Magnetische Eigenschaften der Lanthanid-Nickel-Verbindungen LnNi5

Zusammenfassung Im Temperaturbereich 4–300° K werden Messungen der magnetischen Suszeptibilität in Abhängigkeit von der Temperatur an sechsLnNi₅-Verbindungen angegeben (Ln=Ce, Pr, Nd, Gd, Ho bzw. Tm). Die paramagnetischen Momente von Gd, Ho und Tm inLnNi₅-Verbindungen stimmen gut mit den für eine Anordnung aus freien dreifach positiven Ionen zu erwartenden Werten überein. TmNi₅ zeigt bei tiefen Temperaturen Tendenz zuVan Vleck-Paramagnetismus. Ferromagnetische Ordnungen treten bei GdNi₅, HoNi₅ und TmNi₅ im Temperaturbereich 22–33° K auf. Die Sättigungsmomente der Ho- und Tm-Verbindungen sind — vermutlich wegen der Kristallfeld-wechselwirkung — kleiner als gJ. Nickel scheint in PrNi₅ und NdNi₅ magnetisch zu sein.

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Magnetic characteristics of lanthanide-nickel (LnNi ₅) compounds

Magnetic susceptibility-temperature measurements are presented for sixLnNi₅ compounds, withLn=Ce, Pr, Nd, Gd, Ho and Tm, for the temperature range 4 to 300° K. Paramagnetic moments of the Gd, Ho and Tm in theLnNi₅ compounds are in good agreement with those expected for an assemblage of free tripositive ions. TmNi₅ exhibits a tendency towardVan Vleck paramagnetism at low temperatures. Ferromagnetic ordering is exhibited by GdNi₅, HoNi₅ and TmNi₅ at temperatures ranging from 22 to 33° K. The saturation moments of the Ho and Tm compounds are less than gJ presumably because of the crystal field interaction. Nickel appears to be magnetic in PrNi₅ and NdNi₅.

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Mit 4 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet

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Diese Arbeit wurde durch eine Stiftung des Army Research Office, Durham, gefördert     

Specific heat and neutron diffraction study on quaternary sulfides BaNd2CoS5 and BaNd2ZnS5

Magnetic and electrical properties are investigated for quaternary neodymium sulfides BaNd₂TS₅ (T=Co, Zn) through the specific heat, neutron diffraction, and electrical conductivity measurements. Their electrical conductivities show semiconductive behavior, which follows the Arrhenius temperature dependence with the activation energy of E_a=1.46 eV for BaNd₂ZnS₅ and E_a=1.19 eV for BaNd₂CoS₅. The specific heat of BaNd₂ZnS₅ has a λ-type anomaly at 2.8 K due to the antiferromagnetic ordering of the Nd³⁺ moments and a Schottky-type anomaly at around 60 K, which results from the crystal field splitting of the ⁴I_9/2 ground state of the Nd³⁺ ion. The specific heat of BaNd₂CoS₅ shows two λ-type anomalies at 5.7 K due to the antiferromagnetic ordering of Nd³⁺ and at 58 K due to the antiferromagnetic ordering of Co²⁺. The latter overlaps with the Schottky-type anomaly due to the crystal field splitting of the Nd³⁺ ion. Neutron diffraction measurements for BaNd₂CoS₅ show that a magnetic arrangement of the Co²⁺ moments has a collinear antiferromagnetic structure, while that of the Nd³⁺ moments has a noncollinear one.     

Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

The ⁵⁷Fe Mössbauer effect in two samples (A and B) of [Fe(papt)₂] and in its solvates with CHCl₃ and C₆H₆ has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (⁵T₂) ? low-spin (¹A₁) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔE_Q = 2.03 mm s^?1 and δIS = +0.87 mm s^?1, characteristic of a ⁵T₂ ground state. At 257 K, a second doublet, typical for a ¹A₁ ground state, is observed and its intensity increases as the transition progresses but levels off below ～ 100 K. At 4.2 K, 83% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.56 mm s^?1 and δ^IS(¹A₁ = +0.32 mm s^?1. In an applied magnetic field, V_zz(¹A₁) < 0 and η ≈ 0.7 have been determined, whereas for the ^sT₂ ground state, V_zz(^sT₂) > 0, η ≈ 0.75, and an internal hyperfine field H_n ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5_T2 and f1_A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the ⁵T₂ and constant υ_cff for the ¹A₁ ground state. The temperature dependence of ?In f1_A1 closely follows the Debye model with Θ1_A1 = 165 K, whereas the same applies to ?ln f5_T2 only above ～ 210 K and Θ5_T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.     

Ferromagnetic ordering in ThSi2 type CeAu0.28Ge1.72

The compound CeAu_0.28Ge_1.72 crystallizes in the ThSi₂ structure type in the tetragonal space group I4₁/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu_0.28Ge_1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μ_B/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K². The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.     

Preparation, structural and magnetic characterization of DyCrMnO5

The title compound has been first synthesized by a citrate technique followed by thermal treatments under moderate oxygen pressure conditions, and characterized by X-ray and neutron powder diffraction (NPD) and magnetization measurements. The crystal structure of DyCrMnO₅ has been refined from NPD data in the space group Pbam; a=7.2617(6) Å, b=8.5161(6) Å, and c=5.7126(5) Å at 295 K. This oxide is isostructural with RMn₂O₅ oxides (R=rare earths) and it contains infinite chains of (Cr, Mn)⁴⁺O₆ octahedra-sharing edges, linked together by (Mn, Cr)³⁺O₅ pyramids and DyO₈ units. The high degree of antisite disordering exhibited by DyCrMnO₅ is noteworthy. The octahedral positions are occupied by roughly 50% of Mn and Cr cations, and the pyramidal groups contain two thirds of Mn and one third of Cr cations. We assume that Mn and Cr cations at the octahedral positions exhibit a tetravalent oxidation state, whereas the metals at the pyramidal positions are trivalent, in order to preserve the electroneutrality of this oxide. The susceptibility vs temperature curve of DyCrMnO₅ does not suggest the establishment of a long-range magnetic structure even at low temperatures; the NPD technique does not provide any signal of magnetic ordering, since the reflections do not show any magnetic contribution.     

The binary gallide Eu5Ga9 - crystal structure, chemical bonding and physical properties

The title compound was prepared from the elements by reaction in a sealed tantalum tube at 1320 K followed by slow cooling to 970 K or, alternatively, in glassy carbon crucibles with HF melting. The crystal structure of Eu₅Ga₉ was refined from single-crystal data: Cmcm, a=4.613(1) Å, b=10.902(3) Å, c=26.097(6) Å, Z=4, R_F=0.036, 811 structure factors and 46 variables. The structure is described as a three-dimensional network formed by gallium atoms with europium atoms embedded in the cavities. The bonding analysis (LMTO, ELF) confirmed this representation of the structure. Magnetic susceptibility measurements show Curie-Weiss behavior above 60 K with a magnetic moment per Eu atom of 8.12(1) μ_B, indicating divalent europium. Eu₅Ga₉ orders antiferromagnetically at 19.0(5) K with re-ordering at 6.0(5) K. The electrical resistivity shows a metallic temperature dependence and magnetic scattering. ¹⁵¹Eu Mössbauer spectroscopic experiments are compatible with divalent europium and show complex magnetic hyperfine field splitting below the ordering temperature.     

New germanates RCrGeO5 (R=Nd-Er, Y): Synthesis, structure, and properties

The new complex germanates RCrGeO₅ (R=Nd-Er, Y) have been synthesized and investigated by means of X-ray powder diffraction, electron microscopy, magnetic susceptibility and specific heat measurements. All the compounds are isostructural and crystallize in the orthorhombic symmetry, space group Pbam, and Z=4. The crystal structure of RCrGeO₅, as refined using X-ray powder diffraction data, includes infinite chains built by edge-sharing Cr⁺³O₆ octahedra with two alternating Cr−Cr distances. The chains are combined into a three-dimensional framework by Ge₂O₈ groups consisting of two edge-linked square pyramids oriented in opposite directions. The resulting framework contains pentagonal channels where rare-earth elements are located. Thus, RCrGeO₅ germanates present new examples of RMn₂O₅-type compounds and show ordering of Cr⁺³ and Ge⁺⁴ cations. Electron diffraction as well as high-resolution electron microscopy confirm the structure solution. Magnetic susceptibility data for R=Nd, Sm, and Eu are qualitatively consistent with the presence of isolated 3d (antiferromagnetically coupled Cr⁺³ cations) and 4f (R⁺³) spin subsystems in the RCrGeO₅ compounds. NdCrGeO₅ undergoes long-range magnetic ordering at 2.6 K, while SmCrGeO₅ and EuCrGeO₅ do not show any phase transitions down to 2 K.     

Computing boiling temperatures and vapor pressures in homologous series of perfluorinated oligomers

Correlation relationships are proposed for computing boiling temperatures of T _b oligomers with the general formula R₁(CF₂) _n R₂ at normal pressure, where R₁ and R₂ are arbitrary end groups and the increment of the CF₂ fragment is 20.4 K. The dependences of coefficients A and B in the Clausius-Clapeyron equation (ln P = A ? B/T) on the length of the oligomer chain are determined by computing critical temperatures and pressures using an additive scheme.     

Raman scattering study of CaB6 and YbB6

Phonon spectra of CaB₆ and RB₆ (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. E_g mode shows the doublet spectra for only the divalent crystals of CaB₆ and YbB₆. The doublet spectra are understood by the two-dimensional charge distribution on B₆ without lattice distortion. In addition, the scattered intensities of the phonons change at around the ferromagnetic Curie temperature for YbB₆ and at T?600 K for CaB₆. These are the characteristic temperatures due to the change of the electronic system.     

Synthesis, structures and magnetic properties of pseudo-hollandite chromium sulfides

The pseudo-hollandite chromium sulfides, ACr₅S₈ (A=K, Rb, Cs) and A′_0.5Cr₅S₈ (A′=Sr, Ba), have been synthesized and investigated in structural and magnetic properties. All the compounds crystallize in the isostructure with a monoclinic C2/m. Its crystal structure has triangular lattices and double chains made of Cr³⁺ (d³; S=3/2) triangles. The magnetic susceptibilities of all compounds behave as Curie-Weiss types in high temperature region. From magnetic susceptibility and specific heat measurements, all the compounds have antiferromagnetic ground states. The Néel temperatures are rather low compared to Weiss temperatures, reflecting magnetic frustration in the triangular lattices and double chains. The magnetic transitions in KCr₅S₈ and Ba_0.5Cr₅S₈ are a two-step transition around 50 and 60 K, respectively, while RbCr₅S₈ shows a sharp magnetic transition at 42 K, accompanied by magnetoelastic behavior. CsCr₅S₈ shows a structural transition around 100 K, followed by a magnetic transition at 10 K. In Sr_0.5Cr₅S₈, ferromagnetic interaction develops below 100 K and then a three-dimensional antiferromagnetic order takes place at 30 K. These magnetic properties are discussed from A-cation dependences of local structures and a model of magnetic structure for RbCr₅S₈ is proposed on the basis of the arguments of magnetic interactions and neutron diffraction data. It is different from the known magnetic structure of TlCr₅Se₈.     

Anomalous low-temperature behavior of the Co dimers in the oxo-halide CoSb2O3Br2

We report the synthesis, crystal structure determination, magnetic and low-temperature structural properties of a new cobalt antimony oxo-bromide. CoSb₂O₃Br₂ crystallizes in the triclinic crystal system, space group P−1, with the following lattice parameters: a=5.306(3) Å, b=7.812(4) Å, c=8.0626(10) Å, α=88.54(3)°, β=82.17(3)°, γ=80.32(4)°, and Z=2. The crystal structure was solved from single crystal X-ray data and refined on F², R₁=3.08. The structure consists of layers made up by three building blocks, [CoO₄Br₂], [SbO₃Br], and [SbO₃] that are connected via edge- and corner-sharing so that structural Co-Co dimers are formed. The layers have no net charge and are only weakly connected by van der Waals forces to adjacent layers. Above ∼25 K the magnetic susceptibility is independent of the magnetic field and can be very well described by a Curie-Weiss law. Below 25 K the susceptibility passes through a maximum and decreases again that is typical for the onset of long-range antiferromagnetic correlations. Long-range antiferromagnetic ordering is observed below T_N∼9 K indicating substantial inter-dimer exchange coupling between Co-Co dimers within the layers. However, according to the heat capacity results only a minute fraction of the entropy is associated with the long-range ordering transition. The phonon anomalies observed for T<6 K in Raman scattering and an anomaly in the specific heat point to a structural instability leading to a loss of inversion symmetry at lowest temperatures.     

Fe3PO7, Un cas de coordinence 5 du fer trivalent,etude structurale et magnetique

The structure of Fe₃PO₇ is established from a single crystal. The cell is trigonal R3m, with, in hexagonal reference a = 8,006(5), c = 6,863(5)Å, Z = 3. The structure is determined with the direct method and refined to R = 0,027. The PO₄ tetraedra are isolated; the iron atoms are in a five coordinated site constituted of trigonal bipyramids sharing 2 edges and forming groups of 3 hexaedra; through these shared edges the iron atoms are at 3.13 Å a distance rather short which brings a repulsion causing the off centering of the cation along the pseudo axis of the bipyramid. The magnetic measurements and the Mössbauer spectroscopy show antiferromagnetism behaviour and, in the paramagnetic state, outstanding parameters (μ = 6.45 μ_B, θ_p = ?1707 K) recalling however Ca₂Fe₂O₅ or LaFeO₃. The Mössbauer spectroscopy gives, at room temperature, parameters classical for trivalent iron; through cooling, it points to a magnetic transition temperature of 160 ± 3 K below which the spectrum displays at least 2 hyperfine patterns. The magnetic interactions are discussed.     

Low-temperature heat capacity of heptacopper(II) complex [Cu7(μ3-Cl)2(μ3-OH)6-(d-pen-disulfide)3]

A single crystal calorimetry of a heptacopper(II) complex of [Cu₇(μ₃-Cl)₂(μ₃-OH)₆-(d-pen-disulfide)₃] which has a double-cubane structure supported by d-penicillaminedisulfides has been performed at low-temperature region below 8 K. This compound is a metal complex which contains seven Cu(II)s in a cluster unit. These Cu(II)s are magnetically coupled each other by strong intra-complex interactions. The heat capacities under magnetic fields exhibit Schottky type anomalies explained by the Zeeman splitting of the doublet ground state of the complex. The g-value of the ground state is evaluated as 1.86 from the systematic analysis of the Schottky peak under magnetic fields. The first excited state of the cluster seems to be separated at least by several Kelvins, which is consistent with the theoretical calculations and magnetic susceptibility results.     

Zur Elektronenstruktur von metallorganischen Komplexen der f-elemente

The temperature dependence of the magnetic susceptibility of Uranocene ((C₈H₈)₂U(IV)) between 1.25 and 298 K has been measured for the first time and the results correlated with a systematic study of the crystal field splitting. Independent estimates of the three crystal field parameters B ₂ ⁰ , B ₄ ⁰ and B ₆ ⁰ were obtained by adopting the purely electrostatic approach, the angularoverlap-model and the MHW-MO-method. Subsequently the crystal field splitting pattern was calculated by a simultaneous diagonalisation of the complete f ²-matrix. Assuming rigorous D _8h-crystal field symmetry, a crystal field splitting pattern involving a singlet ground state and a low lying first excited doublet state (ΔE=17cm^?1, ¦ΔJ₂¦ = 1) gives the best agreement with both the MO-results and the experimental findings. The experimental l/χ-vs-T-curve is reproduced to a good approximation by a calculation employing the optimal parameter set: B ₂ ⁰ = ?5610, B ₄ ⁰ = ?1426,B ₆ ⁰ = ?730cm^?1. A crystal field of slightly lowered symmetry having a significantly split doublet ground state and hence positive B ₂ ⁰ -values cannot be completely ruled out.     

5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in [Fe(pythiaz)₂] (BF₄)₂ (I) and [Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.     

The ferric chloride α-DI-imine systemVI: Electronic structure of the dimethyl formamide addcut of mono-1,10-phenanthroline ferric chloride and the by-product,tris-1,10-phenanthroline ferrous μ-oxo-bis[trichloro ferrate(III)]; field induced slow paramagnetic relaxation in monomeric ferric complexes

Magnetic susceptibility (1.45–300 K), zero and high field Mössbauer spectroscopy, IR and solution conductivity measurements were used to characterize the title DMR adduct, Fe(phen)Cl₃·DMF. The absence of magnetic hyperfine splitting in zero external field is correlated with D positive corresponding to an isolated, rapidly relaxing ground (M_s = ± $\text{1}\text{2}$ Kramers doublet. X-ray data show that the ferric ion centres are apparently sufficiently close (～ 7 Å) to allow for rapid spin-spin relaxation via dipolar interactions. The application of longitudinal magnetic fields (0–5 Tesla for 1.7–4.2 K) slows relaxation and results in fully resolved Zeeman split Mössbauer spectra for which the effective internal field, H_n = 354 kG at 1.7 K for H₀ = 5T. The complex [Fe(phen)₃] [O(FeCl₃)₂] contains an unusual binuclear anion based on tetrahedral FeOCl₃ units as recently found in [pyridinium]₂[O(FeCl₃)₂] and is obtained as a trace by-product in our preparation of Fe(phen)Cl₃·DMF. Further magnetic characterization of the latter pyridinium compound is presented.     

EPR and IR spectra of Ni(ClO4)2)·6H2O between 98 and 298 K

EPR measurements between 98 and 298 K on single crystal of Ni(ClO₄)₂·6H₂O have indicated the appearance of a rhombic component in the axial crystal field at Ni²⁺ sites at T_c = 224 K, confirming a phase transition first reported by Chaudhuri from magnetic susceptibility measurements. Temperature variations of g, D and E parameters were determined. IR spectra at room and liquid nitrogen temperatures are consistent with our EPR results.     

An observation of electron phase transition in SmB6 at low temperatures

In order to reveal the nature of the ground state of archetypal intermediate-valence compound SmB₆, a comprehensive study of its transport and magnetic properties was carried out on high-quality single crystals at temperatures of 1.8-300 K in magnetic fields up to 7 T. A drastic enhancement of negative magnetoresistance was observed below 14 K, with the maximum absolute value of Δρ/ρB²∼2.2×10⁻³ T⁻² at T≈5.2 K. This effect seems to be attributable to anomalous magnetic scattering of many-body (exciton-polaronic) complexes induced by fast valence fluctuations on Sm sites. The observed anomalies of magnetotransport, thermoelectric and magnetic characteristics are discussed in terms of electron phase transition to the coherent state of interacting many-body complexes occurring at T^*∼5 K.     

Mössbauer effect study and magnetic properties of a ferromagnetic homologue of bromobis(diethyldithiocarbamato)iron(III)

Mossbauer spectroscopy and magnetic susceptibility properties of a crystal modification of the title complex, denoted Fe(dtc)₂Br, show that the iron atoms in this material become ferromagnetically ordered with T_c = 1.52 × 0.02 K. This result is consistent with a magnetic specific heat anomaly recently observed in similar material by Yoshikawa et al. Mössbauer effect hfs show that the lower Kramers doublet from the orbital singlet, spin quartet ground term is of composition |S = $\text{3}\text{2}$, M_S = ± $\text{1}\text{2}$).     

Neutron diffraction studies of RSn1+xGe1−x (R=Tb-Er) compounds

The magnetic structures of RSn_1+xGe_1−x (R=Tb, Dy, Ho and Er, x≈0.1) compounds have been determined by neutron diffraction studies on polycrystalline samples. The data recorded in a paramagnetic state confirmed the orthorhombic crystal structure described by the space group Cmcm. These compounds are antiferromagnets at low temperatures. The magnetic ordering in TbSn_1.12Ge_0.88 is sine-modulated described by the propagation vector k=(0.4257(2), 0, 0.5880(3)). Tb magnetic moment equals 9.0(1) μ_B at 1.62 K. It lies in the b-c plane and form an angle θ=17.4(2)° with the c-axis. This structure is stable up to the Nèel temperature equal to 31 K. The magnetic structures of RSn_1+xGe_1−x, where R are Dy, Ho and Er at low temperatures are described by the propagation vector k=(1/2, 1/2, 0) with the sequence (++−+) of magnetic moments in the crystal unit cell. In DySn_1.09Ge_0.91 and HoSn_1.1Ge_0.9 magnetic moments equal 7.25(15) and 8.60(6) μ_B at 1.55 K, respectively. The moments are parallel to the c-axis. For Ho-compound this ordering is stable up to T_N=10.7 K. For ErSn_1.08Ge_0.92, the Er magnetic moment equals 7.76(7) μ_B at T=1.5 K and it is parallel to the b-axis. At T_t=3.5 K it tunes into the modulated structure described by the k=(0.496(1), 0.446(4), 0). With the increase of temperature there is a slow decrease of k_x component and a quick decrease of k_y component. The Er magnetic moment is parallel to the b-axis up to 3.9 K while at 4 K and above it lies in the b-c plane and form an angle 48(3)° with the c-axis. In compounds with R=Tb, Ho and Er the magnetostriction effect at the Nèel temperature is observed.