Tantalum‐Niobium Mixing in Ta3–xNbxTeI7 (0 ≤ x ≤ 3)

A substitutional study of the layered, trinuclear metal cluster system, Ta_3–xNb_xTeI₇ (0 ≤ x ≤ 3), has been performed. Synthetic, crystallographic, and spectroscopic results are presented for starting compositions corresponding to the x values: 1, 1.5, and 2. For the entire composition range studied, Ta(Nb) could readily substitute into the Nb(Ta)₃TeI₇ structure, but with changes in the observed stacking arrangements of the layers as x varies. For tantalum‐rich (x ≤ 1.8) phases, the structure conformed to the Nb₃SeI₇ structure type, also adopted by Ta₃TeI₇ and one polytype of Nb₃TeI₇. Niobium‐rich (i. e. x ≥ 1.7) phases were observed to adopt two structure types according to X‐ray powder diffraction, but crystals could only be obtained for the Nb₃SBr₇ structure type, which is a second modification of Nb₃TeI₇. Extended Hückel calculations are used to discuss the distribution of metal clusters in this system.     

1,2,2,2-Tetrafluor-1-(trifluormethyl)ethylat als Nukleophil und oxydierendes Fluorierungsmittel für 2-Halogen-4,4,5,5-tetrakis(trifluormethyl)-1,3,2-dioxaphospholan

Syntheses and Crystal Structures of the Phases 6R? Cu_xM_1+yS₂ (M = Nb, Ta) Thermal decomposition of 2H? Cu_0.66MS₂ (M = Nb, Ta) results in the formation of 6R? Cu_xM_1+yS₂. Crystals can be obtained by chemical vapour transport reactions with iodine in a temperature gradient (1320–1220 K). The structures of four phases with trigonal symmetry (R3 m, Z = 6) were determined from single crystal x-ray diffraction data. Nb and Ta, resp., of the MS₂ partial structures have a trigonal prismatic coordination. The additional metal atoms are distributed at random only in S-octahedra (Nb, Ta) and -tetrahedra Cu sharing faces with MS₆-prisms. The sequence of the layers can be rationalized on the assumption of stabilizing interactions between metal atoms in adjacent layers.     

Transition metal coated fullerenes

Compound clusters of fullerene molecules and transition metal atoms having the composition C₆₀M_x and C₇₀M_x with x = 0..150 and M ∈ {Ti, Zr, V, Y, Ta, Nb} were produced using laser vaporisation in a low-pressure inert gas aggregation cell. Intensity anomalies in the mass spectra correlate with the atomic radii of the different metals indicating the formation of complete metal layers around the central fullerene molecule. Using high laser intensities the metal-fullerene clusters can be transformed into metcars and metal-carbides. Photofragmentation spectra of preselected C₆₀Ta_x indicate that the fullerene cage is destroyed for x ≥ 3.     

Tantalcluster in oxidischer Matrix – Synthese und Strukturen der gemischtvalenten Oxotantalate M2–δTa15O32 (M = K,Rb (δ = 0); M = Sr (δ = 0.15), Ba (δ = 0.12))

Tantalum Cluster in an Oxidic Matrix – Synthesis and Structures of Mixed-Valence Oxotantalates M_2–δTa₁₅O₃₂ (M = K, Rb (δ = 0); M = Sr (δ = 0.15), Ba (δ = 0.12)) The mixed-valent oxides Sr_1.85Ta₁₅O₃₂ ( 1 ), Ba_1.88Ta₁₅O₃₂ ( 2 ), K₂Ta₁₅O₃₂ ( 3 ), Rb₂Ta₁₅O₃₂ ( 4 ) were prepared from appropriate mixtures of Ta₂O₅, tantalum and the corresponding carbonate at 1520–1670 K in sealed tantalum tubes. According to X-ray single crystal structure analyses the oxides crystallize in the space group R3¯, Z = 1. The lattice parameters in the hexagonal setting are a = 777.36(11), c = 3516.2(7) pm for 1 , a = 778.87(11), c = 3548.1(7) pm for 2 , a = 780.7(2), c = 3573.1(11) pm for 3 , and a = 781.90(11), c = 3593.0(7) pm for 4 . The oxide ions form a defect dense packing with the layer sequence chhhh. Anti-cuboctahedral sites are completely occupied by the alkali metal cations. The alkaline earth cations occupy 92 to 94% of such sites; they are displaced from the centres. Smaller voids are located in the centres of the cuboctahedral Ta₆O₁₂ clusters forming the characteristic structural unit of these low-valent oxotantalates. In case of 3 and 4 the clusters have 13 electrons, in case of 1 and 2 they have close to 15 electrons available for Ta–Ta-bonding. Moreover, the structures of the alkali and alkaline earth metal compounds differ notably with respect to the spectrum of Ta–O and Ta–Ta distances in the Ta₃O₁₃ octahedra triples forming another characteristic structural unit for these oxides. Such differences are traced back to distinct local charge balances for the uni- and divalent cations. The oxides 2 , 3 are semiconductors with band gaps ranging from 130 to 360 meV.     

Synthese,Struktur und Eigenschaften der tantalreichen Silicidchalkogenide Ta15Si2QxTe10–x (Q = S,Se)

Synthesis, Structure, and Properties of the Tantalum‐rich Silicide Chalcogenides Ta₁₅Si₂Q_xTe_10–x (Q = S, Se) The quaternary tantalum silicide chalcogenides Ta₁₅Si₂Q_xTe_10–x (Q = S, Se) are accessible from proper, compacted mixtures of the respective dichalcogenides, silicon and elemental tantalum at 1770 K in sealed molybdenum tubes. The structures were determined from the strongest X‐ray intensities of fibrous crystals with cross sections of about 3 μm and confirmed by fitting the profile of single phase X‐ray diffractograms. The phases Ta₁₅Si₂S_3.5Te_6.5 and Ta₁₅Si₂Se_3.5Te_6.5 crystallize in the monoclinic space group C2/m with two formula units per unit cell, a = 2393.7(1) pm, b = 350.08(2) pm, c = 1601.2(1) pm, β = 124.700(4)°, and a = 2461.3(2) pm, b = 351.70(2) pm, c = 1601.7(1) pm, β = 124.363(5)°, respectively. Tri‐capped trigonal prismatic Ta₉Si clusters stabilized by encapsulated Si atoms can be seen as the characteristic unit of the structure. The clusters are fused into twin columns which are connected by additional Ta atoms, thus forming corrugated layers. The remaining valences at the surfaces of the layered Ta–Si substructure are saturated by those of chalcogen atoms which are coordinated only from one side by three, four or five Ta atoms. Few bridging covalent Ta–S–Ta and Ta–Se–Ta bonds and, otherwise, dispersive interactions between the Q atoms hold these nearly one nanometer wide slabs together. The phases are moderate metallic conductors. There is no evidence for any electronic instability within 10–310 K in spite of the high anisotropy of the structures.     

Effects of experimental parameters on characterization of AgNb1?xTaxO3 materials by a modified sol–gel route

Nano-sized AgNb_1?xTa_xO₃ (x?=?0.4) ceramic powders were synthesized by a modified citrate sol?Cgel route. Homogeneous Ag?CNb?CTa precursor gel was prepared with silver citrate, niobium citrate, and tantalum citrate as source of Ag, Nb and Ta, respectively. Citrate acid and ethylene glycol were used as the complexing agents. The structural variation of the AgNb_1?xTa_xO₃ powder with annealing temperature was studied by TG?CDTA and X-ray diffraction. The precursor gel calcinated at 680?°C produced a pure perovskite phase. The effects of experimental parameters including pH value of the solution and the proportion of citric acid to the metal ions on the formation of homogeneous and microstructure of the powders were also investigated. The results indicated that a homogeneous Ag?CNb?CTa precursor gel with no precipitate was formed at about pH?=?6 and the optimum molar ratio of citric acid and the metal ions were 2:1?C3:1. The XRD data was analysis and the correlation between dielectric properties and (Nb,Ta)O₆ octahedra was discussed.     

Röntgenographische und schwingungsspektroskopische Untersuchung der Mischkristallreihen Cu3MxM′;1-xX4 (M,M′ = V,Nb, Ta; X = S,Se) mit Sulvanitstruktur

X-ray and Vibrational Spectroscopical Investigation of the Mixed Crystal Series Cu₃M_xM′_1-xX₄ (M, M′ = V, Nb, Ta; X = S, Se) with Sulvanite Structure Solid solutions Cu₃M_xM′_1-xX₄ (M, M′ = V, Nb, Ta; X = S, Se) with Sulvanite structure have been prepared in the range 0 ≤ x ≤ 1 by solid state reaction between 600°C and 900°C. The lattice constants decrease linearly with x. The UR active antisymmetrical as well as the Raman active symmetrical M–X stretching vibrations may be attached to independently vibrating MX₄ and M′X₄ tetrahedrons.     

Na0.74Ta3O6, ein niedervalentes Oxotantalat mit einer Ta–Ta‐Mehrfachbindung

Na_0.74Ta₃O₆, a Low‐Valent Oxotantalate with Multiple Ta–Ta Bonds The title compound was prepared in a sealed tantalum tube through the reaction of Ta₂O₅, tantalum and Na₂CO₃ in a NaCl flux at 1570 K within 5 d. The crystal structure of Na_0.74Ta₃O₆ (a = 713.5(1), b = 1027.4(2), c = 639.9(1) pm, Immm, Z = 4) was determined by single crystal X‐ray means. The structure is isomorphous with NaNb₃O₅F [1]. The characteristic structural units are triply bonded Ta1₂ dumb‐bells with eight square‐prismatically co‐ordinated O ligands. Four Ta2, each octahedrally surrounded by O atoms, are side‐on bonded weakly to the binuclear Ta₂O₈ complex, thus forming a Ta₆ propellane‐like cluster. The lattice parameters of three additional M_xTa₃O₆ phases, M = Li, Mn, and Yb, are reported.     

MAl2Ta35O70 (M = Na,K, Rb), niedervalente Oxotantalate mit isolierten kuboktaedrischen Ta6O12-Clustern

MAl₂Ta₃₅O₇₀ (M = Na, K, Rb), Low-Valent Oxotantalates with Discrete Cuboctahedral Ta₆O₁₂ Clusters The title compounds were prepared by reducing Ta₂O₅ with tantalum and aluminium in the presence of alkali metal carbonates at 1650 K. NaAl₂Ta₃₅O₇₀ was characterized by means of a single crystal X-ray structure determination: space group P 3, lattice parameters a = 780.15(7) pm, c = 2621.7(8) pm, Z = 1, 167 variables, R_F = 0.048. The structure can be described in terms of a close packing of oxide ions with specific defects. The sequence of the layers is hhcchchcchh. The characteristic structural units are Ta₆O₁₂ clusters being substantially stabilized by Ta–Ta bonding (d_Ta–Ta = 279.3–283.3 pm, 14 electrons per cluster). The sodium cations occupy acentrically and statistically half of the anti-cuboctahedral sites. The compounds are semiconductors with band gaps E_a of 0.2 to 0.3 eV.     

AuTa14S2 — Zentrierte (Au,Ta)13-Ikosaeder organisiert nach dem Motiv einer kubisch dichten Packung

AuTa₁₄S₂ – Centred (Au,Ta)₁₃ Icosahedra Organized According to the Motif of a Cubic Close Packing The tantalum-rich phases of composition Au_xTa_15?xS₂ (0.4 < x < 1.1) were prepared by arc-melting of appropriate compressed mixtures (Au, Ta, Ta_1.35S₂) and subsequent annealing of the samples in sealed molydenum crucibles. Brittle crystals with silver lustre were grown in tantalum ampoules at 1 700 K within two days using iodine as a chemical transport agent. In contrast to Vegard's rule the lattice parameters of the rhombohedral phases which are isostructural with Pd₁₅P₂, shrink with increasing gold content. The structures were determined from Rietveld fits of powder X-ray diffraction spectra and confirmed by a crystal structure analysis of a merohedral twinned crystal of Au_0.7Ta_14.3S₂: a = 747.7(2) pm, α = 59.84(2)°, R3 , Z = 1, 760 reflections (F² > 2σ(F²)), 30 variables, R(F) = 0.048. The parts of the volumina of the domains with distinct orientations are 0.346(4) and 0.654. Topologically the structure corresponds to a cubic close packed arrangement of Ta₁₂-icosahedra with all non-tetrahedral interstices being filled. Gold accumulates preferentially in the centres of the icosahedra. The remaining metal atoms – two per formula and site – populate the ?octahedral sites”? which are encased in stretched polyhedra limited by 32 triangulated faces. The ?tetrahedral sites”? are occupied by the sulfur atoms which themselves have a ninefold, triangulated tetrakaidecahedral coordination. The complete occupation of interstices together with the specific orientation of the gold stabilized icosahedral (Au,Ta)₁₃-clusters ensure a tetrahedral close packing of all atoms. The distortions of the packing are quantitatively analysed in terms of dihedral angles and deviations of the tetrahedral edge lengths from the mean and are compared with those of other tcp structures.     

Nitrido-Sodalithe. II. Synthese,Struktur und Eigenschaften von M(6+(y/2)–x)H2x[P12N24]Zy mit M = Fe,Co, Ni,Mn; Z = Cl,Br, I; 0 ≤ x ≤ 4; y ≤ 2

Nitrido-Sodalites. II. Synthesis, Crystal Structure, and Properties of M_{(6+(y/2)–x)}H_2x[P₁₂N₂₄]Z_y with M = Fe, Co, Ni, Mn; Z = Cl, Br, I; 0 ≤ x ≤ 4; y ≤ 2 The nitrido sodalites M_{(6+(y/2)–x)}H_2x[P₁₂N₂₄]Z_y with M = Fe, Co, Ni, Mn; Z = Cl, Br, I; 0 ≤ x ≤ 4; y ≤ 2 are obtained by the reaction of HPN₂ or [PN(NH₂)₂]₃ with the metal halogenide MZ₂ (T = 700°C). The compounds are isotypic to Zn_(7–x)H_2x[P₁₂N₂₄]Cl₂. An increase of the ionic radii of the cations or anions results in an expansion of the lattice which is caused by an increase of the P? N? P angle. The influence of the cation is more dominant than that of the anion. By reacting [PN(NH₂)₂]₃ with metal halogenide (MZ₂) hydrogen free, X-ray amorphous products are obtained. The formation of the chloride-containing P? N-sodalite in this reaction begins at temperatures below 450°C.     

[Cl5Ta(μ‐O)TaCl3{iPrS(CH2)2SiPr}] and [(TaCl4)2(μ‐O)(μ‐Me2Se2)]: two chalcogenoether complexes of Ta2OCl8 with very different geometries

The title compounds, [1,2‐bis(isopropylsulfanyl)ethane‐2κ²S,S′]octachlorido‐1κ⁵Cl,2κ³Cl‐μ‐oxido‐ditantalum(V), [Ta₂Cl₈O(C₈H₁₈S₂)], (I), and μ‐dimethyldiselane‐κ²Se:Se′‐μ‐oxido‐bis[tetrachloridotantalum(V)], [Ta₂Cl₈O(C₂H₆Se₂)], (II), contain six‐coordinate Ta^V centres linked by a nonlinear oxide bridge. Compound (I) contains one Ta^V centre bonded to a chelating dithioether and three terminal chloride ligands, with the second Ta^V centre bonded to five terminal chloride ligands. In (II), two tetrachloridotantalum(V) residues are bridged by the diselenide, giving a puckered five‐membered Ta/O/Ta/Se/Se ring. The Ta—O distances in the bridges are short in both compounds, indicating that significant multiple‐bond character is retained despite the deviation from linearity, and the bond lengths reveal a clear trans influence order of O > Cl > S > Se on the hard Ta^V centre. The structures are compared with the [Ta₂Cl₁₀O]²⁻ anion, which contains a linear oxide bridge.     

Physical properties and electronic structure of TaC-HfC solid solutions

The results of investigations of the physical properties and electronic structure in the Hf _x Ta_1?x solid solutions have been generalized. The N ₃ X-ray emission spectra Ta in hafnium and tantalum carbides have been investigated for the first time. Peak locations and intensities have been compared with the results of calculations of the electron band structure of Hf _x Ta_1?x C alloys with x = 0, 0.25, 0.5, and 0.75. It has been found that the inversion of peak intensities with increasing hafnium concentration is due to a nonmontonic variation in the density of Ta 5d states in the valence band of these compounds.     

Die Struktur der Komplexnitride NbCrN und Ta1−x Cr1+x N

The crystal structure of the isotypic compounds NbCrN and Ta_1?x Cr_1+x N has been determined from X-ray powder patterns. The tetragonal unit cell contains 12 atoms and belongs to the space group P4bm. The lattice parameters are for NbCrN:a=4.283 Å,c=7.360 Å, for Ta_0.8Cr_1.2Na=4.249 Å,c=7.334 Å. The structure is characterized by relatively close packed double layers of Nb(Ta)-atoms and Cr-atoms parallel to the base plane. The nitrogen atoms are within the octahedral interstitial sites of the niobium(tantalum) double layer.     

Synthesis,Single Crystal Growth and Crystal Structure of Ta7Cu10Ga34 – a 8 × 4 × 2 Super Structure of CsCl

Single crystals of Ta₇Cu₁₀Ga₃₄ were grown from the elements in a Cu/Ga melt. Ta₇Cu₁₀Ga₃₄ represents the first ternary compound of the system Ta/Cu/Ga. The crystal structure (Cmmm, oC102, Z = 2, a = 23.803(1), b = 12.2087(4), c = 5.7487(2) Å, 1291 refl. 78 parameters, R₁ = 0.037, wR₂ = 0.070). The crystal structure is characterized by rods of pentagonal prisms MGa₁₀, which are alternatingly occupied by Ta and Cu. Four of these rods are connected to columns running in direction (001). These columns are linked by cubic units TaGa₈, CuGa₈, and GaGa₈. According to the characteristic structural elements and the size of the unit cell Ta₇Cu₁₀Ga₃₄ represents a 8 × 4 × 2 super structure of CsCl or bcc. With respect to the underlying CsCl structure the formula can be written as [Ta₇Cu₁₀Ga₂□₁₃]Ga₃₂, i.e. a cubic primitive packing of 32 Ga atoms with Ta, Cu, and Ga in cubic voids and 13 vacancies. The pentagonal‐prismatic coordination of Ta and Cu can formally be obtained from the cubic primitive packing of Ga atoms by a 45° rotation of a part of the Ga₈ cubes. There is a close similarity to the binary compounds Ta₈Ga₄₁ and Ta_2–xGa_5+x. The first one is also related to a CsCl‐like structure, the latter one contains rods of pentagonal prisms, which form the same columns. There are also relations to the ternaries V₂Cu₃Ga₈ and V₁₁Cu₉Ga₄₆, whose cubic structures are more or less complex variants of CsCl.     

Transition‐Metal–Boron Intermetallics with Strong Interatomic d–sp Orbital Hybridization for High‐Performance Electrocatalysis

A theoretical and experimental study gives insights into the nature of the metal–boron electronic interaction in boron‐bearing intermetallics and its effects on surface hydrogen adsorption and hydrogen‐evolving catalytic activity. Strong hybridization between the d orbitals of transition metal (T_M) and the sp orbitals of boron exists in a family of fifteen T_M–boron intermatallics (T_M:B=1:1), and hydrogen atoms adsorb more weakly to the metal‐terminated intermetallic surfaces than to the corresponding pure metal surfaces. This modulation of electronic structure makes several intermetallics (e.g., PdB, RuB, ReB) prospective, efficient hydrogen‐evolving materials with catalytic activity close to Pt. A general reaction pathway towards the synthesis of such T_MB intermetallics is provided; a class of seven phase‐pure T_MB intermetallics, containing V, Nb, Ta, Cr, Mo, W, and Ru, are thus synthesized. RuB is a high‐performing, non‐platinum electrocatalyst for the hydrogen evolution reaction.     

Die Oxochlorotantalate (PPh4)2[Ta2OCl9]2 · 2 CH2Cl2, (PPh4)2[Ta2OCl10] · 2 CH3CN und (K-18-Krone-6)4[Ta4O6Cl12] · 12 CH2Cl2

The Oxochlorotantalates (PPh₄)₂[Ta₂OCl₉]₂ · 2 CH₂Cl₂, (PPh₄)₂[Ta₂OCl₁₀] · 2 CH₃CN, and (K-18-crown-6)₄[Ta₄O₆Cl₁₂] · 12 CH₂Cl₂ (K-18-crown-6)₄[Ta₄O₆Cl₁₂] · 12 CH₂Cl₂ was obtained from a reaction of tantalum pentachloride, K₂S₅ and 18-crwon-6 in dichlormethane. According to its crystal structure analysis it is tetragonal (space group I 4 2d) and contains [Ta₄O₆Cl₁₂]^4– ions that have an adamantane-like Ta₄O₆ skeleton. Each K⁺ ion is coordinated by the oxygen atoms of the crown ether molecule from one side and with three Cl atoms of one [Ta₄O₆Cl₁₂]^4– ion from the opposite side. (PPh₄)₂[Ta₂OCl₁₀] · 2 CH₃CN was a product from PPh₄Cl and TaCl₅ in acetonitrile in the presence of Na₂S₄. Its crystals are monoclinic (space group P2₁/c) and contain centrosymmetric [Ta₂OCl₁₀]^2– ions having a linear Ta–O–Ta grouping with short bonds (Ta–O 189 pm). TaCl₅ and H₂S formed a solid substance (TaSCl₃) from which a small amount of (PPh₄)₂[Ta₂OCl₉]₂ · 2 CH₂Cl₂ was obtained by the reaction with PPh₄Cl in CH₂Cl₂. The anions in the monoclinic crystals (space group P2₁/n) consist of two Ta₂OCl₉^– units which are joined by chloro bridges; each Ta₂OCl₉^– unit has a nearly linear Ta–O–Ta group with differing bond lengths (179 and 202 pm). The oxygen in the compounds probably was introduced by traces of water in the crown ether, acetonitrile or H₂S, respectively.     

Peroxofluorokomplexe der Übergangsmetalle. III,Darstellung, Kristallstruktur und Schwingungsspektren von K6Ta3(O2)3OF13 · H2O mit einem μ-Oxo-diperoxo-octafluoroditantalat(V)-Anion

Transition Metal Peroxofluoro Complexes. III. Preparation, Crystal Structure, and Vibrational Spectra of K₆Ta₃(O₂)₃OF₁₃ · H₂O Containing a m?-Oxo-diperoxo-octafluoroditantalate(V) Anion K₆Ta₃(O₂)₃OF₁₃ · H₂O has been prepared from solution and his crystal structure was determined by X-ray single crystal investigation: Space group Pnma, lattice constants a = 1 653.6 pm, b = 883.5 pm, c = 1 365.8 pm, Z = 4, R = 0.033. The compound yields [Ta(O₂)F₅]^2? groups as well as m?-oxo-bridged [Ta₂O(O₂)₂F₈]^4? anions with very diffrent O? O distances within the peroxo groups (139 pm vs. 164 and 175 pm) correlating well with the i.r. and Raman spectra. The different bonding in connection with an oxo-bridge is discussed.     

Tantalum Clusters in Oxidic Matrices — Synthesis,Crystal Structure and Magnetic Properties of Eu1.83Ta15O32

The mixed‐valent oxotantalate Eu_1.83Ta₁₅O₃₂ was prepared from a compressed mixture of Ta₂O₅ and the metals in a sealed Ta ampoule at 1400 °C. The crystal structure was determined by means of single crystal X‐ray diffraction: space group R3¯, a = 777.2(6) pm and c = 3523.5(3) pm, Z = 3, 984 symmetrically independent reflections, 83 variables, R_F = 0.027 for I > 2σ (I). The structure is isotypic to Ba₂Nb₁₅O₃₂. The salient feature is a [Ta(+8/3)₆O₁₂ⁱO₆^a] cluster consisting of an octahedral Ta₆ core bonded to 12 edge‐bridging inner and six outer oxygen atoms. The clusters are arranged to slabs which are sandwiched by layers of [Ta(+5)₃O₁₃] triple octahedra. Additional Ta(+5) and Eu(+2) atoms provide the cohesion of these structural units. Twelve‐fold coordinated Eu(+2) atoms are situated on a triply degenerate position 33 pm displaced from the threefold axis of symmetry. A depletion of the Eu(+2) site from 6 to 5.5 atoms per unit cell reduces the number of electrons available for Ta‐Ta bonding from 15 to 14.67 electrons per cluster. Between 125 and 320 K Eu_1.83Ta₁₅O₃₂ is semi‐conducting with a band gap of 0.23 eV. The course of the magnetization is consistently described with the Brillouin function in terms of a M_mol/(N_Aμ_B) versus B/T plot in the temperature range 5 K — 320 K and at magnetic flux densities 0.1 T — 5 T. At moderate flux densities (< 1 T) the magnetic moment agrees fairly well with the expected value of 7.94 μ_B for free Eu (2+) ions with 4f⁷ configuration in ⁸S_7/2 ground state. Below 5 K, anisotropic magnetization measurements at flux densities B < 1 T point to an onset of an antiferromagnetic ordering of Eu spins within the layers and an incipient ferromagnetic ordering perpendicular to the layers.     

Trennung und Charakterisierung der metallgemischten Cluster [(Nbn Ta6–n) Cl]2+, n = 0–6

Separation and Characterization of Mixed-Metal Clusters [(Nb_n Ta_6–n)Cl ]²⁺, n = 0–6 By reaction of NbCl₅ with Ta or TaCl₅ with Nb in fused NaCl the mixed-metal compounds [(Nb_nTa_6–n)Cl]²⁺, n = 0–6, are obtained. The anions formed in NaF solution by coordination of F^?? are kinetically stable at lower temperatur (–5°C). They have been separated by repeated ion exchange chromatography on DEAE cellulose to give the mixed-metal clusters, for n = 1 and 5 as pure compounds, for n = 2, 3, 4 as pairs of geometric isomers according to statistical distribution. The clusters are distinguishable by intense charge transfer bands shifting on metal substitution by steps of about 12 nm from 327 (Ta₆) to 396 nm (Nb₆). The IR spectra (80 K) exhibit only in the region of the antisymmetric metal–metal vibration distinct band patterns, which are assigned to the components of the degenerated T_1u vibration of the octahedral homonuclear clusters at 233 (Nb₆) and 209 cm^?1 (Ta₆), due to the lower symmetry D_4h, C_4v, and C_2v of the mixed-metal clusters. Along with the substitution of Nb by Ta the metal-Clⁱ vibrations are systematically shifted to lower frequencies, whereas all deformation modes remain uninfluenced.