Introduction

Strontium and europium have almost similar radii. To give an example, Shannon [1] lists values of 118 (Sr2+) and 117 (Eu2+) pm for coordination number 6. Thus, it is not surprising that one observes a large crystal chemical similarity between divalent europium and strontium compounds. The simplest example concerns the rocksalt-type phases SrO and EuO. Another prominent example concerns the structure of Eu3O4, where a complete substitution of the divalent europium by Sr2+ leads to SrEu2O4 [2].

This isovalent substitution is interesting with respect to the magnetic properties. Sr2+ is a diamagnetic cation while Eu2+ with its [Xe]4f 7 configuration is strongly paramagnetic. To keep the example of the rocksalt phases, SrO is a simple diamagnet, whereas EuO is a strong Curie–Weiss paramagnet which orders ferromagnetically below TC = 70 K [3, 4]. These magnetic characteristics offer the possibility to study solid solutions. Due to the close size of the cations, these solid solutions are mostly strain free and one can investigate the force of the magnetic coupling as a function of the europium-strontium substitution.

Interesting results have been obtained for solid solutions of metal hydrides. Eu2IrH5 orders ferromagnetically at TC = 20 K and the Curie temperature decreases with increasing strontium content: 11 K for Eu1.5Sr0.5IrH5, < 7 K for Eu1.0Sr1.0IrH5 and < 5 K for Eu0.5Sr1.5IrH5 [5]. Similar behavior was reported for the solid solutions Eu2–xSrxRuH6 [6] and Eu1 − xSrxMg2H6 (TC = 21 K for x = 0.2 and 15 K for x = 0.4) [7]. In the three series, TC decreases with decreasing number of neighboring europium cations, i. e. a decrease of the strength of the Eu–Eu nearest neighbor exchange interactions. A different behavior was observed for the fullerides Eu6 − xSrxC60 [8] where the Curie temperature shows little variation: 11.6 K for Eu6C60, 12.8 K for Eu3Sr3C60, and 10.4 K for EuSr5C60. It was concluded that the ferromagnetic interaction is mediated through the conduction carriers.

We have recently started a systematic investigation of europium-strontium substitution in the field of intermetallic compounds. The first complete solid solution was Eu1–xSrxPtIn2 with MgCuAl2 type structure [9]. The course of the cell parameters showed Vegard type behavior. EuPtIn2 shows a comparatively high Curie temperature of 32.5 K and it is remarkable that the ferromagnetic ground state even remains in the Eu0.1Sr0.9PtIn2 sample with TC = 3.1 K with a gradual decrease of the Curie temperature within the complete solid solution. This was monitored in parallel by 151Eu Mössbauer spectroscopy.

In continuation of these studies we have now investigated the solid solution Eu1–xSrxAu4Cd2 with YbAl4Mo2 type structure which also shows adjustable Curie temperatures. The crystal chemistry and the magnetic behavior of this solid solution are reported herein.

Results and discussion

Crystal chemistry

EuAu4Cd2 and SrAu4Cd2 [10] crystallize with the YbAl4Mo2 type structure [11], space group I4/mmm. The slightly larger radius of strontium (see “Introduction”) leads to a small increase of the a and c lattice parameters (Table 1) and consequently a larger cell volume for SrAu4Cd2. We have studied the complete solid solution Eu1 − xSrxAu4Cd2 for samples with x = 0.2, 0.4, 0.6, and 0.8. Within the standard deviations, we do not strictly observe Vegard-type behavior. The small differences observed for the two single crystals and the Eu0.2Sr0.8Au4Cd2 sample can be ascribed to the Cd/Au mixing (formation of small homogeneity ranges) on the 4d sites (vide infra).

Table 1 Refined lattice parameters for the compounds of the solid solution Eu1–xSrxAu4Cd2 based on Guinier powder data
Full size table

The prototype YbAl4Mo2 [11] has more than 40 isotypic representatives [12]. The crystal chemistry and chemical bonding of YbAl4Mo2 type phases has repeatedly been discussed in the original contributions [13,14,15,16,17,18,19,20]. Herein we therefore concentrate only on the structural peculiarities of the solid solution Eu1 − xSrxAu4Cd2. As an example, we present the unit cell of Eu0.66Sr0.34Au4.12Cd1.88 in Fig. 1. The projection onto the ab plane (Fig. 1, top) shows the Au4 squares with 279 pm Au–Au. These squares condense with adjacent squares that are shifted by half the translation period c and form the Au12 clusters that surround the europium/strontium atoms. The Au–Au distances between the clusters of 299 pm are slightly longer. In average, these Au–Au distances compare well with fcc gold (12 × 288 pm) [21].

Fig. 1
figure 1

The structure of Eu0.66Sr0.34Au4.12Cd1.88: (top) projection onto the xy plane and (bottom) presentation of the polyhedra and the cadmium substructure. Europium (strontium), gold, and cadmium atoms are drawn as green, blue, and white circles, respectively; mixed occupied sites are emphasized by segments. Relevant interatomic distances are given in units of pm (color figure online)

Full size image

The second striking motif concerns the linear cadmium chains that extend in c direction. The Cd–Cd distances of 276 pm are indicative of substantial Cd–Cd bonding given that they are shorter than in hcp cadmium (6 × 298 and 6 × 329 pm) [21]. Each cadmium atom is surrounded by eight gold atoms in strongly distorted square prismatic coordination. These polyhedra are capped by cadmium atoms on the rectangular faces in c direction.

The refinements of the occupancy parameters of the 2a sites (vide infra) clearly revealed the Eu/Sr mixing, in approximate agreement with the nominal starting compositions of the samples. These deviations indicate that all samples from the solid solution definitely contain quantities of domains with compositions that slightly deviate from the starting ones. The mixed Eu/Sr occupancy for Eu0.66Sr0.34Au4.12Cd1.88 is presented by light and dark green segments.

Another important feature concerns the cadmium chains. For both crystals, we observed small degrees of Cd/Au mixing on the 4d position, similar to the original work on EuAu4.80Cd1.20 [10]. This is a general feature of the YbAl4Mo2 type. We have recently studied the solid solution CeAu4+xCd2 − x towards CeAu5Cd [20]. Other examples are SrAu4.76In1.24 [13], CaAu4.89Zn1.11 [17], EuAu4.75In1.25 [19], LaAg4.4Mg1.6 [14], or CaAu4.08Zn1.92 [15]. The driving force for these mixed occupancies is the covalent bonding Au–Cd, Au–In, Au–Zn, and Ag–Mg within the chains.

Summing up, the present X-ray powder and single crystal data clearly manifest the solid solution Eu1–xSrxAu4Cd2 with Vegard type behavior, but also show Cd/Au mixing on the chains, underpinning that the bulk samples all have distributions of domains with slightly varying compositions, most likely occurring for all YbAl4Mo2 type phases.

Magnetic properties

Results of the magnetic property measurements of the solid solution Eu1–xSrxAu4Cd2 are summarized in Table 2. Measurements of M(T,H) of Eu0.4Sr0.6Au4Cd2 are shown as an example in Fig. 2.

Table 2 Magnetic properties of the solid solution Eu1–xSrxAu4Cd2 with the Curie temperature TC, the effective magnetic moment μeff, the saturation magnetization μsat, the Weiss constant θp, and the magnetic susceptibility χ at 300 K
Full size table
Fig. 2
figure 2

Temperature dependence of the magnetic susceptibility of Eu0.4Sr0.6Au4Cd2 measured from 2.5 to 100 K in the zero-field-cooled/field-cooled (ZFC/FC) mode with an applied field of 100 Oe (top). Magnetization isotherms of Eu0.4Sr0.6Au4Cd2 measured at 3, 10, and 50 K with fields up to 80 kOe (middle). Measurement of the temperature dependence of the magnetic susceptibility in the zero-field-cooled (ZFC) mode from 3 to 300 K with an applied field of 10 kOe (bottom)

Full size image

Measurements of M(T) performed on the samples of the solid solution in zero-field-cooled/field-cooled (ZFC/FC) mode from 2.5 to 100 K, with an applied field of 100 Oe, show ferromagnetic ordering for all members of the solid solution at low temperatures, which is in accordance with the properties of EuAu4Cd2 [10]. As expected, the Curie-temperature TC decreases (Fig. 3) with decreasing europium content (increasing Eu/Sr disorder). Similar behaviour has been observed in other solid solutions of europium and strontium such as Eu1 − xSrxPtIn2 [9].

Fig. 3
figure 3

Course of the Curie temperature in the solid solution Eu1–xSrxAu4Cd2

Full size image

The effective magnetic moments determined from zero-field-cooled (ZFC) measurements with an applied field of 10 kOe in a temperature range of 3–300 K are in good accordance with the theoretical value of 7.94 μB per Eu2+ ion [22] for Eu0.8Sr0.2Au4Cd2 and Eu0.4Sr0.6Au4Cd2. The slight deviations observed for Eu0.2Sr0.8Au4Cd2 and Eu0.6Sr0.4Au4Cd2 most likely result from minor impurity phases. In the case of the Eu0.2Sr0.8Au4Cd2 sample this might by a Eu(III) impurity phase which lowers the experimental moment. All compounds exhibit Curie–Weiss behaviour at high temperatures. The Weiss-constants of all compounds are positive, indicating ferromagnetic interactions in the paramagnetic regime.

Magnetization isotherms were measured for every member of the solid solution at 3, 10, and 50 K with applied fields of up to 80 kOe. All magnetization isotherms at 50 K show a linear course as expected for paramagnetic compounds. The isotherms at 3 K, which are below or close to the ordering temperatures of the compounds, show typical behaviour for soft ferromagnetic compounds as the magnetization rises rapidly at low fields. The saturation magnetization of Eu0.8Sr0.2Au4Cd2 and Eu0.4Sr0.6Au4Cd2 at 3 K and 80 kOe is in good accordance with the theoretical value of 7 μB per Eu-atom, calculated by gJ × J. The higher saturation magnetization of Eu0.6Sr0.4Au4Cd2 could again be caused by a ferromagnetic impurity. The smaller value for the Eu0.2Sr0.8Au4Cd2 sample, in line with the reduced moment in the paramagnetic range is indicative of a Eu(III) impurity contribution.

A ZFC measurement characterizes SrAu4Cd2 as a Pauli paramagnetic compound. The magnetic susceptibility stays nearly constant until ca. 100 K and a weak Curie tail, caused by a trace of paramagnetic impurities is evident at low temperature. The room temperature values of 7.9(2) × 10–4 emu/mol is in the typical range for Pauli paramagnetic intermetallics [23, 24].

151Eu Mössbauer spectroscopy

The europium valence and the magnetic ordering within the solid solution Eu1 − xSrxAu4Cd2 were further studied through temperature dependent 151Eu Mössbauer spectra. Figure 4 presents the 151Eu Mössbauer spectra of these samples at 78 and 6 K, respectively. The corresponding fitting parameters are listed in Table 3. The extremely high absorption caused by the high content of highly absorbing heavy elements leads to a low resolution of the experimental spectra. The 78 K 151Eu Mössbauer spectroscopic investigations confirm divalent europium in the Eu1 − xSrxAu4Cd2 samples. Only the Eu0.6Sr0.4Au4Cd2 sample revealed a small degree of an additional Eu(III) component, most likely resulting from surface oxidation/surface hydrolyses of the polycrystalline material, frequently observed for intermetallic europium compounds (even if samples are handled under inert conditions, small amounts of Eu(III) deterioration products occur; this is decomposition, not intrinsic trivalent europium of the main sample) [25, 26]. The isomer shifts values are in the usual range observed for structurally related intermetallics [25, 26]. The small quadrupole splitting parameters account for the non-cubic site symmetry of the europium atoms.

Fig. 4
figure 4

Experimental and simulated 151Eu Mössbauer spectra of samples from the solid solution Eu1 − xSrxAu4Cd2 with x = 0.2–0.8 at 78 (left) and x = 0.2–0.4 at 6 K (right)

Full size image
Table 3 Fitting parameters of 151Eu Mössbauer spectroscopic measurements of the compounds Eu1 − xSrxAu4Cd2 (x = 0.2–0.8) at 78, 15, and 6 K. δ = isomer shift (mm s–1)
Full size table

The magnetic susceptibility data (vide ultra) showed a decreasing TC with increasing strontium content. The Mössbauer spectrometer allowed data collections only above a temperature of 6 K. Thus, in Fig. 4 we only present spectra for the x = 0.2 and 0.4 samples. The increasing strontium content leads to a dilution of the magnetic centers and this is expressed in the 151Eu spectra in a decrease of the magnetic hyperfine field: 26.2 T for the x = 0.2 vs. 21.9 T for the x = 0.4 sample. The Mössbauer spectroscopic data thus fully support the magnetic ones.

Conclusion

EuAu4Cd2 and SrAu4Cd2, both crystallizing with the tetragonal YbAl4Mo2 type structure, space group I4/mmm form a complete solid solution Eu1–xSrxAu4Cd2. The dilution of the europium substructure with diamagnetic Sr2+ reduces the Eu–Eu magnetic exchange and leads to a drastic decrease of the Curie temperature, although ferromagnetic ordering is still present in the x = 0.8 sample with TC = 2.9(2) K.

Experimental

Synthesis

Starting materials for the synthesis of samples of the solid solution Eu1 − xSrxAu4Cd2 (in x = 0.2 steps) were europium ingots (American Elements, 99.99%), dendritic pieces of strontium (Onyxmet, 99.95%), pieces of gold droplets (Agosi, 99.9%), and a cadmium rod (Sigma Aldrich, 99.999%). First, the europium pieces were cleaned from surface contaminations under dry cyclohexane (over sodium) and cut into small pieces. Also, strontium pieces were cut into smaller pieces under paraffin oil and washed with dry cyclohexane. These pieces were stored in Schlenk tubes prior to the reactions. The elements where then weighed in the atomic ratios of 0.82:0.21:4:2, 0.62:0.41:4:2, 0.41:0.62:4:2, and 0.21:0.82:4:2 and arc-welded [27] in small tantalum tubes under an argon atmosphere of ca. 800 mbar. The argon (Westfalen, 99.998%) was purified over titanium sponge (870 K), silica gel, and molecular sieves. A slight excess of europium and strontium was necessary to minimize the amount of by-products.

The tantalum ampoules were then sealed in quartz tubes for protection against oxidation and were subsequently heated within one hour to 1273 K. This temperature was held for five minutes before the samples were cooled to 1073 K within 20 min, kept at that temperature for another three hours and finally cooled to room temperature at a rate of 10 K h–1. The polycrystalline samples were powdered in an agate mortar under dry cyclohexane, cold pressed into pellets (∅ 6 mm) and thermally post-treated in a muffle furnace (680 K for four days, followed by slow cooling to room temperature at a rate of 5 K h–1) to increase the phase purity and crystallinity. The samples were mechanically separated from the ampoules. No reactions with the ampoule material were evident. All samples are stable in air over several weeks and have metallic luster, ground powders are dark grey.

X-ray diffraction on powders and single crystals

The polycrystalline Eu1 − xSrxAu4Cd2 samples were ground to fine powders and studied by X-ray powder diffraction using the Guinier technique (Enraf–Nonius FR 552 camera) with CuKα1 radiation and α-quartz (a = 491.30 and c = 540.46 pm) as an internal standard. The diffraction patterns were collected through imaging plate technique (Fuji film, BAS-READER 1800). The lattice parameters (Table 1) were obtained from least-squares refinements. Intensity calculations (Lazy-Pulverix routine [28]) facilitated the correct indexing of the diffraction lines.

Conchoidally fractured crystal splinters were selected from the crushed samples with the nominal compositions Eu0.6Sr0.4Au4Cd2 and Eu0.2Sr0.8Au4Cd2 and glued to glass fibers with bees wax. The crystals were first investigated on a Buerger precession camera (white Mo radiation, Fuji-film imaging plate) in order to check their quality for intensity data collection. Complete data sets were collected at room temperature by using a Stoe IPDS-II image plate system (graphite monochromatized Mo radiation; λ = 71.073 pm) in oscillation mode. Numerical absorption corrections were applied to the data sets. Details on the crystallographic data are given in Table 4.

Table 4 Crystal data and structure refinement parameters for the compounds Eu0.66Sr0.34Au4.12Cd1.88 and Eu0.22Sr0.78Au4.10Cd1.90 of the solid solution Eu1 − xSrxAu4Cd2, YbAl4Mo2 type, space group I4/mmm, Z = 2
Full size table

EDX data

The Eu0.66Sr0.34Au4.12Cd1.88 and Eu0.22Sr0.78Au4.10Cd1.90 single crystals were semiquantitatively analyzed by EDX in a Zeiss EVO® MA10 scanning electron microscope (variable pressure mode (60 Pa) and LaB6 cathode) using EuF3, SrF2, Au and Cd as standards. The measurements (8 ± 1 at.% Eu: 3 ± 1 at.% Sr: 62 ± 1 at.% Au: 27 ± 1 at.% Cd, for the Eu0.66Sr0.34Au4.12Cd1.88 and 3 ± 1 at.% Eu: 7 ± 1 at.% Sr: 62 ± 1 at.% Au: 28 ± 1 at.% Cd, for the Eu0.22Sr0.78Au4.10Cd1.90 crystal) confirmed the compositions refined from the single crystal X-ray data of 9.4:4.9:58.9:26.9 for the Eu0.66Sr0.34Au4.12Cd1.88 and 3.1:11.1:58.6:27.1 for the Eu0.22Sr0.78Au4.10Cd1.90 crystal. Slight deviations as well as the standard deviations result from the irregular crystal surface (conchoidal fracture) and the coating with beeswax. No impurity elements (especially with respect to the tantalum tubes) were detected.

Structure refinements

Both data sets showed tetragonal body-centered lattices with high Laue symmetry and no further extinctions. Similar to EuAu4Cd2 and SrAu4Cd2 [10], the centrosymmetric space group I4/mmm was found to be correct. The positional parameters of SrAu4Cd2 [10] were taken as starting values and the structures were refined on F2 with the Jana2006 [29] software package using anisotropic displacement parameters for all atoms. Both data sets showed mixed occupancies. Besides the Eu/Sr mixing on the 2a sites we observed small degrees of Cd/Au mixing on the 4d sites, similar to EuAu4.8Cd1.2 reported earlier [10]. These site occupancies were refined as least-squares variables. The final difference Fourier syntheses were flat. The final atomic coordinates, displacement parameters and interatomic distances of both structures are listed in Tables 5, 6.

Table 5 Atomic coordinates, anisotropic and equivalent isotropic displacement parameters (pm2) for Eu0.66Sr0.34Au4.12Cd1.88 and Eu0.22Sr0.78Au4.10Cd1.90 of the solid solution Eu1 − xSrxAu4Cd2 (YbAl4Mo2 type, I4/mmm)
Full size table
Table 6 Interatomic distances (pm) for Eu0.66Sr0.34Au4.12Cd1.88 and Eu0.22Sr0.78Au4.10Cd1.90
Full size table

CCDC-1995352 (Eu0.66Sr0.34Au4.12Cd1.88) and CCDC-1995314 (Eu0.22Sr0.78Au4.10Cd1.90) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Magnetic characterization

The samples of the solid solution Eu1 − xSrxAu4Cd2 (x = 0.2, 0.4, 0.6, and 0.8) were investigated using the vibrating sample magnetometer (VSM) option of a physical property measurement system (PPMS) by Quantum Design. The samples were ground to a fine powder and each was filled into a polypropylene capsule, which was then inserted into a brass sample holder, which was attached to the sample holder rod of the VSM. The samples of the solid solution with x ranging between 0.2 and 0.8 were investigated through measurements of M(T,H) in a temperature range from 2.5 to 300 K and fields up to 80 kOe. Measurements of M(T) were performed for the end member of the solid solution, SrAu4Cd2, in the temperature range from 3 to 300 K and a field of 10 kOe.

Mössbauer spectroscopy

The samples of the solid solution Eu1 − xSrxAu4Cd2 (x = 0.2, 0.4, 0.6, and 0.8) were investigated via 151Eu Mössbauer spectroscopic investigations. For the measurements a 151Sm:EuF3 source (the 21.53 keV transition was utilized, 55 MBq, 1% of the total activity) was used in either a continuous flow helium cryostat system (Janis Research Co LLC) equipped with a resistance thermometer (accuracy ± 0.5 K) or a commercial liquid nitrogen bath cryostat in the usual transmission geometry. The source was kept at room temperature while the samples were cooled to the respective temperature (Table 3). The sample amounts were adjusted to the ideal absorber thickness [30] and mixing with α-glucose assured homogeneous coverage. The samples were then placed in thin-walled PMMA containers. The high gold and europium content (see also the high absorption coefficients in Table 4) caused long measuring times between one and two weeks with a barely satisfactory resolution of the spectra especially for the spectra that were recorded below the magnetic ordering temperature. Fitting of the spectra was done with the WinNormos for Igor6 program package [31].