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.     

Mineral identification in Colombian coals using Mössbauer spectroscopy and X-ray diffraction

Minerals were identified in three Colombian coal samples from the Southwest of the country using M?ssbauer spectroscopy and X-ray diffraction. Original and sink separated coal fractions of specific gravity 1.40 and 1.60 with particle size less than 600 μm were used in the study. Using M?ssbauer spectroscopy, the minerals identified in the original coal samples were pyrite jarosite, ankerite, illite and ferrous sulfate, whereas by means of X-ray diffraction, minerals identified were kaolinite, quartz, pyrite, and jarosite. Differences in mineral composition were found in the original and sink separated fractions using both techniques. M?ssbauer spectra show that the mineral phases in low concentrations such as illite, ankerite and ferrous sulfate do not always appear in the spectra of sink coals, despite of those minerals occurring in the original coal, due to the fact that they are associated with the organic matter and not liberated in the grinding process. X-ray results show that the peak intensity grows as the specific gravity is increased indicating that the density separation method could be an effective process to clean coal. This revised version was published online in July 2006 with corrections to the Cover Date.     

X-ray diffraction and Mössbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (～10¹⁵/m²) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ?1?1?1? screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.     

In situ high-pressure study of LiNbO3-type FeTiO3: X-ray diffraction and Mössbauer spectroscopy

The structure of LiNbO₃-type FeTiO₃ and the oxidation state of Fe have been investigated using X-ray diffraction and Mössbauer spectroscopy in the diamond anvil cell up to 18 GPa at room temperature. A structural phase transition is observed at 15.7 GPa from LiNbO₃-type to perovskite-type, accompanied by a volume collapse of 1.5%. LiNbO₃-type FeTiO₃, which is shown to contain only ferrous iron up to this pressure, and no charge transfer is observed. In addition to the c/a axial ratio that has been used to distinguish between ilmenite and LiNbO₃-type FeTiO₃, the hyperfine parameters (isomer shift and quadrupole splitting) provide an efficient way to discriminate between these two phases.     

Mössbauer spectroscopy,X-ray diffraction and magnetic measurements of iron-nickel ultrafine particles

Iron-nickel ultrafine particles with a composition in the Invar region (38–50% Ni) were prepared by the gas-evaporation-coalescence technique. The chemical composition was checked by electronprobe microanalysis, while X-ray diffraction Rietveld refinement was used to characterize the structure as well as to estimate the particle size. The temperature and field dependence of the magnetizationM(B, T) was measured for 0B25 kOe in the temperature range 4.2 KT400 K. Transmission Mössbauer spectra were taken at room temperature and at liquid helium temperature. The results obtained show that the predominant phase is a disordered Ni-rich alloy.On leave from Physics Department, University of Khartoum, P.O. Box 321, Khartoum, Sudan.     

A Mössbauer and X-ray diffraction study of nonstoichiometry in magnetite

The paper deals with the applicability of Mössbauer spectroscopy and X-ray diffraction methods for the determination of the deviation of magnetite from stoichiometry. The results show that among the data obtainable by both methods, the ratio of intensities of two partial spectra composing the Mössbauer spectrum of magnetite enables to evaluate the deviation of magnetite from stoichiometry quantitatively.The authors express their gratitude to Prof. Dr. Ing. J.Cirák who enabled them to perform all measurements of Mössbauer spectra at the Department of Nuclear Physics and Technics, Slovak Technical University, Bratislava. The authors are also indebted to Ing. P.Holba (Institute of Solid State Physics, Czechoslovak Academy of Sciences) and to Ing. Z.Drbálek (Research Institute of Sound and Picture) for the preparation of magnetite samples, and to Mr. P.Chaloupek (Faculty of Mathematics and Physics, Charles University, Praha) for computer calculation of lattice constants. The aid provided by members of the G. V. Akimov State Research Institute for the Protection of Materials, Dipl. Chem. K.Jendelová who carried out chemical analysis of the samples and Ing. K.Turecká who took part in X-ray diffraction measurements, is gratefully acknowledged.     

Effect of titanium ion substitution in the barium hexaferrite studied by M?ssbauer spectroscopy and X-ray diffraction

A series of M-type barium hexaferrite has been synthesized in a glass melt by partially substituting the Fe₂O₃ with TiO₂ for investigation of their structure. The glass melt has the basic composition (mol%): 40 BaO + 33 B₂O₃ + (27-x) Fe₂O₃ + x TiO₂ with x =?0, 3.6, 5.4 and 7.2 mol% TiO₂. The substituted ferrites were studied by means of X-ray diffraction, Mössbauer spectroscopy and vibration sample magnetometer. X-ray diffraction studies revealed that not all samples have a single ferritic phase, a small second phase corresponding to BaTi₆O₁₃ was also observed to form. The Mössbauer spectra changed from magnetically ordered (x =?0) to magnetically ordered with strong line broadening. Moreover, the broadening increases with TiO₂ content. The Mössbauer parameters suggested that Ti^4?+? occupies the 2a and 12k crystal sites, and the Ti^4?+? substitution on the 2b and 4f₂ site also occurs at high melt dopings. Therefore, coercivity and saturation magnetization decreased.     

Iron-metal multilayers studied by Mössbauer spectroscopy,RBS and X-ray diffraction

Fe/M (M = Ag, Zn and Sn) multilayers prepared by a vacuum evaporation method are studied by Mössbauer spectroscopy (MS), Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). For the case of an M = Ag multilayer, MS reveals that Fe in the multilayer remains as an-phase down to the layer thickness of 10 nm. This result is in agreement with the RBS result that Fe and Ag form a completely discrete layer structure without any mutual mixing. For the case of M = Zn and Sn, RBS reveals that a considerable mixing has taken place between Fe and Sn during the specimen preparation. MS on Fe/Sn specimens with different layer thickness shows that an alloy phase of about 5 nm thickness is formed at the interface. Structural as well as magnetic properties of the alloy phase are discussed based on MS at different temperatures and on reported results of the intermetallic compound FeSn.     

Celtic gold coins in the light of Mössbauer spectroscopy, electron microprobe analysis and X-ray diffraction

Celtic gold coins found in Southern Germany were studied by Mössbauer spectroscopy, electron microprobe analysis and X-ray diffraction with special attention to coins rich in silver and copper. In such coins the electron microprobe analyses reveal a gold enrichment in a surface layer of more than 100 μm thickness. ¹⁹⁷Au conversion electron Mössbauer spectroscopy also shows that the surface of the coins consists of two phases, one of which is strongly enriched in gold compared to the bulk composition. In comparison with laboratory experiments the observed phenomena suggest that coin production in Celtic times may have involved deliberate heating and etching steps to enrich the surface layer in gold by depleting it of silver and copper.     

Accelerated mechanosynthesis of high-nitrogen stainless steel: Mössbauer and X-ray diffraction studies

A completely austenitic structure has been obtained ten times faster via the mechanical alloying (MA) of high-nitrogen 25Cr10Mn1N alloy from metal components and manganese nitride than during the MA of similar steels in a nitrogen atmosphere. A mechanism for the bcc → fcc phase transformation that occurs during MA, and where deformation stacking faults of the layer fault type on the {211} atomic planes of the bcc phase play a key role, is considered and proposed.     

Characterization of Maghsail meteorite from Oman by Mössbauer spectroscopy, X-ray diffraction and petrographic microscopy

The meteorite found at Maghsail (16 55 70 N–53 46 69 E) west of Salalah Oman, has been studied by ⁵⁷Fe Mössbauer spectroscopy, X-diffractometry and petrographic microscopy. In the polished section the meteorite exhibits a porphyritic texture consisting of pyroxene and olivine phenocrysts in a fine to medium grained ground mass in addition to minor phases possibly skeletal chromite, troilite and minute amount of iron oxides. X-ray diffraction supports the existence of these compounds. The Mössbauer spectra of powdered material from the core of the rock at 298 K and 78 K exhibit a mixture of magnetic and paramagnetic components. The paramagnetic components are assigned to the silicate minerals olivine and pyroxene. On the other hand, the magnetic spectra reveal the presence of troilite and iron oxides. The petrographic analyses indicate that the iron oxides are terrestrial alteration products.     

Energy dependence of ion beam mixing at Fe-Al and Fe-Cu interface: A conversion electron Mössbauer spectroscopic study

The energy dependence of ion beam mixing in the Fe-Al and Fe-Cu bilayer configuration on the energy of bombarding Kr⁺ ions has been investigated by using the technique of conversion electron Mössbauer spectroscopy. The ion energy has been varied over a range between 30 keV and 120 keV by keeping the dose value fixed at 10¹⁶ ions/cm². In both the cases the extent of mixing shows an interesting variation with increase in the value of ion energy. This behaviour is discussed in the light of the calculated energy deposition distributions for the case of the given bilayer systems. The variations in the microstructural details of the mixed region with change in the incident ion energy have also been brought out via determination of hyperfine interaction parameters.     

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.     

X-ray diffraction and Mössbauer spectroscopy of high energy ball-milled α-Fe2O3/TiO2 composite powders

α–Fe₂O₃/TiO₂ Composite powders have been prepared by high energy ball-milling for different times. The composites were studied using Mössbauer Spectroscopy (MS) and X-ray diffraction (XRD). The patterns of XRD show broadening in the diffraction peaks, indicating a decrease in the particle size of the composites with milling time. Also, the XRD patterns show an evolving new structural phase correlated with an evolving Titanium ferrite species with milling time. Mössbauer Spectroscopy shows the evolving titanium ferrite species characterized by a quadrupole doublet at the expense of the α–Fe₂O₃ represented by the magnetic sextet. The doublet corresponding to the Ti-ferrite phase dominates the Mössbauer spectra at long milling time (greater than 100 h of milling).     

Mössbauer effect and X-ray diffraction investigation of Si–Fe thin films

An X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy investigation of Si_{100– x} Fe _x (0?<?x?<?80) thin films prepared by combinatorial sputtering methods is reported. Resulting Mössbauer spectra were fit to Voigt-based distributions of quadrupole doublets for paramagnetic spectral components and Zeeman split sextets for ferromagnetic spectral components. In conjunction with the X-ray measurements, these results show that the Si-rich films are a mixture of dilute Fe in amorphous Si and an approximately equiatomic amorphous SiFe phase. Fe-rich films show the presence of a ferromagnetically ordered phase. For x?<?73, this ferromagnetic phase is amorphous or nanostructured and for x?≥?73, the phase is shown to be a crystalline bcc phase. Results are discussed in terms of short-range structural ordering in these alloys.     

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.     

Mössbauer spectroscopy of lanthanum and holmium ferrites

PrRh₂Si₂ is highly anisotropic Ising-type antiferromagnetic system. The ordering temperature (T _N~ 68 K) of PrRh₂Si₂ is exceptionally high on the de-Gennes scale in the family of RRh₂Si₂ (R = rare earths). The reason for this anomalous behaviour is not clear. It is believed that the presence of uniaxial anisotropy assists in enhancing the T _N. The Mössbauer study was performed on a 10% Fe-doped PrRh₂Si₂ sample to understand the magnetic coupling between different sites of PrRh₂Si₂. The Mössbauer spectra together with the magnetic susceptibility data suggest that the magnetic coupling in PrRh₂Si₂ is provided mainly by the RKKY interaction between Pr-moments.     

Mössbauer and X-ray diffraction study of structural phase transitions in LaMnO<Subscript>3 + δ</Subscript>

The influence of the conditions of thermal treatment on the formation and mutual transitions of structural modifications of the LaMnO_{3 + δ} compound doped with ⁵⁷Fe (4%) have been investigated by Mössbauer spectroscopy and X-ray diffraction analysis. It is found that heat treatment in vacuum and in air ensures reversible structural transformations of two orthorhombic and one rhombohedral phases (space groups Pnma and \(R\bar 3c\), respectively). Changes in the configuration of the oxygen environment of cations at reversible phase transitions are considered.     

Mechanosynthesis of nanocrystalline MgFe2O4—neutron diffraction and Mössbauer spectroscopy

The evolution of nanocrystalline n-MgFe₂O₄ by high-energy milling a mixture of MgO and α-Fe₂O₃ for periods of between 0 h and 12 h has been investigated by neutron diffraction in addition to previous Mössbauer, XRD and HRTEM measurements. Complete transformation of the milled products to n-MgFe₂O₄ only occurs on milling to ～8 h even though the average particle size decreases to <?～10 nm after milling for 2 h. The applied field Mössbauer spectra of n-MgFe2O4 can be well described by two subspectra representing core and shell regions with different cation distributions and spin canting angles. The neutron pattern of nanocrystalline MgFe₂O₄ is described well by two components comprising nanoparticles of core and shell dimensions ～7(1) nm and ～0.7(1) nm, respectively, in support of the Mössbauer core-shell model.