Magnetische Messungen anHeusler-Phasen (Co,Mn)2 XY (X=Ti,Zr,Hf und Mn;Y=Ge und Sn)

Magnetic measurements onHeusler alloys (Co,Mn)₂ XY have been performed. With a few exceptions there is no enhancement of ferromagnetism while substituting cobalt by manganese. The results can be explained by a partially antiferromagnetic ordering of the manganese atoms.     

Magnetische Messungen anHeusler-Phasen (Co,Ni)2 XY (X=Ti,Zr, Hf,V und Mn;Y=Ge und Sn)

Magnetic measurements onHeusler alloys (Co, Ni)₂ XY are performed. The transitions from ferromagnetic to paramagnetic behaviour in some systems are of special interest.

---

---

Mit 4 Abbildungen     

Evolution structurale sous haute pression des phases K2MO3 (M = Zr,Hf, Sn,Pb)

The study of the high-pressure modifications of the oxides K₂MO₃ (M = Zr, Hf, Sn, Pb) confirms the tendency of potassium to adopt the trigonal prismatic coordination in layer oxides: The β form is characterized by layers of (K, M)O₆ octahedra with common edges, between which other potassium atoms are inserted with prismatic coordination. For M = Zr and Hf, an increase of pressure or temperature transforms β to the γ or δ variety, both derived from the NaCl structure. The influence of the electronegativity of M on the potassium coordination in layer structures is discussed.     

Crystal chemistry of Li4XO4 phases: X = Si,Ge, Ti

The polymorphism of Li₄SiO₄, Li₄GeO₄ and Li₄TiO₄ has been studied. All three phases are isostructural above 700–750°C, but undergo phase transformations on cooling. At lower temperatures only Li₄GeO₄ and Li₄TiO₄ are isostructural. Subsolidus phase equilibria within the system Li₄SiO₄Li₄GeO₄Li₄TiO₄ have been investigated.     

Synthesis and structure of the Zintl compounds Na2BaTt4 (Tt = Si, Ge)

The isotypic title compounds ate obtained from reactions of stoichiometric mixtures of the elements in welded Ta containers at 700–800 °C. The structure of Na₂BaSi₄ was determined by single-crystal X-ray diffraction (orthorhombic, Ima2 (No. 46), Z = 8, a = 9.524, b = 17.980(3), c = 8.570(3) Å). The compound crystallizes in a novel structure type with Na⁺ and Ba²⁺ cations and two crystallographically-independent isolated Si₄⁴⁻ anions. The anions are nearly regular tetrahedra with bond distances of 2.385(3) to 2.448(4) Å and bond angles between 59.2 and 61.7°. The overall structure may be described as a distorted face-centered-cubic array of Si₄⁴⁻ anions in which Na⁺ and Ba²⁺ atoms occupy all tetrahedral and octahedral holes in an ordered way. The result is closely related to the topology of f.c.c. Li₃Bi type but with ordering of the two cations. Magnetic measurements showed that Na₂BaSi₄ is diamagnetic, and its family can be formulated in terms of oxidation states as Zintl phases (Na⁺)₂Ba²⁺(Tt₄⁴⁻).     

Vibrational assignment of some X3MCo(CO)4 molecules (M = Si,Ge, Sn and X = Cl,Br, I)

The infrared and Raman vibrational spectra of X₃MCo(CO)₄ compounds (M = Si, Ge, Sn and X = Cl, Br, I) including depolarization measurements are presented. These spectra result in complete vibrational assignments which are different from those reported previously.     

Magnetic Susceptibilities of ThM2X2 Compounds (M = Cr,Mn, Fe,Co, Ni and Cu; X = Si and Ge)

Magnetic susceptibilities of a series of ternary intermetallic compounds of the general composition ThM₂X₂(M = Cr, Mn, Fe, Co, Ni and Cu; X = Si and Ge) were determined by means of FARADAY method in the temperature interval 100–570°K. The results confirm the previously proposed magnetic behaviour of these compounds, based on the observation of the basic crystallographic parameters as a function of the atomic number of the transition metal element. The compounds were found to be alternatively ferromagnetics and antiferromagnetics depending on the atomic number of the transition metal atom.     

Formation of phases with the Ca3Ga2Ge4O14 structure in the AO-TeO3-Ga2O3-XO2 (A = Pb, Ba, Sr; X = Si, Ge) and PbO-TeO3-MO-GeO2 (M = Zn, Co) systems

Variable-composition phases with the Ca₃Ga₂Ge₄O₁₄ structure with a wide homogeneity region were synthesized in quaternary silicate and germanate systems. In (Pb,Ga)-containing systems, sections of the homogeneity region were studied and their boundaries were determined. Cationic vacancies were found. In the 3e position, their concentration can be as high as 25%. The synthesized phases melt incongruently or experience solid-phase decomposition.     

Crystal structure and thermal expansion of the low- and high-temperature forms of BaM(PO4)2 compounds (M=Ti, Zr, Hf and Sn)

The crystal structure of β-BaZr(PO₄)₂, archetype of the high-temperature forms of BaM(PO₄)₂ phosphates (with M=Ti, Zr, Hf and Sn), has been solved ab initio by Rietveld analysis from synchrotron X-ray powder diffraction data. The phase transition appears as a topotactic modification of the monoclinic (S.G. C2/m) lamellar α-structure into a trigonal one (S.G. ) through a simple mechanism involving the unfolding of the layers. The thermal expansion is very anisotropic (e.g., −4.1<α_i<34.0×10⁻⁶ K⁻¹ in the case of α-BaZr(PO₄)₂) and quite different in the two forms, as a consequence of symmetry. It stems from a complex combination of several mechanisms, involving bridging oxygen rocking in M-O-P linkages, and “bond thermal expansion”.     

Protolyses of (CH3)3SnM(CH3)3 (M = Sn,Ge, Si,C)

Product and kinetic studies on the reactions of hydrogen chloride in methanol solution with the substrates (CH₃)₃SnM(CH₃)₃ (M = Sn; Ge and Si) show that both SnM and SnCH₃ cleavage reactions occur, at similar rates, and are followed by other reactions giving complex but explicable mixtures of products. Similar behaviour is observed for trifluoroacetolysis in carbon tetrachloride solution, and some intermediates are observable. Trifluoroacetolysis of (CH₃)₃SnC(CH₃)₃ results in exclusive SnCH₃ cleavage. The very slow apparent solvolysis in acetic acid solution is thought to involve reaction with dissolved oxygen.     

Electronic Structure of Fe+X Alloys (X = Si, Ge, Co)

The electronic structure of Fe–Ge, Fe–Si, and Fe–Co alloys has been investigated by X-ray photoelectron spectroscopy. In Fe–Ge alloys with less than 10 at.% Ge, the Fe–Ge bond is mainly formed by the Fe 4sp- and Ge 4p-electrons. The results obtained for this system are identical to those for the Fe–Si system. The form of the valence band reflects the density distribution of both iron d-electronic states the and p-electronic states of the second component, having more extended density distribution of valence electrons. In FeCo alloys, strong spatial localization of d-electron density takes place in the vicinity of the corresponding atoms, which is stronger on the iron atoms compared to pure iron; the valence band has a two-band structure reflecting the density distribution of the d-states of each component. X-ray photoelectron spectroscopy data are in good agreement with kinetic data for the alloys.     

Structural and electronic properties of Al12X+ (X=C, Si, Ge, Sn, and Pb) clusters

Using the first-principles method with the generalized gradient approximation, the authors have studied the structural and electronic properties of Al(12)X(+) (X=C, Si, Ge, Sn, and Pb) clusters in detail. The ground state of Al(12)C(+) is a low symmetry C(s) structure instead of an icosahedron. However, the Si, Ge, Sn, and Pb atom doped cationic clusters favor icosahedral structures. The ground states for Al(12)Si(+) and Al(12)Ge(+) are icosahedra, while the C(5nu) structures optimized from an icosahedron with a vertex capped by a tetravalent atom have the highest binding energy for Al(12)Sn(+) and Al(12)Pb(+) clusters. The I(h) structure and the C(5nu) structure are almost degenerate for Al(12)Ge(+), whose binding energy difference is only 0.03 eV. The electronic properties are altered much by removing an electron from the neutral cluster. The binding strength of a valence electron is enhanced, while the binding energy of the cluster is reduced much. Due to the open electronic shell, the band gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital are approximately 0.3 eV for the studied cationic clusters.     

Electronic structure and vibrational spectra of X20H20 (X=C, Si, Ge, Sn): A theoretical study

Optimized geometries, HOMO–LUMO gaps, vertical ionization potentials and electron affinities are obtained using HF, and B3LYP methods with 6-311G** basis set for C₂₀H₂₀, Si₂₀H₂₀ and Ge₂₀H₂₀. For germanium and tin analogues, B3LYP calculations are performed with LANL2DZ effective core potential. Electron correlation is included by doing MP2 calculation. The harmonic frequencies of all the compounds are obtained using B3LYP with 6-311G** and/or LANL2DZ basis sets. The force field and vibrational spectra are analyzed and 74 symmetry unique non-redundant local force constants are evaluated. Probable assignments are proposed for all the fundamentals based on the potential energy distribution.     

杂硼原子簇XB6+ (X=C,Si, Ge,Sn, Pb)的稳定性和芳香性

利用从头算MP2方法和密度泛函理论B3LYP和B3PW91方法, 研究了杂硼原子簇XB₆⁺ (X=C, Si, Ge, Sn, Pb)的结构、稳定性及化学键合情况. 对C, Si, Ge, B使用6-311+G(d)基组, 对Sn和Pb使用LANL2DZ赝势基组. 研究结果表明, 具有C_s对称性的假平面XB₆⁺ (X=C, Si, Ge, Sn, Pb)结构是势能面上的全域极小点, 其稳定性要高于C_6v对称性的锥形结构和C₂对称性的假锥形结构. 在B3LYP水平上, 对这些异构体的势能面的极小点进行了自然键轨道(NBO)的分析; 对最稳定构型的最高占据分子轨道(HOMO)和最低空轨道(LUMO)能级差、分子轨道(MO)和核独立化学位移(NICS)进行了计算和讨论. 分析了杂原子和硼原子间、相邻硼原子间的键合情况, 讨论了最稳定构型的芳香性质.     

Metallorganische verbindungen des 1,2-diethinylbenzols vom typ o-C6H4(CCMR3)2 (M = Si,Ge, Pb)

The preparation of the compounds o-C₆H₄(CCMR₃)₂ (M = Si, Ge, Pb; R = CH₃; M = Pb; R = C₆H₅) is described. Their properties are compared with those of o-C₆H₄(CCSnR₃)₂ (R = CH₃, C₆H₅) and those of their p-isomers. The structures and bonding conditions proposed for these molecules are supported by dipole measurements, mass spectroscopy, IR, Raman, ¹H NMR and ¹³C NMR data.     

XVI. (2,5-C4Bu2RHN)MCl3 (M = Ti, Zr, Hf; R = H, SiMe3)—Azacyclopentadienyl-Komplexe der Gruppe 4-Metalle

The azacyclopentadienyl compounds (2,5-C₄^tBu₂RHN)MCl₃ (M = Ti, Zr, Hf; R = H, SiMe₃) have been prepared as stable solids from the lithiated pyrroles and MCl₄. The π-coordination of the azacyclopentadienyl ligands, as suggested from ¹³ C NMR data, has been confirmed for (2,5-C₄^tBu₂H₂N)TiCl₃ by an X-ray diffraction study.     

Vibrational properties and first hyperpolarizability of cyclopentadiene homologues C4H4XH2 (X=C, Si, Ge, Sn): ab initio HF and DFT investigation

IR and Raman spectra and static electronic and vibrational first hyperpolarisabilities, β, of the cyclopentadiene homologues C₄H₄XH₂ (X=C, Si, Ge, Sn) have been calculated by conventional ab initio and DFT-B3LYP methods using Sadlej POL and correlation consistent Dunning basis sets to investigate on the role of the heteroatom on the response property. The pure vibrational contribution to β has been evaluated within the double harmonic oscillator approximation. A clear heavy atom effect was found in the vibrational frequencies and intensities localised on the XH₂ and C–X–C fragments, which decrease and increase, respectively, as the size of the heteroatom increases. Both β^e and β^v show an increase along the series, the vibrational contribution to β being modest in cyclopentadiene, but substantial in metalloles, especially for silole, where it is about two times greater than the electronic counterpart. The β^v/β^e ratio is strongly dependent on the basis set, decreasing as the quality of the basis set increases. Both β^v and total β^e+β^v values of the cyclopentadiene homologues are higher than the corresponding ones in the furan series, except for stannole and tellurophene which have close total β^e+β^v values.     

First-principle studies of Ca-X (X=Si,Ge,Sn,Pb) intermetallic compounds

The structural properties, elastic properties, heats of formation, electronic structures, and densities of states of 20 intermetallic compounds in the Ca-X (X=Si, Ge, Sn, Pb) systems have been systematically investigated by using first-principle calculations. Our computational results indicated that with increasing atomic weight of X, the bulk modulus of Ca-X intermetallic compounds decreases gradually. It was also found that Ca₃₆Sn₂₃ and CaPb are mechanically unstable phases. Results on the electronic energy band and densities of states also indicated that Ca₃Si₄ is an indirect band gap semiconductor with a band gap of 0.598 eV, and Ca₂Si, Ca₂Ge, Ca₂Sn, and Ca₂Pb are direct band gap semiconductors with band gaps of 0.324, 0.265, 0.06, and 0.07 eV, respectively. In addition, it is found that the absolute values of heats of formation for all Ca-X intermetallics are larger than 30 kJ/mol atom.