Unique [Mn6Bi5]â nanowires in KMn6Bi5: a quasi-one-dimensional

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Unique [MnBi] nanowires in KMnBi: a quasione-dimensional antiferromagnetic metal Jin-Ke Bao, Zhang-Tu Tang, Hee Joon Jung, Ji-Yong Liu, Yi Liu, Lin Li, Yu-Ke Li, Zhu-An Xu, Chun-mu Feng, Haijie Chen, Duck Young Chung, Vinayak P. Dravid, Guang-Han Cao, and Mercouri G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00465 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Journal of the American Chemical Society

Unique [Mn6Bi5]− nanowires in KMn6Bi5: a quasi-onedimensional antiferromagnetic metal Jin-Ke Bao1, 2, Zhang-Tu Tang1, Hee Joon Jung3, Ji-Yong Liu4, Yi Liu1, Lin Li5, Yu-Ke Li5, Zhu-An Xu1, 6, 7, Chun-Mu Feng1, Haijie Chen8, Duck Young Chung2, Vinayak P. Dravid3, Guang-Han Cao1, 6, 7, *, and Mercouri G. Kanatzidis2, 8, *

1

Department of Physics, Zhejiang University, Hangzhou 310027, China, 2Materials Science

Division, Argonne National Laboratory, Argonne, Illinois 60439, United States, 3Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States, 4Department of Chemistry, Zhejiang University, Hangzhou 310027, China, 5Department of Physics, Hangzhou Normal University, Hangzhou 310036, China, 6State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, 7China Collaborative Innovation Centre of Advanced Microstructures, Nanjing University, Nanjing 210093, China, 8Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

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ABSTRACT: We report a new quasi-one-dimensional compound KMn6Bi5 composed of parallel nanowires crystallizing in a monoclinic space group C2/m with a = 22.994(2) Å, b = 4.6128(3) Å, c = 13.3830(13) Å and β = 124.578(6)°. The nanowires are infinite [Mn6Bi5]− columns each of which is composed of a nanotube of Bi atoms acting as the cladding with a nanorod of Mn atoms located in the central axis of the nanotubes. The nanorods of Mn atoms inside the Bi cladding are stabilized by Mn−Mn bonding and are defined by distorted Mn-centered cluster icosahedra of Mn13 sharing their vertices along the b axis. The [Mn6Bi5]− nanowires are linked with weak internanowire Bi−Bi bonds and charge balanced with K+ ions. The [Mn6Bi5]− nanowires were directly imaged by high resolution transmission electron microscopy and scanning transmission electron microscopy. Magnetic susceptibility studies show one-dimensional characteristics with an antiferromagnetic transition at ~ 75 K and a small average effective magnetic moment (1.56 µB/Mn for H || b and 1.37 µB/Mn for H ⊥ b) of Mn from Curie-Weiss fits above 150 K. Specific heat measurements reveal an electronic specific heat coefficient of 6.5(2) mJ K−2(mol-Mn)−1, and a small magnetic entropy change ∆Smag ≈ 1.6 J K−1 (mol-Mn)−1 across the antiferromagnetic transition. In contrast to a metallic resistivity along the column, the resistivity perpendicular to the column shows a change from a semiconducting behavior at high temperatures to a metallic one at low temperatures, indicating an incoherent-to-coherent crossover of the inter-column tunneling of electrons.


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INTRODUCTION One-dimensional (1D) systems exhibit many exotic states such as charge density wave induced by Peierls instability1, Luttinger liquid with spin-charge separation2, and topological spin excitation states3. While ideally 1D materials do not exist, quasi-one-dimensional (Q1D) materials, such as compounds with atomic or molecular chains in the structure4, do exist and can show states that are able to approach an ideal 1D system. Therefore, it is of great interest to study new Q1D materials to further explore such novel states of matter. When the atomic chains in Q1D materials contain transition metals bonded to other elements they can create thicker atomic columns and even nanowires, and then they can exhibit much richer structural variations and physical properties. Some notable examples with different chain and columnar structures are shown in Figure 1, which are classified by the number n of transition-metal atoms located in the same structural plane of a column. NbSe3 has monatomic Nb chains coordinated by Se atoms in the structure and exhibits charge density wave orders at low temperatures.5 BaFe2S3 consists of bi-atomic Fe chains Fe2S3 and can evolve from an antiferromagnetic insulator to a superconductor under physical pressures.6 XMo3Se3 (X = Na, K, In, Tl)7 and A2Cr3As3 (A = K, Rb, Cs)8-10 have Mo3Se3 or Cr3As3 chains with Mo or Cr triangles extended along the chains. Among them, XMo3Se3 family is predicted to exhibit “cubically dispersed Dirac semimetal” behavior,11 and A2Cr3As3 (A = K, Rb, Cs) are unconventional superconductors with significant electron correlations.8-10 When the number n of the transition metals located in the same plane is 4, as in M4ZTe4 (M = Ta, Z = Si, Cr, Fe, Co, Ni; M = Nb, Z = Si, Fe)12, square-M and square-Te units form and stack into M4Te4 chains. The additional Z atoms, sandwiched by two neighboring square-M units, form a single-atom chain in the center of the M4Te4 chain. Because of the weak coupling between the chains in M4ZTe4, nanowires with a large aspect ratio can be obtained to investigate their 1D transport properties.13 There is also a series of compounds [Ni8Bi8S]Ix (x = 0, 1, 2)14-16 featuring nanowire Ni4Bi4 chains with S atoms along their central axis, similar to M4ZTe4. The number of the valence electrons in the [Ni8Bi8S]x+ chain can be tuned by adding or removing I atoms.14-16 Similar to the square-M unit in M4ZTe4, Na2.8Cu5Sn5.6 has Cu5 and Sn5 pentagon units occupying the same plane.17 These pentagons stack along the c axis forming a Cu5Sn5.6 column with partially occupied Sn atoms located in the center. Compounds having



transition-metal columns with a larger number n in the same plane, and thus greater diameters, are still rare and need further exploration on their structures and properties. The discovery of the superconducting family A2Cr3As3 (A = K, Rb, Cs) with a Q1D structure810

, prompted us to explore the Mn-analogs such as K2Mn3As3 for possible superconductivity.

While we could not observe any A2Mn3As3 (A = K, Rb, Cs) compounds, but noticed that Saparov et al18 obtained unknown needle-like crystals in their study of heavily K-doped BaMn2Bi2 which, if Ba1-xKxMn2Bi2, should have a plate-like morphology. In their energy dispersive x-ray spectroscopy (EDS) analysis18, the crystals contained three elements K, Mn and Bi. Our reinvestigation identified the new Q1D material KMn6Bi5 by obtaining crystals using Bi flux. The compound has infinite [Mn6Bi5]− columns with a Bi−Bi bonded shell surrounding a Mn−Mn bonded metallic core, creating nanowires. The Mn-cluster core column consists of distorted Mncentered icosahedra Mn13, which share their vertices along the column direction. The presence of [Mn6Bi5]− nanowires in KMn6Bi5 were further confirmed by high resolution transmission electron microscopy and scanning transmission electron microscopy (HRTEM and HRSTEM). The new compound exhibits antiferromagnetic order at ~ 75 K, supported by magnetic susceptibility, resistivity and specific heat measurements. Based on the sharp drop of the magnetic susceptibility around the transition and the temperature-dependent behavior below the transition, the antiferromagnetism in KMn6Bi5 is likely to be a spin-density wave. The resistivity is strongly anisotropic and points to a Q1D electronic structure with an incoherent-to-coherent crossover of the inter-column tunneling of electrons at ~ 40 K.

EXPERIMENTAL SECTION Crystal growth KMn6Bi5 single crystals were grown in Bi flux. High purity of K (99.5%) pieces, Mn (99.98%) granules and Bi (99.999%) shots were used as reagents. All the procedures handling the reagents were done in a glove box filled with highly pure argon gas. 0.1245 g K (3 mmol), 0.3499 g Mn (6 mmol) and 5.3239 g Bi (24 mmol) with an atomic ratio K : Mn : Bi = 1 : 2 : 8 were loaded into an 8 mm alumina crucible. The alumina crucible was sealed in a 13 mm fused silica tube at a vacuum of 10-3 mbar. Quartz wools, used to filter the Bi flux during the centrifugation, were covered on top of the alumina crucible. The tube was slowly heated to 1003 K, held for 12 h, 4 ACS Paragon Plus Environment

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and then cooled down to 703 K at a rate of 2 K h−1. At this temperature, the excess liquid Bi flux was removed by centrifugation. Large shiny silver-color needle-like crystals with dimensions up to 1 × 1 × 4 mm3 were harvested, see Figure 2(a). The crystals are malleable and air sensitive. They can be easily cleaved along the rod direction, creating stripe patterns and revealing fibers on the surface, see Figure 2(b), which indicates the Q1D bonding character. Freestanding crystals with suitable dimensions (~ 0.24 × 0.05 × 0.04 mm3) for single x-ray diffraction were selected from the batch where a stoichiometric mixture of K, Mn and Bi to form KMn6Bi5 was annealed at 823 K for 24 hours. Crystal growth under an atomic ratio K : Mn : Bi = 1 : 6 : 24 also leads to large rod-like crystals. Increasing the K quantity of crystal growth to a ratio K : Mn : Bi = 1 : 1 : 4, however, leads to K-Bi mixtures coated on the surface of as-grown crystals. This is likely the reason of the inconsistent composition data observed in EDS analysis by Saparov et al.

18

The

compositions of clean single crystals obtained from different growth conditions were essentially the same, indicating a stoichiometric single phase. Further increasing the K quantity to a ratio K : Mn : Bi = 2 : 1 : 4, plate-like crystals were formed and identified as a stoichiometric compound KMnBi with a tetragonal layered structure.19 The data in the main text of this article were obtained from crystals of the batch with the ratio K : Mn : Bi = 1 : 2 : 8 for crystal growth.

Structure and Composition Determination The composition analysis of this compound was done by x-ray analysis with an energy dispersive spectroscopy (EDS) in a field-emission scanning electron microscope, operated at 25 kV. Crystals from different batches were also examined and the EDS analysis was performed on the fresh surface cleaved from the crystals selected. EDS analysis confirmed that they contain three elements K, Mn and Bi, see Figure 2(c), in requisite proportion. Calibrated by using the standard stoichiometric compound KMnBi19 with a detailed process shown in the Supporting Information, the average composition is K1.05(11)Mn6Bi5.07(16), close to the stoichiometric one KMn6Bi5. A needle-like crystal of KMn6Bi5 was inserted in a glass capillary (ID = 0.3 mm, thickness = 0.01 mm), which was sealed by candle wax. The crystal was fixed by Apiezon H grease. X-ray diffraction (XRD) data collection from this single crystal was performed on the diffractometer STOE IPDS 2T at room temperature. Data collection, integration and absorption correction were 5 ACS Paragon Plus Environment


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done in the X-area software package.20 The structure of KMn6Bi5 was solved by direct method and refined by full-matrix least-squares based on F2 using a SHELXTL program package.21 The refinement results of KMn6Bi5 are shown in Table 1. The low values of R1 (0.0396) and wR2 (0.0698) under I > 2σ(I) indicate a good structure solution. KMn6Bi5 adopts a C-centered monoclinic structure with a space group C2/m. Table 2 shows one, six and five independent atomic sites for K, Mn and Bi along with corresponding isotropic displacement parameters. The total composition of one unit cell is K4Mn24Bi20, consistent with the EDS results. The atomic site Mn6 is a special position. Atoms Mn1 to Mn5 and Bi1 to Bi5 are in the same plane, forming Mn5 and Bi5 pentagons, respectively. These pentagons have slight distortions. Detailed bond lengths, bond angles and anisotropic displacement parameters are given in the Supporting Information.

Physical Property Measurements Magnetic properties were performed on a commercial Quantum Design magnetic property measurement system (MPMS-5T). Anisotropic temperature-dependent magnetic susceptibilities were measured by applying fields (H = 2000 Oe) parallel and perpendicular to the rod direction. Resistivity and specific heat measurements were done on a commercial Quantum Design physical property measurement system (PPMS-9T). A standard four-probe method was adopted to obtain the resistivity. Gold wires were attached onto the rod-like sample by the silver paste Dupont 4929N. Due to the geometric uncertainty of cross section areas in the rod-like crystals, the absolute values of resistivity may have an error within a factor of 50%. Three pieces of crystals with a total mass of 17.4 mg were used for the specific heat measurements. A very thin film of Apeizon N grease was applied on the surface of crystals to avoid possible deterioration in the air during loading them onto the sample holder of specific heat measurement. Because of the air sensitivity of KMn6Bi5, all sample preparation procedures were done in the glove box filled with argon gas before the measurements. Different crystals from the same batch were employed to perform different physical properties in the main article. For example, sample A1 and A2 in the main article for magnetic property measurements are different samples from the same crystal batch A of KMn6Bi5. Crystals from other batches were also examined and gave similar results to those reported here. The label information of all crystals and the experimental results from the other batches are given in the Supporting Information. 6 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Crystal Structure The structure of KMn6Bi5 features parallel clearly defined infinite [Mn6Bi5]− columns extended along the b axis with K+ cations filled between the columns, see Figure 3(a) and 3(b). The columns are parallel along the b axis. Every [Mn6Bi5]− column is an easily recognizable nanowire and consists of a Bi−Bi bonded nanotube (diameter ~ 8.7 Å) acting as the cladding and it is filled with a narrower column of Mn−Mn bonded atoms acting as the core, see Figure 3(c), 3(d) and 3(e). This Mn-cluster type core column is made of stacked pentagons of Mn atoms with a singleatom chain of Mn (Mn6 site) running through the center of the pentagons with the shortest Mn−Mn bond length of 2.30640(15) Å. Mn6 atoms are sandwiched between neighboring Mn pentagons. The pentagons are stacked and are symmetry related by a crystallographic 21 screw axis along the Mn6 chain. The Mn−Mn bond lengths in the cluster column range from around 2.3 Å to 2.8 Å, close to that in α-Mn metal.22 Note that the [Mn6Bi5]− nanowires are weakly bonded with each other to form a three-dimensional crystalline structure of KMn6Bi5. Each [Mn6Bi5]− nanowire consists of a Bi nanotube bonded tightly with an inserted Mn-cluster core column via Mn-Bi bonding (∼ 2.8−3.0 Å). The structure of KMn6Bi5 is similar to Bi5.6Ni5I23. The Bi nanotubes (diameter ~ 8.6−8.7 Å) acting as the cladding are almost the same in those compounds. In Bi5.6Ni5I23 the Ni atoms occupy the same configuration as the Mn pentagons do in KMn6Bi5 and a partially occupied Bi atom chain replaces the central Mn6 chain of KMn6Bi5. [Mn6Bi5]− nanowires are negatively charged in KMn6Bi5 while [Bi5.6Ni5]+ nanowires are positively charged in Bi5.6Ni5I23. There are essential differences between the Mn-filled Bi nanotubes found in KMn6Bi5 and those in [Ni8Bi8S]Ix (x = 0, 1, 2)14-16 or those in the single or double walled hollow Bi nanotubes which are obtained by a low temperature synthesis method.24-25 For example, the diameter of tubes in the title compound is larger than those in Ni8Bi8S (~ 7.1−7.5 Å) and much smaller than the single or double walled hollow Bi nanotubes (~ 48−56 Å). 7 ACS Paragon Plus Environment


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The local coordination of Mn6 is a distorted icosahedron, see Figure 4(a). The infinite Mncluster core column can also be viewed as vertex-sharing distorted Mn-centered icosahedra of Mn13 extended along the chain direction, see Figure 3(c). The structure of Mn-cluster core column has strong geometrical frustrations for antiferromagnetic interactions from the triangular geometrical connection between Mn atoms. Yet the antiferromagnetic frustration is partially removed by the squeezed Mn-centered icosahedron Mn13 along the column direction due to the short Mn6−Mn6 bonds. The coordination environments of five Mni (i = 1, 2, 3, 4, 5) atomic sites are similar. The local geometry of these Mni is distorted icosahedra as defined by four Bi and eight Mn atoms as nearest neighbors, see Figure 4(b). Every Bi nanotube is formed by stacked Bi5 pentagons which are staggered approximately by 36 degrees’ rotation so that when viewed down the nanotube axis they create a decagonal projection, see Figure 3(a) and 3(d). This is similar to the case of Mn pentagons in the core column. The Bi−Bi bond lengths within or between Bi nanotubes in KMn6Bi5 at 3.52−3.66 Å are similar to that between the puckered Bi layers in Bi metal at 3.52 Å. Also, the Bi−Bi bond distances within the Bi nanotube in KMn6Bi5 range from 3.52 Å to 3.61 Å, slightly larger than the case (~ 3.4 Å) in Bi5.6Ni5I23, while the shortest Bi−Bi bonds between the Bi nanotubes in KMn6Bi5 are Bi2−Bi2 (3.5687(12) Å) and Bi3−Bi3 (3.6517(13) Å), slightly smaller than the case (~ 3.7−3.8 Å) in Bi5.6Ni5I23. Thus, the Bi nanotubes in KMn6Bi5 are elongated along the tube direction compared with the case in Bi5.6Ni5I23, leading to larger Bi−Bi bond distances within the nanotubes and smaller Bi−Bi bond distances between the nanotubes. By comparison, in Bi metal, the Bi−Bi bond distances within and between the puckered Bi layers are 3.07 Å and 3.52 Å, respectively.26

The inter-column interaction between [Mn6Bi5]− occurs via Bi2−Bi2 and

Bi3−Bi3 bonding distorts the column in the perpendicular direction, which is indicated by the larger bond distance of Bi1−Bi4 (3.6115(7) Å), see Figure 3(d). This leads to a low symmetric monoclinic crystal structure. The coordination environment of the K atoms is close to a tricapped trigonal prism, shown in Figure 4(c). In order to confirm the direct cross-sectional morphology and the packing of these [Mn6Bi5]− nanowires it is necessary to observe them along the [010] zone. This is very challenging since it is essential to prepare thin section normal to the natural length of these nanowires for scanning 8 ACS Paragon Plus Environment

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transmission and transmission electron microscopy (S/TEM) analysis. This was accomplished by “cross-sectional” sample preparation by focused ion beam (FIB) thinning. The S/TEM results with HRTEM and selected area diffraction (SAD) presented in Figure 5 were obtained with JEOL GrandARM at 300 kV. The details are provided in the Supporting Information. Also, high angle annular dark field - high resolution scanning transmission electron microscopy (HAADFHRSTEM) was performed using an aberration-corrected JEOL ARM 200cF at 200 kV instrument with 90-200 mrad of collection angle of annular dark field for HAADF. The inter-planar nominal d-spacings of (001), (200) and (201) planes in HRTEM (Figure 5(a)) are 1.10 nm, 9.46 Å and 1.08 nm, respectively, which are in good agreement with the XRD results. The selected area electron diffraction (SAED) of [010] zone axis in Figure 5(b) matches very well with the simulated diffraction pattern as articulated in the Supporting Information. The observed magnified HRTEM and associated simulation of images using the QSTEM27 software are shown in Figure 5(c) and 5(d), respectively. The experimental and simulated images collectively exhibit almost the same configuration of [Mn6Bi5]− nanowires which are aligned parallel to each other along the [010] direction with a brighter single-Mn-core column being visible. This is due to different electron channeling effects from different stacking situations of center-core Mn and outer Mn5 atoms along the projection view direction.28-30 On the one hand, the center-core Mn atoms having a straight alignment on projection view give a brighter contrast. On the other hand, the outer Mn5 atoms forming a pentagon in the plane are stacked by a 21 screw axis parallel to the projection view. That means that adjacent Mn5 pentagons are not eclipsed but staggered on the projection view, thus giving a darker contrast. No stacking or other defects were observed. Furthermore, HAADF-HRSTEM, a technique mapping the intensity proportional to the atomic number of atoms in the image, clearly resolves bright Bi (the highest atomic number in the structure) nanotubes with a decagonal projection along [010] direction with less bright Mn-cluster cores inside, directly confirming the array of [Mn6Bi5]− nanowires in KMn6Bi5, see Figure 5(e). All these observations are consistent with [Mn6Bi5]− nanowires in the structure of KMn6Bi5. In addition to the cross-section view, the TEM and diffraction analyses of “side-view” of KMn6Bi5 needle-like crystals provide further structural evidence indicating that [010] direction is aligned with the needle long axis albeit with a small rotation of individual needles in the ensemble (see Supporting Information). Similarly, chemical maps using scanning 9 ACS Paragon Plus Environment


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transmission electron microscopy-x-ray energy dispersive spectroscopy (STEM-EDS) confirm the homogenous and nominally uniform distribution of K, Mn and Bi (see Supporting Information). The geometric configuration of the infinite Mn-cluster core columns with their Mn−Mn metallic bonding in KMn6Bi5 is unique. The bonding is reminiscent of elemental Mn itself which structurally is the most complex among all transition elements. There exist several Mn allotropes with different structures and magnetic ground states. For example, the complex body-centeredcubic α-Mn has a very complicated crystal structure containing 58 Mn atoms with four different atomic positions in a single cubic unit cell.22 It has a non-collinear antiferromagnetic order at ~ 95 K with different magnetic moments on different Mn sites.31 As a result, α-Mn can be viewed as an “intermetallic” compound with magnetic Mn atoms (2−3 µB) and almost non-magnetic Mn atoms (0.2−0.6 µB).31-32 When the bulk Mn metal is reduced to a lower dimensional structure such as finite Mn clusters, there are rich geometric configurations and magnetic orders depending on the size of the clusters. This is of great interest in the problem of the understanding the relationship between magnetism and spatial confinements on Mn atoms.33-36 Thus, KMn6Bi5 with infinite Mn-cluster columns is an exotic compound to study its structure, magnetism and dimensionality. Magnetic Susceptibility In order to explore the magnetism of KMn6Bi5, we performed anisotropic magnetic susceptibilities χ|| and χ⊥, see Figure 6. The magnetic susceptibilities of KMn6Bi5 are almost isotropic (χ|| ≈ χ⊥) at high temperatures and obey a modified Curie-Weiss behavior above 150 K, Figure 6(a) and 6(c). The data can be well described by the formula χ = χ +

where χ0 is the

sum of temperature-independent contributions including diamagnetic orbital magnetism and Pauli paramagnetism of conduction electrons; C is the Curie constant from which the effective magnetic moment can be calculated; θ is the Curie-Weiss temperature that indicates the effective interaction between magnetic moments.37 The fitting gives χ0 = 0.00108(1) emu/mol-Mn, θ = −214(4) K, µeff = 1.56(1) µB/Mn and χ0 = 0.00126(1) emu/mol-Mn, θ = −185(4) K, µeff = 1.37(1) µB/Mn for χ|| and χ⊥, respectively. The negative values of θ indicate an antiferromagnetic interaction between magnetic moments of Mn. The effective magnetic moment of each Mn is 10 ACS Paragon Plus Environment

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rather small, even smaller than a spin- 3d magnetism with orbital magnetic components quenched, which will give an effective moment of 1.73 µB per Mn atom. It is probable that different Mn sites in KMn6Bi5 have different magnetic moments just as the case in α-Mn.31 Because of the short distances between Mn atoms in KMn6Bi5, a large portion of 3d electrons in Mn can be delocalized by orbital overlap. This leads to a rather small magnetic moment in KMn6Bi5, as observed in several intermetallic compounds with short Mn−Mn bond length such as Ti4MnBi2.38 Below 150 K, the magnetic susceptibilities for both directions gradually deviate from a modified Curie-Weiss behavior and reach a maximum value at ~ 80 K. The hump in the magnetic susceptibilities indicates that 1D short-range spin correlation first appears before longrange antiferromagnetic order is established in KMn6Bi5.39 Magnetic susceptibilities of KMn6Bi5 for both directions drop rapidly at ~ 75 K, see Figure 6(a) and 6(c), indicating an antiferromagnetic order. The ordering temperature is only around one third of the Curie-Weiss temperature θ, consistent with, but not evident of, geometrical frustration in the Mn-cluster columns of KMn6Bi5.40 The magnetic susceptibility drop ratio (

) between 75 K and 60 K for parallel and

perpendicular to the column direction is 22% and 45%, respectively. The larger drop in χ⊥ indicates that the Mn magnetic moments in the ordered state are mainly perpendicular to the b axis. The detailed magnetic structure of KMn6Bi5 should be further investigated by neutron diffraction. A broad maximum at ~ 40 K was observed in χ|| shown in the inset of Figure 6(a) while there is almost no detectable anomaly for χ⊥ in this temperature range. This hump is repeatable in χ|| data from several different samples shown in the Supporting Information, indicating that it is probably intrinsic. Because of the absence of obvious anomaly at this temperature range in the specific heat of KMn6Bi5 (shown below), the magnetic susceptibility anomaly may arise from rearrangements or rotations of Mn spins because of different magnetic interactions between the various types of Mn atoms. It may also be related to an incoherent-tocoherent crossover of the inter-column tunneling of electrons, which also happens in the same temperature range shown below.



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The magnetizations for both directions increase linearly with magnetic field going from 2 K to 300 K, see Figure 6(b) and 6(d). This is consistent with the paramagnetic state at high temperatures and an antiferromagnetic order at low temperatures with no ferromagnetic impurities. The small upturn in χ|| at low temperatures is probably from paramagnetic impurities. The drop of χ⊥ is very sharp at the transition and χ⊥ is almost temperature-independent at low temperatures. This behavior is different from the case of local magnetic moments with antiferromagnetic

exchange

interaction

generally

observed

in

insulators.41

The

antiferromagnetism in KMn6Bi5 is likely an itinerant spin-density wave. Specific Heat To confirm the antiferromagnetic phase transition at ~ 75 K and further investigate its thermodynamic properties, we performed temperature-dependent specific heat measurements on samples of KMn6Bi5. The sharp λ-shaped peak observed at ~ 75 K confirms the magnetic phase transition in KMn6Bi5, see Figure 7(a). The specific heat jump is very high ∆C ≈ 180 J K−1 mol−1 and there is no thermal hysteresis in the specific heat data between warming and cooling runs, see the inset of Figure 7(a), suggesting a second order phase transition. The specific heat data at ~ 40 K are smooth and have no detectable anomaly, supporting the notion that the anomaly observed in χ|| is probably not a phase transition. The specific heat below 4 K can be well described by the equation = + shown in the inset of Figure 7(b), where represents electron specific heat and comes from phonon and antiferromagnetic spin wave contribution42. The fit gives = 6.5(2) mJ K−2(molMn)−1 (39(1) mJ K−2(mol-formula)−1) and =0.0155(1) J K−4 mol−1. The electron specific heat coefficient in KMn6Bi5 is smaller than the case in α-Mn ( = 12.8 mJ K−2(mol-Mn)−1).43 The specific heat data of KMn6Bi5 above 150 K slightly exceed the Dulong-Petit limit44 even when the electron contribution is considered, see Figure 7(a). This is attributed to the contribution of a very thin layer of N grease45 used for protecting the samples. The existence of N grease does not influence the calculation of γ since it is an insulator and has no contribution to the electronic specific heat. In order to calculate the magnetic entropy change across the transition, we used a polynomial to describe the electron, phonon and N grease contribution of the total specific heat, see Figure 12 ACS Paragon Plus Environment

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7(a). The magnetic entropy change ∆Smag across the transition is ~ 1.6 J K−1 (mol-Mn)−1, which is

much smaller than the magnetic entropy change (Rln2 = 5.76 J K−1 mol−1) when a spin- magnet

46

enters from a paramagnetic to an ordered state. This result is in accordance with the magnetic susceptibilities shown above, which show that a large portion of magnetic entropy is lost when KMn6Bi5 falls into a short-range spin correlation regime.

Anisotropic resistivity Figure 8 shows temperature-dependent anisotropic resistivities ρ|| and ρ⊥ of KMn6Bi5 with the currents applied parallel and perpendicular to the column direction, respectively. The schematic arrangements for those two measurements are shown in the insets of Figure 8(a) and 8(c). The magnetic fields are applied perpendicular to the currents. The resistivity of KMn6Bi5 along the column direction ρ|| saturates in the high temperature regime where the mean free path of electrons is comparable to the interatomic distance, reaching the so-called Mott-Ioffe-Regel (MIR) limit, see Figure 8(a).47-48 Mott-Ioffe-Regel limit is the behavior where the resistivity of a normal metal saturates at high temperatures.47-48 In normal metals, the resistivity increases linearly with temperatures in the high temperature range where electron-phonon scattering dominates. In the quasiparticle framework, electron mean free path l becomes smaller when the temperature increases and electron-phonon scattering becomes stronger. That means that the average distance a quasiparticle travels between two collisions becomes smaller with increasing temperature. However, the electron mean free path l should not become shorter than the inter-atomic distance a. As a result, there is a maximum resistivity called the Mott-Ioffe-Regel limit in a normal metal. The room temperature resistivity ρ|| of KMn6Bi5 is ~ 0.21 mΩ⋅cm, which is a typical experimental value where the MIR criterion takes effect.49-51 Assuming that all the 3d electrons of Mn contribute to the conductivity, we obtain a mean free path l ≈ 9 Å from the measured electron specific heat coefficient γ by using Drude model44, see Supporting Information. The value is close to the interatomic distance (2.3 − 2.6 Å) in the [Mn6Bi5]− nanowire, fulfilling the condition for MIR limit.47-48 The residual resistivity ratio (RRR) ρ||300 K /ρ||2 K for the crystal is ~ 19. The data above 90 K can be well described by the phenomenological model called the parallel-resistor formula52 13 ACS Paragon Plus Environment


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1 1 1 = + ( sat ideal shown in Figure 8(a), where sat is the saturation resistivity, ideal = r + aT is the ideal resistivity at high temperatures. The fit gives r = 0.1445(6) mΩ cm, sat = 0.2451(1) mΩ cm and a = 0.00519(1) mΩ cm/K. A small kink is observed at ~ 75 K where the resistivity drops faster and deviates from the high temperature fitting curve, corresponding to the antiferromagnetic transition discussed above. It can be clearly seen as a peak at ~ 75 K from the derivative of resistivity dρ||/dT, see Figure 8(b). There is almost no magnetoresistance above 16 K for ρ|| with the field µ0H = 8 T, see Figure 8(a). The magnetoresistance

( ( (

reaches 80% at 2 K with a

magnetic field µ0H = 8 T. The temperature dependence of the resistivity perpendicular to the nanowire direction ρ⊥ is very different from ρ||, pointing to an anisotropic electronic behavior, see Figure 8(c). First, ρ⊥ gradually increases with cooling in the high temperature range and then shows an abrupt increase at TN ~ 75 K. This anomaly corresponds to the kink observed in ρ||. With continued cooling ρ⊥ further increases with decreasing temperatures and reaches a maximum value at Tmax ~ 40 K. Finally, below this temperature, ρ⊥ enters a metallic-like state with a positive temperaturedependent coefficient. The hump at ~ 40 K in ρ⊥ indicates a dimensional crossover, which is often observed in low-dimensional materials.53-55 Carriers confined in the [Mn6Bi5]− nanowires are incoherently tunneled between the nanowires at high temperatures, giving thermally activated semiconductor-like behavior. At low temperatures the inter-column tunneling of carriers becomes coherent, forming a threedimensional metallic state.54 The electron mean free path perpendicular to the column, which is calculated by the same method shown above, is ~ 0.7 Å at ~ 40 K where a three-dimensional metallic state forms. This is much smaller than the inter-column distance (~ 11.49 Å) or the Bi−Bi bond length (~ 3.6 Å), suggesting hoping conductivity between the nanowires dominates in the incoherent range. The anomaly of ρ⊥ at ~ 75 K is attributed to the antiferromagnetic phase transition. Applying a magnetic field of µ0H = 8 T has a negligible change in the behavior of ρ⊥ , the phase transition or crossover temperature. The magnetoresistance is ~ 150% at 2 K with a magnetic field µ0H = 8 T. 14 ACS Paragon Plus Environment

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So why does ρ|| decrease in contrast to the sudden increase of ρ⊥ at the antiferromagnetic transition in KMn6Bi5? On the one hand, part of the electronic states at the Fermi level are gapped out due to the antiferromagnetic order, resulting in a reduced carrier density. On the other hand, the magnetic scattering of carriers also decreases when KMn6Bi5 enters from a magnetic disordered state to an antiferromagnetic ordered state. The actual resistivity change of KMn6Bi5 at the phase transition reflect those factors together, just as in the antiferromagnetic phase transition in AeFe2As2 (Ae = Ba, Sr, Ca).56 As for resistivity along the column ρ|| of KMn6Bi5, those two factors nearly compensate with each other, leading to a small kink at the phase transition. The inter-column tunneling of electrons is still incoherent, however, near the antiferromagnetic transition. So the electron hopping process between the columns remains almost unchanged even when KMn6Bi5 forms a three-dimensional antiferromagnetic state. As a result, the carrier density dominates ρ⊥ in the incoherence regime, causing an abrupt increase of

ρ⊥ at the transition. The anisotropy ratio r = ρ⊥/ρ|| for KMn6Bi5 is ~ 3 at room temperature and gradually increases with cooling saturating at ~ 20 at low temperatures, Figure 8(d). There is also a steep rise in the ratio at ~ 75 K, consistent with the phase transition. Crystals from another synthesis batch obtained with different growing conditions, see Supporting Information, exhibit a similar behavior in resistivity. The values of RRR, magnetoresistance and anisotropic ratio r, however, for crystals of different batches are different, due to different qualities of the crystals. At low temperatures the resistivity of KMn6Bi5 for both directions can be fitted to the power law + ! " with the value of α close to 2, indicating electron-electron scattering dominant at low temperatures.57 The above data show that though the anisotropic ratio is not exceptionally high in KMn6Bi5, it does exhibit Q1D transport properties with a phase transition at ~ 75 K. The low anisotropic ratio r may be due to direct Bi−Bi bonding between [Mn6Bi5]− nanowires that could facilitate transport. CONCLUSIONS A new Q1D compound KMn6Bi5 with infinite one-dimensional [Mn6Bi5]− weakly interacting columnar nanowires features an antiferromagnetic order at ~ 75 K. The [Mn6Bi5]− nanowire columns are made of a Bi nanotube outside acting as the cladding and inside a Mn-cluster core 15 ACS Paragon Plus Environment


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column formed by vertex-shared distorted Mn-centered icosahedra of Mn13. The existence of defect-free, parallel [Mn6Bi5]− nanowires in KMn6Bi5 is clearly confirmed by HRTEM and HRSTEM. The Mn−Mn bonding inside the Mn-cluster core column is metallic. The structure of KMn6Bi5 consisting of [Mn6Bi5]− nanowires with Mn-cluster core columns is unique in how the Mn atoms cluster reside inside the Bi tubes. It can be used as a platform to explore Mn-Mn interactions in low-dimensional conditions.” “KMn6Bi5 provides a unique platform to study the link of magnetism, electronic transport and dimensionality in the same material. The compound exhibits a low-dimensional magnetic behavior with a reduced average magnetic moment for each Mn atom in KMn6Bi5. Anisotropic resistivities along the columns and perpendicular to them indicate a Q1D electronic structure in KMn6Bi5. The specific heat measurements on KMn6Bi5 further proves the presence of short-range magnetic correlations above the antiferromagnetic transition temperature. The details of the antiferromagnetic configuration of KMn6Bi5 will need to be further resolved by future neutron diffraction experiments. KMn6Bi5 provides a unique platform to study the link of magnetism, electronic transport and dimensionality in the same material. Considering the small average magnetic moment per Mn atom and probably itinerant nature of antiferromagnetism in KMn6Bi5, the magnetism is not as strong as in other manganese compounds such as BaMn2Bi218 and BaMn2As241. Thus, it is promising to search for possible superconductivity in this system by applying chemical doping or physical pressure to suppress the antiferromagnetic order as in iron-pnictides37 and MnP58. Finally, the fact that KMn6Bi5 is a crystalline pure phase makes it a well-defined source of precise identical nanowires without size dispersity. The synthesis of hollow Bi nanotubes is very difficult and typically done through low-temperature synthetic routes, owing to the relatively low melting point of Bi. In contrast, in the KMn6Bi5 case the Bi nanotubes form by templating around the Mn cluster column despite the high temperatures used.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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Detailed energy dispersive x-ray spectrum analysis, bond lengths, bond angles and anisotropic displacement parameters, powder x-ray diffraction, TEM analysis, electron mean free path calculations, magnetic susceptibility and resistivity data from other crystals. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT Work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work made use of the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Work in China was supported by the National Natural Science Foundation of China (grant number 11674281) and National Key Research and Development Program of China (No. 2017YFA0303002). The authors thank Prof. M. Y. Yuan for his assist in EDS measurements, Dr. F. Han at ANL for his advice in single crystal diffraction and Dr. Q. Tao and H. Bai for their helps in magnetic property measurements. The authors thank Dr. D. Phelan at ANL for his helpful discussions.

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Table 1. Crystal data and structure refinement for KMn6Bi5. Empirical formula

KMn6Bi5

Formula weight Temperature

1413.64 g/mol 293 K 0.71073 Å Monoclinic C2/m

Wavelength (Mo Kα) Crystal system Space group

a = 22.994(2) Å Unit cell dimensions

b = 4.6128(3) Å

Volume

c = 13.3830(13) Å 1168.74(19) Å3

Z Density (calculated) Absorption coefficient F(000) Crystal size

α = 90° β = 124.578(6)° γ = 90°

4 8.034 g/cm3 81.513 mm-1 2336 0.240 × 0.050 × 0.040 mm3 19 ACS Paragon Plus Environment


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Theta range for data collection Index ranges Reflections collected Independent reflections

−34 ≤ h ≤ 34, −6 ≤ k ≤ 6, −19 ≤ l ≤ 19 7043 2207 [Rint = 0.0937]

Completeness to theta = 25.242° Absorption correction Max. and min. transmission Refinement method Data / restraints / parameters Goodness-of-fit on F2

99.7 % Numerical 0.3395 and 0.0112 Full-matrix least-squares on F2 2207 / 0 / 73 0.979

Final Ra indices [I>2σ(I)] R indices (all data) Largest diff. peak and hole

R1 = 0.0396, wR2 = 0.0698 R1 = 0.0588, wR2 = 0.0756

a

3.545 to 31.802°

-3

2.952 and −3.026 e.Å

R = ∑∥Fo| – |Fc∥/∑|Fo|, wR = {∑[w(|Fo|2 – |Fc|2)2]/∑[w(|Fo|4)]}1/2 and w = 1/[σ2(Fo2) + (0.0269P)2], where P =

(Max(Fo2,0)+ 2Fc2)/3.


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Table 2. Atomic coordinates and equivalent isotropic displacement parameters for KMn6Bi5. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq) (Å2)

K

0.8687(2)

0

0.1143(4)

0.040(1)

Mn1 Mn2 Mn3 Mn4 Mn5 Mn6 Bi1 Bi2 Bi3 Bi4 Bi5

0.1877(1) 0.3344(1) 0.6373(1) 0.2371(1) 0.1292(1) 0.2500 0.6331(1) 0.0942(1) 0.0398(1) 0.2771(1) 0.5241(1)

0 0 0 0 0 0.2500 0 0 0 0 0

0.2867(2) 0.4539(2) 0.3185(2) 0.6616(2) 0.4180(2) 0.5000 0.1062(1) 0.5866(1) 0.1631(1) 0.2063(1) 0.3448(1)

0.018(1) 0.018(1) 0.018(1) 0.017(1) 0.018(1) 0.015(1) 0.026(1) 0.025(1) 0.026(1) 0.025(1) 0.025(1)



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Figure captions

Figure 1 Crystal structures of NbSe3, BaFe2S3, XMo3Se3 (X = Na, K, In, Tl), A2Cr3As3 (A = K, Rb, Cs), M4ZTe4 (M = Ta, Z = Si, Cr, Fe, Co, Ni; M = Nb, Z = Si, Fe) and Na2.8Cu5Sn5.6 with transition-metal chains. The number n of transition-metal atoms located in the same plane varies. All structures are viewed along the chain direction.


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Figure 2 (a) Photo of as-grown KMn6Bi5 single crystals on a 1-mm-grid paper under an optical microscope. (b) A scanning electron microscopy image of KMn6Bi5 crystals, showing a strongly fibrous nature. (c) A typical energy dispersive x-ray spectrum collected on a flat clean surface of crystals (shown in inset) showing elemental composition.



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Figure 3 (a) Crystal structure of KMn6Bi5 viewed along the column direction [010]. Only Mn−Mn and Bi−Bi bonds are added to exhibit the [Mn6Bi5]− column structure clearly. Direct inter-column connections via Bi2−Bi2 (3.5687(12) Å) and Bi3−Bi3 (3.6517(13) Å) bonding are present. (b) Side view of the crystal structure perpendicular to the column direction. (c) Mncluster column structure with metallic Mn−Mn bonding. (d) Bi nanotube with Bi−Bi bonding between neighboring Bi5 pentagons extended along the column direction. (e) [Mn6Bi5]− nanowires consisting of one Bi nanotube acting as the cladding which is filled with one Mncluster core column.


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Figure 4 (a) Coordination environment of Mn6: twelve Mn atoms forming a squeezed icosahedron. (b) Coordination environment of Mni (i = 1, 2, 3, 4, 5): four Bi and eight Mn atoms. (c) Coordination environment of K: nine Bi atoms located in three planes.



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Figure 5 (a) Cross-section HRTEM and (b) selected area diffraction of KMn6Bi5 at [010] zone. Some lattice planes and their corresponding distances are marked in (a). (c) A magnified HRTEM at [010] zone and (d) TEM image simulation of a super cell of [010] zone. (e) HAADFHRSTEM showing [Mn6Bi5]− nanowire bundles at [010] zone. A projection of the crystal structure along the [010] direction is inserted in the inset of (d) and (e). Please note that HRTEM (a), diffraction (b) and HAADF-HRSTEM (e) have same direction.


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Figure 6 (a) Temperature-dependent magnetic susceptibility of KMn6Bi5 with the magnetic field H applied parallel to the column direction. The lower inset shows the expanded area at low temperatures. The blue dashed line is the Curie-Weiss fit above 150 K. (b) Isothermal magnetization M versus magnetic fields H applied along the column direction at different temperatures. (c) Temperature-dependent magnetic susceptibility of KMn6Bi5 with the magnetic field H applied perpendicular to the column direction. The blue dashed line is the Curie-Weiss fit above 150 K. (d) Isothermal magnetization M versus magnetic field H applied perpendicular to the column direction at different temperatures.



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Figure 7 (a) Temperature dependence of specific heat of KMn6Bi5. The dashed horizontal line corresponds to the Dulong-Petit limit value 3NR at high temperatures, where N is the number of atoms per formula and R is the ideal gas constant. The green line is the polynomial fitting attributed to electron, phonon and N grease contributions. The inset shows the specific heat data around 75 K under both cooling and warming runs. (b) Magnetic entropy change across the magnetic phase transition. The inset shows a C/T versus T2 plot to extract the electron specific heat coefficient γ. The blue line is the linear fit.


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Figure 8 (a) Temperature dependence of resistivity (ρ||) along the column direction of KMn6Bi5 under magnetic fields µ0H = 0 and 8 T. Because there is almost no magnetoresistance above 60 K for ρ|| at 8 T, the data above 60 K is not shown to make the kink at ~ 75 K more clear. The blue dashed line is the parallel-resistor formula fit. The schematic picture shows the configuration of contacts, current and magnetic field. (b) The derivative of resistivity dρ||/dT along the column direction near 75 K. (c) Temperature dependence of resistivity perpendicular to the column direction ρ⊥ under magnetic fields µ0H = 0 and 8 T. The configuration of the applied current and magnetic field is shown. (d) Temperature dependence of anisotropic resistivity ratio r = ρ⊥/ρ|| under zero field for KMn6Bi5.



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Unique [Mn6Bi5]â nanowires in KMn6Bi5: a quasi-one-dimensional

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