Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic

Nov 29, 2016 - Synopsis. Fe32+δGe33As2 (δ ≤ 0.136) shows a new variant of a rare two-dimensional recurrent intergrowth structure. It is formed by ...
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Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic Behavior of Intermetallic Compound Fe32+δGe33As2 Roman A. Khalaniya,† Andrei V. Mironov,† Valeriy Yu. Verchenko,†,‡ Anton Jesche,§ Alexander A. Tsirlin,§ and Andrei V. Shevelkov*,† †

Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia § Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany ‡

S Supporting Information *

ABSTRACT: A new phase Fe32+δGe33As2 (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe32+δGe33As2 crystallizes in its own structure type (space group P6/mmm, Z = 1, a = 11.919(3) Å, c = 7.558(4) Å) that can be described as a recurrent two-dimensional intergrowth of two intermetallic structure types, MgFe6Ge6 and Co2Al5. Their blocks are represented by infinite columns in the structure. No visible structural changes were observed in the temperature range from 10 to 300 K. At 125 K, Fe32+δGe33As2 undergoes an antiferromagnetic-like transition, while above 150 K it shows a typical Curie−Weiss paramagnetic behavior. Below the transition temperature, a peculiar field-dependent magnetic susceptibility, that shows a significant increase of the susceptibility upon increasing the magnetic field, and a change in transport properties have been observed. Above 140 K, Fe32+δGe33As2 reveals a metallic behavior, in agreement with electronic structure calculation, while below this point the resistivity nonmonotonically increases upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers, and shows a broad maximum around 57 K.

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INTRODUCTION Recurrent or periodic intergrowth structures are widely known among ionic compounds. The principles behind their formation are rather simple: similar periodicities of the initial structures along at least two crystallographic directions, chemical coexistence of structural elements in the resulting structure, and its overall electroneutrality. These structures provide a powerful tool to design new compounds and tailor their properties or to combine properties of several compounds in one material. This approach produced remarkable results in the case of cuprate superconductors;1 in particular, the highest temperature of 133 K of the superconducting transition2 under normal pressure was achieved by varying the ratio and composition of copper-containing perovskite-type layers and rock-salt-type layers between them. The simple approach of building intergrowth structures generally fails in intermetallics, despite the fact that many of the intermetallic compounds have similar structure motifs and same coordination polyhedra. The main reason for this is the prevailing metallic nature of chemical bonding, which renders basic structural blocks much more flexible than in ionic compounds and in most cases leads to a formation of new structures with lower lattice energies instead of the expected © XXXX American Chemical Society

intergrowth structures. Nevertheless, one could expect that a larger difference between electronegativities of the elements, which implies a larger ionic contribution to the chemical bonding, will favor the formation of intergrowth structures. Indeed, many of the compounds with the intergrowth structures are located on the border between intermetallic and ionic compounds such as borides, gallides, silicides, and germanides with a fairly big content of the p-element. NbCoB is an example of a compound of this type.3 Its crystal structure can be described as a combination of two alternating units: the first one appears as a half of the unit cell of the TiNiSi-type structure, and the second one is a commensurable structural unit taken from the ZrNiAl-type structure. Relatively small amounts of large atoms, such as alkaline-earth or rare-earth metals, in the crystal structure can also trigger the formation of intergrowth structures as in the case of the CaCu5-Laves type of intergrowth,4−6 where large atoms are located inside cavities of a tetrahedron-based skeleton formed by atoms of smaller radii. All of the above is mostly related to a conventional linear intergrowth, where 2-dimensional blocks or slabs stack on top Received: October 4, 2016

A

DOI: 10.1021/acs.inorgchem.6b02412 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of each other along the main crystallographic axis. However, specific features of the chemical bonding in intermetallics may favor the formation of unconventional types of intergrowth structures, such as a two-dimensional intergrowth.6 In this paper, we report on a new compound, Fe32+δGe33As2 (δ ≤ 0.136), that crystallizes in its own structure type and represents a new example of two-dimensional intergrowth. We present and discuss its synthesis, crystal growth, crystal and electronic structures, and thermodynamic and transport properties.

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EXPERIMENTAL SECTION Synthesis and Characterization. During our preliminary study of the Fe−Ge side of the ternary As−Fe−Ge system, a new phase with the approximate Fe12Ge12As composition was found. Synthesis of a powder sample of the Fe12Ge12As composition was carried out by the standard ampule technique. Fe powder (Sigma-Aldrich, 99.99%), Ge chips (Sigma-Aldrich, 99.999%), and As polycrystalline lump (Alfa Aesar, 99.999%) were used as starting materials. The elements were mixed in a stoichiometric ratio and placed in quartz ampules, which were evacuated to the residual pressure of 10−3 Torr, then sealed, and annealed at 900 °C for 2 days. After first annealing, specimens were ground and pressed into pellets and then annealed again in sealed and evacuated quartz ampules at 650 °C for 7 days. Phase composition of the Fe12Ge12As powder and the crystal structure of the main phase were investigated by X-ray diffraction using Cu Kα radiation (BRUKER D8 Advance diffractometer, λ = 1.54056, 1.54439 Å). Aside from the new phase, this sample contained a significant amount of Fe-rich phases FeGe and η-Fe1.3Ge. A starting model for the crystal structure of the new phase and its corresponding rough composition Fe32.2Ge33As2 were determined using the Superflip option and the Rietveld method, implemented in the Jana2006 package.7 To obtain single crystals of Fe32+δGe33As2, a powdered Fe12Ge12As sample (0.5 g) and with a freshly recrystallized small (3 × 3 mm) plate-like crystal of iodine (approximately 0.005 g) were annealed for 7 days at 650 °C in an evacuated quartz ampule. Single-crystal X-ray diffraction experiments were performed using a Enraf Nonius CAD-4 diffractometer (Ag Kα radiation, λ = 0.56083 Å). Composition of the selected single crystal was confirmed by EDX spectroscopy, using a JSM JEOL 6490LV scanning electron microscope equipped with INCA xSight EDX detection system and operated at 30 kV. The crystal structure of the single crystal of Fe32+δGe33As2 was solved by SIR20028 and refined using Jana2000.9 First, all corresponding atoms were marked as Ge. The site at (1/2, 1/2, 0) appeared to have extremely high in-plane atomic displacement parameters (ADP), namely, U11 = 0.477(22) and U22 = 0.127(5). A Fourier map (Figure 1), obtained by the maximum entropy method (Dysnomia program10), showed its separation in two symmetrically related positions about 1 Å apart with the occupancy of 1/2. Difference Fourier synthesis revealed a small positive peak at (0, 0, 0.5) lying 2.53−2.59 Å from Ge atoms. It was interpreted as an Fe site with partial occupation. Although it is impossible to distinguish As and Ge in the X-ray experiment, nevertheless, occupancies of different Ge sites were refined. All of them but one showed occupancy variation within 1% with stable ADP. Only for the (0, 0, 0.166) site occupancy variation was more than 2% with large changes of ADP. Considering full occupancy of this site by As, the phase composition appeared to be Fe32.136(12)Ge33As2, which is the same as from the EDX

Figure 1. Fourier map of XY-section of the Fe32+δGe33As2 crystal cell at z = 0.

analysis within standard deviations. So, in the final refinement this atom was considered as As. The crystal data collection and refinement parameters are shown in Table 1. Atomic parameters and selected interatomic distances for Fe32+δGe33As2 single crystal are presented in Tables 2 and 3, respectively. Table 1. Crystallographic and Refinement Parameters for Fe32+δGe33As2 Single Crystal refined composition composition from EDX mol wt cryst syst space group a, Å c, Å V, Å3 Z dcalc, g/cm3 radiation/wavelength temp, K cryst form abs corr θ range range of h, k, l Rint/Rσ total no. of reflns no. of params GoF Δρmax positive/negative, e/Å3 R/Rw [I > 3σ(I)]

Fe32.136(12)Ge33As2 Fe32.0(7)Ge32.6(7)As2.4(4) 4340 g/mol hexagonal P6/mmm (191) 11.919(3) 7.558(4) 929.9(6) 1 7.779 Ag Kα/0.56083 Å 293 needle Gaussian (cryst shape) 2.00−22.00 −6 ≤ h ≤ 0, 0 ≤ k ≤ 15, 0 ≤ l ≤ 10 0.055/0.023 505 48 1.08 1.06/−0.64 0.0168/0.0194

Since the observed deviation from the Fe 32 Ge 33 As 2 composition is small and cannot be properly controlled at the stage of weighing the initial chemicals, this idealized composition was chosen to obtain a single-phase sample. Synthesis of the powder sample of the Fe 32 Ge 33 As 2 composition was carried out by the same technique as the Fe12Ge12As powder sample. Phase composition of the Fe32Ge33As2 powder and the crystal structure of Fe32+δGe33As2 in the temperature range from 10 to 300 K were investigated by X-ray diffraction using synchrotron radiation (ID22 beamline, ESRF, 0.41066 Å). Aside from Fe32+δGe33As2, this sample (Figure 2) also contained a tiny amount of FeGe (relative intensity of the diffraction peaks did not exceed 1%). The crystal structure was refined using the Rietveld method in Jana2006. Crystallographic data obtained from the Fe32Ge33As2 powder are contained in the Supporting Information. Single B

DOI: 10.1021/acs.inorgchem.6b02412 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Atomic Parameters for the Fe32+δGe33As2 Single Crystal atom

site

x

y

z

Uani, Å2

occupancy

As1 Fe1 Fe2 Fe3 Fe4 Fe5 Ge1 Ge2 Ge3 Ge4 Ge5

2e 6m 2c 12n 12o 1b 6k 6m 12o 6j 6l

0 0.21227(5) 1/3 0 0.12318(4) 0 0 0.43188(4) 0.24445(3) 0.27503(7) 0.47506(11)

0 2x 2/3 0.38559(6) 2x 0 0.22901(7) −x 2x 0 −x

0.16573(18) 1/2 0 0.27833(11) 0.23375(10) 1/2 1/2 1/2 0.19685(8) 0 0

0.0108(3) 0.0072(3) 0.0114(5) 0.0102(3) 0.0084(2) 0.010(6) 0.0080(2) 0.0095(2) 0.0161(2) 0.0100(2) 0.0270(8)

1 1 1 1 1 0.136(12) 1 1 1 1 1/2

crystals of Fe32+δGe33As2 for the magnetic measurements were prepared by the aforementioned technique using the Fe32Ge33As2 powder. Electronic Structure Calculations. Crystal structure parameters of Fe32+δGe33As2 at 10 K obtained from the Rietveld refinement of powder X-ray diffraction data were used for electronic structure calculations. The value of δ was set to 0, because such a low occupancy of the Fe5 position should not contribute significantly to the overall DOS and should not hinder its qualitative analysis. Moreover, the Ge5 position was moved to a higher symmetry position 3f (0; 0.5; 0) in order to perform calculations in the P6/mmm space group. Since there are only three Ge5 atoms per formula unit, and the shift of Ge5 has almost no impact on average distances in the crystal structure, we assume that moving Ge5 to the 3f position does not affect the results of calculations. Further, we examined the electronic structure of Fe32+δGe33As2 for six different scenarios, in which two As atoms are present at only one of six possible sites, along with the electronic structure of the hypothetical Fe32+δGe35 compound for a comparison. Calculations were performed within the density functional theory approach, as implemented in the FPLO code (version 14.00-47)11 Local density approximation functional “Perdew Wang 92” was used for the exchange-correlation energy.12 The reciprocal-space integration was performed by an improved

Table 3. Selected Interatomic Distances for the Fe32+δGe33As2 Single Crystal atom

atom

distance, Å

As1

As1 × 1 Fe4 × 6 Fe5 × 1 Fe1 × 2 Fe3 × 2 Fe4 × 4 Fe5 × 1 Fe1 × 2 Fe3 × 4 Fe1 × 1 Fe2 × 1 Fe3 × 2 Fe4 × 1 Fe3 × 2 Fe4 × 4 Fe3 × 4 Ge5 × 1 Fe3 × 4 Fe4 × 1 Fe3 × 1 Fe4 × 2 Fe4 × 2

2.505(3) 2.5944(19) 2.5264(14) 2.437(2) 2.5080(17) 2.438(2) 2.730(2) 2.302(2) 2.5777(17) 2.386(2) 2.3624(15) 2.6071(18) 2.519(2) 2.482(2) 2.4280(18) 2.559(2) 1.030(3) 2.8721(17) 2.726(2) 2.7273(17) 2.7312(13) 2.543(2)

Ge1

Ge2 Ge3

Ge4 Ge5 Fe1 Fe3 Fe4

Figure 2. Powder synchrotron X-ray diffraction pattern for the Fe32 Ge33As2 powder sample. The upper black dotted line represents the experimental diffraction pattern, the upper black ticks denote peaks of hexagonal FeGe, the lower black ticks show peaks of Fe32+δGe33As2, and the lower black line is the difference between the experimental and calculated patterns. C

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Inorganic Chemistry tetrahedron method13 on a grid of 16 × 16 × 16 k-points in the scalar-relativistic calculation. Thermodynamic and Transport Measurements. Magnetization of the Fe32Ge33As2 powder was measured with the VSM setup of the Physical Property Measurement System (PPMS, Quantum Design) in external magnetic fields between 0.1 and 5 T in the temperature range from 2 to 380 K. Magnetization of the Fe32Ge33As2 single crystals was measured using the SQUID (MPMS 3, Quantum Design) in the temperature range from 2 to 300 K in external magnetic fields between 1 and 7 T. Heat capacity measurements were performed with the HC option using a relaxation-type calorimeter (PPMS, Quantum Design) in zero magnetic field between 2 and 150 K and in 5 T field from 100 to 150 K. Thermoelectric properties were measured using the four-probe method with the TTO setup of PPMS (Quantum Design) in the temperature range of 4−400 K in zero magnetic field. Thermal and electrical contacts (gold-plated Pb strips) were fixed on a rectangular shaped pellet with a size of 8 × 3 × 1 mm3 using silver-containing epoxy resin (Epotek H20E) hardened at 100 °C. The pellet was cold pressed from the powder at an external pressure of 100 bar. Density of the obtained pellet was estimated from its linear dimensions and mass to be 74% of the theoretical value.

Figure 3. Polyhedral view of the crystal structure of Fe32+δGe33As2. Atoms from different blocks and their coordination polyhedra are shown in different colors to highlight the intergrowth structure.

Figure 4. Crystal structures of MgFe6Ge6 (a) and Co2Al5 (b).

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RESULTS AND DISCUSSION Synthesis, Crystal Growth, and Homogeneity Range. Fe32+δGe33As2 was obtained from the elements by two-stage synthesis, which included annealing at 900 °C, grinding and pressing into pellets, and second annealing at 650 °C. At the first stage, the initial mixture melts, and an ingot is obtained after cooling. According to the PXRD results, the ingot contains FeGe, FeGe2, η-Fe1.3Ge, and FeAs1−xGex. The second stage yields the desired compound. Single crystals of Fe32+δGe33As2 could be obtained by isothermal recrystallization at 650 °C of a powder with addition of iodine as a transport agent. Crystals of Fe32+δGe33As2 are formed after 1 week of annealing. Fe32+δGe33As2 has a narrow homogeneity range. The value of δ was observed to vary from 0.07 for the powder specimen to 0.136 for the single crystal. The latter does not pertain to the equilibrium conditions in the As−Fe−Ge system, because the synthetic method involved iodine as an additional component. However, the maximum value of δ is not expected to differ notably from 0.07−0.136, since the sample of the Fe12Ge12As composition contained significant amounts of Fe-rich phases, such as FeGe and η-Fe1.3Ge. Crystal Structure. Fe32+δGe33As2 crystallizes in the space group P6/mmm in its own structure type shown in Figure 3. Its crystal cell contains the entire fomula unit and 11 symmetry nonequivalent positions (5 for the Fe atoms, 5 for the Ge atoms and 1 for the As atoms). This makes the regular approach of describing crystal structures somewhat complicated and even inefficient. However, the crystal structure of Fe32+δGe33As2 can be easily described in terms of the crystallographic intergrowth as a recurrent intergrowth of two intermetallic structure types: MgFe6Ge614 (Figure 4a) and Co2Al515 (Figure 4b). The first structure type, MgFe6Ge6, is represented in the crystal structure by separated hexagram-shaped infinite columns (Figure 5a), which are a part of the original Kagome lattice. In these columns, Fe atoms are placed in the form of curved hexagrams, while Ge atoms occupy trigonal prismatic voids between them. This atomic arrangement creates a hexagonal

Figure 5. MgFe6Ge6-type column (a) and Co2Al5-type column (b) in the crystal structure of Fe32+δGe33As2.

tunnel in the center of each column. The tunnel is filled with E2 (E = As, Ge) dumbbells, which alternate with the partially occupied Fe5 site. The dumbells are believed to consist almost entirely of As atoms, since Ge2 dumbbells in MgFe6Ge6-related compounds do not form without significant amounts of the nearby Fe5-like atoms. Such a situation occurs in FeGe16 and Cr0.8Fe5.3Ge5.2Sb0.8.17 An opposite situation is observed when the Sb−Sb dumbbells, the As2 isoelectronic counterpart, are present in the crystal structure, as in Fe3Ge2Sb18 and Co3Ge2Sb.19 In these compounds, the formation of the E−E dumbbells is not accompanied by the presence of atoms in the nearby Fe5-like position. In the crystal structure of Fe32+δGe33As2, the MgFe6Ge6-type columns alternate with the Co2Al5-type columns. Each of them contains about a half of the unit cell of a hypothetical Fe2Ge5 compound (Figure 5b). This kind of a block arrangement forms a two-dimensional intergrowth structure since an alteration of MgFe6Ge6-type and Co2Al5-type blocks occurs in the ab plane. The crystal structure of Fe32+δGe33As2 is fully commensurate, and no signs of any modulations are seen in the powder and single-crystal diffraction patterns. However, the c unit cell parameters of the original MgFe6Ge6 phase (8.045 Å) and of Fe32+δGe33As2 (7.558 Å) do not match, whereas the latter is D

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Figure 6. A comparison between the MgFe6Ge6-type columns in Fe32+δGe33As2 (a) and in MgFe6Ge6 (b).

surpisingly close to the c parameter of Co2Al5 (7.605 Å). A closer look at the MgFe6Ge6-type columns in Fe32+δGe33As2 reveals that they are significantly distorted in comparison to those in MgFe6Ge6 (Figure 6). The columns not only are compressed along the c direction but also became wider as the diameter of these columns is increased by more than 0.4 Å. Moreover, while in MgFe6Ge6 the Fe layers are perfectly flat, in Fe32+δGe33As2 they are bent, resulting in the formation of 4 different Fe−Fe distances (see Figure 6). Ge layers are also noticeably different from each other. Ge atoms in the dumbbell layer are spread far more widely (d(Ge4−Ge4) = 3.278 Å) in Fe32+δGe33As2 than in MgFe6Ge6 (d(Ge−Ge) = 2.925 Å), while the Fe5-containing layer is more compact (d(Ge1−Ge1) = 2.730 Å). This difference comes from the fact that the Ge1 atom is directly bonded through the Fe1 atom to the strained sixmembered Ge2−Fe1 ring, which features one of the shortest known Fe−Ge/As separations of 2.302 Å, and pushes Ge1 away to prevent formation of even shorter distances (see Figure 7). Such short Fe−Ge/As distances are extremely rare and

Co2Al5-type block: Ge3−Fe1 (2.390 Å) and Ge3−Fe2 (2.366 Å), which can be compared to the shortest Fe−Ge distance of 2.380 Å in the Fe−Ge system (cubic FeGe).22 All these facts indicate that the deformation of the MgFe6Ge6-type columns is rooted not only in the size mismatch of the original columns along the c direction but also in the fact that the trigonal arrangement of the undistorted MgFe6Ge6-type columns does not provide the space required for the Co2Al5-type columns in the ab plane. The disparity between the two parent structure blocks is not restricted to the size mismatch. Their space groups P6/mmm (MgFe6Ge6) and P63/mmc (Co2Al5) are mutually exclusive in the case of two-dimensional intergrowth that occurs in Fe32+δGe33As2 because the c parameters of the parent structures are not doubled. However, the structure blocks take the form of columns, and this symmetry mismatch affects the Ge5 site only. This site is located at the joint of two Co2Al5-type columns. Since the columns are essentially the same, this atom can be located in either of them, but not in both columns simultaneously, which is prohibited by the short (around 1 Å) interatomic distance formed in that case. This symmetry mismatch yields the splitting of the Ge5 site. As a consequence, the crystal structure does not lower its symmetry and inherits the P6/mmm space group of MgFe6Ge6. It should be noted that in the HRPXRD experiments no sign of ordering of the Ge5 atoms was observed on cooling down to 10 K, and other kinds of structural symmetry breaking were not observed either. Moreover, from 55 to 300 K the temperature dependence of unit cell parameters was almost linear with expected deviation at low temperature (Figure 8). The partial occupancy of the Ge5 site is not its only peculiarity. Since this site is located at the joint of the two MgFe6Ge6-type columns and two Co2Al5-type columns, its atomic environment is largely distinct from that in MgFe6Ge6 and Co2Al5. It rather resembles the atomic environment of Ge in FeGe2, although bonding to an additional Ge atom is absent (Figures 9a and 9b).23 This incompleteness of the coordination polyhedron could be responsible for a huge atomic displacement factor for the Ge5 site compared to other atoms in the structure. Other atoms at the boundary of the two structural blocks are rather similar to the original structure types. However, some of their coordination polyhedra also resemble those in other compounds of the Fe−Ge system, such as the ηphase24 (8 coordinated Ge1 atom, Figures 9c and 9d) and Fe6Ge5 (3 + 2 coordinated Ge3 atom, Figures 9e and 9f).25

Figure 7. A fragment of the crystal structure of Fe32+δGe33As2 at z = 0.5 at the joint of the MgFe6Ge6-type and Co2Al5-type blocks through the Ge1−Fe1 bond.

mostly observed in the case of partially occupied Fe sites. Only two examples with the fully occupied Fe positions are known in the literature: LaFeGe320 and CeFeGe321 with the Fe−Ge distances of 2.299 and 2.307 Å, respectively. Moreover, the decrease of the outer diameter of the MgFe6Ge6-type columns would result in the shortening of the Ge2−Fe1 distance assuming that the Fe3−Fe3 distance between the columns is constant. Also there are two relatively short distances in the E

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troscopy and neutron diffraction studies for some of these compounds show the absence of Fe atoms at this site.27−29 Taking into account that the typical Fe−Ge distances in the first coordination sphere vary from about 2.4 Å (2.380 Å in cubic FeGe, 2.408 Å in NdFe2Ge2,30 2.399 Å in YFe2Ge230) to about 2.55 Å (2.556 Å in FeGe2), while the shortest distances in the coordination polyhedra of the FeGe- or MgFe6Ge6related compounds start from 2.742 Å (Mg−Ge distance in MgFe6Ge6), we suggest that the size of this cavity is too big for the Fe atom. Even if the size of the cavity in Fe32+δGe33As2 is significantly decreased (see Figure 10), the occupancy of the

Figure 8. Cell volume (black circles) versus temperature. The red line is a guide for the eye.

Figure 10. Comparison between the Fe5 coordination polyhedron in Fe32+δGe33As2 (a) and Mg coordination polyhedron in MgFe6Ge6 (b).

Fe5 site remains relatively small. This is likely due to the presence of nearby As2 dumbbells that would be oversaturated and destabilized when the occupation of the Fe5 site increases. Indeed, the As−As distance in the dumbbell (2.505 Å) is almost equal to that for a single covalent As−As bond (2.517 Å).31 Electronic Structure. Crystal structure parameters of Fe32+δGe33As2 obtained from the Rietveld refinement of the synchrotron X-ray powder diffraction data of Fe32Ge33As2 at 10 K were used for electronic structure calculations. The Ge5 site was moved to a higher symmetry position 3f (0; 0.5; 0) in order to perform calculations in the P6/mmm space group. Spinnonpolarized calculations show high density of states of 103.1 states/eV per formula unit at the Fermi level, indicating the metallic ground state and suggesting the possible magnetic instability in the system. The density of states plot versus energy with respect to the Fermi energy (EF) for Fe32Ge33As2 is shown in Figure 11. The Fermi level crosses the peak of DOS.

Figure 9. Coordination of the Ge5 atom in Fe32+δGe33As2 (a) and of Ge atoms in FeGe2 (b); Ge1 coordination polyhedra in the crystal structure of Fe32+δGe33As2 (c) and a corresponding fragment from the crystal structure of the η-phase (Fe6.5Ge4) (d); two adjacent Ge3 coordination polyhedra in the crystal structure of Fe32+δGe33As2 (e) and a corresponding fragment of the Fe6Ge5 crystal structure (f), with the Fe atoms of the Fe6Ge5 fragment, which are not present in the crystal structure of Fe32+δGe33As2, shown in the pale color.

The Fe5 site, which is located inside a 20-vertex cavity in the MgFe6Ge6-type block, also shows partial occupation. The occupancy factor of Fe5 varies from 0.070(13) for the powder specimen to 0.136(12) for the single crystal. These values do not contradict each other, since the initial stoichiometry of samples and synthetic methods were different. The presence of the Fe atom in this polyhedron was never confirmed before in any FeGe- or MgFe6Ge6-related compounds, and it was proposed only in Mn4Fe3Ge6.26 Moreover, Mossbauer spec-

Figure 11. Calculated electronic density of states versus E − EF for Fe32Ge33As2. F

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ature does not look like a conventional antiferromagnetic one, since the magnetic susceptibility in this temperature region depends heavily on the external field and rises continuously with increasing H. The phase transition is also visible in the temperature dependence of heat capacity both at zero field and at 5 T field (Figure 13) as a kink located at 125 K. Unlike the magnetic susceptibility, heat capacity does not depend on the magnetic field either above the transition temperature or below it.

There is a pseudo gap with approximately 25 states/eV per formula unit at its minimum, which is located slighly higher in energy than EF. An integration from the Fermi level to this minimum shows that the doping of almost 4.7 electrons would be needed to reach the minimum. To check the effect of the Ge/As distribution in the structure, we repeated the electronic structure calculations for other scenarios, in which As atoms were placed on the other five possible sites. Electronic structure calculations for the hypothetical Fe32Ge35 compound were also performed. All these variants produce qualitatively similar electronic structures. The DOS at the Fermi level varies from 74.5 states/eV per formula unit for Fe32Ge35 to 132.0 states/eV per formula unit for Fe32Ge33As2 with 2 As atoms at the Ge3 site. Moreover, for the unsubstituted Fe32Ge35 the addition of seven electrons is required to reach the minimum, which indicates that the DOS changes as in the rigid-band-shift approximation. Magnetic Properties. Temperature dependence of the magnetic susceptibility χ = M/H of nonoriented Fe32+δGe33As2 single crystals was measured in the temperature range of 2−300 K in different magnetic fields from 1 to 7 T (Figure 12). Above

Figure 13. Temperature dependence of the heat capacity of the Fe32+δGe33As2 powder in zero field (red dots) and in applied field μ0H = 5 T (green dots). The transition temperature is marked by a blue vertical line.

The magnetic transition temperature is significantly higher than the absolute value of the Weiss constant, derived in the paramagnetic region, which may indicate a competing nature of magnetic interactions within the structure. Below 60 K, magnetic susceptibility increases again. The lowtemperature magnetic susceptibility is also field-dependent. However, at 25 K this dependency inverts and below this temperature the magnetic susceptibility decreases upon increasing the field. The low-temperature upturn of the magnetic susceptibility appears in the same way in both single crystal and powder data and hence is likely to be an intrinsic property of the compound. Magnetization curve of the Fe32+δGe33As2 powder at 2 K (upper inset in Figure 12) reveals a weak bend around 2 T that is typical for the saturation of paramagnetic impurities. The absolute value of the magnetization, 0.05 μB/Fe at 14 T, is very low and confirms antiferromagnetic nature of the compound. No field-induced effects can be seen, suggesting that field-induced changes in the susceptibility at low temperatures are relatively small (about 5%) and not easily detectable in field-dependent magnetization. While Co2Al5, the close sibling of Fe32+δGe33As2 within the Co2Al5-type, is a Pauli paramagnet and lacks any kind of magnetic ordering,32 MgFe6Ge6-related compounds, such as XFe6Ge6 (where X = Mg, Ti, Yb, Hf, Nb, etc.)29,33−35 and hexagonal FeGe,36 show a somewhat similar magnetic behavior to Fe32+δGe33As2. XFe6Ge6 and FeGe possess simple layered collinear antiferromagnetic structure just below the Neel temperature, with Fe moments directed along the c direction. Upon cooling, these moments depart from the c direction, as observed in FeGe,35 MgFe6Ge6,27 TiFe6Ge6,28 and YbFe6Ge6.34,35 This deviation from an ideal antiferromagnetic structure results in the increase of the magnetic susceptibility at low temperatures. However, unlike Fe32+δGe33As2, these compounds undergo antiferromagnetic transitions with rela-

Figure 12. Temperature dependence of the magnetic susceptibility of Fe32+δGe33As2 single crystals in various applied fields of μ0H = 1−7 T. Upper inset: magnetizaion curves of the Fe32+δGe33As2 powder at 2 K. Lower inset: temperature dependence of the inverse magnetic susceptibility of the Fe32+δGe33As2 powder in 5 T field (blue dots) and extrapolated Curie−Weiss approximation for the paramagnetic region (red line).

150 K, the susceptibility obeys Curie−Weiss law. Due to the small size of the single crystals, another measurement of χ(T) was performed on a larger powder sample (lower inset in Figure 12). Both measurements show identical temperature dependence within the error of the measurement. The absolute value of χ of the single crystals was obtained by scaling to the powder data. An effective magnetic moment of 2.9 μB/Fe and a large antiferromagnetic Weiss temperature of θW = −414 K are obtained. The latter reveals strong antiferromagnetic interactions within the system, which are likely to be a result of predominant Fe−Ge−Fe AFM superexchange interactions. Below 150 K, magnetic susceptibility decreases, and a phase transition is seen at ∼125 K from the divergence of the curves measured in different magnetic fields. Despite the fact that the magnetic susceptibility continues to decrease upon further cooling, the magnetic ordering below the transition temperG

DOI: 10.1021/acs.inorgchem.6b02412 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry tively high Neel temperatures, higher than 400 K.29,33,35 They also feature positive Weiss constant as a result of complementary contributions of ferromagnetic Fe−Fe and antiferromagnentic Fe−Ge−Fe interactions. Moreover, since the crystal structure of Fe32+δGe33As2 departed significantly from the crystal structure of MgFe6Ge6, it remains unclear whether magnetic ordering within the MgFe6Ge6-type columns in Fe32+δGe33As2 resembles that in parent MgFe6Ge6. Further studies involving neutron diffraction experiments are highly desirable in order to determine the magnetic structure of Fe32+δGe33As2. Temperature dependence of heat capacity was measured in zero magnetic field between 2 and 150 K and in 5 T field from 100 to 150 K (Figure 13). A previously mentioned phase transition can be seen at these curves both at zero field and at 5 T field as a kink located at 125 K, which, in addition to the temperature dependence of unit cell parameters (Figure 8), confirms the continuous nature of the phase transition. Unlike the magnetic susceptibility, heat capacity does not depend on the magnetic field either above the transition temperature or below it. Transport Properties. Temperature dependencies of the electrical resistivity and Seebeck coefficient are presented in Figure 14. In the temperature range from 400 to 150 K, the

Fe3−xETe2 (E = Ge, As).37,38 Despite these peculiarities in the temperature dependence of the Seebeck coefficient, its values are still typically low for metals.

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CONCLUSIONS Fe32+δGe33As2 (δ ≤ 0.136) crystallizes in its own structure type that belongs to a rare two-dimensional type of intergrowth. This crystal structure is formed by an alternation of infinite columns of two different structure types, MgFe6Ge6 and Co2Al5. The imperfect size match of these blocks results in a significant deformation of the MgFe6Ge6-type block along with the short Fe−Ge distances in the Co2Al5-type block. The structure is stable and does not undergo any visible changes in the temperature range from 10 to 300 K. From electronic structure calculations, one expects metallic behavior, which is indeed observed down to 150 K, while at lower temperature the resistivity increases upon decreasing temperature. An antiferromagnetic-like transition is observed around 125 K, and a peculiar field-dependent magnetic susceptibility, which is expressed in the significant increase of the susceptibility upon increasing the magnetic field, has been recorded below the transition temperature. The intergrowth nature of the crystal structure results in nontrivial physical properties that await detailed investigation in future studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02412. X-ray crystallographic files for Fe32Ge33As2 powder (CIF) X-ray crystallographic files for Fe32+δGe33As2 single crystal (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Valeriy Yu. Verchenko: 0000-0002-8000-425X Andrei V. Shevelkov: 0000-0002-8316-3280

Figure 14. Temperature dependencies of the electrical resistivity (green dots) and Seebeck coefficient (blue dots) of the Fe32+δGe33As2 pellet.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank ESRF for granting the beam time at ID22 and acknowledge the experimental support by Carlotta Giacobbe during the measurements. The work in Moscow was supported by the Russian Science Foundation, Grant No. 14-13-00089. The work in Tallinn was supported by the Estonian Research Council (Grants PUT210 and TK134). The work in Augsburg was supported by the Federal Ministry for Education and Research under the Sofja Kovalevskaya Award of the Alexander von Humboldt Foundation, and by the German Research Foundation (DFG) under Grant JE 748/1.

compound reveals metallic type of conductivity, with resistivity decreasing almost linearly with temperature upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers. Near 140 K, the resistivity starts to increase upon cooling. At lower temperatures, the resistivity does not match a simple activation behavior and rises nonmonotonically. The proximity of the upturn in resistivity to the magnetic transition temperature indicates that these phenomena are likely related to each other. Temperature dependence of the Seebeck coefficient also changes its slope near the phase transition temperature. The Seebeck coefficient reaches its maximum of 12.5 μV/K at 57 K and rapidly decreases to 3 μV/K at 6.6 K upon further cooling. Such low-temperature behavior is likely caused by a phonon drag, which often occurs in this temperature range. However, since the upturn of the Seebeck coefficient is also correlated with the phase transition, it could be a result of some more complicated interactions similar to those observed for

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DOI: 10.1021/acs.inorgchem.6b02412 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02412 Inorg. Chem. XXXX, XXX, XXX−XXX