Ines Dürr

Cadmium-Auride AIICdxAu2– x (AII = Ca, Sr) Synthese, Kristallstruktur, chemische Bindung/ Cadmium-Aurides AIICdxAu2−x (AII = Ca, Sr) – Synthesis, Crystal Structure, Chemical Bonding

Following the observation that the binary dicadmides and diaurides of calcium and strontium (A) both form the KHg2 structure type, the two sections AIICdxAu2−x have been studied systematically by means of synthetic, X-ray structural and theoretical investigations. The binary border compound CaCd2 is dimorphic forming the KHg2 structure at elevated temperatures (orthorhombic, space group Imma, a = 488.63(9), b = 754.1(2), c = 851.3(2) pm, Z = 4, R1 = 0.0498) and the MgZn2-type Laves phase at ambient conditions (hexagonal, space group P63/mmc, a = 599.71(9), c = 962.7(2) pm, Z = 4, R1 = 0.0309). Starting from the known binary calcium auride CaAu2 only a very small amount of Au can be replaced by Cd. Around the 1 : 1 ratio of Au and Cd the TiNiSi structure type (orthorhombic, space group Pnma), an ordered variant of the KHg2 type, has a small homogeneity range (CaCdxAu2−x with x = 1/0.76(2): a = 735.0(1)/731.7(1), b = 433.66(6)/431.43(7), c = 873.7(2)/869.9(2) pm, Z = 4, R1 = 0.0482/0.0539). The analogous structure type is also observed in the Sr compounds with the difference that in this case a continuous transition from the KHg2 type of SrAu2 (i. e. x = 0) towards the distorted TiNiSi structure type (up to x = 0.86) is observed in the series SrCdxAu2−x (for x = 0.86(1)/0.45(1): a = 764.0(1)/758.4(1), b = 458.07(7)/474.6(1), c = 872.16(12)/829.2(2) pm, Z = 4, R1 = 0.0446/0.0410). Attempts to prepare the Ca compounds of intermediate composition around a Cd content of x ≈ 0.5 resulted in the formation of the Aurich phase Ca5Cd2Au10 crystallizing with the Zr7Ni10 structure type (orthorhombic, space group Cmca, a = 1390.6(4), b = 1015.7(3), c = 1025.6(2) pm, Z = 4, R1 = 0.0657). In this compound, Cd and Ca occupy common crystallographic sites, which are occupied by In in the isotypic ternary compound Ca4In3Au10. Similarly, at the Cd-rich parts of the sections AIICdxAu2−x no simple phase width of the KHg2 structure type exists. In the case of the calcium series the new compound Ca11Cd18Au4, which shows only a very small phase width, is formed instead. This compound crystallizes with a new structure type (Ca11Cd18+xAu4−x with x = 0.6/0: tetragonal, space group I41/amd, a = 1030.83(6)/1029.39(6), c = 3062.5(3)/3051.0(3) pm, Z = 4, R1 = 0.0475/0.0379) exhibiting a complicated Cd/Au polyanion with four-, five- and six-bonded Cd/Au atoms. The results of FPLAPWband structure calculations are used to explain the electronic stability of the compounds. The calculated Bader charges of cadmium and gold atoms (and In and Au atoms for comparison) are used to discuss the transition between Cd-rich cadmides (like CaCd2 and Ca11Cd18Au4), auridocadmates (like CaCdAu) and the Cd-poor cadmium aurides (like Ca5Cd2Au10). Graphical Abstract Cadmium-Auride AIICdxAu2– x (AII = Ca, Sr) Synthese, Kristallstruktur, chemische Bindung/ Cadmium-Aurides AIICdxAu2−x (AII = Ca, Sr) – Synthesis, Crystal Structure, Chemical Bonding

New members of the A{sub 2}M′M{sub 2}{sup ″} structure family (A=Ca, Sr, Yb, La; M′=In,Sn,Pb; M″=Si,Ge)

Abstract The new mixed tetrelides Sr 2 PbGe 2 and Yb 2 SnGe 2 , several mixed Ca/Sr (AII) germanides A 2 II ( Sn , Pb ) Ge 2 and two polymorphs of La 2 In Si 2 represent new members of the general structure family of ternary alkaline-earth/lanthanoid main group silicides/germanides A 2 M ′ M 2 ″ ( M ′ = In , Sn , Pb ; M ″ = Si , Ge ) . All compounds were synthesized from melts of the elements and their crystal structures have been determined by means of single crystal X-ray diffraction. Sr 2 PbGe 2 (Cmmm, a=402.36(11), b=1542.3(4), c=463.27(10) pm) crystallizes with the Mn 2 AlB 2 -type structure. In exhibiting infinite planar Ge zig-zag chains, it represents one border of the compound series. The other borderline case, where only [ Ge 2 ] dumbbells are left as Ge building units, is represented by the Ca/Yb tin germanides Ca 2 SnGe 2 and Yb 2 SnGe 2 ( Mo 2 FeB 2 -type ; P4/mbm, a=748.58(13)/740.27(7), c=445.59(8)/435.26(5) pm). In between these two border structures compounds with variable Si/Ge chain lengths could be obtained by varying the averaged size of the AII cations: Ca 0.45 Sr 1.55 PbGe 2 (new structure type; Pbam, a=791.64(5), b=2311.2(2), c=458.53(3) pm) contains planar six-membered chain segments [ Ge 6 ] . Tetrameric pieces [ Ge 4 ] are the conspicuous structure elements in Ca 1.16 Sr 0.84 SnGe 2 and La 2 In Si 2 ( La 2 InNi 2 -type ; Pbam, a=781.01(2)/762.01(13), b=1477.95(3)/1494.38(6), c=457.004(9)/442.1(3) pm). The tetragonal form of ’ La 2 In Si 2 ′ (exact composition: La 2 In 1.07 Si 1.93 , P4/mbm, a=1309.11(12), c=443.32(4) pm) also crystallizes in a new structure type, containing only [ Si 3 ] trimers as cutouts of the planar chains. In all structures the Si/Ge zig-zag chains/chain segments are connected by In/Sn/Pb atoms to form planar M layers, which are separated by pure A layers. Band structure calculations within the FP-LAPW DFT approach together with the Zintl formalism, extended by the presence of hypervalent bonding of the heavier M ′ elements, give insight into the chemical bonding of this series of p-block metallides. An analysis of the band structure for the border phases Sr 2 PbGe 2 and Ca 2 SnGe 2 shows the considerable π bonding contributions within the Ge building units, which also become apparent from the short Ge–Ge bond lengths.

Neue Lanthan-Stannide/Germanide: La3Sn4.4Ge0.6, La3Sn3.1Ge0.9 und La9Sn6.7Ge3.3 / New Lanthanum Stannide/Germanides: La3Sn4.4Ge0.6, La3Sn3.1Ge0.9 and La9Sn6.7Ge3.3

Three new ternary mixed lanthanum stannide/germanides have been synthesized and characterized in the course of a systematic study of the phase formation at the 3 : 5, 3 : 4 and 1 : 1 pseudo-binary sections of the ternary system La - Ge - Sn, i. e. a study of the ’coloring’ in mixed tetrelides. The structures of the title compounds have been determined using single-crystal X-ray data, and the electronic structure was analyzed by means of simple electron counting rules and FP-LAPW band structure methods. Even a very minor substitution of Sn by Ge in the 3 : 5 stannide La3Sn5 already changes the structure from the Pu3Pd5 type of the binary stannide towards the Tl4PbTe3 type (La3Sn4.4Ge0.6: tetragonal, space group I4/mcm, a = 861.35(10), c = 1211.48(13) pm, Z = 4, R1 = 0.0420). According to the anionic building blocks present, isolated [Ge/Sn]4− anions and heavily puckered 4.82 nets of three-bonded tin atoms (i. e. [Sn4]4− Zintl ions), a small formal electron excess (+9/−8) occurs, which is also apparent from the position of the minimum in the calculated tDOS. In contrast, starting from the known binary stannide La3Sn4 (orthorhombic, space group Cmcm, Er3Ge4 structure type) a substitution of tin by germanium is possible without a change of the structure type up to the border composition La3Sn3.1Ge0.9 (a = 448.61(5), b = 1170.68(14), c = 1556.0(2) pm, Z = 4, R1 = 0.0380). Germanium preferentially occupies the sites of the [Ge/Sn]3 trimers, whereas the site of the square-planar coordinated Sn(3), which is stabilized by hypervalent bonding, is not populated by germanium. Taking the respective partial bonding into account, the Zintl electron count is also nearly exact (+9/−8). The new complex mixed tetrelide La9Sn6.7Ge3.3 (tetragonal, space group P42/ncm, a = 1602.04(3), c = 1724.42(5) pm, Z = 8, R1 = 0.0706) exhibits nine crystallographically different tetrel positions, which are occupied by isolated Sn and Ge atoms, [Sn/Ge]2 dumbbells, bent [Sn3] trimers, planar four-membered rings [Ge4], and planar six-membered rings [M6]. In this compound, the electron count following the Zintl concept also reveals only a very small formal excess of electrons (+54/−52). Graphical Abstract Neue Lanthan-Stannide/Germanide: La3Sn4.4Ge0.6, La3Sn3.1Ge0.9 und La9Sn6.7Ge3.3 / New Lanthanum Stannide/Germanides: La3Sn4.4Ge0.6, La3Sn3.1Ge0.9 and La9Sn6.7Ge3.3

Lanthan-Triel/Tetrel-ide La(Al,Ga)x(Si,Ge)1-x. Experimentelle und theoretische Studien zur Stabilität intermetallischer 1 : 1-Phasen / Lanthanum Triel/Tetrel-ides La(Al,Ga)x(Si,Ge)1−x. Experimental and Theoretical Studies on the Stability of Intermetallic 1:1 Phases

Systematic studies of the phase formation at the binary sections LaSi - LaGa and LaGe - LaAl have been carried out by means of synthetic, crystallographic and bond theoretical methods. The hightemperature forms of the two binary monotetrelides LaSi and LaGe crystallize with the FeB structure type, whereas LaGa forms the related CrB type and LaAl the significantly different CeAl type. Starting from LaSi/LaGe, the FeB type (orthorhombic, space group Pnma, Z = 4, a = 839.2(1)/842.7(1), b = 399.9(1)/412.3(1), c = 606.2(2)/612.2(1) pm, R1 = 0.0356/0.0298) remains stable only down to a valence electron number per M atom (M = Si, Ge, Al, Ga) of 6.9 (LaGa0.10Si0.90: a = 840.14(7), b = 404.12(12), c = 608.5(2) pm, R1 = 0.0513; LaAl0.15Ge0.85: a = 845.40(7), b = 414.08(13), c = 614.08(14) pm, R1 = 0.0213). In the system LaGaxSi1−x, the stability range of the CrB type (orthorhombic, space group Cmcm, Z = 4) starts at a gallium proportion of 25% (LaGa0.25Si0.75: a = 450.03(8), b = 1140.5(2), c = 406.05(6) pm, R1 = 0.0163) and extends to the border compound LaGa (v. e./M = 6). The CrB type also occurs in the system La-Al-Ge, but is in this case only formed around the 1 : 1 composition in between LaAl0.42Ge0.58 (a = 455.90(12), b = 1161.1(3), c = 418.05(9) pm, R1 = 0.0474) and LaAl0.61Ge0.39 (a = 454.89(10), b = 1168.8(2), c = 420.41(11) pm, R1 = 0.0447). These stability ranges, the variations of several key geometric parameters such as the M-M distances or the heights of the trigonal prisms, and the main aspects of the chemical bonding in these lanthanum monometallides are analyzed using FP-LAPWband structure methods. The structure of the new compound La2Al2Ge (V2B3 structure type, orthorhombic, space group Cmcm, a = 416.69(4), b = 2719.7(4), c = 450.46(5) pm, Z = 2, R1 = 0.0458) combines the structural elements of the CrB/FeB structure family (two-bonded M atoms) with the trigonally planar bonded M atoms of the ThSi2 type in a fully ordered Al and Ge atom distribution and thus without phase width Graphical Abstract Lanthan-Triel/Tetrel-ide La(Al,Ga)x(Si,Ge)1-x. Experimentelle und theoretische Studien zur Stabilität intermetallischer 1 : 1-Phasen / Lanthanum Triel/Tetrel-ides La(Al,Ga)x(Si,Ge)1−x. Experimental and Theoretical Studies on the Stability of Intermetallic 1:1 Phases