Large Crystal Field Research Articles

Zur Elektronenstruktur hochsymmetrischer Verbindungen der f-Elemente, XXV / The Electronic Structure of Highly Symmetrical Compounds of thef-Elements, XXV

A simple method is described which allows the growth of large transparent single crystals of tris[bis(trimethylsilyl)amido]lanthanide(III) complexes (lanthanide = Pr, Nd, Sm, Eu, Er, Tm, Yb). We have used these single crystals as starting material for a number of chemical reactions and detected no contamination with THF containing by-products. The superior quality of the crystals grown is demonstrated by optical polarization experiments. From the spectra a large negative crystal field parameter B2 0 could be concluded

EPR structural investigations of transition metal fluoride glasses (TMFG)

Abstract The results of an EPR study of TMFG separately doped with Ti3+, Cr3+, Fe3+, Mn2+, Co2+ and Cu2+ ions are presented. These ions are octahedrally coordinated; for Ti3+ and Cu2+ the octahedra are tetragonally distorted. Spin-spin interactions give rise to a strong concentration and temperature dependence of the EPR spectra for Cr3+, Fe3+ and Mn2+; moreover, for these ions a zero field absorption is observed (consistent for Cr3+ with a large single crystal field parameter distribution).

Linear and magnetic circular dichroism in the Ce 4d X-ray absorption spectroscopy of CeRh 3B 2

We have measured linear and magnetic circular dichroism in the Ce 4d X-ray absorption spectra of CeRh 3B 2, which has a highly anomalous ferromagnetic state. The result indicates an unusually large axial crystal field of order 0.1–0.2 eV along the hexagonal c-axis.

Giant crystal field and ferromagnetism in the Kondo system CeRh3B2: Soft-x-ray linear-dichroism study.

We have studied the electronic structure of ${\mathrm{CeRh}}_{3}$${\mathrm{B}}_{2}$, which has an anomalously high ferromagnetic ordering temperature, by linear dichroism in the Ce 4d core-level x-ray-absorption spectra. The result indicates an unusually large axial crystal field of order 0.1--0.2 eV along the hexagonal c axis. We propose a possible origin of the giant crystal field and the anomalous ferromagnetism in terms of the strong interatomic Ce 4f--Ce 5d hybridization.

Sterische Cyclopentadienyl‐Äquivalente in der Chemie der f‐Elemente: Monomere, homoleptische Lanthanid(III)‐tris[N,N′‐bis(trimethylsilyl)‐benzamidinate

Steric Cyclopentadienyl Equivalents in f‐Element Chemistry: Monomeric, Homoleptic Lanthanide(III) Tris[N,N′‐bis(trimethylsilyl)benzamidinates]Anhydrous lanthanide trichlorides react with N‐silylated sodium benzamidinates, Na[4‐RC6H4C(NSiMe3)2], (1a–d), to give the monomeric, homoleptic lanthanide(III) benzamidinates [4‐RC6H4C(NSiMe3)2]3Ln (2–23, R = H, MeO, CF3, Ph). The molecular structure of [4‐MeOC6H4C(NSiMe3)2]3Pr (11) has been determined by X‐ray diffraction. Absorption and emission measurements reveal that the three benzamidinate ligands produce an unusually large crystal field which is comparable with that of cyclopentadienyl.

Magnetic susceptibility and electron paramagnetic resonance studies of tetravalent terbium ions in BaTbO 3 and SrTbO 3

Tetravalent terbium perovskites, BaTbO 3 and SrTbO 3, were prepared, and their magnetic susceptibilities and electron paramagnetic resonance (EPR) spectra were measured. Both the compounds showed antiferromagnetic transitions at 32.7 and 30.5 K, respectively. The effective magnetic moment of terbium is 7.96 μ B, which is very close to the theoretical value of the Tb 4+ ion. The EPR spectra could be measured at room temperature. The spectra were broad and the g-values of BaTbO 3 and SrTbO 3 were 2.00 and 1.97, respectively. These results also show that the oxidation state of terbium is not trivalent, but tetravalent. The deviation of the g-value from 2.00 for SrTbO 3 is due to the large crystal field distortion from octahedral symmetry around the terbium ion.

Zur elektronenstruktur hochsymmetrischer Verbindungen der ⨍-elemente–XXIII. Wie groß sind die Kristallfeldaufspaltungseffekte bei den Tris (bis (trimethylsilyl) amido)-Komplexen der Lanthanoiden?

The σ and π absorption spectrum of the 3 H 4→ 3 P 0 transition has been measured at room temperature on an oriented single crystal of tris(bis(trimethylsilyl)amido)praseodymium (III) as well as the room temperature magnetic circular dichroism spectrum of the compound dissolved in a mixture of toluene and methylcyclohexane in the ratio 1:1. From these data the crystal field (CF) splitting of the ground 3 H 4 manifold was derived. An approximate set of CF parameters was obtained which qualitatively reproduces the experimental levels. The magnitude of the CF parameters demonstrates that the three bis(trimethylsilyl)amido ligands produce an unusually large crystal field.

Search for intermultiplet transitions in USb

We report a search for high energy intermultiplet transitions in antiferromagnetic USb using the HET and MARI spectrometers at the Rutherford-Appleton Laboratory. The large spin-orbit and crystal field interactions, and the hydridisation of the 5f electrons, in the UX rocksalt-structure compounds makes investigation of these transitions more difficult than for the rare earth elements. No unambigous intermultiplet transitions have been observed in these systematic measurements for USb. Comparisons are made with recent measurements on UPd 3.

Charge transfer redistribution in ErBa 2Cu 3O 6+x determined by neutron crystal field spectroscopy

A new parametrisation model which takes into account oxygen 2p-4f rare earth hybridization energies is tested in order to understand the unusually large total crystal field splitting in the RBa 2Cu 3O 6+ x (R= rare earth) compounds. This model gives also a quantitatively good picture of the charge-transfer mechanism involved by doping with oxygen.

Magnetism and electronic structure of 3d and 4d ions in metals

The magnetism and electronic structure of 3d and 4d ions in metals exhibit qualitative differences in sp metal hosts compared to hosts with d band electrons. 3d and 4d ions in sp metal hosts, in particular in alkali metals, reflect the phenomena of well-defined ionic ground states, large orbital magnetism, mixed valence, and crystal field splittings smaller than the LS coupling. Magnetic 3d and 4d states in sp metals cannot be described by one-electron approaches based on Anderson type models but require an analysis based on ionic configurations where the tendencies towards reduced magnetism can be well reproduced by increasing spin fluctuation rates. The behavior of d ions in sp metals exhibit basic common features to the physics of stable and unstable f states in metals. In contrast, the local moment formation of Fe, Co and Ni ions in transition metal hosts, in Cu, Ag, Au, and even in Hg is governed by inter-atomic interactions of the magnetic d states with host d band electrons. As the leading contribution to magnetic stability an effectively ferromagnetic interaction between impurity d states and host d band electrons has been suggested, which in certain hosts successfully suppress spin fluctuations arising from antiferromagnetic d-sp exchanges. All host dependent trends of moment stability of 3d and also of 4d ions in metals are consistent with this proposal.

Magnetic circular dichroism of atomic rhenium in a krypton matrix: Large ground state crystal field splitting

The absorption and magnetic circular dichroism (MCD) spectra of the rhenium atom isolated in a krypton matrix at 10 K are presented. Assignment of all optical bands observed in the 510–230 nm range is made. Expressions are developed to determine the ground state crystal (matrix) field splitting using the MCD saturation technique on nonoverlapped optical bands. A large crystal field (CF) splitting of 7.5±3.0 cm−1 is found and attributed to two sources, both of which must be present for the splitting to occur: (1) the spin–orbit mixing of the low-lying 4P5/2 state (11 600 cm−1) into the 6S5/2 ground state and (2) the crystal (matrix) field effect on this mixed state by the tetradecahedral substitutional matrix site. Band moment expressions are extended to account for a large ground state CF splitting and are used to determine the excited z 6PJ state spin–orbit splitting parameter (−102 cm−1). Finally, evidence for Jahn–Teller activity in the z 6PJ state is presented from an analysis of the modified MCD/absorption band moment expressions.

Magnetization jumps and critical currents in HoBa 2Cu 3O 7 single crystals, up to 18 T

The magnetization of a large single crystal of HoBa 2Cu 3O 7 exhibits several new features associated to the anisotropy (and sample size) : at low temperature large magnetization jumps are observed which depend on the field variation rate and thermalization of the sample. At high temperature the hysteresis loops, M(H), are convex lens shaped which gives a maximum of the current density J c(H). The anisotropic critical currents can be compared with values obtained in small crystals. The anisotropic critical currents can be compared with with value obtained in small superconducting state. A large crystal field anomaly is observed around 80 K for H applied along the c direction.

A bonding model for strong generalized Lewis acid-base interactions in intermetallics

Abstract

The temperature dependence of the anomalous effective g value of BiTm:YIG

Bi- and Tm-substituted YIG have shown [V. R. K. Murthy et al., in Digest of INTERMAG Conference (IEEE, New York, 1983)] anomalously low effective g values. The experimentally measured temperature dependence of BiTm:YIG shows that the effective g value decreases with decreasing temperature whereas the simple spin-wave theory predicts the opposite behavior. A recent paper [A. Lehmann-Szweykowska et al., J. Magn. Magn. Mater. 54, 1183 (1986)] predicts that the single ion anisotropy and spin-wave damping also make important contributions to the effective g value. These effects are due to the large crystal field effects of Tm in the garnet crystal host. The ground state of the Tm is J=6, L=5, and S=1. The large orbital angular momentum states enhance the spin-orbit coupling which is related to the crystal field effect. The temperature dependence of the effective g value was measured by FMR (ferromagnetic resonance) on BiTm:YIG. The experimental temperature dependence is fitted to the theoretical model and excellent agreement is obtained.

Influence of hydrogen on the magnetic characteristics of R 2Fe 14B (R  Ce, Pr, Nd, Sm or Y) systems

The effect of absorbed hydrogen on the magnetic behavior of R 2Fe 14B (R  Ce, Pr, Nd, Sm and Y) has been investigated. These compounds are found to absorb 4.4 to about 6 H per formula unit. Hydrogen absorption does not change the structure of these compounds but is accompanied by an expansion in unit cell volume ranging from 4% to 6%. The measurements show that the hydrogen absorption leads to an increase in the iron moment in all the hydrides investigated and also a pronounced change in the room temperature easy direction of magnetization of the yttrium- and praseodymium-containing compounds. Anisotropy fields for Nd 2Fe 14B and Ce 2Fe 14B at 300 K are significantly reduced on hydrogenation. The results are taken to imply a weakening of 4f-3d exchange interaction with the existence of reasonably large 4f crystal field interactions at low temperatures.

Anisotropic rare-earth spin-glasses

We present magnetic measurements (ac and dc susceptibilities and magnetization) on $Y\mathrm{Er}$ and $Y\mathrm{Gd}$ single crystals. The longitudinal (i.e., along the $c$ axis) susceptibility of $Y\mathrm{Er}$ shows a typical spin-glass-like cusp, whereas no cusp is observed in the transverse directions. This suggests that only the longitudinal spin components are frozen, in agreement with the theoretical predictions of Cragg and Sherrington for a spin-glass with a large uniaxial crystal field. The $Y\mathrm{Er}$ system appears to represent an almost perfect Ising type of spin-glass. Surprisingly the $Y\mathrm{Gd}$ system is also definitely anisotropic. A spin-glass-like cusp is observed only for the transverse susceptibility. This anisotropy cannot be ascribed to the crystal field and we discuss its possible origins. Finally, we present some data on the remanence properties of the $Y\mathrm{Er}$ and $Y\mathrm{Gd}$ alloys.

Superconductivity in the neptunium Chevrel phase Np 1+ xMo 6Se 8

The neptunium based Chevrel phases Np 1.0Mo 6Se 8 have been synthesized and their magnetic susceptibility and electrical resistivity have been measured from 3 K to room temperature. These compounds are superconductors with a critical temperature T c = 5.6 (0.1) K. The magnetic susceptibility shows large crystal field effects with probably an important non-cubic component.

On the magnetism of a so-called one-dimensional crystal, dimethylammonium trichlorocuprate(II), DMACuCl 3

Magnetic anisotropy and susceptibility of a so-called one-dimensional crystal, dimethylammonium trichlorocuprate(II), DMACuCl 3, measured between 300 and 77 K can be explained on the basis of a Cu 2+ ion in a large tetragonal crystal field, although the shortest Cu-Cu distance is 3.443 Å. Measurements of anisotropic susceptibilities below 77 K are needed to ascertain the real magnetic behaviour in this system.

Absorption spectra of noble metal atoms isolated in molecular matrices

Absorption spectra for Cu, Ag and Au atoms isolated in H 2, D 2, N 2 and CH 4 matrices are shown and the matrix influence on these spectra is evaluated. Unlike the inert gases, these matrices cause an unusually large crystal field splitting of the metal atom nP 3 2 level. Distinct differences in the spectra for isolation in H 2 and D 2 are found and discussed.

Neutron diffraction studies of Pr 2C 3, Nd 2C 3, and Dy 2C 3 at 300 to 1.6°K

Neutron diffraction measurements have shown that the body-centered cubic Pr 2C 3, Nd 2C 3, and Dy 2C 3 become antiferromagnetic below 8, 24, and 22°K, respectively, all exhibiting the Tb 2C 3-type magnetic structure. In the uniaxial moment model having two antiferromagnetic and two paramagnetic body diagonals, the saturation order moments per metal atom are 1.3, 3.0, and 9.5 Bohr magnetons, respectively, being 41, 92, and 95% of the respective free ion values. Pr 2C 3 shows an exceptionally large crystal field effect. The antiferromagnetic alignment is uninfluenced by the applied field of up to 21 kOe. The crystal structure data at 300 to 1.6°K are also given. A brief review is presented on the physical properties of the rare earth sesquicarbides.