Polar Transition-Metal Chalcogenide: Structure and Properties of the

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Polar transition metal chalcogenide: structure and properties of the new pseudo-hollandite Ba0.5Cr5Se8 Robin Lefèvre, David Berthebaud, Olivier Pérez, Denis Pelloquin, Sylvie Hébert, and Franck Gascoin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02933 • Publication Date (Web): 18 Sep 2015 Downloaded from http://pubs.acs.org on September 25, 2015

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Chemistry of Materials

Polar transition metal chalcogenide: structure and properties of the new pseudohollandite Ba0.5Cr5Se8

Robin Lefèvre, David Berthebaud*, Olivier Perez, Denis Pelloquin, Sylvie Hébert, Franck Gascoin Laboratoire CRISMAT, UMR 6508 CNRS/ENSICAEN, 6 bd du Maréchal Juin, F-14050 CAEN Cedex 4, France

ABSTRACT: Single crystals of the new ternary selenide Ba0.5Cr5Se8 were synthesized using a self-flux method in fused silica tubes. This new phase was first considered to crystallize in the usual monoclinic symmetry for pseudo-hollandite compounds. However, single crystal X-ray diffraction study showed unambiguously that Ba0.5Cr5Se8 crystallizes in triclinic space group with cell parameters a = 9.5084(4) Å, b = 7.1788(3) Å and c = the 1 8.9296(4) Å; α = 89.9979(16)°, β = 104.3958(22)° and γ = 100.8869(17)°, Z=2. Bulk samples were prepared by solid-state reaction and sintered using spark-plasma sintering device. A combination of powder X-ray diffraction and transmission electron microscopy was used to perform structural analysis on SPS prepared samples. Ba0.5Cr5Se8 orders antiferromagnetically with TN = 58 K and shows a semiconducting behavior with ρ300K = 0.35 Ω.cm and S300K = 230 µV.K-1. A maximum of the Seebeck coefficient of 315 µV.K-1 occurs at 635 K with ρ635K = 0.14 Ω.cm , while the thermal conductivity remains low and constant at about 0.8 W.m-1.K-1 from room temperature up to 873 K leading to a maximum ZT of 0.12 around 800 K. A remarkable large increase of thermal conductivity is observed in the antiferromagnetic state.

INTRODUCTION 1 ACS Paragon Plus Environment


While the original Zintl concept was rather constricted to few compounds, electronically 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

balanced and solely made of main group metals,1 the family of polar intermetallic compounds now encompasses a great variety of phases including transition metal ions with partially filled d-orbitals.24

With such a variety, evidently come various interesting physical properties2,5 and of particular

interest is the advent of polar compounds in the field of thermoelectric materials. In fact, Zintl phases typically being small band gap semiconductors with complex structure, they have naturally appeared as ideal candidates for thermoelectric materials.6 Indeed, with this renewed interest, efficient thermoelectric Zintl-materials have been recently unveiled, Yb14MnSb11 with a ZT above one at high temperature being the greatest discovery of all.7 Motivated by both the exploratory research of new polar intermetallic compounds and by the systematic characterization of the thermoelectric properties of this family of compounds, we have recently reported the thermoelectric properties of the known pseudo-hollandite TlCr5Se8.8 This compound indeed exhibits a rather complex structure thought responsible for a very low thermal conductivity (about 0.7 W.m-1.K-1 from RT to 800 K) and a large Seebeck peaking at around 300 µV/K at 800 K, thus leading to a ZT of about 0.5 at 800 K. Thus, stimulated by this first investigation and by the numerous combinations of elements that can be accommodated by this structure type, both by the framework and by the channels,9-21 we have embarked on the systematic studies of pseudo-hollandite compounds. In this contribution, we report the synthesis, crystal structure resolution and physical properties of the new compound Ba0.5Cr5Se8. Although crystallizing in the 1 space group, this new compound belongs to the pseudo-hollandite family, the lowering of symmetry (C2/m to 1) being due to the peculiar cationic arrangements observed in Ba0.5Cr5Se8. Transport and magnetic properties of this new compound are discussed and compared to the related pseudo-hollandites compounds. EXPERIMENTAL SECTION In order to prevent oxidation of the reactants and of the products, all manipulations were performed under inert gas or vacuum (glove box or sealed containers). The elements, Ba (rod, 99+%), Se (shots, 99.999%) and Cr (powder -325 Mesh, 99%) all from Alfa Aesar, were used as received. First, 2 ACS Paragon Plus Environment

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attempts to synthesize Ba0.5Cr5Se8 were made by directly loading stoichiometric mixture of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

corresponding elements in, evacuated and flame sealed, fused silica tubes. This method was rapidly abandoned as it appears extremely challenging to obtain the desired phase most likely because of the reaction of elemental barium with the fused silica tube. It was thus chosen to first prepare the binary BaSe and then use it as a precursor for attempting to make the title compound. In order to prevent the adventitious reaction of Ba with the fused silica containers, BaSe was prepared by mechanical alloying using appropriate amounts of the elements (10 g total) loaded in 20 ml tungsten carbide jars and using 7 balls of the same material. The mechanical alloying synthesis was achieved using a Fritsch Pulverisette 7 PL, with a program of 15 cycles of 2 minutes at a speed of 700 rpm. Ba0.5Cr5Se8 was then prepared from appropriate amount of BaSe and elemental Cr and Se. Warning: attempts to make the analogue SrSe and CaSe binaries using the same mechanical alloying process resulted in uncontrolled reactions and damage and/or destruction of the milling jars. Powder synthesis: in order to obtain 5 g of the title phase, precursors were ground, mixed and placed in an alumina boat in a sealed silica tube, and heated to 873 K in 10 hours, temperature at which it remained for two days. The mixture was then slowly cooled to room temperature within 10 hours and a homogeneous dark grey powder was obtained. Crystal synthesis: In order to obtain single crystals, 0.5 g of the prepared powder (see above) was mixed with an excess of elemental selenium (0.15 g), and reheated to 1373 K (at 20 K/hour) for one day and then slowly cooled at 5 K/hour. Obtained crystals were shiny black platelets, with a micrometer-size for their larger dimension. These could be manually selected and extracted from the batch. Powder densification: Powders were compacted using spark plasma sintering (FCT HP D 25/1) in order to produce dense samples for physical property measurements. About 5 g of sample were inserted in high density graphite dies (Carbonloraine) with an inner diameter of 15 mm. The temperature was raised in 45 minutes to 973 K, this temperature plateau lasted 40 minutes before a 45 minutes ramp down to room temperature. The pressure was raised from 28 MPa to 50 MPa 3 ACS Paragon Plus Environment


during the heating step, kept constant over the temperature plateau and release during the cooling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

step. Bulk densities of each sample were determined by the Archimedes method using ethanol as the flooding liquid and were found to be over 95% of the theoretical densities. The samples were stored in air and were checked multiple times using XRD measurements and showed no sign of degradation or oxidation after several weeks. Single-crystal X-Ray diffraction: Single-crystal X-Ray diffraction analyses were performed using Mo-Kα radiation produced with microfocus Incoatec IµS sealed X-ray tube on a KappaCCD (Bruker-Nonius) four circles diffractometer equipped with a 2D dimensional CCD detector. Refinements were done with the following softwares: Apex2,22 Crysalis,23 cell-now,24 twinabs,25 superflip,26 and Jana2006.27 Powder X-ray Diffraction and scanning electron microscopy: The sample quality was checked at each step (after the reaction and densification) by means of X-Ray powder diffraction using X-Pert Pro Panalytical diffractometer using Cu Kα1/Kα2 radiations (λ = 1.540598 Å, 1.544426 Å) and equipped with PIXCEL detector. Rietveld refinements were performed using Jana2006 program27 on X-Ray powder diffraction pattern performed on a D8 Advance diffractometer working with the Cu Kα1 radiation (λ = 1.540598 Å) over the 2θ range of 5-120°. Energy Dispersive X-Ray Spectroscopy analyses were also performed to check the composition using scanning electron microscope (SEM; ZEISS Supra 55, 15 kV). Transmission electron microscopy: In order to test the local chemical homogeneity and possible disorder, the densified samples have been studied by transmission electron microscopy (TEM). Electron diffraction patterns have been recorded with a 2010 JEOL (tilt=+/−60°) working at 200 kV while HREM images have been collected with a TECNAI G2UT30 working at 300 kV (Cs=0.7). Both are equipped with an Energy Dispersive Spectroscopy (EDS) analyzer. Simulated images have been calculated with the JEMS software. Transport property measurements: High temperature measurements were performed from room temperature to 873 K. High temperature resistivity (ρ) and Seebeck coefficient (S) were measured 4 ACS Paragon Plus Environment

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with a ZEM-3 system (Ulvac) by using the four-probe method and differential method respectively 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

on approximately 2.5 × 2.5 × 10 mm3 bars. The measurements were done under a partial pressure of helium and 2 cycles (heating-cooling) were performed. The high temperature thermal diffusivity was measured by laser flash method using a LFA-457 apparatus (Netzsch). Samples size for thermal diffusivity was 6 × 6 × 0.7 mm. The specific heat was determined with Dulong-Petit law (Cp). Finally, the thermal conductivity (κ) was calculated from measured thermal diffusivity, specific heat and bulk density. Low temperature resistivity, Seebeck coefficient and thermal conductivity were measured using a Quantum Design Physical Properties Measurement System. The resistivity was measured from 325 to 10 K, in sweep mode at 1 K/min, below 10 K the sample becomes too resistive to permit the measurement. Seebeck coefficient was measured every 5 K, from 325 to 25 K in settle mode using a home-made sample holder. Electrical contacts were made of indium. Thermal conductivity measurement was performed using the Thermal Transport Option (TTO – Quantum Design). The temperature range was from 325 to 5 K, with a 0.25 K/min rate. Data above 100 K were deleted due to an increase of the radiation effects. All these transport property measurements were performed perpendicularly to the pressing direction. Magnetic susceptibility measurements: The magnetic susceptibility was measured using a SQUID MPMS (Quantum design) on a 25.5 mg densified sample from 2.5 to 325 K in settle mode. Magnetic susceptibilities were recorded as a function of temperature using a zero-field-cooled and field-cooled procedure (B=0.1 T). A magnetization loop has been measured under a field range from -5 to 5 T, at 5 K on the same device on a 8.7 mg sample. RESULTS AND DISCUSSION After the recent report of the promising thermoelectric properties of TlxCr5Se8,8 and with the aim to further investigate the transport properties of related compounds, we have embarked in the preparation of Ba0.5Cr5Se8. First, polycrystalline samples with a nominal composition Ba0.5Cr5Se8 were prepared by solid state reaction and densified by spark plasma sintering (see experimental part 5 ACS Paragon Plus Environment


for further details). At first glance, a powder diffraction analysis of the title compound patterned 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(Figure 1) after the densification evidenced the presence of extra diffraction peaks (marked with stars in the inset of Figure 1).

Figure 1. Le bail fitting plot for Ba0.5Cr5Se8 with the monoclinic symmetry. The inset shows the unindexed reflections marked with blue stars. Plot is represented as follow, observed (black line) and calculated (red dot) intensities. Bragg positions are indicated by green vertical bars

These peaks could not be assigned to any known phase. As a matter of fact, Petricek et al. reported that compounds belonging to the pseudo-hollandite family are known to exhibit various cations arrangement.28 For example the sulfide BaxCr5S8 has been found to crystallize in two different phases were the barium ions form 1D domain structure (x~0.56) with a nine-fold superperiod along the b axis, or 2D superstructure of 2b (x~0.5) with ordered half-occupation of the channels coupled in the c direction (2b, c).28 However, the refinements of the structures have been carried out considering an average structure isotype to TlCr5Se8. It appeared necessary to attempt to synthesize single crystals of Ba0.5Cr5Se8 in order to precisely ascertain the nature of their crystalline structure. Several techniques were used, including the mixture of Ba, Cr and Se at high temperature, which only produced bundle of crystals impossible to sort. However, only the use of a slight excess of elemental selenium during the synthesis led to the formation of crystals suitable (both in terms of size and quality) for further XRD analysis. 6 ACS Paragon Plus Environment

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Structural refinement 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Suitable crystals were thus selected using a Zeiss V20 stereomicroscope. Preliminary X-ray diffraction investigations were performed at room temperature, using Mo Kα radiations produced by a microfocus Incoatec IµS sealed X-ray tube on a Kappa CCD (Bruker-Nonius) diffractometer equipped with an Apex2 CCD detector. Large Ω scans evidenced different crystals of high quality. The Apex2 software22 package was used to index the collected data in a monoclinic symmetry with the following cell parameters: a = 18.6582(7) Å, b = 3.5885(1) Å, c = 8.9212(4) Å and β = 104,604(2)°. These parameters are in agreement with those observed for the isostructural TlCr5Se8 phase.8 Experimental frames were assembled using Crysalis23 and oriented planes of the reciprocal space were produced. (h0l)* and (0kl)* planes are in agreement with the previous monoclinic cell. The (hk0)* plane shown in Figure 2 reveals extinction rules that conflict with the monoclinic system or higher crystalline systems. An accurate observation of this plane and of these extinction rules unveils the occurrence of a pseudo-merohedral twinning, meaning that the real space-group belongs to another crystal system than the observed one. The description of the whole reciprocal space including the unexpected observed extinction rules can be done using the cell parameters a1 ≃ 9.5 Å, b1 ≃ 7.2 Å, c1 ≃ 8.9 Å, α1 ≃ 90°, β1 ≃ 104° and γ1 ≃ 101° and a two-fold axis parallel to b as twin operator (see Figure 2). This assumption was confirmed using the program cell_now.24 Data integration was processed with Apex222 using the matrices corresponding to the two twin domains.

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Figure 2. Reconstruction with the Crysalis software of the (hk0)* plane from single crystal diffraction, reflections of each twinning domains (blue and red) and shared reflections (purple)

Absorption and scale corrections including the twin assumption were performed using the Twinabs program;25 two reflection files were created with hklf4 and hklf5 formats. The hklf4 file containing merged reflections is used for the structure solution, while the hklf5 file including both overlapping reflections and single reflections of the two domains is used for the refinement. The structure is determined with the ”charge flipping” algorithm included in the superflip program.26 A first structural model was obtained and refined using Jana2006,27 a difference Fourier analysis allowed the location of all missing atomic sites. In a second step the refinement was done with the hklf5 data files; the refined twin fraction led to the existence of two domains in a nearly equivalent quantity, i.e 0.4950(13) for domain 1. In order to compare easily with other pseudo-hollandite compounds, the following crystal structure description uses the pseudo-hollandite usual characteristic and the structure will not be standardized regarding origin and coordinates. The crystal structure can be described with one Ba site at the cell origin (on the 1 inversion center), 8 Se and 5 Cr sites in general positions. The refinement of the site occupancy of Ba led to a value of 8 ACS Paragon Plus Environment

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0.9122(6), revealing the existence of ≃ 8.8 % of vacancies randomly distributed over this site. A 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

final Fourier difference calculation evidenced a small residue in the electron density centered around the special position (0,½,0); This position was found to stand in the channels within compatible distances, regarding Se environment, for an atom occupation. Therefore a second Ba position (Ba(2)) was introduced and the refinement of the site occupancy led to a partial occupancy of ≃ 7.7 %. Anisotropic displacement parameter of Ba(2) was fixed equal to Ba(1) one, since Ba(2) is highly minority compared to Ba(1). Thus, according to the refinement, the barium stoichiometry is equal to 0.484, nonetheless, in order to simplify the discussion, the compound will still be written as Ba0.5Cr5Se8. Final agreement factor was equal to 0.0693 for a goodness of fit of 1.51. The refinement led to the following cell parameters a = 9.5084(4), b = 7.1788(3) and c = 8.9296(4) Å; α = 89.9979(16), β = 104.3958(22) and γ = 100.8869(17)°. All information on the Ba0.5Cr5Se8 single crystal analysis is gathered in Table 1. Table 2 reports atomic coordinates and equivalent isotropic displacement parameters and the Table 3 lists inter-atomic distances. Structural description The lowering of symmetry from monoclinic to triclinic doubles atom positions which are not in a special position. All atoms were found in a 2i position, except for the two Ba(1) and Ba(2) positions which are positioned in 1a and 1c, respectively. Chromium atoms are found in five different positions and selenium atoms are located on eight sites. Figure 3 shows a view of the Ba0.5Cr5Se8 structure. Despite the symmetry change, the structure framework remains similar to previously reported pseudo-hollandite compounds. The parameters are related to the usual parameters of pseudo-hollandite, with 2a ≈ a+, ½ b = b+, where a+ and b+ are the monoclinic cell parameters. The γ angle differs also from the 90° of the monoclinic cell. Nevertheless, the following structural description is given taking into consideration the new positions of atoms as defined in the triclinic symmetry. All Cr atoms are coordinated by six selenium atoms to form CrSe6 octahedra. The Cr(1)Se6, Cr(3)Se6 and Cr(5)Se6 octahedra form a 2D hexagonal pattern on ab plane by sharing edges. While, Cr(2)Se6 and Cr(4)Se6 octahedra are arranged in a 1D-along b double chain, with a 9 ACS Paragon Plus Environment


zigzag shape, by sharing edges. The ab octahedron planes are bridged by the octahedron double 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

chain. These two fragments are connected by sharing octahedron faces following the connection between Cr(2)Se6 and Cr(5)Se6; but also between Cr(3)Se6 and Cr(4)Se6. This disposition creates infinite channels running along the crystallographic axis b in which barium atoms are located. These octahedra are formed by Cr-Se bonds, which a length ranging between 2.470(2) and 2.6233(18) Å. These distances are in agreement with those found in other chromium selenides with octahedral Cr such as the pseudo-hollandite TlCr5Se8,8 with dCr-Se = 2.564 Å , Cr2Se3 (dCr-Se = 2.539 Å)29 or AgCrSe2 (dCr-Se = 2.46 Å). 30 Se-Se distances range from 3.4250(12) to 3.7131(12) Å.

Figure 3. Refined crystal structure from single crystal analysis of Ba0.5Cr5Se8, with the triclinic cell represented in red. In-plane and double-chain octahedra CrSe6 share edges, while the connection is made by face-sharing. The bariums are located in the channels formed by the octahedra. In order to compare cell parameters, the black cell represent usual pseudo-hollandite. The γ+ angle of the monoclinic cell should be equal to 90°, but has been set to the γ value for the representation

As described previously, the connection between planes and chains is made by face-sharing. Cr-Cr distances in this connection are dCr(2)-Cr(5) = 3.0750(10) Å and dCr(3)-Cr(4) = 3.0575(9) Å. In-plane CrCr distances are in the range 3.4708(14) Å to 3.4806(14) Å. Shortest in-chain Cr-Cr distances are 10 ACS Paragon Plus Environment

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dCr(2)-Cr(4) = 3.4637(17) Å. Cr-Cr distances are found shorter in the inter-chain connection than inplane or in-chain distances. This observation is in agreement with previous reports on TlCr5Se8.11 The barium lies in the channels and is found in a 10 Se bicapped distorted cube coordination.8 Ba-Se distances range from 3.3936(9) Å to 3.6354(5) Å. Cation-Se distances are similar to those of other thallium, alkaline and alkaline-earth pseudo-hollandites such as TlCr5Se8 (dTl-Se_min = 3.520 Å),11 TlV5Se8 (dTl-Se_min = 3.394 Å),11 Ba0.51Cr5S8 (dBa-S_min = 3.257 Å),28 KCr5Se8 (dK-Se_min = 3.444 Å)9 and RbCr5Se8 (dRb-Se_min = 3.493 Å).15 X-ray powder diffraction study was carried out on a regrind densified sample. At first, a Le Bail refinement has been performed with the triclinic structural model, issued from the single crystal study. It was noticed that unindexed reflections using the monoclinic symmetry were now fully indexed (Figure 4).

Figure 4. Rietveld refinement plot for bulk Ba0.5Cr5Se8 with the triclinic symmetry.

Plot is

represented as follow: observed (black line), calculated (red dot) and difference (blue line) intensities. Bragg positions are indicated by green vertical bars

This is why a Rietveld refinement was carried out on the same structural model. 1 space group was considered, and profile, preferential orientation, cell, atomic parameters were refined. However, the constraints of the two barium positions displacement parameters were kept as it has been discussed earlier for the single crystal study. Refinement trials led to the best following satisfactory reliability 11 ACS Paragon Plus Environment


factors: Rp = 2.41% and RB = 7.11%. However, some isotropic displacement parameters were found 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

to be systematically negative. In order to keep a steady framework of the structure, it was decided to constrain isotropic displacement parameters of each equivalent position in the monoclinic symmetry. This method gave satisfactory results, indicated by the improved reliability factors Rp = 2.17 %, RB = 5.90 %, Rwp = 2.82 % and RF = 4.60 %. Results of the refinement are given in Table 4. After refining 72 parameters, the following lattice parameters were found a = 9.51384(22), b = 7.18935(13) and c = 8.93444(17) Å; α = 89.9400(29), β= 104.3482(12) and γ = 100.9290(42)°. Table 5 reports the refined atomic coordinates, ADPs, and standard deviations. A refinement of the Ba occupancies led to a global occupancy of 0.511. As for the single crystal, Ba(1) position was found have a greater occupancy ratio than the Ba(2) position. Interatomic distances are in agreement with the single crystal structure analysis and are given in supplementary information. To judge the quality of this structural determination, but also to image the twinning effects revealed by the single crystal diffraction analyses, an additional transmission electron microscopy study has been carried out. This approach has also allowed to confirm the chemical homogeneity of Ba0.5Cr5Se8 powder. For this last point, no significant chemical deviation has been detected during the EDS measurements performed on about ten crystallites. The electron diffraction study has confirmed the actual triclinic lattice with existence of twins but the latter are sometimes absent or modulated according to the crystallites as illustrated by the Figures 5a and 5b respectively. The two images have been recorded with a defocus value close to 45nm in which the main barium rows (Ba1 site) are highlighted. We can notice that the simulated image inserted in the figure 5a – and calculated with a thickness ranging from 3 to 5 nm- fits well with the experimental one. More interesting, the twinning mechanism is clearly visible in the Figure 5b (white arrows) and is often disturbed at the nanoscale leading to the diffuse lines parallel to a+ direction on the experimental ED patterns.


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Figure 5. experimental [001] oriented HREM images with corresponding FFT without twinning phenomena (a) and with random nanotwinning effects (b). Simulated image from structural model shown in tables 1 & 2 is inserted in Figure 5a. Magnetic properties The magnetic susceptibility χ of Ba0.5Cr5Se8 is plotted in Figure 6 as a function of temperature.

Figure 6. Temperature dependence of magnetic and reciprocal magnetic susceptibility for Ba0.5Cr5Se8 measured under a magnetic field of 0.1 T. An antiferromagnetic transition is observed at 58 K, where the derivative of the curve is maximum. The straight line corresponds to the CurieWeiss fit. The inset shows the magnetic moment of each chromium between -5 and 5 T at 5 K.

The susceptibility continuously increases from 400 K to 68 K, where an antiferromagnetic transition occurs. Following this transition, χ decreases upon reaching lower temperature.

The Neel

temperature (TN) of the magnetic transition has been defined at the maximum value of the χ 13 ACS Paragon Plus Environment


derivative, and is then TN = 58 K. The Ba0.5Cr5Se8 TN is comparable to the TlCr5Se8 parent 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

compound which was reported to be TN = 55 K by Bensch et al.20 and TN = 52 K by Ohtani et al.9 χ-1 = f(T) has been plotted in the same Figure. Following the same procedure as in Bensch et al.20 a fit of this curve using the Curie-Weiss law ((χ = C/(T – θp), where C is the Curie constant, T the absolute temperature, and θp the Curie-Weiss temperature) from 400 to 140 K is used to extract the effective paramagnetic moment µeff and the Curie-Weiss temperature θp. They are 3.95 µB and -179.9 K respectively. Ba0.5Cr5S8 magnetic properties were studied, and showed TN = 63 K, θp = -650 K and µeff = 4.12 µB. This sulfide phase values are similar to what was found here, but the |θp| is much smaller than in sulfide pseudo-hollandite compound. Bensch et al. reported the evolution of µeff and of θp versus the Tl content x in TlxCr5Se8.20 The stoichiometric compound showed a µeff = 4.14µB. In terms of charge balance, Ba0.5 is equivalent to Tl1, the value of Ba0.5Cr5Se8 µeff is in agreement with what was previously reported for TlCr5Se8. Moreover, this value is also in agreement with the spinonly value of Cr3+ (µeff = 3.87 µB). As shown in the inset of Figure 6, the compound shows a strong antiferromagnetism with very low moments of ~ 4.10-2 µB/Cr at 5 T, which cannot be destabilized by field less than 5 T. Transport properties Transport properties have been performed on polycrystalline dense sample. In low-dimensional materials, the in-plane direction usually shows the best combination of transport properties, thus leading to a higher ZT,31-33 this has also been seen for SPS processed powder.34 Since our material has the potential to show anisotropic properties, as said in the experimental section, all the measurements have been performed in the in-plane direction, e.g. perpendicularly to the pressing direction. The resistivity ρ of this sample is plotted against temperature, as well as the Seebeck coefficient S, in Figure 7. The resistivity decreases from 5 to 873 K, with a typical semiconducting behavior. A slight change in the slope occurs around 635 K. The maximum value for the Seebeck coefficient is around 315 µV.K-1 at 635 K. At higher temperature, the thermal activation of minority carriers induces a reduction of S till 873 K. S decreases from 635 K to 58 K, the magnetic transition, 14 ACS Paragon Plus Environment

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and a change of the sign of the slope is occurring reaching lower temperature. Room temperature 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

value of resistivity is about ρ300K = 0.35 Ω.cm, more than one order of magnitude of the resistivity of TlCr5Se8 (ρ300K =0.02 Ω.cm). Seebeck coefficient S300K = 230 µV.K-1 is almost equal to the S300K = 215 µV.K-1 of TlCr5Se8.8

Figure 7. Temperature dependence of resistivity and Seebeck coefficient for Ba0.5Cr5Se8 from 5 to 873 K.

Figure 8. Temperature dependence of thermal conductivity from 5 to 873 K, the spots correspond to the total thermal conductivity, upside and downside triangle correspond to electronic and lattice contributions respectively In Figure 8 is plotted the total thermal conductivity κ vs temperature, with lattice κlatt and electronic κel contributions from 5 to 873 K (where κ = κel + κlatt and with κel evaluated from the WiedemannFranz law κel = L0T/ρ, where the Lorenz number L0 is 2.4x108 W.Ω.K-2). Low temperature measurement data above 100 K have been deleted due to an increase of the radiation effects. At high T, the thermal conductivity is small, and almost constant from 300 up to 873 K, close to ~ 0.8 W.m15 ACS Paragon Plus Environment

Chemistry of Materials 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

.K-1, a value very close to the one measured in TlxCr5Se8. This confirms that the pseudo-hollandite

structure has an intrinsic low thermal conductivity. The calculated ZT value are reported in Figure 9, the figure of merit increase up to ~835 K to reach a value of 0.12, and ZT value decreases up to 873K. At low T, a huge increase of κ is observed at the antiferromagnetic transition, with a maximum of κ close to 2 W.m-1.K-1 at 25K. The intensity of this κ peak does not change with the magnetic field up to 9 T, this observation confirms the previous statement that Ba0.5Cr5Se8 is strongly antiferromagnetic. Even if TlxCr5Se8 and BaxCr5Se8 exhibit very similar magnetic susceptibilities, the impact of the antiferromagnetic transition on electronic and thermal transport is very different. In TlxCr5Se8, a peak of ρ and of S was observed at TN,8 while κ was not affected, whereas, in BaxCr5Se8, κ is strongly enhanced, while ρ and S do not show any clear accident (except for a minimum of S at TN). The lattice part (phonon spectra) must thus be impacted by the magnetic transition in BaxCr5Se8, contrary to the case of TlxCr5Se8. In order to better understand the impact of the magnetic transition on transport, a neutron diffraction study will be performed to determine the magnetic structures.

Figure 9. Temperature dependence of the figure of merit ZT from 300 to 873 K.

CONCLUSIONS

The new compound Ba0.5Cr5Se8 has been evidenced. Its structure has been determined by means of X-ray single crystal diffraction. Densified bulk sample was successfully synthesized and its 16 ACS Paragon Plus Environment

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structural analyses were done by means of X-ray powder diffraction and transmission electron 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

microscopy. The structure framework remains similar to pseudo-hollandite networks, despite the structure being refined in a triclinic cell, with γ=100.8869(17)°. Moreover, the unit cell can be describe as ½ a+, 2 b+ and 1 c+ where a+, b+ and c+ are parameters from the usual pseudo-hollandite. The powder, after being sintered, was measured in terms of magnetic and transport properties. The phase is antiferromagnetic with TN = 58 K. Similarly to TlCr5Se8, as expected from the low dimensional framework, a small thermal conductivity has been measured which remains constant at 0.8 W.m-1.K-1 in the high temperature region. A sharp peak in the thermal conductivity occurring at the magnetic transition is observed. A final ZT of 0.12 at 835 K is determined, lower than for TlCr5Se8 due to a higher resistivity of Ba0.5Cr5Se8.

AUTHOR INFORMATION Corresponding Author * David BERTHEBAUD, [email protected]

ACKNOWLEDGEMENTS Authors would like to thank Fabien Veillon for magnetic measurements, Xavier Larose for assistance in transmission electron techniques, Stéphanie Gascoin for support concerning the powder x-ray diffraction and Guillaume Renouf for technical help regarding the syntheses.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1. Crystallographic data for the single crystal Ba0.5Cr5Se8 Physical, crystallographic, and analytical data Formula

BaxCr5Se8, x = 0.484

Molecular weight (g.mol-1)

958.09

Crystal system

Triclinic

Space group

1 (No.2)

Cell parameters a (Å)

9.5084(4)

α (°)

89.9979(16)

b (Å)

7.1788(3)

β (°)

104.3958(22)

c (Å)

8.9296(4)

γ (°)

100.8869(17)

V (Å3)

579.05(4)

Z=2

Calc. density (g.cm-3)

5.4952

Twin fraction

0.4951(13) / 0.5049(13)

Data collection Temperature (K)

293

Diffractometer

Kappa CCD (Bruker-Nonius)

Radiation

MoKα (0.71069 Å)

Crystal color

Black

Crystal description

Platelets

Crystal size (mm3)

0.628 × 0.054 × 0.028

Linear absorption coeff. (cm-1)

312.41

Scan mode

ωφ


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recording range 2θ (°)

4.5° < 2θ < 72.22°

hkl range

-15 ≤ h ≤ 15

N° of measured reflections

9287

-11 ≤ k ≤ 11

Data reduction N° of independent reflections

6979

Rint (%)

5.55

Absorption correction

Numerical method (SADABS)

Transmission coeff.

0.228717 - 0.437962

Independent reflections with I > 3.0 σ(I)

4061

Refinement R (%)

9.91

RWP (%)

6.93

GOF

1.51

No° of refined parameters

128

Difference Fourier residues (e-.Å-3)

[ - 5.4 , + 4.4]


0 ≤ l ≤ 14


Page 20 of 30

Table 2. Refined coordinates, atomic displacement parameters (ADPSs), and their estimated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

standard deviations for single crystal Ba0.5Cr5Se8. Wyckoff Atom

Occ

x

y

z

Uiso

Ba(1) 1a

0.890(2)

0

0

0

0.01953(16)a

Ba(2) 1c

0.0774(14)

0

0.5

0

0.01953(16)a

Cr(1) 2i

1

0.99995(5)

0.75023(7)

0.50006(6)

0.0085(3)

Cr(2) 2i

1

0.59178(7)

0.3940(3)

0.16427(8)

0.0078(3)

Cr(3) 2i

1

0.68628(7)

-0.0788(3)

0.51794(8)

0.0085(2)

Cr(4) 2i

1

0.41144(7)

0.1054(3)

-0.16426(7)

0.0080(2)

Cr(5) 2i

1

0.68645(7)

0.4221(3)

0.52009(8)

0.0084(2)

Se(1) 2i

1

0.85265(4)

0.9625(2)

0.33673(5)

0.00809(18)

Se(2) 2i

1

0.47849(4)

0.3721(2)

0.65786(5)

0.00748(17)

Se(3) 2i

1

0.66604(4)

0.1663(2)

0.00592(5)

0.00848(18)

Se(4) 2i

1

0.83082(4)

0.7089(2)

0.68111(5)

0.00898(17)

Se(5) 2i

1

0.85197(4)

0.4625(2)

0.33938(5)

0.00824(18)

Se(6) 2i

1

0.47883(4)

-0.1298(2)

0.65658(5)

0.00759(17)

Se(7) 2i

1

0.33606(4)

0.3319(2)

-0.00558(5)

0.00843(18)

Se(8) 2i

1

0.83051(4)

0.2078(2)

0.68065(5)

0.00891(17)

Atom U11

U22

U33

U12

U13

U23

Ba(1) 0.0206(3)

0.0247(3)

0.0110(2)

0.0048(2)

-0.00041(17)

-0.0004(2)

Ba(2) 0.0206(3)

0.0247(3)

0.0110(2)

0.0048(2)

-0.00041(17)

-0.0004(2)

Position


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr(1) 0.0086(4)

0.0092(6)

0.0076(4)

0.0019(12)

0.0019(3)

0.0005(13)

Cr(2) 0.0078(3)

0.0072(4)

0.0078(3)

0.0000(8)

0.0020(3)

-0.0001(9)

Cr(3) 0.0080(3)

0.0085(4)

0.0092(3)

0.0006(8)

0.0031(3)

-0.0013(10)

Cr(4) 0.0079(3)

0.0074(4)

0.0079(3)

-0.0006(8)

0.0019(3)

0.0000(9)

Cr(5) 0.0076(3)

0.0085(4)

0.0090(3)

0.0005(8)

0.0027(3)

-0.0014(10)

Se(1) 0.0063(2)

0.0089(3)

0.0088(2)

0.0021(6)

0.00105(16)

0.0010(7)

Se(2) 0.0075(2)

0.0078(3)

0.0082(2)

0.0021(6)

0.00341(16)

0.0018(7)

Se(3) 0.0082(2)

0.0098(3)

0.0080(2)

0.0031(6)

0.00222(17)

0.0002(7)

Se(4) 0.0083(2)

0.0096(3)

0.0087(2)

0.0008(6)

0.00221(16)

-0.0012(7)

Se(5) 0.0064(2)

0.0091(3)

0.0088(2)

0.0020(6)

0.00082(16)

0.0009(7)

Se(6) 0.0074(2)

0.0080(3)

0.0083(2)

0.0020(6)

0.00349(16)

0.0017(7)

Se(7) 0.0083(2)

0.0098(3)

0.0077(2)

0.0029(6)

0.00200(17)

0.0002(7)

Se(8) 0.0083(2)

0.0098(3)

0.0086(2)

0.0016(6)

0.00224(16)

-0.0003(7)

a. fixed as equal



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

Table 3. Interatomic distances and their standard deviation in Ba0.5Cr5Se8 given by the single crystal study. Crystal At 1

At 2

Crystal At 1

data

Tunnelled Ba-Se

At 2 data

Ba(1) Se(1) 3.6095(5)

Ba(2) Se(3) 3.6103(8)

Se(3) 3.6094(8)

Se(4) 3.4027(9)

Se(4) 3.3936(9)

Se(5) 3.6354(5)

Se(7) 3.6183(8)

Se(7) 3.6340(8)

Se(8) 3.4025(9)

Se(8) 3.4011(9)

Cr(1) Se(1) 2.4905(12) Cr(2) Se(2) 2.5962(19) Se(1) 2.4966(12)

Se(3) 2.470(2)

Se(4) 2.5299(7)

Se(5) 2.5347(7)

Se(5) 2.4909(12)

Se(6) 2.5809(19)

Se(5) 2.4855(12)

Se(7) 2.4836(7)

Se(8) 2.5305(7)

Se(7) 2.515(2)

In-plane and in-chain Cr-Se

Cr(3) Se(1) 2.5109(9)

Cr(3) Se(6) 2.5493(9)

Se(2) 2.6233(18)

Se(6) 2.6091(18)

Se(4) 2.4758(19)

Se(8) 2.4838(19)

Cr(1) Cr(1) 3.5929(8)

Cr(2) Cr(4) 3.580(3)

In-plane and In-chain

Cr(1) 3.5859(8)

Cr(4) 3.599(3)

Cr-Cr

Cr(3) 3.4708(14) Cr(3) Cr(3) 3.8577(17) Cr(3) 3.4743(13)


Cr(5) 3.583(3)

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr(5) 3.4755(13)

Cr(5) 3.596(3)

Cr(5) 3.4806(14)

Cr(5) 3.8538(16)

Cr(2) Cr(2) 3.5248(18) Cr(4) Cr(4) 3.4982(18) Cr(4) 3.4637(17) Bridging connexion Cr-Cr

Cr(2) Cr(5) 3.0750(10) Cr(3) Cr(4) 3.0575(9) Se(1) Se(1) 3.4658(6)

Principal Short Se-Se

Se(4) Se(5) 3.5100(12)

Se(2) Se(2) 3.4321(12) Se(5) Se(5) 3.4431(6) Se(3) Se(3) 3.5594(11) Se(6) Se(6) 3.4250(12)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

Table 4. Details on the structure refinement of bulk Ba0.5Cr5Se8 Compound

Ba0.5Cr5Se8

Sample refined composition

Ba0.511Cr5Se8

Diffractometer radiation

CuKα, 1.5406 Å

2θ range (deg.), number of points

5-120°, 10455

Step size (deg.), counting time (sec.)

0.011, 0.061

Background function

9 term Legendre polynom

Profile function

Pseudo-Voigt

Preferential orientation correction

March & Dollase

Number of refined parameters

72

Unit-cell parameters (Å,°)

a

9.51384(22)

α

89.9400(29)

b

7.18935(13)

β

104.3482(12)

c

8.93444(17)

γ

100.9290(42)

Cell Volume (Å3)

580.62(2)

Reliability factors (obs / all)

RF (%)

4.49 / 4.60

RB (%)

5.84 / 5.90

Rp (%)

2.17

Rwp (%)

2.82

χ²

1.14


Page 25 of 30


Table 5. Atomic coordinates (x,y,z), atoms occupancy, isotropic displacement parameters (A²) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and their estimated standard deviations found with the Rietveld refinement of bulk Ba0.5Cr5Se8 Atom

s.o.f

x

y

z

Uiso

Ba1

0.886(9)

0

0

0

0.025(4) a

Ba2

0.136(8)

0

0.5

0

0.025(4) a

Cr1

-

0.996(2)

0.749(3)

0.509(2)

0.005(4)

Cr2

-

0.5894(18)

0.397(5)

0.1543(17)

0.010(2) b

Cr3

-

0.684(2)

-0.083 (6)

0.5283(19)

0.013(3) c

Cr4

-

0.407(2)

0.095(5)

-0.1768(18)

0.010(2) b

Cr5

-

0.686(2)

0.419(6)

0.515(2)

0.013(3) c

Se1

-

0.8524(15)

0.969(4)

0.3325(13)

0.006(2) d

Se2

-

0.4830(14)

0.375(4)

0.6544(13)

0.0047(19) e

Se3

-

0.6633(13)

0.169(4)

-0.0009(14)

0.0086(17) f

Se4

-

0.8293(15)

0.711(3)

0.6795(14)

0.0072(19) g

Se5

-

0.8530(15)

0.463(4)

0.3467(13)

0.006(2) d

Se6

-

0.4770(14)

-0.130(4)

0.6586(12)

0.0047(19) e

Se7

-

0.3319(13)

0.328(4)

-0.0116(12)

0.0086(17) f

Se8

-

0.8315(16)

0.201(3)

0.6833(13)

0.0072(19) g

a, b, c… fixed as equal.



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Polar Transition-Metal Chalcogenide: Structure and Properties of the

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