Aluminium-Germanide der zweiwertigen Lanthanoide Eu und Yb: Synthese, Strukturchemie und chemische Bindung

Abstract

In the course of attempts to substitute Ca by Yb and Sr by Eu in known alkaline earth Al-germanides, the four new ternary compounds Eu3Al1.8Ge2.2, Eu3 Al2Ge4, Yb2 AlGe3, and Yb17Al8Ge19 have been synthesized from mixtures of the elements and their crystal structures determined by means of single-crystal X-ray data. The two europium compounds Eu3Al1.8Ge2.2 (Ta3B4 structure type, orthorhombic, space group Immm, a = 417.68(3), b = 470.70(3), c = 1897.2(2) pm, Z = 2, R1 = 0.0439) and Eu3Al2Ge4 (Sr3Al2Ge4 structure type, monoclinic, space group C2/m, a = 1235.9(6), b = 416.8(2), c = 878.4(4) pm, β = 110.615(13)°, Z = 2, R1 = 0.0978) are isotypic with the corresponding strontium phases. After ionic decomposition, the layers [Al2- Ge4- ]6− in Eu3Al2Ge4 with four-bonded Al and three-bonded Ge atoms can be interpreted as electron-precise Zintl anions. In contrast, the planar ribbons 1∞[Al2/2Ge2Al2/2] of condensed six-membered rings in Eu3Al1.8Ge2.2 exhibit considerably shorter Al-Ge bonds and an Al-Al bond length of only 251 pm. Yb2AlGe3 (orthorhombic, space group Pnma, a = 682.20(10), b = 417.87(9), c = 1813.9(3) pm, Z = 4, R1 = 0.0415) crystallizes with the Y2AlGe3 structure type. Folded [Al2Ge2] ladders, also found in Eu3Al2Ge4 and the known compound Yb7Al5Ge8, are connected by planar cis/trans chains of Ge atoms. The total density of states calculated within the FP-LAPW|DFT band structure approach shows a distinct minimum at the Fermi level for the electron precise Zintl compound Eu3Al2Ge4, whereas π-bonding contributions are evident from the band structures of Eu3Al2Ge2 and Yb2AlGe3. In full accordance, the tDOS of both compounds exhibits no minimum at EF, small phase widths are possible for Eu3Al2Ge2 and related alkaline earth compounds, and Yb2AlGe3 is isotypic with several other more electron-rich LnIII compounds. The complicated structure of the new compound Yb17Al8Ge19 (tetragonal, space group P4/nmm, a = 1542.50(2), c = 788.285(8) pm, Z = 2, R1 = 0.0282) contains three different building blocks: distorted [Al4Ge4] heterocubane units are interconnected by four-bonded Ge atoms to form columns running along the c axis. Secondly, eight-membered rings are formed by alternating Al and Ge atoms, each being in a trigonal-planar Al/Ge coordination. The rings are terminated by Ge atoms (bonded to Ge of the ring) and linked to the first structural unit by a further Ge atom (bonded to Al of the ring). Thirdly, inside the large channels, which are formed by the packing of the eightmembered rings, Ge2 dumbbells are interspersed as a third structural element.

Correlations between Chemical Bonding and Magnetic Exchange Interactions: Synthesis, Crystal Structures, and Magnetic Properties of the New Family RE2AlGe2 (RE = Tb–Tm, Lu)

“…The Al atoms are in atypical planar four-coordinate environment with Al–Ge distances in the range of 2.6620(9) Å to 2.6733(8) Å. Similar distances are known for structurally related compounds such as Gd 2 AlGe 2 , RE 2 AlGe 3 , RE 2 Al 3 Ge 4 , Eu 3 Al 2 Ge 2 , and Yb 7 Al 5 Ge 8 , among others.…”

“…The “square” unit is commonly seen in binary RE-germanides with the α-ThSi 2 - and ZrSi 2 -type structures (and their derivatives), as well as in the REAlGe compounds with the YAlGe-type . The “hexagonal rings” are a hallmark in the crystal chemistry of binary RE-germanides with the AlB 2 -type structure and its derivatives, as well as the Eu 3 Al 2 Ge 2 structure …”

Crystal structures of the compounds GdAl0.09Ge2.07 and Gd2AlGe3

“…The cell parameters of the isotypic compounds R 2 AlGe 3 , listed in Table 7, decrease with decreasing radius of the trivalent rare-earth metal [20] from Sm to Tm. In the compound Yb 2 AlGe 3 , Yb shows the valency 2+ [19].…”