Antiferromagnetic ordering in the plumbide EuPdPb

1 Introduction

Among the huge number of ternary intermetallic rare earth (RE) compounds [1], the equiatomic RETX phases (T=transition metal; X=element of the 3^rd, 4^th, or 5^th main group) have deeply been investigated and are the subject of several hundred publications. Numerous structural aspects and the broadly varying physical properties have been reviewed [2], [3], [4], [5], [6]. Especially the cerium [7], [8], [9], europium [10], and ytterbium [11] containing members have attracted the interest of solid state chemists and physicists when searching for materials with valence instabilities, i.e. Ce^III/Ce^IV, Eu^II/Eu^III and Yb^II/Yb^III. The synthesis conditions for the RETX compounds depend on the boiling temperature of the elements. If elements with high and similar boiling temperatures are used, simple arc-melting leads to larger sample quantities. Europium and ytterbium samples cannot be synthesized in such a quasi-open setup, since substantial evaporation changes the starting composition, leading to multi-phased samples. The same holds true for magnesium, zinc, cadmium, and lead as X components. Such samples need to be prepared in sealed high-melting metal tubes.

Besides the restrictions in synthesis techniques, especially the RETPb plumbides [12] are sensitive to moisture and need to be handled under inert condition. For these reasons, only few equiatomic plumbides have been characterized with respect to their magnetic and transport properties [12]. In extension of our recent studies on the equiatomic EuTMg [13], EuTZn [14] and EuTCd [15] compounds, we have now focused on the EuTPb plumbides. So far, X-ray powder data have been reported for EuTPb with T=Mg, Zn, Pd, Ag, Cd, Au and Hg [16], [17], [18], [19], [20]. Ordering of the T and X atoms was postulated for EuMgPb and EuPdPb (TiNiSi type) as well as EuCdPb and EuHgPb (LiGaGe type), while the remaining plumbides were described with T/X statistics (KHg₂ type).

Small single crystals of EuPdPb were now obtained from an induction-melted sample. Herein we report on the structure refinement and a combined study by magnetic susceptibility and ¹⁵¹Eu Mössbauer spectroscopic measurements.

2 Experimental

3 Crystal chemistry

The single crystal structure refinement of EuPdPb fully confirms the Rietveld powder data reported by Sendlinger [18]; however, the single crystal data are by two orders of magnitude more precise. EuPdPb crystallizes with the orthorhombic TiNiSi type structure, space group Pnma with complete palladium-lead ordering. Each palladium atom has four lead neighbors in strongly distorted tetrahedral coordination with Pd–Pb distances ranging from 280–289 pm, close to the sum of the covalent radii [29] for Pd+Pb of 282 pm. This is consistent with covalent Pd–Pb bonding within the three-dimensional [PdPb] network. A cutout of this network is presented in Fig. 1, highlighting the europium near-neighbor environment. Each europium atom is coordinated by two tilted and puckered Pd₃Pb₃ hexagons, where the inter-layer Pd–Pb distance of 289 pm is only slightly longer than the intra-layer distances of 280 and 283 pm, underlining the three-dimensional character of the [PdPb] network. The puckering leads to a distorted europium coordination. Of the 12 atoms from the two coordinating Pd₃Pb₃ hexagons, one palladium atom is significantly shifted off the first coordination sphere (390 pm Eu–Pd) as compared to the Eu–Pd distances of 312–339 pm for the five closer neighbors. Each europium atom has two europium neighbors above and below the hexagons at Eu–Eu distances of 393 pm and two further neighbors at 390 pm at the open side of the [PdPb] network. Besides intra-layer Pd–Pb bonding we also observe weaker Pb–Pb contacts. The Pb–Pb distance of 365 pm within the Pd₂Pb₂ rhombs is close to that in fcc lead [30] with 350 pm Pb–Pb. These bonding characteristics fit into the whole family of TiNiSi type intermetallic compounds. For further details we refer to review articles [2], [9], [31], [32], [33], [34], [35].

Fig. 1:

(left) View of the EuPdPb structure approximately along the crystallographic b axes. Europium, palladium and lead atoms are drawn as medium grey, blue and red circles, respectively. (right) Coordination of the europium atoms in EuPdPb. Relevant interatomic distances are given in pm.

Finally we compare EuPdPb with the complete series of REPdPb plumbides [36], [37], [38]. The phases with RE=Y, La–Nd, Sm and Gd–Yb crystallize with the hexagonal ZrNiAl type. Except YbPdPb, the cell volumes follow the lanthanide contraction. The positive deviation for YbPdPb is pronounced (the cell volume is even larger than that of GdPdPb) pointing towards intermediate-valent or divalent ytterbium. EuPdPb shows a stable divalent ground state (vide infra) and a switch in structure type with a cell volume per formula unit larger than that for LaPdPb. This is typically observed also in other series of equiatomic RETX intermetallics [1], [6].

For the closely related plumbides EuAgPb [19] and EuAuPb [20] only subcell data (KHg₂ type) with Ag/Pb and Au/Pb statistics has been reported on the basis of powder X-ray diffraction and single-counter diffractometer data. In view of the well-resolved palladium-lead ordering observed in the present study, we have reinvestigated the EuAgPb and EuAuPb structures and indeed observed transition-metal lead ordering. The complicated modulated EuAgPb and EuAuPb structures will be reported in a forthcoming publication.

4 Magnetic properties

Figure 2 (top) shows the susceptibility (χ and χ⁻¹ data) measured in zero-field-cooled mode of a polycrystalline EuPdPb sample at an external magnetic flux density of 10 kOe in the temperature range of 3–300 K. EuPdPb exhibits Curie Weiss behavior above ca. 100 K. Fitting the reciprocal susceptibility in the temperature range of 120–300 K leads to an experimental magnetic moment of 7.35(1) μ_B/Eu which is lower than the theoretical value of the free Eu²⁺ ion of 7.94 μ_B/Eu [39]. The paramagnetic Curie temperature obtained from the Curie Weiss fit is θ_P=23(1) K, indicating ferromagnetic correlations in the paramagnetic range.

Fig. 2:

Magnetic properties of EuPdPb. (top) Temperature dependence of the magnetic susceptibility (χ and χ⁻¹ data) at 10 kOe; (middle) zero-field/field-cooled data at 100 Oe in the low-temperature regime and (bottom) magnetization isotherms at T=3, 10, 50 and 100 K.

In addition, we performed a zero-field-cooled/field-cooled measurement with an applied magnetic field of 100 Oe in the temperature range from 2.5 to 100 K (Fig. 2, middle). The pronounced anomaly is the antiferromagnetic ground state of EuPdPb below a Neél temperature of T_N=13.8(5) K. Trace amounts of EuO were ascertained by its characteristic Curie temperature of T_C=71 K [40], [41], [42]. This anomaly is already seen in the high-field measurement (Fig. 2; top).

Magnetization isotherms (Fig. 2, bottom) were measured at 3, 10, 50 and 100 K with magnetic field strengths of up to 80 kOe. At 50 and 100 K, well above the Néel temperature of EuPdPb, we observe a linear increase of the magnetization, as expected for a paramagnetic material. The 10 K isotherm (slightly below the Néel temperature) shows a steeper increase. At 3 K the magnetization isotherm shows a clear metamagnetic transition at a critical field of 15 kOe, fully underlining the antiferromagnetic ground state. The highest observed magnetization at 3 K and 80 kOe is 6.4(1) μ_B/Eu, slightly smaller the theoretical value of g×J=7 μ_B for Eu²⁺ [39]. Similar saturation magnetization values were observed for many other equiatomic EuTX compounds [10].

5 ¹⁵¹Eu Mössbauer spectroscopy

The ¹⁵¹Eu Mössbauer spectra of EuPdPb at 78 and 6 K are presented in Fig. 3. The corresponding fitting parameters are listed in Table 4. The 78 K spectrum shows a main spectral component at an isomer shift of δ=–10.04(1) mm s⁻¹, subjected to quadrupole splitting of ΔE_Q=3.56(9) mm s⁻¹, a consequence of the non-cubic site symmetry around the europium atoms. The isomer shift is consistent with divalent europium. The small signal at δ=0.6(1) mm s⁻¹ indicates an Eu(III) contribution, most likely arising from surface oxidation. This signal was included as a simple Lorentzian within the fitting procedure. The Eu(II) signal shows full magnetic hyperfine field splitting at 6 K with a hyperfine field of 21.1(1) T. Almost similar hyperfine fields of 22.2 T were observed for EuPdIn [43] and EuPdSn [44].

Fig. 3:

Experimental and simulated ¹⁵¹Eu Mössbauer spectra of EuPdPb at 78 (top) and 6 K (bottom).

EuPdPb belongs to the large family of EuTX intermetallic compounds [10]. The ¹⁵¹Eu Mössbauer spectroscopic data have been relationized. The isomer shifts δ correlate with the valence electron count (VEC) [13], [44]. EuPdPb with VEC=16 and δ=–10.04(1) mm s⁻¹ perfectly matches this δ vs VEC plot [13].