G. Michels

Pressure effects on the electronic properties and the magnetic ground state of γ′-Fe4N

Abstract The effects of pressure on the electronic properties and the magnetic ground state of γ′-Fe4N have been investigated using both the 57Fe Mossbauer effect (up to 6.1 GPa at 4.2 K) and X-ray diffraction (up to 8.0 GPa at 300 K) techniques. The analysis of the X-ray diffraction data under pressure shows a linear decrease of the volume with pressure with a compressibility of 5.1(1) × 10−3 GPa−1. From the measured difference (ΔS = 0.06 mm/s) between the isomer shifts of the Fe(I) and Fe(II) sites at ambient pressure, a charge transfer from N-2p to Fe(II)-3d is proposed, which is supported by recent band structure calculations. The pressure-induced decrease of S of Fe(I) and Fe(II) is found to be nearly the same, indicating that the increase of the s-electron density at the 57Fe nucleus is governed by the compression of the 4s-electrons rather than by charge transfers. Regarding the pressure dependence of the magnetic moments of the Fe(I) and Fe(II) sites, we find in contrast to the results of very recent high-pressure NMR experiments on γ′-Fe4N, which reveal almost a comparable pressure-induced decrease of the 57Fe NMR frequency of the two Fe sites, that the magnetic hyperfine (hf) field (Beff) at the Fe(I) site decreases more rapidly with pressure than that at the Fe(II) site. This finding is inconsistent with recent band structure calculations on γ′-Fe4N which show that the magnetic moment of the Fe(I) site is more localized than that of the Fe(II) site. We suggest that this discrepancy between theory and our experimental results is due to the contribution of a transferred hf field to the pressure dependence of Beff, which is sensitive to the variation of Fe-N interactions with pressure.

Mixed valency and first order phase transition in EuPdAs

The intermetallic alloy EuPdAs exhibits a first order phase transition at 180 K. The transition is seen by anomalies in lattice parameters, susceptibility, resistivity, Moessbauer effect and X-ray absorption. It is accompanied by an increase of the Eu valence from 2.15 to 2.40 between 300 K and 4 K. The nature of the non-integral valence and the phase transition are discussed.

Final-state effects in divalent Eu pnictides

From Mossbauer effect, lattice constant and susceptibility measurements we show that Eu is nearly divalent in Eu(Cu,Ag,Au)(P,As,Sb). However, the LIII X-ray absorption spectra consist of a pronounced double structure, suggesting a non-integral mean Eu valence. The high-energy structure in the LIII-edge spectra is interpreted as a final-state (shake-up) effect. This finding strongly supports the existence of final-state effects in the LIII X-ray absorption spectra of divalent Eu compounds as previously reported by Sampathkumaran et al (1985) in EuPd2P2.

First-order phase transitions in the ThCr2Si2-type phosphides ARh2P2 (A = Sr, Eu)

Abstract We observed for the first time a first-order phase transition with strong and extremely anisotropic changes of the lattice parameters in compounds crystallizing in the ThCr2Si2 structure type. The phase transition occurs SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On the basis of single-crystal data of ARh2P2 (A = Ca, Sr, Ba, Eu) at ambient pressure and temperature we discuss the PP distance in the framework of band-structure calculations at the first-order phase transition the PP state changes from a ‘no-bond’ to a ‘single-bond’ state.

Temperature induced valence instabilities in ternary Eu-pnictides: a comprehensive view

We report on measurements of the lattice constants, magnetic susceptibility, LIII X-ray absorption and Mössbauer-effect on EuNiP and EuPdP, which crystallize in the hexagonal layered Ni2In structure. In both compounds the Eu valence above 510 K is 2.33. With decreasing temperature they show one (EuPdP) or two (EuNiP) first order phase transitions with a valence increase of about 0.16. At the same time thec-axis shrinks while the a-axis even increases. From Mössbauer measurements we show that the nature of the valence mixing is static. In contrast, the valence mixing in isostructural EuPtP is static at low temperatures, too, but it becomes homogeneous valent above a first order phase transition at 235 K. The behaviour of these compounds (as well as that of EuPdAs) is explained in a new model of electrostatically charged layers. In this model we can explain the temperature dependence of the lattice constants, the static valence mixing and the occurrence of preferred valences of the order 2 6/n. Together with the compression shift model of Hirst we can also understand the mechanism of the phase transitions. A comparison with EuT2Si2 compounds in ThCr2Si2 structure shows that in EuTX compounds only the electronic structure of the transition elements is relevant for the occurrence of mixed valency.

Kristallstrukturen und Eigenschaften neuer ternärer iridiumphosphide

Four new ternary iridium phosphides were prepared by reaction of the elements at 900–1100 °C. Their structures were determined by means of single-crystal X-ray methods. EuIrP (a = 6.272(1) A) and BaIrP (a = 6.531(1) A) crystallize in a SrSi2-type derivative structure (LaIrSi-type structure; P213; Z = 4). EuIr2P2 (a = 6.671(1), c = 7.055(1) A) and SrIr2P2 (a = 6.693(1), c = 7.061(1) A) form a new structure (P3221; Z = 3). All the P atoms and half of the Ir atoms build up a three-dimensional framework with the Eu(Sr) atoms and the remaining Ir atoms in the cavities. The latter atoms form threefold screws along [001] with short IrIr distances. Magnetic measurements of EuIrP and EuIr2P2 show that europium is divalent in both compounds.

The anomalous valence state of Eu and magnetic order in

According to x-ray absorption, M?ssbauer effect measurements, lattice constants and static susceptibility, Eu in the novel compound is non-integral valent with a valence of the order of 2.2. On the other hand, the compound orders antiferromagnetically below 50 K, the M?ssbauer isomer shift is rather independent of temperature, and the resistivity shows no anomalies in the paramagnetic region. Hence, according to existing theories the Eu 4f level cannot be strongly hybridized with the conduction band as in usual intermediate-valent compounds. We therefore propose a considerable participation of the 4f electrons in a covalent Eu - P bond. However, the absence of the ESR signal of as well as the small but finite temperature dependence of the valence clearly demonstrate that the hybridization of the 4f electrons with the conduction band is not negligible in this compound.

Three magnetic order transitions in mixed valent EuPtP

We report three magnetic order transitions, at 25K, 8K and 0.9 K for the ternary intermediate valent europium system EuPtP in addition to the two first order phase transitions, reported in [1]. For comparison Eu0.5Sr0.5PtP was also studied by Lm-X-ray absorption and susceptibility measurements. The two first order transitions do not exist in Eu0.5Sr0.5PtP and only one magnetic order transition was found. The behaviour of EuPtP and Eu0.5Sr0.5PtP is discussed in terms of the domain picture, proposed in [1].

Tuning the valence in ternary Eu-pnictides: the series EuPd1-xAgxP and EuPd1-xAuxAs

The series of ternary Eu-pnictides EuPd1−xAgxP and EuPd1−xAuxAs were synthesized by heating the elements and characterized by means of X-ray powder diffraction. The unusual behaviour of the lattice constants are explained by geometric and electronic factors, in particular the change of the Eu valence. In EuPdP and EuPdAs, the Eu atoms adopt a temperature-dependent mixed valent state which tends towards divalency while substituting Pd by Ag or Au. This is investigated by means of temperature-dependent X-ray diffraction. TB-LMTO-ASA band structure calculations and resonant photoemission (PE) experiments using synchrotron radiation. The calculated density of states (DOS) was compared with the results of the PE experiments. The measured partial DOS, as well as the LMTO-DOS, of EuPdP is marked by a high DOS at the Fermi level. A van Hove singularity in the band structure leads to a logarithmic DOS-peak. If the Fermi level coincides with this peak, valence instabilities are expected. The results of the LMTO band calculations are in good agreement with the experimental facts, leading to the conclusion that the range of valence instabilities of europium is limited to x < 0.4 in EuPd1−xAgxP and x < 0.15 in EuPd1−xAuxAs.

Magnetic relaxation spectra of EuNi2P2

We report on inelastic neutron scattering experiments on EuNi2P2 prepared from153Eu-isotopes. The sample was checked by Mössbauer spectroscopy; a “satellite” phase of about 4% was found. The neutron scattering experiment was performed with thermal neutrons. AboveT=50 K a strong quasielastic magnetic line was detected with a nearly temperature independent width of about 6 meV. At low temperatures the spectrum changes to an inelastic character with a distinctQ-dependence in the intensity. For comparison static susceptibility and magnetization measurements were performed down to 250 mK.