably determine the magnetic moment on the Mn atoms, an additional neutron diffraction study will be necessary. At this point, we would like to note that the reported temperature dependence of magnetic susceptibility for Sr2MnSb2 also follows an approximately linear trend in the high temperature region.20 The magnetic moment of 4.85 μB determined from the Curie−Weiss fit of the data in this region appeared to be smaller than the expected value, and the absolute value of the Weiss constant |θ| = 309 K exceeded the highest measured temperature. This may indicate that, similarly to Sr2MnBi2, Sr2MnSb2 also displays magnetic frustration. The magnetic correlations between the Mn centers within the layers remain strong up to high temperatures, preventing the magnetic susceptibility from entering the Curie−Weiss regime.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01851. Details of data collection, atomic coordinates, and atomic displacement parameters, TG-DSC plot, magnetic structures, DOS plots, and linear fit of inverse magnetic susceptibility (PDF) Accession Codes

CCDC 1563382−1563384 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (320)831-8720. Fax: (320) 831-6335. ORCID

Sheng-Qing Xia: 0000-0002-6199-2491 Svilen Bobev: 0000-0002-0780-4787 Funding

US Department of Energy, Office of Science, Basic Energy Sciences. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award #No. DE-SC0008885. I

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