Laser-produced iron nitrides seen by Mössbauer spectroscopy

Mössbauer spectroscopy is a very powerful tool to investigate technological processes performed mainly at the surface of materials. Nitriding of metals and steel is well established in surface engineering, and gas nitriding is used most frequently. Laser nitriding, i.e. the nitrogen take-up from the ambient gas upon irradiation of a steel surface with short laser pulses, is presented in its application to iron, stainless steel and plain carbon steels. It will be demonstrated how Mössbauer spectroscopy in combination with complementary methods (Rutherford backscattering spectroscopy, Resonant nuclear reaction analysis, Nanoindentation) can help to reveal basic mechanisms in these processes.     

Observation of spin-lattice relaxations of dilute Fe3+ in MgO by Mössbauer spectroscopy

We present a method to describe the temperature dependence of emission Mössbauer spectra showing slow spin-lattice relaxations of Fe^3?+? in MgO single crystals, obtained after implantation of ⁵⁷Mn at ISOLDE/CERN. The analysis is based on the Blume-Tjon model for the line-shape of relaxing paramagnetic sextets with the spin relaxation rate, τ ^???1 as a parameter. The temperature dependent spin relaxation rate of Fe^3?+? in MgO is found to increase to ~10⁸ s^???1 at 647 K by assuming a relaxation rate of τ ^???1?< 10⁶ s^???1 at 77 K. The results are in accordance with those obtained by electron paramagnetic resonance spectroscopy demonstrating the possibility of retrieving spin-lattice relaxation rates of dilute Fe^3?+? from emission Mössbauer spectroscopy of Mn/Fe-implanted oxides.     

Dynamics of iron in Fe-porphyrin aggregates studied by X-ray absorption and Mössbauer spectroscopy

The iron-porphyrin aggregates were studied by optical absorption and fluorescence method, infrared spectroscopy, X-ray absorption and Mössbauer spectroscopy. The aggregation of porphyrin molecules strengthens the Fe-ligands bonds and accelerates the spin-spin relaxations. A significant speeding-up of relaxation was observed with lowering the temperature down to 25 K. The comparison of the EXAFS (Extended X-ray Absorption Fine Structure) and Mössbauer spectroscopy results enabled some separation of the individual Fe vibration from its collective movement with ligands.     

Mössbauer spectroscopy of Al substituted Fe in holmium iron garnet

A Mössbauer spectroscopy study was made on Ho₃Fe_5-xAl_xO₁₂ (x=0.0, 0.05, 0.7). X‐ray diffraction patterns indicate that the samples with x=0.0 and 0.05 have the garnet structure, while the sample with x=0.7 has an additional noncubic structural phase. The room temperature spectrum for samples with x=0.0 and 0.05 consists of two magnetic components corresponding to the octahedral and tetrahedral sites with hyperfine magnetic fields (B_hf) of 50 T and 40 T, respectively. For x=0.7 we observe a new magnetic component with B_hf= 45 T, a reduction in the intensity and broadening of the tetrahedral component, and the evolution of a nonmagnetic central component. These variations are evidently due to the addition of aluminium to the system. At liquid nitrogen temperature the samples with x=0.0 and 0.05 are nearly identical. It was also observed that the increase in B_hf for the octahedral site is smaller than that for the tetrahedral site as the temperature is lowered to 80 K.     

Study on spin configuration in photoresponsive iron mixed-valence complexes by Mössbauer spectroscopy

We have investigated magnetic properties in a series of photoresponsive dithiooxalato (dto)-bridged iron mixed-valence complexes, (SP-R)[Fe^IIFe^III(dto)₃] (SP-R = R-substituted pyridospiropyran cation; R = Me, Et, and Pr; abbreviated as 1 ^Me, 1 ^Et, and 1 ^Pr, respectively). As for our previous reports, 1 ^Me and 1 ^Et show two-step succeeding ferromagnetic transitions at T _C?=?25 & 8 K and 22 & 5 K, respectively. However, 1 ^Et has no hysteresis in the magnetic susceptibility, while 1 ^Me undergoes the charge transfer phase transition with thermal hysteresis around 75 K. To elucidate the two-step transitions of them, we measured ⁵⁷Fe Mössbauer spectra of 1 ^Et. The spectra of Fe^II (S?=?2) and Fe^III (S?=?1/2) in the HTP were observed in the magnetically ordered state as well as the paramagnetic state, and revealed that only HTP exists in a temperature range up to 5 K. The result is consistent with that of 1 ^Pr, where one ferromagnetic phase transition occurs at T _C?=?10 K. ⁵⁷Fe Mössbauer spectroscopy is useful to clarify the origin of the succeeding magnetic transition for these systems.     

Mössbauer spectroscopy study of iron corrosion underneath painting system

The effect of pigments on the development of corrosion products between the painting system and metal surface when exposed to marine environments has been discussed. The pigments studied were; Red Mud Zinc chromate, Zinc chromate, Red oxide Zinc Phosphate, Manganese Phosphate Barium chromate and Basic Lead Silico Chromate. Mossbauer Spectroscopy revealed that the upper rust layer in all the cases consisted of-Fe203,-FeOOH and-FeOOH. The lower rust layer immediately in contact with the metal surface consisted of an asymmetrical doublet due to -FeOOH.     

Mössbauer study of iron hydride produced under high pressure

The hexagonal iron hydride formed at a hydrogen pressure of about 7 GPa was studied by⁵⁷Fe Mössbauer spectroscopy. At 80 K its spectrum consists of two magnetic patterns with hyperfine fields of 33.6 and 29.0T, respectively. Both have an isomer shift of +0.50mm/s with respect to-iron. A tentative explanation for these results is that iron hydride is stoichiometric FeH with a dhcp crystal structure.     

Corrosion studies of iron and its alloys by means of57Fe Mössbauer spectroscopy

Some of the advantages and limitations of Mössbauer spectroscopy when used in corrosion research are shown by using three examples taken from the work of the authors on (i) the passive layer of iron, (ii) the corrosion of weathering steels by SO₂-polluted atmospheres and (iii) the performance of rust converters.     

Study of archaeological iron objects by PGAA,Mössbauer spectroscopy and X-ray diffraction

Archaeological iron objects often corrode rapidly after their excavation, even though they have survived long times of burial in the ground. Chlorine that accumulates during burial is thought to play a major role in this destructive post-excavation corrosion. It is therefore important for the conservation of such objects to determine the chlorine content in a non-destructive manner and, if necessary, to remove the chlorine from the artefacts by appropriate methods. Such methods are leaching in alkaline solutions or heating in a reducing atmosphere at temperatures up to 800 ^°C. We have studied the efficiency of the heating method using prompt gamma activation analysis (PGAA) for monitoring the Cl content and Mössbauer spectroscopy at room temperature (RT) and 4.2 K as well as X-ray diffraction to study the mineralogical transformations of the rust layers. The heat treatments were performed a N₂/H₂ (90/10) mixture at temperatures up to 750 ^°C. As test specimens sections of iron rods from the Celtic oppidum of Manching (Bavaria) were used. The initial Cl contents of the pieces varied in the range of several hundred ppm, referring to the iron mass. Annealing for 24 h at 350, 550 and 750 ^°C was found to reduce the Cl contents of the specimens, to about 70, 30 and 15 % of the original values, respectively. The rust consists mainly of goethite with admixtures of magnetite, lepidocrocite and akaganeite, which is thought to be a major carrier of chlorine, probably together with iron chlorides. Much of the goethite is so fine-grained that it does not split magnetically at RT. Annealing converts the rust mainly to maghemite at 350 ^°C, to magnetite at 550 ^°C and to wüstite plus magnetite and metallic iron at 750 ^°C. Pure akaganeite behaves in nearly the same manner.     

Characterisation of synthetic trioctahedral micas by Mössbauer spectroscopy

Trioctahedral potassium micas |K}[M₃]〈T₄〉O₁₀(OH)₂ have been synthesized by hydrothermal techniques with various cationic substitutions in the octahedral and the tetrahedral sheet. Taking annite |K}[Fe ₃ ²⁺ ]〈AlSi₃〉O₁₀(OH)₂ as the reference mineral, [Fe²] was replaced by [Mg²] and [Ni²], 〈Al³⁺〉 by 〈Fe³⁺〉 and finally [Fe²⁺] + 〈Si⁴⁺〉 by [Al³⁺] + 〈Al³⁺〉. Mössbauer spectra were evaluated in terms of quadrupole splitting distributions (QSDs) using three generalized sites for 〈Fe³⁺〉, [Fe³⁺] and [Fe²]. Annites, nominally free of 〈Fe³⁺〉, show a lower limit of [Fe³⁺]/Fe _tot of 0.10, which stabilizes the structure. The ferrous iron, [Fe²], QSD consists of two main components. In some of the solid solution series, there is strong experimental evidence for a third ferrous component, particularly at higher [Al³⁺] contents. This third component is centered at low quadrupole splittings and may be assigned to a defect [Fe²] site, forming 1:2 structures with two neighbouring trivalent octahedral cations. For charge compensation one OH^? is replaced by O^2? for each [M³⁺] cation. The ferrous QSDs vary systematically with chemical composition. Compared to those of annite, the QSD parameters (mean quadrupole splitting 〈QS〉 and quadrupole splitting with maximum probability, QS _peak ) are shifted towards higher values with increasing [Mg²] and [Ni²] contents, and decrease slightly with increasing content of trivalent cations. These trends can be interpreted in terms of changes in the local environment around the Fe probe nucleus, i.e., in terms of decreasing or increasing distortions from the ideal octahedral configurations.     

Observation of Cooper pairs in high-temperature superconductors by 67Cu(67Zn) Mössbauer spectroscopy

A Mössbauer emission spectroscopy study on the ⁶⁷Cu(⁶⁷Zn) isotope showed that the superconducting transition in the Nd_1.85Ce_0.15CuO₄, La_1.85Sr_0.15CuO₄, and Tl₂Ba₂CaCu₂O₈ compounds is accompanied by an electron density redistribution in the crystal, which is considered evidence of Cooper-pair Bose condensation.     

Mössbauer spectroscopy in physical metallurgy

In this anniversary contribution the natural and intimate match making which occurs between the two star performers-⁵⁷Fe in Mössbauer spectroscopy and iron in physical metallurgy—is described by selecting typical examples reflecting the author's interest: phases, f.c.c. -Fe, defects, diffusion and amorphous metals.     

Mössbauer spectroscopy in molecular magnetism

The aim of this paper is to highlight some selected research activities on molecular magnetic materials using Mössbauer spectroscopy as a technique carried out in our laboratory in recent years. The first part of the present article is devoted to the studies of the various magnetic interactions, metal-to-metal electron-transfer phenomenon, glass transition occurring in molecular magnetic materials, whereas the second part deals with the iron(II) high spin (S = 2)–low spin (S = 0) transition phenomenon occurring in some isoxazole ligand based iron(II) compounds as examples with unusually complicated spin transition behaviour. Also, an example of a dinuclear a spin crossover compound of iron(II) is described, where Mössbauer spectroscopy has most convincingly unraveled the mechanism of the spin transition process. Finally, an example from our most recent studies of spin crossover materials exhibiting both thermal spin crossover and liquid crystalline properties in the same temperature interval near room temperature will be presented.     

Mössbauer spectroscopy of zirconium alloys

Mössbauer investigations of zirconium alloys were examined. Data about the chemical state of iron atoms in the zirconium alloys of different composition has been provided. Mössbauer spectroscopy revealed that small quantities of iron in binary zirconium alloy are in the solid solution α-Zr (up to 0.02 wt.%). Different iron atoms concentration and thermo-mechanical treatments may lead to formation the intermetallic compounds Zr₃Fe, Zr₂Fe, ZrFe₂. Adding tin atoms does not affect the formation and shape of Mössbauer spectra of these compounds. Adding Cr and Nb atoms makes significant changes in the shape of Mössbauer spectra and leads to the formation of complex intermetallic compounds. Adding Cu and W atoms, the shape of the binary alloys spectra (Zr-Fe) remains unchanged, but a change in the temperature dependence behavior of the spectral parameters occurs and also, changes to the properties of the alloys.     

A Mössbauer spectroscopy study of the corrosion of nodular cast iron in mine waters

The corrosion of ductile cast iron in water containing different amounts of chloride ions was investigated under both static and dynamic conditions. Corrosion/time relationships were established for exposure times of up to 30 days. Post-corrosion investigations were performed, employing Mössbauer spectroscopy, optical microscopy and electrochemical techniques. It was found that the nature of the surface corrosion product formed under static conditions differed morphologically and chemically from that formed under dynamic conditions. The latter was a hard layer consisting of a mixture of - and -FeOOH (situated on an underlying cementite layer), whereas the static tests resulted in a soft, spongy corrosion product, identified as -FeOOH.     

Characterization of the photolyase-like iron sulfur protein PhrB from Agrobacterium tumefaciens by Mössbauer spectroscopy

High field Mössbauer spectroscopy has been used to characterize the [4Fe-4S] ^{2 +}cluster of the protein PhrB from Agrobacterium tumefaciens which belongs to the cryptochrome/photolyase family (CPF) and which biological function has previously been shown to be DNA repair. Mössbauer spectra taken of the as prepared protein reveal δ = 0. 42 mms ^{? 1}, and Δ E _Q = 1. 26 mms ^{? 1}as well as an asymmetry parameter of η = 0. 8. These parameters are characteristic for a ferredoxin-type [4Fe-4S] ^{2 +}cluster. In order to investigate whether this cluster is involved in DNA-repair the protein has also been studied in its photoactivated state during DNA binding. The so obtained data sets exhibit essentially the same Mössbauer parameters as those of the non-activated PhrB. This indicates that during DNA repair the [4Fe-4S] ^{2 +}cluster of PhrB has no significant amounts of transition states which have conformational changes compared to the resting state of the protein and which have life times of several seconds or longer.     

Mössbauer study of iron removal in a montmorillonite

⁵⁷Fe Mössbauer spectroscopy in conjunction with atomic absorption spectrometry and X-ray powder differaction analyses have been used to study the iron present in a montmorillonite prior to and after different successive stages of two deferration processes. Fe³⁺ ions occupy mainly octahedrical M(2) sites in the mineral structure; no impurities of iron oxides were detected. The quite efficient deferration by HCl refluxing produced a substantial alteration of lamellar structure of montmorillonite, whilst dithionite/citrate treatment did not induce severe structural changes but had low iron removal efficiency.     

Evaluation of Mössbauer line diffusion broadening by exponential fit

The Mössbauer line broadening measured by R.Lindsey in -iron stabilized with vanadium is evaluated by an exponential fit described in the APPENDIX of this paper. By this procedure the low temperature Mössbauer line width _m ⁰ , the diffusion frequency factorD ₀ and the activation enthalpyH are computed.Knauer's andSørensen-Trumpy's definitions of the time correlation factorf _t , necessary for theD ₀ calculation from Mössbauer line broadening, are applied to the Fe-V diluted BCC solid solutions.     

Magnetism of ErAgSn studied by 119Sn Mössbauer spectroscopy

Antiferromagnetic ErAgSn compound was investigated in detail by ¹¹⁹Sn Mössbauer spectroscopy in a temperature range between 2.2 and 300 K. The ¹¹⁹Sn spectra recorded below 4.2 K can be well fitted with a single main magnetic component in agreement with recent neutron diffraction studies [1]. A broad distribution of magnetic hyperfine fields observed above 4.2 K and enhanced spin correlations among Er³⁺ ions at T > T _N = 5.6 K are the remarkable features of the investigated system.