Albrecht Mewis

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.

Darstellung und kristallstrukturen der verbindungen SEPdAs (SELa-Lu)☆

Abstract The compounds SEPdAs (SEue5fcLa-Lu) have been prepared by heating the elements. With SEue5fcLa-Nd they crystallize in the Ni2In-type structure (P63/mmc, Z=2) with nets of planar PdAs hexagons as the characteristic structure unit. The nets of NdPdAs are a little uneven at low temperatures (e.g. 173 K). This atomic arrangement corresponds to the CaIn2-type structure (P63mc, Z=2) which is built up by SmPdAs already at room temperature. Two different phases of EuPdAs were detected. On cooling, α-EuPdAs transforms at 180 K into β-EuPdAs; this phase transition is connected among other things with a change in the lattice constants. Both phases occur in the form of microdomains. The structures are variants of the Ni2In type ( P 3 m1 , Z=2) and differ from each other by a different distortion of the PdAs hexagons. The compounds with SEue5fcTb-Tm form the TiNiSi-type structure (Pnma, Z=4), whereas YbPdAs and LuPdAs crystallize in a ternary variant of the Fe2P type ( P 6 2m , Z=3).

Neue Arsenide mit ThCr2Si2- oder einer damit verwandten Struktur: Die Verbindungen ARh2As2 (A: Eu, Sr, Ba) und BaZn2As2 / New Arsenides with ThCr2Si2-type or Related Structures: The Compounds ARh2As2 (A: Eu, Sr, Ba) and BaZn2As2

Four new arsenides of rhodium and zinc were prepared by heating mixtures of the elements at high temperatures (1000 - 1200 °C) and investigated by single crystal X-ray methods. EuRh2As2 (a = 4.067(1), c = 11.319(2) Å ) and BaRh2As2 (a = 4.053(1), c = 12.770(3) Å ) crystallize with the well-known ThCr2Si2-type (I4/mmm; Z = 2). Due to the rigid layers of RhAs4 tetrahedra, and to the atomic size of europium and barium, the As-As distances between the layers with values of 2.97 and 3.66 Å, respectively, are very long. SrRh2As2 is polymorphic and undergoes two phase transitions at about 190 and 282 °C. Main features of the three crystal structures are also layers of RhAs4 tetrahedra. At room temperature α-SrRh2As2 (a = 5.676(1), b = 6.178(2), c = 11.052(2) Å ) probably crystallizes with the BaNi2Si2-type (Cmcm; Z = 4), whereas β -SrRh2As2 (a = 5.760(3), b = 6.067(4), c = 11.264(5) A° , Fmmm, Z = 4) forms a new orthorhombically distorted variant of the ThCr2Si2-type. Single crystals grown in a flux of lead and quenched at high temperature show that the γ -phase (a = 4.112(1), c = 11.431(6) Å ) crystallizes with the ThCr2Si2-type. The same is true for the high temperature modification of BaZn2As2 (β -phase; a = 4.120(1), c = 13.578(1) Å ), whereas the already known α-BaZn2As2 forms the α-BaCu2S2-type (Pnma; Z = 4) consisting of a 3D-network of edge- and vertex-sharing ZnAs4 tetrahedra with Ba atoms in the voids of this network.

Darstellung und Kristallstruktur der Verbindungen SEPdP (SE = Seltenerdelement) / Preparation and Crystal Structure of SEPdP Compounds (SE = Rare Earth Element)

The new compounds SEPdP (SE = La—Gd), which were prepared by heating the elements, crystallize with a modified Ni2In-type structure. The phosphides of SE = Dy—Lu attain the orthorhombic TiNiSi-type structure. A comparison of the cell volumes suggests that europium has intermediate valence in EuPdP.

ScPtP und LaPtP – Zwei Phosphide mit ,,inverser“ TiNiSi-Struktur / ScPtP and LaPtP – Two Phosphides with “Inverse” TiNiSi-Type Structure

Single crystals of ScPtP (orthorhombic, a = 6.437(1), b = 4.291(1), c = 7.550(2) Å ) were grown by reaction of the elements in molten lead (1000 °C), whereas LaPtP (orthorhombic, a = 7.268(1), b = 4.532(1), c = 7.864(2) Å ) was prepared by heating mixtures of the elements at 900 °C. Both phosphides were investigated by single crystal X-ray diffraction. Their crystal structures belong to the TiNiSi-type (Pnma; Z = 4), but the positions of the Ni and Si atoms are exchanged. Therefore the Pt atoms are located in the centers of trigonal prisms and the P atoms are coordinated by four Pt atoms in the shape of distorted tetrahedra.

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.

MgNi2P - das erste ternäre Phosphid mit geordneter Fe3C-Struktur / MgNi2P - The First Ternary Phosphide with an Ordered Fe3C Type Structure

MgNi2P was prepared by heating a mixture of the elements and investigated by means of single crystal X-ray methods. The compound crystallizes in an ordered Fe3C-type structure (Pnma; a = 6.477(1) Å, b = 6.364(1) Å, c = 5.001(1) Å; Z = 4). Ni and P atoms are linked together to a three-dimensional framework with octogonal tubes running along [010], which are occupied by Mg atoms in the form of stretched zigzag chains.

NixP12-Käfige. Die Kristallstrukturen der Verbindungen SrNi9P5 (x=18) und BaNi10P6 (x=14)

The new ternary phosphides SrNi9P5 and BaNi10P6 were synthesized by heating the elements at 900° —1100° C. Single crystal X-ray methods revealed that SrNi9P5 is isotypic with BaNi9P5 and crystallizes hexagonally [P63/mmc; Ζ = 2; α = 6.529(1) Ä, c = 10.732(1) A], whereas BaNi10Pö forms a new structure with orthorhombic symmetry [Cmca; Ζ = 4; a = 6.449(1) A, 6 = 16.191(1) A, c = 8.784(1) A], The structures are best described as an arrangement of alkaline-earth metal atoms, which are surrounded by Ni18P12(SrNi9P5) and Ni14P12 cages (BaNii0P6), respectively.

Eu2Pt7AlP~3 und isotype Verbindungen: Eine neue Struktur aus CaBe2Ge2⁻ und Cu3Au-Einheiten / Eu2Pt7AlP~3 and Isotypic Compounds: A New Structure Containing CaBe2Ge2⁻ and Cu3Au-Type Units

Eu2Pt7AlP~3 (a = 4.046(1) Å, c = 26.850(2) Å, Z = 2) and some isotypic compounds were prepared by heating the elements. They crystallize tetragonally (I 4/m m m) in a new intergrowth structure where segments occur with atom arrangements as in the CaBe2Ge2⁻ and Cu3Au-type.