Effect of Transition Metal Substitution on the Structure and Properties of a Clathrate-Like Compound Eu7Cu44As23

A series of substitutional solid solutions—Eu7Cu44−xTxAs23 (T = Fe, Co, Ni)—based on a recently discovered clathrate-like compound (Eu7Cu44As23) were synthesized from the elements at 800 °C. Almost up to 50% of Cu can be substituted by Ni, resulting in a linear decrease of the cubic unit cell parameter from a = 16.6707(1) Å for the ternary compound to a = 16.3719(1) Å for the sample with the nominal composition Eu7Cu24Ni20As23. In contrast, Co and Fe can only substitute less than 20% of Cu. Crystal structures of six samples of different composition were refined from powder diffraction data. Despite very small differences in scattering powers of Cu, Ni, Co, and Fe, we were able to propose a reasonable model of dopant distribution over copper sites based on the trends in interatomic distances as well as on Mössbauer spectra for the iron-substituted compound Eu7Cu36Fe8As23. Ni doping increases the Curie temperature to 25 K with respect to the parent compound, which is ferromagnetically ordered below 17.5 K, whereas Fe doping suppresses the ferromagnetic ordering in the Eu sublattice.


Introduction
Clathrates belong to a peculiar class of inclusion compounds with a complete segregation of guests inside large polyhedral cages forming a framework. More than 250 inorganic/intermetallic clathrate compounds are known. They are grouped into 10 structure types and include over 40 chemical elements as constituents of host and guest substructures [1]. Despite such a structural and chemical variety, the nature of guests is typically limited to alkali and alkali-earth metals for anionic clathrates, and to halogens and chalcogens for inverse clathrates. However, there are several compounds that feature rare-earth elements Ce, Pr, or Eu. These examples are relatively scarce and feature type-I and type-VIII clathrates only. Nevertheless, the presence of a rare-earth cation gives rise to various interesting properties, including the enhancement of thermopower [2] and the formation of magnetic order [3,4] that can trigger a giant magnetocaloric effect [5,6].
A combination of europium, copper, and a group 15 metals gives rise to a broad family of ternary compounds with a great variety of crystal structures and properties. The structures range from pseudo-layered-related to the types known for Fe-As based superconductors [7,8]-to truly threedimensional structures, in which europium occupies large voids and displays high coordination numbers. The vast majority of these compounds contain divalent europium, with the 4f 7 ground state configuration giving rise to strong paramagnetic response and, eventually, magnetic ordering. It is worth noting that the pseudo-layered compounds typically feature antiferromagnetic (AFM) ordering [7], whereas ferromagnetic (FM) ordering is rare-Eu2Cu6P5 and EuCu4P3 being the only examples [9]. On the contrary, compounds with 3D structures frequently display FM ordering, as in the clathrate compound EuxBa8-xCu16P30, exhibiting a superstructure of the type-I clathrate [10], and in the recently discovered clathrate-like compound Eu7Cu44As23 [11]. Whereas the former is a typical clathrate compound whose structure and properties can be rationalized using the Zintl-Klemm approach, the latter phase is an unbalanced metallic compound. Its resemblance to clathrates is ensured by a high coordination number of the Eu 2+ cations. In its crystal structure (Figure 1), two types of Eu 2+ cations alternate, one of which resides in a cubic environment of eight arsenic atoms, whereas the other one sits in the center of a 20-vertex polyhedral void. Low-temperature thermodynamic measurements revealed ferromagnetic behavior of Eu7Cu44As23 below 17.5 K, owing to the interaction between the localized 4f 7 Eu 2+ cations, presumably through the conducting Cu-As framework. Given the electronic imbalance and the corresponding metallic behavior of Eu7Cu44As23, we considered that extended solid solutions could be formed by substituting Cu with its neighboring 3delements possessing lesser number of valence electrons than copper. In this paper, we present synthesis and the investigation of solid solutions formed by substituting Fe, Co, or Ni for copper in Eu7Cu44As23, and discuss the influence of such substitutions on structural and magnetic properties. Given the electronic imbalance and the corresponding metallic behavior of Eu 7 Cu 44 As 23 , we considered that extended solid solutions could be formed by substituting Cu with its neighboring 3d-elements possessing lesser number of valence electrons than copper. In this paper, we present synthesis and the investigation of solid solutions formed by substituting Fe, Co, or Ni for copper in Eu 7 Cu 44 As 23 , and discuss the influence of such substitutions on structural and magnetic properties.

Synthesis and Homogeneity Ranges
An optimal synthetic procedure for the solid solutions is almost the same as for the parent compound. The only difference is that we had to increase the annealing time by 48 h in order to reach equilibrium. The largest homogeneity range was observed for T = Ni, with an almost linear ( Figure 2) decrease of the unit cell parameter from a = 16.6707(2) Å [11] for the undoped phase to a = 16.3719(1) Å for the sample with x = 20 (the sample with the nominal composition Eu 7 Cu 24 Ni 20 As 23 contains up to 5% EuNi 5 As 3 ). The homogeneity range for Co and Fe were found to be narrower, with the substitution limit of x = 8.

Synthesis and Homogeneity Ranges
An optimal synthetic procedure for the solid solutions is almost the same as for the parent compound. The only difference is that we had to increase the annealing time by 48 h in order to reach equilibrium. The largest homogeneity range was observed for T = Ni, with an almost linear ( Figure 2) decrease of the unit cell parameter from a = 16.6707(2) Å [11] for the undoped phase to a = 16.3719(1) Å for the sample with x = 20 (the sample with the nominal composition Eu7Cu24Ni20As23 contains up to 5% EuNi5As3). The homogeneity range for Co and Fe were found to be narrower, with the substitution limit of x = 8.

Crystal Structure Refinement and Description
Refinement of powder XRD samples with the crystal structure of Eu7Cu44As23 as a starting model resulted in low residuals for all samples (Tables 1 and 2). This fact indicates that no significant structure distortion occurs during the substitution. In all cases, we observe a gradual decrease of the cubic cell parameter with almost the same increment (about 0.1% per atom) for T = Co and Ni, while for T = Fe, the increment is smaller, about 0.04% per atom.

Crystal Structure Refinement and Description
Refinement of powder XRD samples with the crystal structure of Eu 7 Cu 44 As 23 as a starting model resulted in low residuals for all samples (Tables 1 and 2). This fact indicates that no significant structure distortion occurs during the substitution. In all cases, we observe a gradual decrease of the cubic cell parameter with almost the same increment (about 0.1% per atom) for T = Co and Ni, while for T = Fe, the increment is smaller, about 0.04% per atom.

T = Fe, x = 8
Atom A general view of the crystal structure of Eu 7 Cu 44´x T x As 23 is shown in Figure 1. The 3D framework built of As and T features large voids occupied by Eu, having a 20-fold coordination composed of eight As + 12T atoms ( Figure 3a), with the distances to the neighbors varying from 3.13 to 3.54 Å. The remaining Eu atoms occupy smaller cubic voids formed solely by the As atoms. Within the framework, the T and As atoms occupy three and four crystallographic sites, respectively. The coordination of the framework atoms is quite different; importantly, there are no As-As bonds, which suggests that all As atoms can be considered as As´3 anions. Additionally, each of three independent T atoms has four As neighbors forming slightly distorted tetrahedra, and the coordination is supplemented by five, six, or seven T atoms (Figure 4b-d). In general, the atomic arrangement resembles that found in the crystal structure of BaHg 11 [13], with the doubling of a cubic unit cell parameter, a(Eu 7 Cu 44´x T x As 23 ) « 2a(BaHg 11 ).
A general view of the crystal structure of Eu7Cu44-xTxAs23 is shown in Figure 1. The 3D framework built of As and T features large voids occupied by Eu, having a 20-fold coordination composed of eight As + 12T atoms ( Figure 3a), with the distances to the neighbors varying from 3.13 to 3.54 Å. The remaining Eu atoms occupy smaller cubic voids formed solely by the As atoms. Within the framework, the T and As atoms occupy three and four crystallographic sites, respectively. The coordination of the framework atoms is quite different; importantly, there are no As-As bonds, which suggests that all As atoms can be considered as As −3 anions. Additionally, each of three independent T atoms has four As neighbors forming slightly distorted tetrahedra, and the coordination is supplemented by five, six, or seven T atoms (Figure 4b-d). In general, the atomic arrangement resembles that found in the crystal structure of BaHg11 [13], with the doubling of a cubic unit cell parameter, a(Eu7Cu44-xTxAs23) ≈ 2a(BaHg11).

Mössbauer Spectra
Our X-ray diffraction data did not allow us to distinguish between Cu and the substituting T element. Therefore, we used other methods in order to shed more light onto the distribution of the T atoms. To this end, we chose the sample with the Eu7Cu36Fe8As23 composition and performed the 57 Fe-Mössbauer study at low temperatures. The experimental spectrum presented in Figure 4 consists of a single narrow quadrupole doublet with the hyperfine parameters listed in Table 3. We note that these parameters are not sensitive to temperature in the entire investigated range. The isomer shift of 0.63-0.64 mm·s −1 and quadrupole splitting of 0.17-0.18 mm·s −1 are characteristic of high-spin Fe 3+ cations in a symmetric coordination environment with a high coordination number. We note, however, that the observed value of the isomer shift is higher than those reported for other compounds exhibiting numerous Fe-Fe bonds. For instance, the isomer shift of 0.30-0.43 mm·s −1 was found for Fe3GeTe2 [14], where the coordination number of iron ranges from eight to ten, including up to four Fe-Fe bonds. We believe that the difference might arise from shorter Fe-As bonds in our compound compared to Fe-Ge and Fe-Te bonds in Fe3GeTe2. Table 3. Hyperfine parameters of the 57 Fe Mössbauer spectra of Eu7Cu36Fe8As23 at different temperatures; δ is the isomer shift, Δ is the quadrupole splitting, and W is the linewidth. (7) 0.29 (2) The constant hyperfine parameters in the temperature range of 14.7-41.1 K rule out the possibility of electron transfer between the transition metal atoms, whereas a very low half-width of the doublet points at a single coordination site occupied by the iron atoms. This is because the difference in the coordination numbers of Fe in the T1, T2, and T3 positions would result in essentially dissimilar hyperfine parameters. However, as long as three metal sites possess similar-though not identical-coordination, the 57 Mössbauer spectrum alone cannot distinguish which site is preferred by Fe in the crystal structure of Eu7Cu36Fe8As23.

Mössbauer Spectra
Our X-ray diffraction data did not allow us to distinguish between Cu and the substituting T element. Therefore, we used other methods in order to shed more light onto the distribution of the T atoms. To this end, we chose the sample with the Eu 7 Cu 36 Fe 8 As 23 composition and performed the 57 Fe-Mössbauer study at low temperatures. The experimental spectrum presented in Figure 4 consists of a single narrow quadrupole doublet with the hyperfine parameters listed in Table 3. We note that these parameters are not sensitive to temperature in the entire investigated range. The isomer shift of 0.63-0.64 mm¨s´1 and quadrupole splitting of 0.17-0.18 mm¨s´1 are characteristic of high-spin Fe 3+ cations in a symmetric coordination environment with a high coordination number. We note, however, that the observed value of the isomer shift is higher than those reported for other compounds exhibiting numerous Fe-Fe bonds. For instance, the isomer shift of 0.30-0.43 mm¨s´1 was found for Fe 3 GeTe 2 [14], where the coordination number of iron ranges from eight to ten, including up to four Fe-Fe bonds. We believe that the difference might arise from shorter Fe-As bonds in our compound compared to Fe-Ge and Fe-Te bonds in Fe 3 GeTe 2 .  (7) 0.29 (2) The constant hyperfine parameters in the temperature range of 14.7-41.1 K rule out the possibility of electron transfer between the transition metal atoms, whereas a very low half-width of the doublet points at a single coordination site occupied by the iron atoms. This is because the difference in the coordination numbers of Fe in the T1, T2, and T3 positions would result in essentially dissimilar hyperfine parameters. However, as long as three metal sites possess similar-though not identical-coordination, the 57 Mössbauer spectrum alone cannot distinguish which site is preferred by Fe in the crystal structure of Eu 7 Cu 36 Fe 8 As 23 .
Materials 2016, 9, 587 8 of 13 Taking into account that the Mössbauer data points at a single position of the iron atoms, we believe that Fe most likely occupies the T3 site (Table 3).

Magnetic Properties
Temperature dependences of the magnetic susceptibility for Eu7Cu36Fe8As23and Eu7Cu42Ni2As23 are presented in Figure 6a,b respectively.
The former sample behaves as a typical Curie-Weiss paramagnet (Figure 6c), with the Weiss temperature only slightly exceeding zero, θW = 0.95 K. A deviation from the Curie-Weiss behavior at low temperatures with a visible increase of the magnetic susceptibility likely stems from minor paramagnetic impurities. The calculated magnetic moment Meff = 8.32 μB per Eu-atom is noticeably higher than the expected value for pure Eu 2+ (Meff = 7.94 μB for J = S = 7/2), indicating that some contribution from iron is also present. Assuming additivity of the magnetic moments, where a square of the effective moment is a sum of the squares of individual moments, we obtain = .

= .
− . × = . (2) The presence of the magnetic contribution from Fe atoms is also consistent with their high-spin state inferred from the Mössbauer spectra.

Magnetic Properties
Temperature dependences of the magnetic susceptibility for Eu 7 Cu 36 Fe 8 As 23 and Eu 7 Cu 42 Ni 2 As 23 are presented in Figure 6a,b respectively.
The former sample behaves as a typical Curie-Weiss paramagnet (Figure 6c), with the Weiss temperature only slightly exceeding zero, θ W = 0.95 K. A deviation from the Curie-Weiss behavior at low temperatures with a visible increase of the magnetic susceptibility likely stems from minor paramagnetic impurities. The calculated magnetic moment M eff = 8.32 µ B per Eu-atom is noticeably higher than the expected value for pure Eu 2+ (M eff = 7.94 µ B for J = S = 7/2), indicating that some contribution from iron is also present. Assuming additivity of the magnetic moments, where a square of the effective moment is a sum of the squares of individual moments, we obtain per one Fe-atom, which is in the typical range for Fe-based itinerant magnets (compare to 2 µ B in FeSi [15]). Note that the paramagnetic effective moments are calculated as µ eff = g[S(S + 1)] 1/2 µ B and, for example, µ eff of Eu 2+ is 7.94 µ B assuming g = 2.0 for J = 0 (4f 14 ). On the other hand, Fe only weakly contributes to the ordered moment (Figure 6e) at low temperatures, µ ord pFeq " p7.14´7.0qˆ7 The presence of the magnetic contribution from Fe atoms is also consistent with their high-spin state inferred from the Mössbauer spectra. In contrast to Eu7Cu36Fe8As23, Eu7Cu42Ni2As23 exhibits FM ordering below 25 K, whereas above TC it behaves as a Curie-Weiss paramagnet (Figure 6d) with the effective moment of 7.89 μB, which is only slightly lower than the expected value of 7.94 for Eu 2+ (4f 7 ). The Curie-Weiss temperature extracted (Figure 6d) from the high-temperature paramagnetic susceptibility (θW = 25 K) coincides with TC, showing that the FM ordering stems from localized Eu 2+ cations. It is worth noting that the parent compound Eu7Cu44As23 orders ferromagnetically at 17.5 K; above this temperature, it behaves as a Curie-Weiss paramagnet with the effective moment of 7.94 μB and at 2 K the moment saturates with the saturation moment MS = 7.0 μB. These observations suggest that the localized Eu 2+ (4f 7 ) cations undergo FM ordering. Taking into account that the shortest Eu-Eu separation exceeds 4 Å and the compound displays metallic conductivity, one can assume that the Eu 2+ cations interact through the conduction electrons of the Cu-As clathrate-like framework. The partial substitution of iron for copper in the framework drastically changes the magnetic properties. The FM ordering is lost, and, as long as the Eu-Eu separation does not change substantially (by 0.08 Å only), we believe that the change in the charge carrier concentration within the framework is responsible for the modification In contrast to Eu 7 Cu 36 Fe 8 As 23 , Eu 7 Cu 42 Ni 2 As 23 exhibits FM ordering below 25 K, whereas above T C it behaves as a Curie-Weiss paramagnet (Figure 6d) with the effective moment of 7.89 µ B , which is only slightly lower than the expected value of 7.94 for Eu 2+ (4f 7 ). The Curie-Weiss temperature extracted (Figure 6d) from the high-temperature paramagnetic susceptibility (θ W = 25 K) coincides with T C , showing that the FM ordering stems from localized Eu 2+ cations. It is worth noting that the parent compound Eu 7 Cu 44 As 23 orders ferromagnetically at 17.5 K; above this temperature, it behaves as a Curie-Weiss paramagnet with the effective moment of 7.94 µ B and at 2 K the moment saturates with the saturation moment M S = 7.0 µ B . These observations suggest that the localized Eu 2+ (4f 7 ) cations undergo FM ordering. Taking into account that the shortest Eu-Eu separation exceeds 4 Å and the compound displays metallic conductivity, one can assume that the Eu 2+ cations interact through the conduction electrons of the Cu-As clathrate-like framework. The partial substitution of iron for copper in the framework drastically changes the magnetic properties. The FM ordering is lost, and, as long as the Eu-Eu separation does not change substantially (by 0.08 Å only), we believe that the change in the charge carrier concentration within the framework is responsible for the modification of magnetic properties upon doping. In the case of Eu 7 Cu 42 Ni 2 As 23 , a different substitution picture seems to appear because Ni tends to behave as an effectively d 10 atom in many intermetallic and related compounds [16,17]. As a result, it shows no magnetic moment that could interfere the FM ordering of Eu 2+ cations, which is observed at 25 K.

Discussion
Recently we have discovered two new arsenides, namely Eu 7 Cu 44 As 23 and Sr 7 Cu 44 As 23 , which are the first representatives of a new structure type derived from the intermetallic compound BaHg 11 . As often observed for ternary arsenides of coinage and alkaline earth metals, these two compounds are the only representatives showing that the new crystal structure is sensitive to the radius of the A-cation. This is not surprising as long as the crystal structure is quite complex and demonstrates a clathrate-like environment of 6/7 of Eu(Sr) atoms by 20 distant Cu and As atoms, whereas the rest of the Eu(Sr) atoms reside in the cubic voids built of eight copper atoms. Such a combination of structure elements requires precise matching of atomic sizes. As a result, isostructural compounds with smaller Ca or larger Ba do not form due to an apparent size mismatch. Since Eu 7 Cu 44 As 23 is electronically unbalanced and demonstrates a metallic type of conductivity, we supposed that extended solid solutions could be formed by substituting copper with 3d-elements having lower number of valence electrons but similar atomic radius. Indeed, our assumption proved to be correct and we have observed homogeneity ranges of Eu 7 Cu 44´x T x As 23 (T = Fe, Co, Ni) extended to 50% for Ni and 20% in the cases of Co and Fe. A close location of Cu and the substituting element T in the Periodic Table resulted in a quite challenging task of determining the dopants distribution among copper sites. The Rietveld refinement against powder X-ray diffraction data was not sensitive enough (as expected); nevertheless, it allowed us to analyze changes in interatomic distances with T for Cu substitution and to indicate the most probable position for the T for Cu substitution. This assumption was facilitated by the 57 Fe Mössbauer data for the iron-containing sample, which confirmed that Fe substitutes for Cu only at a single site. Interestingly, the substitution of Cu by Fe leads to suppression of ferromagnetic ordering in Eu-sublattice, while small amounts of Ni increase T C with respect to the parent phase.
The obtained results call for further investigation aimed at expanding our knowledge about this structure type. The main challenge is to rationalize the FM-ordering mechanism in Eu 7 Cu 44 As 23 and to check for possible magnetocaloric effect near the transition temperature. Another important task is to examine the geometrical and electronic limits for the Eu 7 Cu 44 As 23 structure type by partially substituting Eu by Na, Ca, or Ce, and As by Sb, Ge, and Te. The respective research is currently in progress.

Synthesis and Primary Characterization
The starting materials were ingots of Eu, Cu, Fe, Co, and Ni, as well as As powder of at least analytical grade. The procedure was essentially the same as for the previously reported A 7 Cu 44 As 23 (A = Eu, Sr) [11]. Prior to use, the Fe, Co, and Ni powders were annealed in hydrogen to remove surface oxide. All operations were performed in an Ar-filled glovebox (M'Braun, p(O 2 , H 2 O) < 1 ppm). The elements were mixed according to the composition Eu 7 Cu 44´x T x As 23 , pressed into pellets, loaded in carbon-lined silica tubes, evacuated to~0.05 mTorr and annealed at 200, 400, 600, and 800˝C (ramp 1˝C/min, soak 12 h). The obtained samples were ground, pressed, and annealed at 800˝C for 48 h, three times. The phase composition was checked using a Bruker D8/Advance diffractometer (CuKα 1,2 radiation, LynxEye PSD).

Crystal Structure Determination
Phase-pure or nearly phase-pure powder samples were used for the crystal structure refinement. PXRD data were collected on Powder X'Pert diffractometer (CuKα 1,2 radiation, PANalytical, Almelo, The Netherlands) and processed using the Jana2006 package [12] utilizing the Rietveld method with the crystal structure of Eu 7 Cu 44 As 23 as a starting model. At the first step, profile parameters and atomic coordinates were refined, while atomic displacement parameters for all atoms were fixed at the value of 0.01 Å 2 , and the distribution of T (T = Fe, Co, Ni) atoms over three copper sites was set to be random. The attempt to refine the atomic displacement of all atoms simultaneously led to unrealistic values for the As atom (close to zero or negative); consequently, they were fixed, and the T/Cu ratio (T = Fe, Co, Ni) was refined at the Cu sites. Then, this ratio was fixed, and atomic displacement parameters were refined. When the satisfactory values of atomic displacement parameters were obtained, we checked the occupancy of Eu and As atoms-which appeared to be close to unity-and were then fixed at their ideal values. Details of the refinement, refined structural parameters, and selected interatomic distances are collected in Tables 1-3, respectively. A typical Rietveld plot is presented in Figure 7. the crystal structure of Eu7Cu44As23 as a starting model. At the first step, profile parameters and atomic coordinates were refined, while atomic displacement parameters for all atoms were fixed at the value of 0.01 Å 2 , and the distribution of T (T = Fe, Co, Ni) atoms over three copper sites was set to be random. The attempt to refine the atomic displacement of all atoms simultaneously led to unrealistic values for the As atom (close to zero or negative); consequently, they were fixed, and the T/Cu ratio (T = Fe, Co, Ni) was refined at the Cu sites. Then, this ratio was fixed, and atomic displacement parameters were refined. When the satisfactory values of atomic displacement parameters were obtained, we checked the occupancy of Eu and As atoms-which appeared to be close to unity-and were then fixed at their ideal values. Details of the refinement, refined structural parameters, and selected interatomic distances are collected in Tables 1-3, respectively. A typical Rietveld plot is presented in Figure 7. The Rietveld refinement plot for Eu7Cu36Ni8As23. Experimental profile, green; peak positions, black; differential profile, red.

Magnetic Properties
Magnetic susceptibility measurements were carried out with the vibrating sample magnetometer (VSM) setup of a Physical Property Measurement System (PPMS, Quantum Design, San Diego, CA, USA). The data were collected in external magnetic fields between 0 T and 14 T in the temperature range of 2-380 K.

MössbauerStudy
57 Fe Mössbauer spectra of the sample implemented within the closed-cycle refrigerator system were recorded between 10 and 50 K using a conventional constant-acceleration spectrometer MS-1104Em in the transmission geometry. The radiation source 57 Co(Rh) was kept at room temperature. All isomer shifts are referred to α-Fe at 300 K. The experimental spectra were processed and analyzed using the SpectrRelax program [18].

Conclusions
In conclusion, we studied possibilities of copper substitution in the recently discovered clathrate-like compound Eu7Cu44As23. We showed that up to nearly 50% of Cu can be substituted by Ni, and almost 20% can be substituted by Fe and Co. Based on the X-ray structure analysis and Mössbauer spectroscopy, we analyzed the distribution of dopants among Cu sites. We showed that the introduction of even a small amount of Ni increases TС, while Fe doping suppresses ferromagnetic Figure 7. The Rietveld refinement plot for Eu 7 Cu 36 Ni 8 As 23 . Experimental profile, green; peak positions, black; differential profile, red.

Magnetic Properties
Magnetic susceptibility measurements were carried out with the vibrating sample magnetometer (VSM) setup of a Physical Property Measurement System (PPMS, Quantum Design, San Diego, CA, USA). The data were collected in external magnetic fields between 0 T and 14 T in the temperature range of 2-380 K.

Mössbauer Study
57 Fe Mössbauer spectra of the sample implemented within the closed-cycle refrigerator system were recorded between 10 and 50 K using a conventional constant-acceleration spectrometer MS-1104Em in the transmission geometry. The radiation source 57 Co(Rh) was kept at room temperature. All isomer shifts are referred to α-Fe at 300 K. The experimental spectra were processed and analyzed using the SpectrRelax program [18].

Conclusions
In conclusion, we studied possibilities of copper substitution in the recently discovered clathrate-like compound Eu 7 Cu 44 As 23 . We showed that up to nearly 50% of Cu can be substituted by Ni, and almost 20% can be substituted by Fe and Co. Based on the X-ray structure analysis and Mössbauer spectroscopy, we analyzed the distribution of dopants among Cu sites. We showed that the introduction of even a small amount of Ni increases T C , while Fe doping suppresses ferromagnetic ordering in the Eu-sublattice.