Localized and Delocalized Chemical Bonding in the Compounds CaNiGe2, SrNiGe2, and SrNiSn2

The stannides Ca1.692Pt2Sn3.308, SrPtSn2 and EuAuSn2

Polycrystalline samples of the stannides Ca1.692Pt2Sn3.308, SrPtSn2 and EuAuSn2 were synthesized directly from the elements, using sealed tantalum ampoules as crucible material. The reactions were performed in muffle or induction furnaces. The phase purity of the samples was studied by X-ray powder diffraction (Guinier technique). The structures of Ca1.692Pt2Sn3.308 and SrPtSn2 were refined from single-crystal X-ray diffractometer data: NdRh2Sn4 type, Pnma, a = 1887.22(13), b = 441.22(3), c = 742.89(4) pm, wR = 0.0626, 1325 F 2 values, 45 variables for Ca1.692(8) Pt2Sn3.308(8) and CeNiSi2 type, Cmcm, a = 462.59(5), b = 1932.8(2), c = 458.00(5) pm, wR = 0.0549, 481 F 2 values, 18 variables for SrPtSn2. The calcium compound shows a homogeneity range Ca1+x Pt2Sn4−x with substantial Sn4/Ca2 mixing on one of the 4c Wyckoff positions. The [PtSn2] network is characterized by Pt–Sn (269–281 pm) and Sn–Sn (306–336 pm) bonding interactions. SrPtSn2 contains two different tin substructures: (i) Sn1–Sn1 zig-zag chains (282 pm) and (ii) orthorhombically distorted Sn2 squares (326 pm) with stronger and weaker Sn–Sn bonding. Together, the platinum and tin atoms build up a three-dimensional [PtSn2] network in which the platinum atoms have a distorted square-pyramidal tin coordination with Pt–Sn distances ranging from 261–270 pm. EuAuSn2 also crystallizes with the CeNiSi2-type structure with the lattice parameters a = 453.9(1), b = 2018.9(5) and c = 456.8(1) pm. Temperature dependent magnetic susceptibility studies indicate europium(II) with an experimental magnetic moment of 8.28(2) µB per Eu atom. EuAuSn2 is ordered antiferromagnetically at T N  = 14.8(2) K. 151Eu Mössbauer spectra confirm the oxidation state +2 for europium (isomer shift δ = −11.17(2) mm s−1) and the magnetic ordering at low temperature (21.8 T magnetic hyperfine field at 6 K).

The germanides APtGe2 (A = Ca, Sr, Eu)

The germanides APtGe2 (A = Ca, Sr, Eu) were synthesized from the elements in sealed tantalum ampoules in an induction furnace followed by annealing. The polycrystalline samples were characterized by powder X-ray diffraction (Guinier patterns). The structures of CaPtGe2 (CeRhSn2 type, Cmcm, a = 443.45(3), b = 1593.03(12), c = 886.15(6) pm, wR = 0.0464, 673 F 2 values, 30 variables) and EuPt1.043(5)Ge1.957(5) (CeNiSi2 type, Cmcm, a = 445.23(4), b = 1752.5(2), c = 429.63(4) pm, wR = 0.0415, 389 F 2 values, 19 variables) were refined from single crystal X-ray diffractometer data. One of the germanium site in EuPt1.043(5)Ge1.957(5) showed a small Ge/Pt mixed occupancy. SrPtGe2 (a = 451.13(6), b = 1764.8(2), c = 429.60(5) pm) is isotypic with EuPtGe2. The platinum atoms in both germanides have trigonal prismatic coordination: Pt@Ca4Ge2 and Pt@Eu4Ge2. The germanium substructures differ significantly: Ge2 dumb-bells with 246 and 262 pm Ge–Ge distances in CaPtGe2 versus germanium zig-zag chains (253 pm Ge–Ge distances) and a distorted square net (309 pm Ge–Ge distances) in EuPtGe2. CaPtGe2 and SrPtGe2 are diamagnetic. EuPtGe2 is a Curie–Weiss paramagnet with an experimental magnetic moment of 7.88(1) µB Eu atom−1. The purely divalent character of europium is manifested by a single signal at δ = −11.38(1) mm s−1 in the 151Eu Mössbauer spectrum at 78 K. EuPtGe2 orders antiferromagnetically at T N = 5.4(1) K.

Electron-Poor Polar Intermetallics: Complex Structures, Novel Clusters, and Intriguing Bonding with Pronounced Electron Delocalization

Intermetallic compounds represent an extensive pool of candidates for energy related applications stemming from magnetic, electric, optic, caloric, and catalytic properties. The discovery of novel intermetallic compounds can enhance understanding of the chemical principles that govern structural stability and chemical bonding as well as finding new applications. Valence electron-poor polar intermetallics with valence electron concentrations (VECs) between 2.0 and 3.0 e^-/atom show a plethora of unprecedented and fascinating structural motifs and bonding features. Therefore, establishing simple structure-bonding-property relationships is especially challenging for this compound class because commonly accepted valence electron counting rules are inappropriate. During our efforts to find quasicrystals and crystalline approximants by valence electron tuning near 2.0 e^-/atom, we observed that compositions close to those of quasicrystals are exceptional sources for unprecedented valence electron-poor polar intermetallics, e.g., Ca₄Au₁₀In₃ containing (Au₁₀In₃) wavy layers, Li_14.7Mg_36.8Cu_21.5Ga₆₆ adopting a type IV clathrate framework, and Sc₄Mg_xCu_15-xGa_7.5 that is incommensurately modulated. In particular, exploratory syntheses of AAu₃T (A = Ca, Sr, Ba and T = Ge, Sn) phases led to interesting bonding features for Au, such as columns, layers, and lonsdaleite-type tetrahedral frameworks. Overall, the breadth of Au-rich polar intermetallics originates, in part, from significant relativistics effect on the valence electrons of Au, effects which result in greater 6s/5d orbital mixing, a small effective metallic radius, and an enhanced Mulliken electronegativity, all leading to ultimate enhanced binding with nearly all metals including itself. Two other successful strategies to mine electron-poor polar intermetallics include lithiation and "cation-rich" phases. Along these lines, we have studied lithiated Zn-rich compounds in which structural complexity can be realized by small amounts of Li replacing Zn atoms in the parent binary compounds CaZn₂, CaZn₃, and CaZn₅; their phase formation and bonding schemes can be rationalized by Fermi surface-Brillouin zone interactions between nearly free-electron states. "Cation-rich", electron-poor polar intermetallics have emerged using rare earth metals as the electropositive ("cationic") component together metal/metalloid clusters that mimic the backbones of aromatic hydrocarbon molecules, which give evidence of extensive electronic delocalization and multicenter bonding. Thus, we can identify three distinct, valence electron-poor, polar intermetallic systems that have yielded unprecedented phases adopting novel structures containing complex clusters and intriguing bonding characteristics. In this Account, we summarize our recent specific progress in the developments of novel Au-rich BaAl₄-type related structures, shown in the "gold-rich grid", lithiation-modulated Ca-Li-Zn phases stabilized by different bonding characteristics, and rare earth-rich polar intermetallics containing unprecedented hydrocarbon-like planar Co-Ge metal clusters and pronounced delocalized multicenter bonding. We will focus mainly on novel structural motifs, bonding analyses, and the role of valence electrons for phase stability.

From one to three dimensions: corrugated ∞(1)[NiGe] ribbons as a building block in alkaline earth metal Ae/Ni/Ge phases with crystal structure and chemical bonding in AeNiGe (Ae = Mg, Sr, Ba)

The new equiatomic nickel germanides MgNiGe, SrNiGe, and BaNiGe have been synthesized from the elements in sealed tantalum tubes using a high-frequency furnace. The compounds were investigated by X-ray diffraction both on powders and single crystals. MgNiGe crystallizes with TiNiSi-type structure, space group Pnma, Z = 4, a = 6.4742(2) Å, b = 4.0716(1) Å, c = 6.9426(2) Å, wR2 = 0.033, 305 F(2) values, 20 variable parameters. SrNiGe and BaNiGe are isotypic and crystallize with anti-SnFCl-type structure (Z = 4, Pnma) with a = 5.727(1) Å, b = 4.174(1) Å, c = 11.400(3) Å, wR2 = 0.078, 354 F(2) values, 20 variable parameters for SrNiGe, and a = 5.969(4) Å, b = 4.195(1) Å, c = 11.993(5) Å, wR2 = 0.048, 393 F(2) values, 20 variable parameters for BaNiGe. The increase of the cation size leads to a reduction of the dimensionality of the [NiGe] polyanions. In the MgNiGe structure the nickel and germanium atoms build a ∞(3)[NiGe] network with magnesium atoms in the channels. In SrNiGe and BaNiGe the ∞(1)[NiGe] ribbons are separated by strontium/barium atoms, whereas in the known CaNiGe structure the ribbons are fused to two-dimmensional atom slabs. The crystal chemistry and chemical bonding in AeNiGe (Ae = Mg, Ca, Sr, Ba) are discussed. The experimental results are reconciled with electronic structure calculations performed using the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method.

Syntheses and structures of Sc2Nb(4–x)Sn5, YNb6Sn6, and ErNb6Sn5: exploratory studies in ternary rare-earth niobium stannides

Three new rare-earth (RE) niobium stannides, namely, Sc(2)Nb(4-x)Sn(5) (x = 0.37, 0.52), YNb(6)Sn(6), and ErNb(6)Sn(5), have been obtained by reacting the mixture of corresponding pure elements at high temperature and structurally characterized by single-crystal X-ray diffraction studies. Sc(2)Nb(4-x)Sn(5) crystallizes in the orthorhombic space group Ibam (No. 72) and belongs to the V(6)Si(5) type. Its structure features a three-dimensional (3D) network composed of two-dimensionally (2D) corrugated [Nb(2)Sn(2)] and [Nb(2)Sn(3)] layers interconnected via Nb-Sn bonds, forming one type of one-dimensional (1D) narrow tunnels along the c axis occupied by Sc atoms. YNb(6)Sn(6) crystallizes in the hexagonal space group P6/mmm (No. 191) and adopts the HfFe(6)Ge(6) type, and ErNb(6)Sn(5) crystallizes in the trigonal space group R3m (No. 166) and belongs to the LiFe(6)Ge(5) type. Their structures both feature 3D networks based on 2D [Nb(3)Sn], [Sn(2)], and [RESn(2)] layers (RE = Y, Er). In YNb(6)Sn(6), one type of [Nb(3)Sn] layer is interconnected by [Sn(2)] and [YSn(2)] layers via Nb-Sn bonds to form a 3D network. However, in ErNb(6)Sn(5), two types of [Nb(3)Sn] layers are interlinked by [Sn(2)] and [ErSn(2)] layers via Nb-Sn bonds into a 3D framework. Electronic structure calculations and magnetic property measurements for "Sc(2)Nb(4)Sn(5)" and YNb(6)Sn(6) indicate that both compounds show semimetallic and temperature-independent diamagnetic behavior.

Structure and Properties of Gd3Ge4: The Orthorhombic RE3Ge4 Structures Revisited (RE = Y, Tb−Tm)

The new binary compound Gd(3)Ge(4) has been synthesized and its structure has been determined from single-crystal X-ray diffraction. Gd(3)Ge(4) crystallizes in the orthorhombic space group Cmcm (No. 63) with unit cell parameters a = 4.0953(11) A, b = 10.735(3) A, c = 14.335(4) A, and Z = 4. Its structure can be described as corrugated layers of germanium atoms with gadolinium atoms enclosed between them. The bonding arrangement in Gd(3)Ge(4) can also be derived from that of the known compound GdGe (CrB type) through cleavage of the (infinity)(1)[Ge(2)] zigzag chains in GdGe and a subsequent insertion of an extra germanium atom between the resulting triangular fragments. Formally, these characteristics represent isotypism with the Er(3)Ge(4) type (Pearson's oC28). However, re-examination of the crystallography in the whole RE(3)Ge(4) series (RE = Y, Tb-Tm) revealed discrepancies and called into question the accuracy of the originally determined structures. This necessitated a new rationalization of the bonding, which is provided in the context of a comparative discussion concerning both the original and revised structure models, along with an analysis of the trends across the series. The temperature dependence of the magnetic susceptibility of Gd(3)Ge(4) shows that it is paramagnetic at room temperature and undergoes antiferromagnetic ordering below 29 K. Magnetization, resistivity, and calorimetry data for several other members of the RE(3)Ge(4) family are presented as well.