Abrupt Europium Valence Change in Eu₂Pt₆Al₁₅ around 45 K

Abstract

EuPtAl has been prepared from the elements via arc-melting and subsequent temperature treatment; the structure was refined from single crystal X-ray diffraction data. The compound crystallizes in an orthorhombic (3 + 1)D commensurately modulated structure (ScPtAl type) with space group Cmcm(α,0,0)0 s0 (α = 2/3). Full ordering of the Pt and Al atoms within the [PtAl] polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T = 41.6(1) K, which was also observed as λ-type anomaly in heat capacity measurements ( T = 40.7(1) K). Temperature dependent powder X-ray diffraction experiments indicated a drastic shortening of the c axis (-18 pm, -1.1%) around 45 K, while the a axis nearly remains the same (-1 pm, -0.2%). Measurements of the electrical resistivity verified the anomaly, indicating a clear change in the electronic structure of the material. The observed anomalies in the physical measurements can be explained by a temperature driven first order valence change from Eu at higher temperatures (>55 K) to Eu at low temperatures. This valence change was proven by temperature dependent Eu Mössbauer spectroscopic investigations. Isostructural EuPtGa was prepared in comparison, and it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at T = 13.1(1) K.

Magnetic Properties of theRE2Pt6X15(RE= Y, La–Nd, Sm, Gd–Lu;X= Al, Ga) Series

“…Unit cell parameters a and c and unit cell volume V of the hexagonal subcell of the ( top ) RE 2 Pt 6 Al 15 ( RE = Y, Ce–Nd, Sm, Gd–Lu) and ( bottom ) RE 2 Pt 6 Ga 15 ( RE = Y, La–Nd, Sm, Gd–Lu) series. The lattice parameters of Y 2 Pt 6 Al 15 , La 2 Pt 6 Al 15 and Lu 2 Pt 6 Al 15 as well as Eu 2 Pt 6 Al 15 and Eu 2 Pt 6 Ga 15 are taken from the literature. A comparison with all other reported unit cell parameters is given in Table 1.…”

“…Yb 2 Pt 6 Ga 15 in contrast, as mentioned before, exhibits a nearly temperature independent susceptibility (Figure ) that can be attributed to the sole presence of the Yb 2+ (4 f 14 ) oxidation state. The differences in the magnetic behavior can most likely be attributed to the differences in the crystal structures, as seen in the case of Eu 2 Pt 6 Al 15 and Eu 2 Pt 6 Ga 15 …”

“…Recently, for one of the compounds adopting the structure of the title compounds, Eu 2 Pt 6 Al 15 , a temperature induced valence phase transitions was reported. This is a very rare case and only Eu(Pd 1– x Au x ) 2 Si 2 has been reported to show a similar transitions.…”

“…In Eu 2 Pt 6 Al 15 , divalent Eu atoms are present above ≈ 50 K, while below the europium valence changes towards the trivalent one upon delocalization of an electron. In contrary to the aluminum compound, isostructural Eu 2 Pt 6 Ga 15 exhibits simple antiferromagnetic ordering at low temperatures . These compounds crystallize in a (3+1)D commensurately modulated structure that was first solved, refined and described for Sc 2 Pt 6 Al 15 [ Cmcm ( α ,0,0)0 s 0; α = 2/3] .…”

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 .…”