Polytypism of LiSr₂Ge₃ and the Solid Solutions LiSr₂Si_xGe_3–x and LiSr_2–xEu_xGe₃ (0 < x < 1)

Abstract

Three modifications of LiSr2Ge3 were prepared by solid state syntheses at high temperatures under inert conditions in sealed niobium ampoules. α‐LiSr2Ge3 [space group Pnnm (No. 58), a = 11.102(1), b = 11.862(1), c = 4.631(1) Å, Z = 2] is isostructural to LiCa2Tt3 (Tt = Si, Ge), containing a one‐dimensional infinite germanium chain in (tttctc)n conformation. β‐LiSr2Ge3 [space group Fmmm (No. 69), a = 8.733(1), b = 8.996(1), c = 15.045(2) Å, Z = 4] crystallizes with the AgCa2Si3 structure. The β‐phase shows a tendency for lithium deficiency, which may be as small as 6 % but can be larger. γ‐LiSr2Ge3 [space group Fddd (No. 70), a = 8.675(3), b = 15.066(5), c = 18.258(5) Å, Z = 8] is isostructural to LiBa2Tt3. The latter two structures contain planar and distorted Ge6‐rings, respectively. Comparison of the structures reveals significant changes of lattice parameters and bond lengths. β‐LiSr2Ge3 may be stabilized by a slightly smaller lithium content due to preparation procedure. This assumption is supported by the structure analysis and total energy calculations. LiSr2Ge3 can be formulated as (Li+)2(Sr2+)4[Ge6]10– according to the Zintl–Klemm concept, with each of the Zintl anions having a partially occupied delocalized π* system. Theoretical investigations indicate metallic properties since π* bands are crossing the Fermi level. Phase widths were explored for Ge/Si and Sr/Eu exchange and complete miscibility was found for both series. Mixed silicide germanides exclusively crystallize in form of the γ‐phase, whereas the Eu‐containing mixed phases prefer the α‐form, both throughout the series.

Synthesis, Crystal, and Electronic Structure of the New Ternary Zintl Phase Eu2–xMg2–yGe3 (x = 0.1; y = 0.5)

“…For the exchange correlation functional, we took LMH, [26] BOP, [27] and PEB [28] within LMTO, Dmol 3 , and CASTEP, respectively; all of them in the generalized gradient approximation. The equivalence of strontium and europium, concerning the chemical bonding, has been proved in previous works; [12,15] therefore, we replace every europium atom by a strontium, in all the calculations here presented. In this way we reduce the computational cost and avoid the complication of dealing with the spin configura- tions.…”

Exploring the Borders of the Zintl‐Klemm Concept: On the Isopunctual Phases Eu5+xMg18–xGe13(x = 0.1) and Eu8Mg16Ge12

“…In this work, we present another peculiarity, which occasionally occurs in Zintl phases: isosteric structural motifs, in which one or more sites are either occupied by a metametal or by a cation. Examples of this kind are found in compoundsLiBa 2 Si 3 ,22 LiBa 2 Ge 3 ,22 and LiSr 2 Ge 3 (both β and γ modifications)23 containing heterographene layers, which are comprised of lithium and tetrelide atoms. The formal Li + cations are located in a surrounding of three lone electron pairs of three planar Ge 6 rings, thus taking part in both three σ bonds as well as in the extended π bond system due to the planarity of the layer, respectively.…”

The Zintl Phases Eu3Mg5Si5 and Eu3Mg5Ge5

“…From the available exchange correlation functionals, we used the PBE29 (CASTEP), Bethe 's one‐parameter BOP30 (Dmol 3 ) and LMH31 (LMTO). We replaced europium by strontium in both compounds, since this has proven to give practically identical bonding structures,32,33 simplifying the treatment of the randomly oriented spins of the europium cores in the paramagnetic state. Beyond the valence states of the isolated atoms, we included the d states of strontium, silicon, and germanium, as well as the p orbitals of magnesium and strontium.…”