Mössbauer studies of valence fluctuations

In the last ten years the Mössbauer technique has made a considerable contribution to the research of the phenomena of mixed valencies, valence instabilities, valence fluctuations and intermediate valencies of transition elements. The sensitivity of the hyperfine interaction parameters and in particular the isomer shift, to the valency of the Mössbauer ion, enabled the research of dynamics of valence fluctuations and the temperature, pressure and composition dependence of the ionic intermediate valence state. Studies of¹⁴⁹Sm,¹⁵²Sm and¹⁵³Eu in Sm_1?xR_xS contributed to the understanding of the outstanding insulator-metal phase transition that occurs in these systems. Studies of⁵⁷Fe and¹⁵¹Eu in mixed valent systems yield the charge fluctuation rates of the “hopping” mechanism, contributing to the conductivity in these systems. Studies of¹⁵¹Eu in EuRh₂, EuCu₂Si₂, EuPd₂Si₂, EuFe₄Al₈, and EuPd₆B₄ as a function of temperature and pressure reveal many aspects of the thermodynamics of intermediate valencies. Studies of systems like Eu_1?xLa_xRh₂, EuA_2?xB_x, EuA_5?xB_x reveal strong local environment dependence of the intermediate valency. Mössbauer spectra of¹⁶⁹Tm in TmSe,¹⁷⁰Yb in YbAl₃ and²³⁷Np in NpOs₂ also display phenomena associated with fluctuating valencies.     

Mössbauer spectra of EuPd3B0.5, an experimental test of a microscopic theory

The Mössbauer spectra of EuPd₃B_0.5 have been interpreted on the basis of the general microscopic theory previously developed by the authors. The theory connects the valence state of the europium ion with the distribution of the neighbouring boron atoms. The general characteristics of the present spectra are fairly well accounted for by theory, and an evolution with temperature of the line intensity ratio, in good agreement with the experimental data, is predicted.     

57Fe and 151Eu Mössbauer studies of magnetoresistive Europium based cobalt perovskites

Eu_0.8Sr_0.2Fe _x Co_1?x O_3?z CMR perovskites with different iron concentrations (x?=?0, 0.025, 0.075, 0.15, 0.3) were investigated by X-ray diffraction, AC magnetic susceptibility, magnetotransport, as well as ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectrometry. The valence state of europium ions was found to be trivalent, independently of the iron concentration. ⁵⁷Fe Mössbauer spectra and magnetic susceptibility of the investigated perovskites presented complementary results for the magnetic transitions.     

Combined Mössbauer and LIII-edge X-ray absorption study of mixed- valent EuPd2Si2 and EuNi2P2

The mean valence of Eu is determined by L_III-edge X-ray absorption (L_III-XA) and ¹⁵¹Eu Mössbauer isomer shift measurementd for the mixed-valent compounds EuPd₂Si₂ and EuNi₂P₂ in the temperature range from 1.7 to 300 K. For EuPd₂Si₂, the behaviour of the satellite line observed in the Mössbauer spectra was investigated in detail. From a comparison of the L_III-Xa and Mössbauer results obtained on identical samples the valence/isomer shift calibration problem as well as possible final-state effects in L_III-XA spectra are discussed.     

A Mössbauer study of La1−xEuxB6 compounds synthesized at high pressure and temperature

A series of hexaborides La_1?xEu_xB₆ (x=0.0–1.0) were synthesized under a pressure of 3.5 GPa and at a temperature of 1600^○C using La₂O₃, Eu₂O₃ and amorphous boron as the starting materials. The products were characterized by X‐ray Diffraction (XRD) and Mössbauer spectroscopy. XRD data analysis shows that all samples crystallize in a cubic CsCl‐type structure, and the cell volume increases with x. Room temperature ¹⁵¹Eu Mössbauer measurements reveal that Eu ions in all samples are in the divalent state, except for the x=1.0 sample where a small amount of Eu³⁺ ions was detected. The quadrupole splitting of the Eu²⁺ ions is positive. Eu ions were reduced from trivalent to divalent during the high‐pressure and ‐temperature processes. The isomer shifts of the Eu²⁺ ions are all smaller than ?12.5 mm/s, suggesting that there is no valence fluctuation in the samples. The hexaborides doped by divalent Eu are not metallic.     

Occurence and correlation of two non-equivalent sites for europium in EuPd3B

Mössbauer spectroscopy has been used to investigate the mechanism which affects the valence state of europium in EuPd₃B. The obtained results are interpreted in terms of a correlation between two non equivalent sites each in a fluctuating valence state.     

Eu valence mixture in amorphous EuxPd1−x alloys

The electronic configuration of Eu in amorphous Eu_x Pd_1−x alloys as determined by Mössbauer spectroscopy is found to change from a trivalent to a divalent state over the concentration range 0.18 ≦ x ≦ 0.35. The intermediate valence state of Eu in these alloys is characterized by a mixture of Eu occuring in two different integral valence states.     

The magnetic properties of the MgZn ferrite system by Mössbauer spectroscopy

Samples of the magnetism-zinc ferrite series Zn_xMg_1?xFe₂O₄ (x = 0.0 to 1.0) have been studied by the Mössbauer effect technique at 77 K. Mössbauer spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octachedral ions (B-sites), while for x=0.7 it shows relaxation behaviour and for x?0.8 it exhibits a paramagnetic quadrupole doublet. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the AB and BB supertransferred hyperfine interactions. Analysis of the average Mössbauer line width as a function of zinc concentration suggests that the relaxation spectrum observed at x=0.7 (77 K) is possibly due to domain wall oscillations.     

On the iron valence states in B2O3-PbO-GeO2 glasses

The Mössbauer effect measurements performed on 20Fe₂O₃ 80 3B₂O₃ (1?x)PbO xGeO₂ glasses show that the ratio between the number of ferrous ions to the total number of iron ions decreases by increasing the GeO₂ content. The Curie constants calculated from the distribution of iron cations obtained by Mössbauer effect data are in agreement with the values determined from magnetic measurements. Finally, we discuss the influence of the glass composition and melting temperature on the iron valence states.     

On the nature of the valence transition in Eu(Pd1−xAux)2Si2

The intermediate valent compound series Eu(Pd_1−xAu_x)₂Si₂ has been studied by Mössbauer effect measurements on ¹⁵¹Eu(T = 4.2−300K) and ¹⁹⁷Au (4.2K) and by X-ray diffraction (10K, 300K). The temperature induced valence transition Eu²⁺ → Eu³⁺ for x < 0.175 is not of first order type, as suggested in a previous phase diagram [1]. The valence change of the Eu-ion is reflected also in the isomer shift of the ¹⁹⁷Au Mössbauer-resonance.     

Investigations of the lanthanum-europium-copper-oxygen system by X-ray powder diffraction,thermal analysis and europium-151 Mössbauer spectroscopy

Lanthanum-europium-copper oxides of composition La_2?x Eu _x CuO₄ with structures related to those of the high temperature superconducting oxides have been prepared by solid state reactions between the component oxides in air. The X-ray powder diffraction data demonstrate that an orthorhombic to tetragonal structural transformation occurs at compositions betweenx=0.5 andx=0.8. The¹⁵¹Eu Mössbauer spectra show that europium is present in all phases as Eu³⁺. Thermal analysis studies in hydrogen show that a two-step reduction process occurs in the lanthanum-europium-copper oxides with the orthorhombic type structure.¹⁵¹Eu Mössbauer spectroscopy shows that the process does not involve the reduction of the lanthanide ion.     

Neutron diffraction and Mössbauer study on FeGa x Cr2−x S4

Ga doped sulphur spinel FeGa _x Cr_2?x S₄ (x = 0.1 and 0.3) have been studied with X-ray, neutron diffraction, and Mössbauer spectroscopy. Rietveld refinement of X-ray, neutron diffraction, and Mössbauer spectroscopy lead to the conclusion that the samples are in inverse spinel type, where most Ga ions are present at octahedral site (B). The neutron diffractions on FeGa _x Cr_2?x S₄ (x = 0.1) above 10 K show long range interaction behaviors and reveal a ferrimagnetic ordering, with the magnetic moment of Fe²⁺(?3.45 μ_B) aligned antiparallel to Cr³⁺ (+2.89 μ_B) at 10 K. Fe ions migrate from the tetrahedral (A) site to the octahedral (B) site with an increase in Ga substitutions. The electric quadrupole splittings of the A and B sites in Mössbauer spectra give direct evidence that Ga ions stimulate an asymmetric charge distribution of Fe ions in the A site.     

151Eu and57Fe Mössbauer effect studies of magnetic interactions in europium transition element oxides solution

The solid state solutions of europium transition element oxides Eu (Fe0.8M0.2)O₃ (M=Sc,Cr,Mn,Co) are synthesized. The X-ray diffraction of the compound shows that all the compounds possess the perovskite structures. Both the¹⁵¹Eu Mössbauer spectra and the⁵⁷Fe Mössbauer spectra are measured. The hyperfine magnetic field and non-axisymmetric electric field gradient are observed in the¹⁵¹Eu Mössbauer spectrum. The⁵⁷Fe Mössbauer spectrum shows that there are four components of hyperfine fields corresponding to four kinds of different neighbours of the Fe ion.     

151Eu Mössbauer spectroscopic, electric, and magnetic measurements of EuAgGe and EuAuGe

The physical properties of EuAgGe and EuAuGe, the structures of which are derived from the CeCu₂ type, have been investigated in detail by means of magnetic susceptibility, electrical conductivity and ¹⁵¹Eu Mössbauer measurements. Above 50 K both germanides show Curie--Weiss behavior with experimental magnetic moments of \mu _exp=7.70(5) \mu _B (EuAgGe) and \mu _exp=7.40(5) \mu _B (EuAuGe) and Weiss constants of -2(1) K (EuAgGe) and 33(1) K (EuAuGe). For EuAgGe, a magnetic phase transition is observed below 18(1) K. Zero-field cooling and field cooling measurements indicate cluster glass behavior (weak ferromagnetism, mictomagnetism). Magnetization measurements at 5 K show a saturation magnetic moment of 3.3(2) \mu _B/Eu at 5.5 T. ¹⁵¹Eu Mössbauer measurements show a Eu(II) valence state (\delta =-10.4 mm/s). While magnetic hyperfine splitting appears in the spectra at temperatures as high as 15 K, complete magnetic ordering is not reached at temperatures down to 4.2 K. EuAuGe orders ferromagnetically at 32.9(2) K. Magnetization measurements at 2 K show a saturation magnetic moment of 6.2(1) \mu _B/Eu at 5.5 T, respectively, indicating that all spins are ordered ferromagnetically at low temperatures. ¹⁵¹Eu Mössbauer measurements show a Eu(II) valence state (\delta =-10.6 mm/s) and two spectral components in an approximate 1:1 ratio, subjected to magnetic hyperfine splitting effects at T₁=32(2) and T₂=18(4) K, respectively. Thus, the transition temperature of 32.9 K observed in the susceptibility measurements appears to be associated with ordering of only one of the two crystallographically distinct europium sites in this compound. Electrical conductivity measurements indicate metallic behavior for both germanides.     

Magnetic properties of metallic glasses of the type Fe80−x Pd x B20 and Fe80−x Pt x B20

Amorphous alloys of the type Fe_80???xPd_xB₂₀ and type Fe_80???xPt_xB₂₀ for 0?≤?x?≤?50 have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy and magnetisation measurements in temperatures from 4.2 up to 300 K. Curie temperatures and crystallisation temperatures are found by DTMG-DTA method. Mössbauer spectroscopy magnetic field is observed to visible increase for x?=?1 and 1.5% at room temperature for Pd, while a decrease is observed for higher x values. Curie temperature for Pd alloys has a maximum at x?=?4 with T _C?=?753 K, which supports enforcing influence of Pd at low concentrations of Pd for magnetic interactions. We discuss different explanations for these measurements and compare with other findings for high Pd concentrations and alloys with Pt instead of Pd.     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

Coexistence of long range magnetic order and intervalent state of Eu in EuCu2(Si x Ge1 − x )2: Evidence from neutron diffraction and spectroscopic studies

Experimental results of the X-ray absorption spectroscopy, Mössbauer spectroscopy (isomer shift) and neutron diffraction are presented for the series of EuCu₂(Si _x Ge_{1 ? x} )₂ polycrystalline samples (0 < x < 0.75). Homogeneous intermediate valence state is established for Eu ions as well as long range magnetically ordered state at the temperatures below 10–15 K. Observation of the ordered magnetic moments at Eu site gives rise to the experimental statement for the coexistence of valence fluctuations and long range magnetic order takes place in the wide range of Ge concentrations for this substance.     

The coordination of Co2+ions in Co substituted magnetites,Fe3−xCoxO4, with x⩽0.04

⁵⁷Fe Mössbauer spectra at room temperature, both with and without external magnetic field, indicate that Co²⁺ ions in Co_xFe_3?xO₄spinels (x?0.04) are situated on the octahedral B sites. The Mössbauer parameters are listed and the existence of unpaired Fe³⁺ ions is evidenced.     

Evidence of magnetic ordering in tellurium substituted FeSb2

We report on a⁵⁷Fe Mössbauer study of tellurium substituted FeSb₂, FeSb_2?x Tc _x (x=0.2, 0.4, 0.6), at temperatures between 4.2 K and 300 K. For all three alloys, the Mössbauer spectra at 4.2 K are characteristic of a magnetically ordered state. The hyperfine field at Fe site increases with increasing tellurium concentration. The magnetic character may be attributed to the existence of a very narrow band gap leading to fairly strong Coulomb and exchange interactions between holes in the valence band and electrons in the conduction band.     

Mössbauer study of the spinel system GexCu1−xFe2O4

The magnetic properties of the spinel series Ge_xCu_1?xFe₂O₄ (X = 0 to 0.8) have been investigated by means of Mössbauer spectroscopy. Mössbauer spectra for X = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to Fe³⁺ octahedral ions (B-sites), while for X = 0.8 it shows the superposition of hyperfine field split spectra from A- and B-site ions and a broad central line spectrum. For 0.2 ? X ? 0.4, fast electron exchange among octahedral iron ions occurs as in Fe₃O₄. The variations of nuclear magnetic fields at the A- and B-sites are explained on the basis of AB and BB supertransferred hyperfine interactions.