EuAu₃Al₂: Crystal and Electronic Structures and Spectroscopic, Magnetic, and Magnetocaloric Properties

Abstract

The intermetallic compound EuAu3Al2 has been prepared by reaction of the elements in tantalum ampules. The structure was refined from single-crystal data, indicating that the title compound crystallizes in the orthorhombic crystal system (a = 1310.36(4), b = 547.87(1), c = 681.26(2) pm) with space group Pnma (wR2 = 0.0266, 1038 F(2) values, 35 parameters) and is isostructural to SrAu3Al2 (LT-SrZn5 type). Full ordering of the gold and aluminum atoms was observed. Theoretical calculations confirm that the title compound can be described as a polar intermetallic phase containing a polyanionic [Au3Al2](δ-) network featuring interconnected strands of edge-sharing [AlAu4] tetrahedra. Magnetic measurements and (151)Eu Mössbauer spectroscopic investigations confirmed the divalent character of the europium atoms. Ferromagnetic ordering below TC = 16.5(1) K was observed. Heat capacity measurements showed a λ-type anomaly at T = 15.7(1) K, in line with the ordering temperature from the susceptibility measurements. The magnetocaloric properties of EuAu3Al2 were determined, and a magnetic entropy of ΔSM = -4.8 J kg(-1) K(-1) for a field change of 0 to 50 kOe was determined. Band structure calculations found that the f-bands of Eu present at the Fermi level of non-spin-polarized calculations are responsible for the ferromagnetic ordering in this phase, whereas COHP chemical bonding coupled with Bader charge analysis confirmed the description of the structure as covalently bonded polyanionic [Au3Al2](δ-) network interacting ionically with Eu(δ+).

From 3D to 2D: Structural, Spectroscopic and Theoretical Investigations of the Dimensionality Reduction in the [PtAl2]δ− Polyanions of the Isotypic MPtAl2 Series (M=Ca–Ba, Eu)

“…The spectrum of EuPtAl 2 recorded at 78 K shows one main signal with an isomer shift of δ =−9.94(1) mm s −1 . This shift is in line with divalent europium, and the magnetic data discussed above. As a consequence of the non‐cubic site symmetry of the europium atom (4 c , m 2 m ), the spectrum shows a quadrupole splitting of Δ E Q =−8.8(1) mm s −1 .…”

“…As a consequence of the non‐cubic site symmetry of the europium atom (4 c , m 2 m ), the spectrum shows a quadrupole splitting of Δ E Q =−8.8(1) mm s −1 . The experimental line width was refined to Γ =2.95(4) mm s −1 and is therefore in the typical range for 151 Eu Mössbauer investigations . In addition, also the asymmetry parameter η =0.57(3) has to be used in order to obtain an appropriate fit of the experimental data.…”

“…This is in line with the asymmetric coordination environment of the Eu atoms (see Figure S5). The spectrum recorded at 6 K, and therefore well below the Curie temperature of T C =54.0(5) K, shows an additional magnetic hyperfine field splitting of B hf =21.7(1) T, which is also in the typical range for intermetallic europium compounds . In addition, an Eu 3+ impurity signal with a total contribution of 1.4(1)% was used to reproduce the experimental data.…”

An Unusual Valence State: Trivalent Europium in Intermetallic Eu2Ir3Al9

“…Small quadrupole splitting parameters of Δ E Q =−2.2(2) mm s −1 (78 K) and −2.1(3) mm s −1 (6 K) were observed, in line with the highly symmetric coordination environment. The experimental line widths Γ =2.34(8) mm s −1 (78 K) and 2.5(1) mm s −1 (6 K) are in the typical range . The isomer shift is clearly indicative for purely trivalent europium, similar to ionic compounds like EuF 3 ( δ =−0.59(2) mm s −1[39a] ), Eu 2 O 3 ( δ =+0.22(1) mm s −1[39a] ) or Eu 2 Ta 2 O 7.1 N 0.6 ( δ =+0.62(1) mm s −1[39b] ) or intermetallic compounds, such as EuRh 2 ( δ =−0.36(2) mm s −1[39c] ), EuPd 3 ( δ =−0.20(2) mm s −1[39c] ), EuFe 2 Si 2 ( δ =−0.7 mm s −1[20c] ) or EuCo 2 Si 2 ( δ =−0.5 mm s −1[20c] ).…”

“…This is somewhat surprising, because intermetallic Eu compounds usually exhibit divalent Eu atoms. Eu and Sr compounds have comparable unit cell dimensions in quite a number of cases (e.g., Sr 2 Au 6 Al 3 and Eu 2 Au 6 Al 3 or SrAu 3 Al 2 and EuAu 3 Al 2 ) due to the similar sizes of Sr 2+ (126 pm; CN=8) and Eu 2+ (125 pm; CN=8).…”