Svilen Bobev

Zintl Phase Variations Through Cation Selection. Synthesis and Structure of A21Cd4Pn18 (A = Eu, Sr, Ba; Pn = Sb, Bi)

Four new Zintl compounds, Ba21Cd4Sb18, Ba21Cd4Bi18, Sr21Cd4Bi18, and Eu21Cd4Bi18, have been synthesized and structurally characterized. Despite the similarity in their chemical formulas and regardless of their identical electronic requirements, the structures of the Ba compounds and the Sr and Eu compounds are subtly different. Due to the cations, a cleavage of a selected pnicogen-cadmium bond occurs and the structures adapt to a novel packing of the resultant heteronuclear anions.

Isolated ∞1[ZnPn2]4− Chains in the Zintl Phases Ba2ZnPn2 (Pn = As, Sb, Bi)—Synthesis, Structure, and Bonding

Reported are the synthesis of three new Zintl phases, Ba2ZnAs2 (I), Ba2ZnSb2 (II), Ba2ZnBi2 (III) and their structural characterization by single-crystal X-ray diffraction. They are isoelectronic and isotypic and crystallize in the orthorhombic space group Ibam, with four formula units per cell (Pearson symbol oI20; K2SiP2 type). Lattice parameters are as follows: a = 13.399(9)/14.133(3)/14.325(6); b = 6.878(5)/7.1919(15)/7.280(3); and c = 6.541(4)/6.9597(15)/7.089(3) A for I/II/III, respectively. The structure can be viewed as polyanionic chains, infinity1[ZnPn2]4- (Pn = As, Sb, Bi), running parallel to the c-axis, with Ba2+ cations separating them. The chains are made of edge-shared ZnPn4 tetrahedra, which are isosteric with the infinity1[SiS4/2] chains in SiS2. This and some other structural parallels with known Zintl phases have been discussed. The experimental results have been complemented by tight-binding linear muffin-tin orbital electronic structure calculations.

Synthesis, Structure, and Bonding of the Zintl Phase Ba3Cd2Sb4

Reported are the synthesis of the new ternary compound Ba3Cd2Sb4 and its structure determination by single-crystal X-ray diffraction. Ba3Cd2Sb4 crystallizes with the monoclinic space group C2/m (No. 12); unit cell parameters a = 17.835(2) A, b = 4.8675(5) A, c = 7.6837(7) A, and beta = 112.214(1) degrees; Z = 4. Its structure can be viewed as made of Ba2+ cations and [Cd2Sb4] double chains that are interconnected through Sb-Sb bonds to form 2D infinity2[Cd2Sb4]6- layers. The bonding arrangement in Ba3Cd2Sb4 can also be derived from other known structure types that feature similar fragments, such as TiNiSi, Ca3AlAs3, and Ca5Al2Sb6. Tight-binding linear muffin-tin-orbital band structure calculations are presented as well and show that the constituent elements have closed-shell configurations, indicative of Ba3Cd2Sb4 being a Zintl phase with poor metallic behavior. Crystal orbital Hamilton population analyses on selected atomic interactions in this structure are discussed within the context of the site preference, manifested in the mixed-cation compounds and Ba3-xAxCd2Sb4, where A = Ca, Sr, Eu, and Yb.

BaGa2Pn2 (Pn = P, As): new semiconducting phosphides and arsenides with layered structures.

Reported are the synthesis, the structural characterization, and the electronic band structures of two new Zintl phases: BaGa2P2 and BaGa2As2. Both compounds are isoelectronic and isotypic and crystallize in a monoclinic system with a new structure type (Pearson symbol mP20). The structures have been established by single-crystal X-ray diffraction, space group P2(1)/c (Z = 4), with lattice parameters as follows: a = 7.3363(13)/7.495(5) Å; b = 9.6648(17)/9.901(6) Å; c = 7.4261(13)/7.643(5) Å; beta = 115.373(2) degrees/115.381(8) degrees for BaGa2P2/BaGa2As2, respectively. The atomic arrangements in both cases are devoid of disorder and are best rationalized as polyanionic layers, (infinity)(2)[Ga2Pn2]2- (Pn = P, As), with Ba2+ cations separating them. The layers, in turn, can be viewed as the result of condensation of Ga2Pn6 units, which are isosteric with the ethane molecule in its staggered conformation. Structural parallels with other known Zintl phases are presented. The electronic structures, computed using the tight-binding linear muffin-tin orbital methods (TB-LMTO), are discussed as well.

Mixed Cations and Structural Complexity in (Eu1-xCax)4In3Ge4 and (Eu1-xCax)3In2Ge3-The First Two Members of the Homologous Series A2[n+m]In2n+mGe2[n+m] (n, m = 1, 2, ...∞; A = Ca, Sr, Ba, Eu, or Yb)

Reported are the synthesis and the structural characterization of two members of a new homologous series of polar intermetallic compounds, which exist only with mixed alkaline-earth and rare-earth metal cations. Crystals of (Eu(1-x)Ca(x))(4)In(3)Ge(4) (0.35(1) <or= x <or= 0.70(1)) and (Eu(1-x)Ca(x))(3)In(2)Ge(3) (0.78(1) <or= x <or= 0.90(1)) have been grown using a molten In metal flux and structurally characterized by single-crystal X-ray diffraction. (Eu(1-x)Ca(x))(4)In(3)Ge(4) adopts the monoclinic Mg(5)Si(6)-type structure (space group C2/m, Z = 2, Pearson symbol mS22) with lattice parameters a = 16.874(1)-17.024(2) A, b = 4.496(3)-4.556(1) A, c = 7.473(4)-7.540(1) A, and beta = 107.306(10)-105.631(3) degrees . (Eu(1-x)Ca(x))(3)In(2)Ge(3) crystallizes with a novel orthorhombic structure (space group Pnma, Z = 4, Pearson symbol oP32) with lattice parameters in the ranges a = 7.382(2)-7.4010(9) A, b = 4.452(1)-4.4640(6) A, and c = 23.684(6)-23.734(3) A, depending on the Eu/Ca ratio. The polyanionic substructures in both cases are related and are based on InGe(4) edge-shared tetrahedra, Ge(2) dimers, and bridging In atoms in a nearly square-planar environment. The (Eu(1-x)Ca(x))(4)In(3)Ge(4) structure can be viewed as a 1:1 intergrowth of Mo(2)FeB(2)-like and TiNiSi-like fragments, whereas (Eu(1-x)Ca(x))(3)In(2)Ge(3) can be rationalized as a 2:1 intergrowth of the same structural motifs. Both phases exhibit fairly wide homogeneity ranges and exist only with mixed cations. The experimental results have been complemented by linear muffin-tin orbital tight-binding band structure calculations, as well as an analysis of the observed cationic site preferences.

Synthesis, structure, chemical bonding, and magnetism of the series RELiGe2 (RE = La-Nd, Sm, Eu).

This article focuses on the synthesis and the crystal chemistry of six members of a series of rare-earth metal based germanides with general formula RELiGe(2) (RE = La-Nd, Sm, and Eu). The structures of these compounds have been established by single-crystal X-ray diffraction (CaLiSi(2) structure type, space group Pnma, Z = 4, Pearson symbol oP16). The chemical bonding within this atomic arrangement can be rationalized in terms of anionic germanium zigzag chains, conjoined via chains of edge-shared LiGe(4) tetrahedra and separated by rare-earth metal cations. The structure can also be viewed as an intergrowth of AlB(2)-like and TiNiSi-like fragments, or as the result of the replacement of 50% of the rare-earth metal atoms by lithium in the parent structure of the REGe monogermanides. Except for LaLiGe(2) and SmLiGe(2), the remaining four RELiGe(2) phases exhibit Curie-Weiss paramagnetism above about 50 K. In the low temperature regime, the localized 4f electrons in CeLiGe(2), PrLiGe(2), and SmLiGe(2) order ferromagnetically, while antiferromagnetic ordering is observed for NdLiGe(2) and EuLiGe(2). The calculated effective magnetic moments confirm RE(3+) ground states in all cases excluding EuLiGe(2), in which the magnetic response is consistent with Eu(2+) configuration (J = S = 7/2). The experimental results have been complemented by tight-binding linear muffin-tin orbital (TB-LMTO) band structure calculations.

Magnesium Substitutions in Rare-Earth Metal Germanides with the Orthorhombic Gd5Si4-type Structure. Synthesis, Crystal Chemistry, and Magnetic Properties of RE5−xMgxGe4 (RE = Gd−Tm, Lu, and Y)

A series of magnesium-substituted rare-earth metal germanides with a general formula RE(5-x)Mg(x)Ge(4) (x approximately = 1.0-2.3; RE = Gd-Tm, Lu, Y) have been synthesized by high-temperature reactions and structurally characterized by single-crystal X-ray diffraction. These compounds crystallize with the common Gd(5)Si(4) type structure in the orthorhombic space group Pnma (No. 62; Z = 4; Pearsons code oP36) and do not appear to undergo temperature-induced crystallographic phase transitions down to 120 K. Replacing rare-earth metal atoms with Mg, up to nearly 45% at., reduces the valence electron count and is clearly expressed in the subtle changes of the Ge-Ge and metal-metal bonding. Magnetization measurements as a function of the temperature and the applied field reveal complex magnetic structures at cryogenic temperatures and Curie-Weiss paramagnetic behavior at higher temperatures. The observed local moment magnetism is consistent with RE(3+) ground states in all cases. In the magnetically ordered phases, the magnetization cannot reach saturation in fields up to 50 kOe. The structural trends across the series and the variations of the magnetic properties as a function of the Mg content are also discussed.

Methanol—inhibitor or promoter of the formation of gas hydrates from deuterated ice?

Abstract Kinetic studies are reported of the effect of methanol on the rate of formation of CO2- and CH4-hydrates by means of in situ time-of-flight neutron powder diffraction. The experiments were carried out at temperatures ranging from 200 to 250 K and pressures up to 7 MPa. The samples were prepared from mixtures of ground, deuterated ice and deuterated methanol (up to 20 vol%), which were transformed in situ into CO2- or CH4-hydrates by pressurizing the systems with the corresponding gas. The observed rates of formation of hydrates are orders of magnitude higher than the rate of formation from pure deuterated ice under the same pressure and temperature conditions. Glycols and alcohols, methanol in particular, are long known as thermodynamic inhibitors of hydrate formation. Our study indicates that methanol can also act as a kinetic promoter for the formation of gas hydrates. Preliminary data suggest that the kinetics also depend strongly on concentration and the isotopic composition.

Are Ba11Cd6Sb12 and Sr11Cd6Sb12 Zintl phases or not? A density-functional theory study

The chemical bonding and the electronic band structures of two isoelectronic and isostructural Zintl compounds, Sr11Cd6Sb12 and Ba11Cd6Sb12, have been studied on the basis of the density‐functional theory (DFT) using the tight‐binding linear‐muffin‐tin‐orbital (TB‐LMTO‐ASA) approach and the local‐density approximation (LDA). These results reveal that the classic Zintl reasoning and the concept of two‐center two‐electron bonds cannot explain the subtleties of this complex structure type. The computations also suggest that the antimony dimers present in these structures play an important role and allow for a greater flexibility in optimization of the bonding with the surrounding d‐metal atoms.

Closely related rare-earth metal germanides RE2Li2Ge3 and RE3Li4Ge4 (RE = La-Nd, Sm): synthesis, crystal chemistry, and magnetic properties.

Reported are the syntheses, crystal structures, and magnetic susceptibilities of two series of closely related rare-earth metal-lithium germanides RE(2)Li(2)Ge(3) and RE(3)Li(4)Ge(4) (RE = La-Nd, Sm). All title compounds have been synthesized by reactions of the corresponding elements at high temperatures, and their structures have been established by single-crystal X-ray diffraction. RE(2)Li(2)Ge(3) phases crystallize in the orthorhombic space group Cmcm (No. 63) with the Ce(2)Li(2)Ge(3) structure type, while the RE(3)Li(4)Ge(4) phases crystallize in the orthorhombic space group Immm (No. 71) with the Zr(3)Cu(4)Si(4) structure type, respectively. Both of their structures can be recognized as the intergrowths of MgAl(2)Cu- and AlB(2)-like slabs, and these traits of the crystal chemistry are discussed. Temperature-dependent direct-current magnetization measurements indicate Curie-Weiss paramagnetism in the high-temperature regime for RE(2)Li(2)Ge(3) and RE(3)Li(4)Ge(4) (RE = Ce, Pr, Nd), while Sm(2)Li(2)Ge(3) and Sm(3)Li(4)Ge(4) exhibit Van Vleck-type paramagnetism. The data are consistent with the local-moment magnetism expected for RE(3+) ground states. At temperatures below ca. 20 K, magnetic ordering transitions have been observed. The experimental results have been complemented by tight-binding linear muffin-tin orbital electronic-band-structure calculations.