Christian Kranenberg

Structure and properties of the compounds LnAl2X2 (Ln=Eu, Yb; X=Si, Ge)

Abstract EuAl2Si2, EuAl2Ge2 and YbAl2Ge2 were synthesized by heating the elements at 1070–1270 K and characterized by single-crystal X-ray methods. They are isotypic and crystallize in the CaAl2Si2-type structure (space group P3m1) with the lattice constants (A): YbAl2Ge2: a=4.179(2), c=7.069(3). EuAl2Ge2: a=4.214(1), c=7.320(1). EuAl2Si2: a=4.181(1), c=7.259(1). Magnetic susceptibility measurements of EuAl2Si2 and EuAl2Ge2 show paramagnetic behavior above 50 K with experimental magnetic moments of 7.82(5) μB/Eu and 7.90(5) μB/Eu indicating divalent europium. Antiferromagnetic ordering is detected at TN=35.5(5) K for EuAl2Si2 and at TN=27.5(5) K for EuAl2Ge2. Both compounds undergo metamagnetic transitions at low temperatures. Previously described YbAl2Si2 shows the typical behavior of an intermediate-valent compound. Between 100 and 300 K the inverse susceptibility linearly depends on temperature with a reduced moment of 2.57(5) μB/Yb and a strongly negative paramagnetic Curie temperature of −382(5) K. Below 100 K the degree of divalent ytterbium increases. YbAl2Ge2 is a Pauli paramagnet with a room temperature susceptibility of 1.2(1)×10−9 m3 mol−1. All compounds are metallic conductors between 8 and 320 K. 151Eu Mossbauer spectroscopic measurements of EuAl2Si2 and EuAl2Ge2 show isomer shifts of −10.3(1) and −10.8(2) mm s−1, respectively, at 4.2 K in accordance with divalent europium. Full magnetic hyperfine field splitting is detected at 4.2 K. LMTO band structure calculations confirm the metallic properties for all compounds and result a fully polarized 4f7 state for EuAl2Ge2 and EuAl2Si2. For the Yb-compounds nonmagnetic 4f14 ground states were predicted, but the high 4f-contribution at the fermi level indicates the tendency to intermediate valency in YbAl2Si2.

The stability range of the CaAl2Si2-type structure in case of LnAl2Ge2 compounds

Abstract AAl2Ge2 ( A = Ca, Y, La, Nd, Gd, Tb, Lu) were synthesized by heating the elements at 1000–1300 K and characterized by single crystal X-ray methods. The isotypic compounds crystallize in the CaAl2Si2-type structure (space group P 3 m1) with the lattice constants (A): CaAl2Ge2: a=4.175(1), c=7.173(2); YAl2Ge2: a=4.205(1), c=6.699(1); LaAl2Ge2: a=4.297(1), c=7.013(2); NdAl2Ge2: a=4.269(1), c=6.832(1); GdAl2Ge2: a=4.253(2), c=6.716(2); TbAl2Ge2: a=4.238(1), c=6.661(1); LuAl2Ge2: a=4.160(1), c=6.615(2). The electronic band structures (LMTO method) of CaAl2Ge2 and YAl2Ge2, as examples of an electrovalent and a nonelectrovalent composition, are discussed with regard to bondings and electrical conductivity. Investigations of GdAl2–xZnxGe2 mixed crystals and of YAlMgGe2 show how the transition from an electrovalent to a nonelectrovalent composition affects the lattice and structural parameters.

Neue ternäre Germanide: Die Verbindungen Ln4Zn5Ge6 (Ln: Gd, Tm, Lu)

Durch Erhitzen entsprechender Elementgemenge auf 750 °C wurden drei neue ternare Germanide erhalten. Gd4Zn5Ge6 (a = 4,249(3), b = 18,663(17), c = 15,423(6) A), Tm4Zn5Ge6 (a = 4,190(1), b = 18,410(5), c = 15,105(5) A) und Lu4Zn5Ge6 (a = 4,179(1), b = 18,368(4), c = 15,050(3) A) sind isotyp und kristallisieren in einem neuen Strukturtyp (Cmc21; Z = 4). Dieser wird von ZnGe4-Tetraedern aufgebaut, die ecken- und kantenverknupft ein dreidimensionales Gerust bilden. In dessen Lucken befinden sich die Seltenerdmetallatome, die uberwiegend oktaedrisch von Germanium bzw. pentagonal prismatisch von Zink und Germanium koordiniert werden. Die ZnGe4-Tetraeder sind so zueinander orientiert, dass jeweils zwei von sechs Ge-Atomen zu Paaren verknupft sind, wahrend die ubrigen keine homonuklearen Kontakte haben. Dies steht im Einklang mit einer ionischen Formelaufspaltung gemas (Ln3+)4(Zn2+)5(Ge3–)2(Ge4–)4. Zur Interpretation der Bindungsverhaltnisse werden Abstandsbetrachtungen sowie LMTO-Bandstrukturrechnungen herangezogen. Messungen des elektrischen Widerstands an Tm4Zn5Ge6 bestatigten die von der elektronischen Bandstruktur her erwartete metallische Leitfahigkeit. New Ternary Germanides: The Compounds Ln4Zn5Ge6 (Ln: Gd, Tm, Lu) Three new ternary germanides were prepared by heating mixtures of the elements. Gd4Zn5Ge6 (a = 4.249(3), b = 18.663(17), c = 15.423(6) A), Tm4Zn5Ge6 (a = 4.190(1), b = 18.410(5), c = 15.105(5) A), and Lu4Zn5Ge6 (a = 4.179(1), b = 18.368(4), c = 15.050(3) A) are isotypic and crystallize in a new structure type (Cmc21; Z = 4), composed of edge- and corner-sharing ZnGe4 tetrahedra. The rare-earth atoms fill channels of the Zn,Ge network running along the a axis and predominantly have an octahedral coordination of Ge atoms or a pentagonal prismatic environment of Zn and Ge atoms. The ZnGe4 tetrahedra are orientated to each other so that two of six Ge atoms form pairs, while the other ones have no homonuclear contacts. This is in accord with an ionic splitting of the formula: (Ln3+)4(Zn2+)5(Ge3–)2(Ge4–)4. LMTO band structure calculations support the interpretation of bondings derived from interatomic distances. The metallic conductivity of these compounds expected from the electronic band structure was confirmed by measurements of the electrical resistance of Tm4Zn5Ge6.

Kristall- und elektronische Struktur von LaAlSi2 / Crystal and Electronic Structure of LaAlSi2

Abstract LaAlSi2 (a = 4.196(2), c = 11.437(7) Å; P3̄ml; Z = 2) was synthesized by arc-melting of preheated mixtures of the elements. The compound was investigated by means of X-ray methods and by neutron diffraction. The crystal structure can be described as a stacking variant of two different segments. The first one corresponds to the CaAl2Si2 structure type (LaAl2Si2), the second one with the A1B2 structure type (LaSi2). The segments are stacked along [001]. The electronic structure of the compound is discussed on the basis of LMTO band structure calculations.

Neue Pnictide im CaAl2Si2-Typ und dessen Existenzgebiet/New Pnictides with the CaAl2Si2 Type Structure and the Stability Range of this Type

Five new compounds were synthesized by heating mixtures of the elements at 600 - 1000 °C and investigated by powder and single crystal X-ray methods. EuMg2P2 (a = 4.280(1), c = 7.164(3) Å), EuMg2As2 (a = 4.393(1), c = 7.321(1) Å),EuMg2Sb2 (a = 4.695(1), c = 7.724(2) Å), YbMg2Sb2 (a = 4.650(1), c = 7.540(2) Å), and SrLiAlSb2 (a = 4.584(3), c = 7.697(9) Å) crystallize with the CaAl2Si2 type structure (P3̄m1; Z = 1). The magnetic susceptibility of EuMg2Sb2 shows Curie-Weiss behavior with an experimental magnetic moment of 7.48(2) μB/Eu atom and a Weiss constant θ = 3.2(1) K. EuMg2Sb2 is ordered antiferromagnetically at 8.2(3) K. Magnetisation measurements at 4.5 K show a linear increase and a saturation for a magnetic moment of 5.9(1) μB/Eu at 5.5 T, indicating an almost parallel spin alignment with increasing field strength. 151Eu Mössbauer spectra at 78 K show an isomer shift of -11.69(5) mm/s, compatible with divalent europium. At 4.2 K we observe full hyperfine field splitting with 23 T. The 121Sb spectrum at 4.2 K shows a transferred hyperfine field of 8(2) T at an isomer shift of -7.9(3) mm/s. From the band structure of EuMg2Sb2 we draw the conclusion, that analogous compounds of trivalent rare-earth metals with CaAl2Si2 type structure should not exist due to electronic reasons.