The isostructural compounds Yb 2 MgSi 2 , La 2.05 Mg 0.95 Si 2 , and Ce 2.05 Mg 0.95 Si 2 , as well as Yb 2 Li 0.5 Ge 2 and Yb 1.75 Mg 0.75 Si 2 , respectively, were synthesized from stoichiometric mixtures of the corresponding elements in sealed Nb-ampoules under argon atmosphere. The structures were determined by single crystal X-ray diffraction: Yb 2 MgSi 2 (P4/mbm (

Two new ternary compounds Eu 5+x Mg 18-x Ge 13 (x = 0.

The discovery of giant magnetoresistance (GMR effect) [1] in magnetic multilayers has led to a rapid technological advance in spintronic research and device building, for example, magnetoresistive random-access memory (MRAM). The operating principle is based on the dependence of the electrical resistivity on the spin alignment in the magnetic domains of the material. [2] All ferromagnetic metals exhibit a finite but small change of the electronic conductivity after application of an external magnetic field. The value of the magnetoresistivity MR under applied magnetic field H is defined as MR = [1(H)À1(0)]/1(0)] 100 % (1 = electrical resistivity) and can be either positive or negative. If the resistance drop is associated with critical ferromagnetic fluctuations, this phenomenon is called colossal magnetoresistance (CMR). This effect was observed for the first time for the manganese perovskites RE 1Àx B x MnO 3 (RE = rareearth metal; B = divalent cation). [3] The discovery of enhanced MR, often described as CMR in the literature, for the ternary Zintl phase [4] Eu 14 MnSb 11 has led to intensive investigations of this class of compounds. [5] The MR effect occurs simultaneously with ferromagnetic ordering due to the parallel alignment of the unpaired 4f electrons of the rare-earth metals. In the case of Eu 2+ ions they should contribute with a local magnetic moment of 7.94 m B.We report herein on the synthesis of the new ternary Zintl phase Eu 5+x Mg 18Àx Si 13 (x = 2.2) [6] which displays an unusual MR effect. The title compound does not show a maximum of the resistivity at low temperatures and no saturation at fields up to 6 T. Furthermore, an inversion of the sign of the MR as a function of applied field and temperature can be observed.The crystal structure of Eu 5+x Mg 18Àx Si 13 (x = 2.2) is depicted in Figure 1. Tetrel compounds such as M 5+x Mg 18Àx T 13 (M = Sr, Ba; Tt = Si, Ge), which crystallize in their own structure type, have been known for some time. [7] The structure usually contains isolated Si 4À anions as well as planar Si 4 clusters for which different valence electron numbers have been found. [7,8] Moreover, the tetrel center of the Tt 4 cluster can be replaced by a metal such as Li or Mg without causing a structural change in the geometrical pattern. Recently, we reported on the synthesis of the phase Eu 5+x Mg 18Àx Ge 13 (x = 0.1), which is isostructural with Sr 6.3 Mg 16.7 Si 13 and isopunctual with Eu 8 Mg 16 Ge 12 . In the new compound the Tt 4 unit collapses into three isolated Tt 4À anions by means of such substitution. [9] This indicates the remarkable flexibility of that structure type, which we have tested now by systematic changes of the composition and investigated the related electronic effects.The electronic structure of the title compound can be interpreted according to the Zintl-Klemm concept as (Eu 2+ ) 5+x (Mg 2+ ) 18Àx (Si 4À ) 9 (Si 4 10À ) with nine isolated silicon anions and a planar [Si 4 ] unit. [6,9] The anisotropic displacement ellipsoid of the central silicon atom...

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A systematic thermochemical investigation of the binary system Yb/Si revealed the phase YbSi1.4 in pure form that is accessible by reacting the elements at 890 °C in sealed niobium ampoules. According to XRD investigations, the basic structure of YbSi1.4 is orthorhombic (space group: Cmcm; a = 4.159(1), b = 23.510(5), c = 3.775(1) Å). The structure contains blocks with three sheets of trigonal Yb6 prisms that are connected to each other by face‐sharing. Two of these prism sheets are fully occupied by Si whereas the middle one is occupied by ∼ 80 % only, as obtained from the electron density distribution by XRD. Transmission electron microscopy (TEM) investigations show the presence of two distinct ordering variants: YbSi1.4‐I has a unit cell with doubled volume (space group of the structural model: Imm2; aI = 4.16, bI = 7.56, cI = 23.51Å). In the structural model derived from HRTEM images, Si fills 3/4 of the partly occupied Yb6 prisms in a regular way. YbSi1.4‐II exhibits an incommensurate modulation that can be described in the 3 + 1 dimensional superspace group Cmcm(10γ) (No. 63.3). The evaluation of HRTEM images of YbSi1.4‐II revealed parallel domains with the structure of YbSi1.4‐I, with the modulation due to a shift of the domains with respect to each other.