Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Electromagnetism wikipedia , lookup
Magnetic field wikipedia , lookup
Lorentz force wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Magnetic monopole wikipedia , lookup
Condensed matter physics wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Electromagnet wikipedia , lookup
Mössbauer spectroscopy Enver Murad, D-95615 Marktredwitz, Germany Introduction In 1958, Rudolf Ludwig Mössbauer, a young German physicist, published two papers on “Nuclear resonance fluorescence [absorption] of gamma radiation in Ir191” 1. Both the radiation source (191Os, the decay of which to 191Ir is accompanied by the emission of gamma rays) and absorbing material (an Ir foil) were bound in solids. By slightly shifting the source energy Mössbauer was able to bring about a strong resonant absorption of the 129 keV gamma ray, which increased as the temperature was reduced. Both papers were written in German, and international acclaim was initially subdued. Mössbauer’s discovery was even met with skepticism by some colleagues, and it was not until scientists at the Los Alamos and Argonne National Laboratories – on the basis of a bet ! – repeated his experiment and later extended the range of known Mössbauer-active nuclides to more common elements such as Fe and Sn that the phenomenon became generally accepted. A further major breakthrough came in 1960 when Kistner and Sunyar 2 showed that the Mössbauer Effect allows measurement not only of the magnetic hyperfine field and quadrupole splitting in hematite, but also of an isomer shift relative to the source. In 1961, Mössbauer was awarded the Nobel Prize in Physics for his discovery. To date, the Mössbauer effect has been observed for 50 elements and 80 nuclides. About two thirds of the publications making use of Mössbauer spectroscopy have been concerned with 57Fe. We will be concerned exclusively with this nuclide here. Physical Background As indicated above, Mössbauer spectroscopy stands for the recoil-free emission and resonant absorption of gamma rays in solid materials. Prior to Mössbauer’s discovery, the observation of gamma-ray resonance had been hampered by the Brownian motion that causes the energies of both the source and absorber to have rather broad distributions of energy in fluids. The maxima of these distributions furthermore differ by the energies of recoil during emission and absorption (see graph at right). Their overlap is therefore only minor, but increases with temperature because the distributions broaden. However, when the emitting and absorbing atoms are fixed in a crystalline structure, there is no thermal broadening and a certain fraction of emission and absorption events occurs without recoil at well-defined energies 3. The degree of absorption furthermore increases as the temperature is reduced 4. Mössbauer parameters Resonant absorption occurs when the atoms in the emitting and absorbing matrices are in identical environments. In the more common case that the emitting and absorbing materials 2 differ, resonant absorption will occur at slightly ´ different energies. The difference, the isomer shift, S δ (also called “center shift”); see the (simplified) I = _3 2 diagram for 57Fe at right, is a function of the sA electron densities at the emitting and absorbing 57Co nuclei 5. The maximum of resonant absorption can 1 2 123456 be determined by moving the source over a range of velocities, making use of the Doppler Effect to 57Fe vary the gamma-ray energy. Energies in _ 1 Mössbauer spectroscopy are therefore given in I = 2 mm/s, and the velocity vs. intensity scan produces a Mössbauer spectrum. The isomer shift is sensitive to the oxidation state and can therefore be used to quantify Fe2+/Fe3+ ratios. Next, the interaction between the electric quadrupole moment of an ellipsoidal nuclear charge distribution and the electric field gradient (EFG) in a non-spherically symmetric electric field splits the degeneracy of the excited state into two levels that are separated by the quadrupole splitting (Δ). The Mössbauer spectrum will then consist of a doublet. The quadrupole splitting can be broken down into two contributions: a valence contribution from the atom itself and a lattice contribution from neighboring atoms. High-spin Fe3+, with a cubically symmetric 3d5 valence electron configuration, lacks an EFG contribution from the d electron shell, so that high-spin Fe3+ ions have a relatively low quadrupole splitting that is 4 proportional to the site distortion 6. High spin Fe2+ th Fe ions have a large EFG contribution from the 6 Fe d electron, giving typical splittings of 1.5-3 mm/s 3 Fe Fe (with a maximum of > 4 mm/s), whereas the splittings are reduced in the low spin states (see Fe 2 Fe Fe(III) figure at right),. Isomer shifts and quadrupole splittings Fe 1 of Fe-bearing phases thus vary systematically as a function of Fe oxidation and spin states and Fe(II) Fe 0 coordination (see figure at right), so that the -0.5 0.5 1.0 1.5 0.0 Mössbauer parameters can be used to Isomer shift (mm/s) “fingerprint” an unknown phase. Finally, under a magnetic field (whether applied or internal), the nuclear levels split into (2I+1) components. The allowed transitions (ΔI = 0, ±1) between the ground and excited levels then generally lead to a sextet for 57Fe. This is one of the most convenient methods for measuring internal, i.e. magnetic hyperfine fields (Bhf). The magnetically-split excited levels can be furthermore shifted by the quadrupole interaction. For Fe3+, the resultant shifts can be considered merely a perturbation of the magnetic splitting (broken lines in the diagram at the top of this page; quadrupole shifts are indicated by the symbol Δ’) 7. 60 At the magnetic ordering temperature (the Curie temperature for ferromagnetic and ferrimagnetic materials and Néel temperature for antiferromagnetic materials), Mössbauer spectra 40 thus change from a singlet or, more commonly, a doublet above to a sextet below the ordering Gt temperature. The variations of magnetic hyperfine 20 fields as a function of temperature can be readily monitored by Mössbauer spectroscopy. The figure Hm at right shows the variations of magnetic 0 0 200 400 600 800 1000 hyperfine fields of hematite and goethite as a T (K) _ 5 2 _ 3 2 _ 1 2 [6] 2+ [8] [5] 2+ 3+ [4] [4] 2+ 3+ [6] [6] 3+ [5] [6] Bhf (T) [sq] -10 -5 0 5 10 -10 -5 0 5 10 2+ 2+ 3 function of temperature. The broken and dotted vertical lines indicate room temperature (295 K) and the boiling point of liquid nitrogen (77 K) and the insets display Mössbauer spectra of hematite and goethite at 295 K. The theoretical line shape for resonance is a Lorentzian function, and this should therefore be the shape for Mössbauer spectra. Deviations from Lorentzian line shape provide an indication of imperfections in the material under study or point to shortcomings in the fitting procedure (for example when the existence of multiple Fe sites in the absorbing material or a complex composition of this are not taken into regard). Non-ideal behavior Many natural materials have non-ideal properties (”real samples”), such as those that might be due to structural defects, small particle size, or foreign element substitution. Such deviations from perfection are often reflected in Mössbauer spectra. Numerous physical phenomena can further complicate Mössbauer spectra, leading to equivocal or controversial interpretations. Magnetically-ordered materials possess nominally static hyperfine fields below their ordering temperatures. In small particles, however, the magnetization can change its direction spontaneously. This collective phenomenon, superparamagnetism, involves all spins in a magnetic domain, and the nucleus senses a reduced or eventually zero magnetic field if it occurs rapidly enough 8. Superparamagnetism causes magnetic order to disappear at room temperature in, for example, goethites smaller than 15 nm and hematites smaller than 8 nm. Mössbauer spectra of these minerals then bear some resemblance to those in the paramagnetic state. The effects of superparamagnetic relaxation 9 can be offset by reducing the sample temperature, and because many minerals formed in the weathering environment (soils and clays) have particle sizes in this range, their Mössbauer spectra must often be taken at low temperatures, e.g. those of boiling liquid nitrogen (77 K) or liquid helium (4.2 K), in order to observe magnetic order. Another widespread “imperfection” that can strongly affect magnetic hyperfine fields is the substitution of foreign elements in a crystal Ionic radii structure. One of the most common and best 3+ 3+ documented substitutions is that of Al for Fe . Their ionic radii are relatively similar (see graph at right) and Al is more common in the earth’s crust than Fe. Al can therefore replace up to ⅓ of the Fe content of goethite and up to 1/6 of the Fe content of hematite. Because Al3+ is diamagnetic, i.e. magnetically “dead”, such substitutions constitute a strong perturbation of the magnetic properties of vi 2+ Fe : 0.78, viFe3+: 0.645 nm ivFe2+: 0.63, ivFe3+: 0.49 nm viAl3+ : 0.535 nm ivAl3+ : 0.39 nm Fe oxides, lowering the ordering temperatures and reducing the hyperfine fields at all temperatures. Initially it was believed that the hyperfine field reduction would allow a determination of the degree of Al substitution in Fe oxides. Magnetic hyperfine fields, however, react similarly to other perturbations: thus small particle size, even in the absence of superparamagnetism, has a similar effect. At Fe concentrations of a few percent or less (“magnetically dilute” systems), reduced energy exchange between neighboring magnetic atoms in paramagnetic materials can lead to a slow electronic relaxation rate. If the relaxation rate is slower than the lifetime of the excited nucleus, the nucleus senses a magnetic field (slow paramagnetic relaxation). At low temperatures, the resultant Mössbauer spectra often resemble spectra of magnetically-ordered systems, but differ from these in the quadrupole shifts and rather broad resonant lines. 4 Measurement of Mössbauer spectra The singular component of all Mössbauer Source Absorber HV spectrometers is the drive, which imparts a Drive Detector range of velocities to the source to modulate Preamp Vel the gamma-ray energy over the desired span. signal Display In the standard (transmission) arrangement Power Amp supply (see graph at right), the resultant range of Drive waveform Sync PC/MCA generator gamma-ray energies falls upon the material SCA to be studied (the “absorber”) and the transmitted radiation is recorded by a detector. Most other constituents of a Mössbauer spectrometer are standard electronic components such as waveform generators, single- and multichannel analyzers, amplifiers and power supplies. Excited Mössbauer sources decay to the ground state by emitting not only gamma rays, but also conversion electrons, associated Auger electrons and X-rays. Electrons have penetration depths to ~ 100 nm, X-rays have penetrations of 1-10 µm and gamma rays have penetrations to 100 µm or more (depending on their energy and the sample). Suitable detectors can therefore simultaneously characterize samples at up to three depths. As mentioned above, it is often necessary to take Mössbauer spectra below room temperature (and less frequently at higher temperatures). Modern cryostats that enable the former measurements to be made are available in several geometries and many can cool samples down to almost any desired temperature. To enable comparison with published data, and also for convenience, spectra are often taken at 77 or 4.2 K (the boiling points of liquid nitrogen and helium, respectively, at a pressure of 1 atmosphere). Occasionally the application of external magnetic fields is necessary to better characterize samples; an example for such an application is given below. The evaluation (fitting, plotting) of Mössbauer data is a complex, occasionally controversial procedure. Numerous computer programs that can do this are currently in use. It must, however, be borne in mind that fitting Mössbauer spectra is a purely mathematical procedure, and models that are used must be supported by the physical realities 10. It is furthermore obvious that straightforward interpretations of 57Fe Mössbauer spectra of any substance are only possible if this contains no other Fe-bearing components that mask the resonances of the material that is to be studied, or if such components can be removed without affecting the material in question. Selected mineral data Fe oxides and oxyhydroxides (Fe oxides sensu lato) 11 Oxygen and Fe are the most abundant and fourth most abundant elements by mass in the earth’s crust, respectively. It is therefore not surprising that compounds consisting of these two elements are common in nature. The oxides, oxyhydroxides and a (single) hydroxide of Fe3+ are often grouped together under the general term “iron oxides”, and the natural occurrence of nine such Fe oxides sensu lato has been reported 11. Most Fe3+ oxides are structurally related, having Fe atoms located in the octahedral interstices of oxygen atom arrangements that are either hexagonal close-packed (α forms, sequence ABAB) or cubic close-packed (γ forms, sequence ABCABC). In magnetite and maghemite, which belong structurally to the spinel group, part of the Fe is also in tetrahedral coordination. A major distinction between Fe oxides and the majority of other Fe-bearing minerals is that the Fe oxides order magnetically at relatively high temperatures. Thus magnetic order can be observed for some Fe oxides at room temperature and for the majority of Fe oxides at 5 Bhf (T) 77 K. Because the Mössbauer parameters of the various Fe oxides in their magnetically ordered states differ significantly 12, these may serve to discriminate between the individual oxides. Deviations from crystalline perfection or ideal chemical composition can usually also be recognized by reduced magnetic ordering temperatures and hyperfine fields, and can lead to distributions of Mössbauer parameters rather than precise, well-defined parameters. Hematite, α-Fe2O3, is a common mineral in nature and perhaps the Fe oxide that has been most frequently studied by Mössbauer spectroscopy. Hematite has the highest magnetic ordering temperature and the highest magnetic Bhf (0) = 54.2 T 60 TM 264 K hyperfine field of all Fe oxides. Between the Curie temperature and the Morin transition the spins lie 40 close to the basal plane but are tilted away from this by a small angle. This results in a weak TC = 955 K magnetic moment and thus weak ferromagnetic afm pm wfm Bhf c Bhf c 20 character. At the Morin temperature the spins change their direction (spontaneously at 264 K for pure, well-crystallized hematite) so that they are 0 nearly parallel to the c axis, leading to 0 200 400 600 800 1000 T (K) antiferromagnetic character. Because the principal axis of the electric field gradient is and remains oriented parallel to the c axis, the quadrupole splitting changes from +0.40 mm/s above the Morin transition to −0.20 mm/s below 7. For poorly crystalline and most foreign-element substituted hematites, the Morin transition is spread out over a range of lower temperatures and becomes completely suppressed at particle sizes below 20 nm or Al substitutions ≥ 10 mol %. Such hematites therefore remain in the weakly ferromagnetic state at all temperatures. Goethite, α-FeOOH, is another very common Fe oxide. Well-crystallized, pure goethite has a Néel temperature of 400 K. Most goethites, however, have lower Néel temperatures and display asymmetrically broadened resonant lines when magnetically ordered. The magnetic hyperfine field of goethite is lower than that of hematite at all temperatures, which allows these two minerals to be readily distinguished in samples, such as soils, that contain both (see graph in section on Mössbauer parameters). Akaganéite, nominally β-FeOOH (actually β-Fe(OH)1-x(Cl,F)x) occurs very rarely in nature. Despite this fact, Mössbauer spectra of akaganéite have attracted a lot of attention because of their complexity in both the paramagnetic and magnetically-ordered states. The Mössbauer spectrum of paramagnetic akaganéite consists of a broad doublet. Careful analysis, however, shows this to consist of several doublets rather than just one doublet broadened due to high site distortion. Mössbauer spectra of magnetically-ordered akaganéite, which have a distinctly asymmetric appearance, similarly need to be fitted with at least three sextets with differing magnetic hyperfine fields and quadrupole splittings. Lepidocrocite, γ-FeOOH, has the lowest Néel temperature of all Fe oxides (77 K). However, even well crystallized, pure lepidocrocites display coexisting doublets and sextets over a relatively wide range of temperatures. This persistence of a doublet has been attributed to various causes, among others particle size effects and excess structural water. The ferrimagnetic Fe oxides, magnetite, (Fe3O4), and maghemite, (γ-Fe2O3), have relatively high Curie temperatures and are therefore often magnetically ordered at room temperature. The Mössbauer spectrum of stoichiometric magnetite (Fe3+[Fe2+Fe3+]O4) consists of two sextets resulting from Fe on the A and B sites; the isomer shifts indicate these to result from Fe3+ and nominally Fe“2.5+” (because of electron dislocation), respectively. Partial oxidation of magnetite leads to a reduction of the “2.5+” site resonance paralleled by a gain in the intensity of a new sextet attributed to octahedrally-coordinated Fe3+. The tetrahedral and octahedral Fe3+ components are difficult to distinguish in the absence of an externally applied magnetic field because their isomer shifts are quite similar. -10 -5 0 5 10 -10 -5 0 5 10 -10 -5 0 5 10 6 Transmission (%) Transmission (%) The A and B site magnetic hyperfine fields in 100 magnetite and maghemite will add to and subtract from an external magnetic field that is applied parallel to the 97 gamma-ray direction, so that the two sites can then be readily distinguished and quantified. The graph at right 94 shows Mössbauer spectra of a non-stoichiometric magnetite taken at 160 K without (top) and with a 91 100 magnetic field of 6 T applied parallel to the gamma-ray direction (bottom). The spectra not only reveal the 98 separation of the A and B site resonances, but also a 96 substantial (albeit incomplete) suppression of the 2nd and 5th (ΔmI = 0) peaks. 94 Below the Verwey transition at ~ 120 K, electron 92 dislocation in magnetite is inhibited. Magnetite then -14 -7 0 7 14 Velocity (mm/s) displays a very complex Mössbauer spectrum consisting of several poorly-distinguishable sextets. Ferrihydrite, ~ Fe5HO8·4H2O, is a classic example of a superparamagnetic mineral. Ferrihydrite is the Fe oxide that has been (and occasionally still is) termed “amorphous iron (hydr)oxide” or, more simply but just as erroneously, “Fe(OH)3”. Ferrihydrite occurs in spherical particles about 7 to 2 nm in size that order magnetically between about 120 and 15 K. These magnetic ordering temperatures are determined by particle size and should not be confused with Néel or Curie temperatures; they are termed magnetic blocking temperatures and defined as the temperatures at which the (super-)paramagnetic and magnetically ordered components of a Mössbauer spectrum have equal areas. Because of its poor crystallinity, ferrihydrite has Mössbauer parameters that tend to be “smeared out”. Mössbauer spectra of ferrihydrite – both when superparamagnetic and magnetically ordered – must therefore be 100 fitted with distributions of parameters rather than specific, well-defined parameters. 96 Ilmenite, Fe2+TiO3, has a structure that is related 92 to that of hematite. These two minerals form a continuous solid solution series at high temperatures (and 88 often occur exsolved at lower temperatures). Ilmenite has 100 a low quadruple splitting of ~ 0.6 mm/s at room temperature which increases markedly with decreasing 96 temperature. At 4.2 K ilmenite is magnetically ordered with a relatively low hyperfine field of about 5 T. 92 Because of the high quadrupole splitting, the two ΔmI ± ½ → ± ³/2 transitions are shifted to the highest velocities, 88 giving the spectrum a skewed appearance (see graph at -3 -2 -1 0 1 2 3 right). Velocity (mm/s) Fe-bearing clay-sized phyllosilicates (clay minerals sensu stricto) By analogy to the Fe oxides, the phyllosilicates are also prone to isomorphous substitution, Fe replacing elements such as Si, Al and Mg. Phyllosilicates, like almost all Fe-bearing silicates, order magnetically at lower temperatures than Fe oxides. Mössbauer spectroscopy can therefore help to distinguish between Fe bound in oxide and clay mineral structures in samples of complex mineralogy. 57Fe Mössbauer spectroscopy can furthermore serve to determine the oxidation state of Fe in phyllosilicates and in favorable cases also the Fe coordination, making this technique suitable for the characterization of their Fe contents. Because the quadrupole splitting of Fe3+ is determined by external charges, its magnitude will furthermore give a direct 7 , (mm/s) indication of the site distortion. In the following discussion, exemplary data on clay minerals from three groups will be presented: 1:1 minerals (kaolinite), non-expandable 2:1 minerals (illite) and expandable 2:1 minerals (the smectite minerals montmorillonite and nontronite). Fe can substitute for Al on the octahedral sites of kaolinite, Al4Si4O10(OH)8. However, because the Fe3+ ion is larger than Al3+ (see graph above), this substitution is more limited than Al substitution in the Fe oxides, although Fe substitutions in kaolinite of up to several mole-percent have been observed (especially in kaolinites formed in soils). The determination of the Mössbauer parameters of Fe in kaolinites is often complicated by their low Fe contents (leading to the development of slow paramagnetic relaxation) and by the presence of associated minerals. Spectral contributions from associated Fe oxides (that can dominate the spectra even when present only in minor amounts) can be prevented by removing these using selective dissolution procedures; alternatively, they can be taken into account separately by taking spectra at low temperatures (an example is given below). Mössbauer spectra taken at 4.2 K have allowed the identification of goethite in kaolins of complex mineralogy at concentrations as low as 0.1 %. Fe3+ dominates the Mössbauer spectra of most kaolinites. “Best values” for Fe2+-free kaolinites are a temperature-independent quadrupole splitting of ~ 0.5 mm/s and an isomer shift at room temperature of 0.35 mm/s relative to metallic Fe, thus resembling many (super)paramagnetic Fe oxides. However, many kaolinites contain noticeable proportions of Fe2+, which has a quadrupole splitting of ~ 2.5 mm/s and an isomer shift of ~ 1.1 mm/s at room temperature. 2.0 The Mössbauer spectra of kaolinite change Neoformed radically upon firing, reflecting properties of both silicates the original sample and the reactions that take 1.5 place during firing. The most striking change is a Kaolinite Metakaolin substantial increase in quadrupole splitting upon 1.0 dehydroxylation of kaolinite, leading to the formation of (highly disordered) metakaolin, and 0.5 the subsequent formation of high-temperature phases (see graph at right). These variations allow 0.0 an assessment of the temperature and redox 0 200 400 600 800 1000 1200 Firing temperature (°C) conditions during firing and can, for example, be used for reconstruction of the conditions during the productions of archaeological ceramics. Illite, a clay-sized 2:1 mica with more Si and less K than muscovite, is a common constituent of soils, clays and shales. Illites may contain Fe in concentrations ranging from < 1 % to over 8 %, and a typical composition is K0.75(Al1.75R0.25)(Si3.5Al0.5)O10(OH,F)2, where R is a cation such as Mg2+ or Fe2+. As in kaolinites, slow paramagnetic relaxation occurs in Fe-poor illites and must be accounted for when fitting their Mössbauer spectra. Octahedrally-coordinated Fe3+ in illite has a quadrupole splitting of ≥ 0.6 mm/s (i.e. higher than that of Fe3+ in kaolinite), and both the Fe3+ and Fe2+ quadrupole splittings vary inversely with the Fe contents. Distinguishing the cis and trans-OH coordinated Fe sites by Mössbauer spectroscopy is not generally possible, but the presence of tetrahedral Fe3+ can be observed in some Fe-rich samples. While the production of high-quality porcelain requires clays to be as free from minerals other than kaolinite as possible, most clays used for the production of structural clay products (bricks, tiles, etc.) contain significant proportions of other phyllosilicates, for example illite. Two properties of illite have a strong effect on the firing behavior of such complex clays: the alkali (K) content, which acts as a flux, and the generally significant Fe content, which lead to the formation of (colored) Fe-bearing silicates and hematite in the final product. The firing reactions of illite can be readily monitored by Mössbauer spectroscopy. Thus the graph below shows features such as the disappearance of Fe2+ and the formation of magnetically ordered 8 Transmission (%) Transmission (%) hematite in an illite containing ~ 5 % Fe as a function of firing temperature. The displayed spectra were taken at room temperature; at 4.2 K part of the intense doublet in the high-firing temperature range is replaced by a sextet, indicating that the doublet contains contributions from superparamagnetic hematite as well as genuinely paramagnetic (silicate-bound) Fe3+. Smectites are clay-sized 2:1 phyllosilicates with negative layer charges between 0.6 and 0.2 per formula unit (O10(OH)2) that can accommodate a variety of materials in their interlayers. Montmorillonite, ideally M(Al2-xMgx)Si4O10(OH)2, is a dioctahedral smectite with a layer charge that arises mainly in the octahedral sheet. Montmorillonites with Fe contents from < 0.05 to > 20 wt.-%, corresponding to 0.01 – 1.70 Fe3+ per formula unit have been described. Montmorillonites with more than 0.6 Fe3+ per formula unit 100 are termed ferrian or Fe-rich. 298 K A feature that is often neglected in Mössbauer 99.8 studies of Fe-poor montmorillonites is – once again – 99.6 the influence of slow paramagnetic relaxation. Paramagnetic relaxation shows up clearly in spectra of 100 Fe-poor montmorillonites at 4.2 K (see figure at right), but is not overtly apparent at room temperature. It is, 99.6 however, conceivable that the outer doublet (i.e. the difference between the dotted doublet fitted to the 99.2 room-temperature spectrum and the measured data in the upper spectrum at right) of two-doublet fits that 98.8 SAz-1 4.2 K have repeatedly been used to fit the Fe3+ resonance of -10 -5 0 5 10 montmorillonites may, in the case of Fe-poor samples, Velocity (mm/s) result from this phenomenon. Nontronite is the Fe3+-rich end-member dioctahedral smectite with the theoretical formula MFe2(Si4-xAlx)O10(OH)2, and thus a predominantly tetrahedral negative charge, although nontronites from different localities may vary widely in chemical composition. Mössbauer spectra taken at low temperatures 100 (≤ 120 K) have revealed a common association of 296 K 90 nontronites with Fe oxides, in particular goethite, that are superparamagnetic at room temperature. The 80 resonant peaks of these minerals will in general be unresolvably superimposed upon each other, making 70 meaningful interpretations of the spectra difficult if not 100 impossible. Thus the room-temperature spectrum of the 95 Garfield nontronite H33a at right shows no evidence of goethite, whereas the sextet that can be observed at 90 77 K indicates ~ 5 % of the Fe content of the sample to be bound in goethite. 85 H33a 77.3 K Magnetic order has been observed in nontronites -10 -5 0 5 10 at temperatures between 4.2 and 1.3 K. Mössbauer Velocity (mm/s) spectra taken under external magnetic fields have indicated Garfield nontronite to have a Néel temperature of about 20 K. Frustration due to tetrahedral Fe3+, however, reduces the magnetic ordering temperature to below 7 K. 9 Intercalation (”pillaring”) of both montmorillonites and nontronites with Fe-rich materials significantly raises the magnetic ordering temperatures. Complex materials Transmission (%) Transmission (%) Transmission (%) Mössbauer studies of materials formed at the earth’s surface (sediments, soils, clays, etc.) is 100 often demanding: besides the complex mineralogy of 295 K the materials themselves, the individual components 98 often display one or more of the features of non-ideal 96 behavior outlined above, making an interpretation of 94 their spectra correspondingly challenging. 92 120 K Soils formed under moderate climatic conditions 100 thus do not only exhibit the expected intricacy of spectra 98 due to complex mineralogy, but may in addition display 96 78 K 100 superparamagnetism down to temperatures well below 98 77 K, indicating the Fe oxides to have particle sizes in 96 4.2 K the nanometer range. As an example, the figure at right 100 shows Mössbauer spectra of a rubified soil from the 98 northern Alpine foothills. Note the increase in sextet 96 area and concurrent attenuation of the doublet between -10 -5 0 5 10 Velocity (mm/s) 78 and 4.2 K. The values of commercial clays are strongly dependent not only on the essential mineral(s), but also on the possible presence of 100.0 ancillary components. Thus kaolins that are used for 99.8 paper coating must be free from pigmenting minerals such as Fe oxides, and possibilities to account for such 99.6 components have been mentioned above. As an 99.4 example, the figure at right shows Mössbauer spectra 100.0 of a commercial kaolin taken at 4.2 K before (top) and 99.8 after (bottom) treatment with Na dithionite. The sextet that disappears as a result of this treatment identifies 99.6 the removed constituent as goethite; the amount of Fe removed by this treatment (0.07 wt.-%) corresponds to -10 -5 0 5 10 a goethite content of 0.11 % (the remaining sextet is Velocity (mm/s) due to paramagnetic relaxation). 100 The distinction of the components contributing 90 to the Mössbauer spectra in the above example was facilitated by the markedly different magnetic fields of 80 oxide and silicate-bound Fe. Such distinctions become 70 Untreated more complicated if the constituents of a mixture have 100 similar magnetic hyperfine fields, and may make 90 physical or chemical treatments necessary. As an 80 example, the graph at right shows Mössbauer spectra of 70 an Fe-rich “pipe stem” (a root void that has been filled 3 x 2h oxalate 60 with Fe oxides) at 4.2 K. Spectra were taken prior to 100 and following different treatments with acid Na oxalate, 98 which selectively removes poorly-crystalline Fe oxides. 96 All samples show complete magnetic order, but even a 24 h oxalate 94 cursory inspection shows distinct differences: 92 increasingly intense treatments cause the hyperfine field -10 -5 0 5 10 Velocity (mm/s) distributions to narrow and the average hyperfine field 10 to increase. A concurrent change of the average quadruple splitting from -0.10 to -0.23 mm/s identifies the removed constituent – not surprisingly – as ferrihydrite (the broad lines of which initially mask the goethite sextet) and the residual component as goethite. Summary: strengths and weaknesses of Mössbauer spectroscopy Strengths Weaknesses Sensitive only to Fe (“sees” only 57Fe) Sensitive only to Fe (no matrix effects) Sensitive to oxidation state Determination of coordination may be controversial Allows distinction of magnetic phases Paramagnetic phase data may be ambiguous Very sensitive to magnetic phases Magnetic parameters sensitive to diamagnetic element substitution and relaxation Non-destructive Relatively slow Resolution limited by uncertainty principle If possible, also use other techniques ▼ Often a combination of Mössbauer spectroscopy with other techniques can help solve problems that cannot be resolved using Mössbauer spectroscopy alone. Useful web sites Mössbauer Effect Data Center, Asheville, N.C., U.S.A. (http://orgs.unca.edu/medc). Comprehensive services to the Mössbauer community including an archive of Mössbauer publications (> 46.000 to date !), data and personnel searches, publication of the “Mössbauer Effect Reference and Data Journal” (10 issues annually) and handbooks on specific materials (e.g. minerals) and nuclides. Mössbauer Effect “Community” Site (http://www.mossbauer.org). Topical collection of data on Mössbauer spectroscopy with a variety of links to relevant web sites. Mars Mineral Spectroscopy Database (http://www.mtholyoke.edu/courses/mdyar/database) Freely accessible collection of Mössbauer spectra of a variety of minerals taken over ranges of different temperatures. IBAME Web Site: (http://pecbip2.univ-lemans.fr/%7emoss/webibame). Introduction to Mössbauer spectroscopy. 11 Appendix: references and notes 1 Mössbauer RL (1958) “Kernresonanzabsorption von Gammastrahlung in Ir191”, Die Naturwissenschaften 45, 538-539; “Kernresonanzfluoreszenz von Gammastrahlung in Ir191”, Zeitschrift für Physik 151, 124-143. 2 Kistner OC & Sunyar AW (1960) “Evidence for quadrupole interaction of Fe57m, and influence of chemical binding on nuclear gamma-ray energy”, Physical Review Letters 4, 413-415. Mössbauer line widths are limited by the natural line width (Γ) that ensues from the Heisenberg uncertainty principle. Γ is given by Γ· ≥ ħ, where is the mean lifetime of excited gamma state and ħ is Planck’s constant divided by 2π. For 57Fe, the mean life of the 14.4 keV excited nuclear level is 141 ns and Γ is each 4.67·10–9 eV for emission and absorption. This corresponds to 0.097 mm/s, leading to a theoretical minimum line width of 0.194 mm/s. Line widths attained in practice (W) are somewhat larger, and values of 0.22 – 0.23 mm/s are considered good. The ratio of the 14.4 keV 57Fe line width to the transition energy amounts to ~ 3·10−13, i.e. this line is extremely well defined (narrow). 4 Conservation of momentum requires some emission and absorption events to occur with recoil. At a first approximation, the “recoil-free” fraction is given by f = exp [–k2 <x2>], where k is the wave vector of the gamma ray and <x2> the mean square amplitude of vibrations along the gamma-ray direction. <x2> decreases and f therefore increases as the temperature is reduced. Typical values of f for selected Fe oxides and silicates range from ~ 0.65 − 0.85 at room temperature and ≥ 0.85 − 0.95 at 80 K 13. 4π 2 2 2 5 Ze [ Ψa(0) Ψs(0) ] RδR The mathematical expression for the isomer shift, δ, is given by δ 52 2 where Z is the atomic number, e is the change of the electron, |a(0)| and |s(0)| are the electron densities at R = 0 inside the nuclei at the absorber and source, respectively, R is the nuclear radius, and R is the change in nuclear radius during the transition from the excited to the ground state. 3 6 Quadrupole splitting. The interaction between the electric quadrupole moment and the electric field gradient splits the excited state into two levels separated by = ½ eQ Vzz (1 + 2/3), where e is the charge of the electron, Q the nuclear quadrupole moment, Vzz the electric field gradient along the principal axis and η an asymmetry parameter defined as (Vxx −Vyy)/Vzz. 7 The combined effects of a high magnetic hyperfine field and a much smaller quadrupole interaction for an axially symmetric electric field gradient is given by Δmag = Δpm (3cos2θ − 1), where Δmag and Δpm are the quadrupole splitting in the magnetically ordered and paramagnetic states, respectively, and θ is the angle between the principal axis of the electric field gradient and the magnetic field. 8 The energy required to reverse the magnetization of a particle depends on its volume V and magnetic anisotropy constant K. As particles become smaller, the energy barrier KV becomes comparable to the thermal energy kBT. The frequency for spontaneous magnetization reversal is then given by f = f0 exp (−KV/(kBT)), where f0 is a poorly-defined frequency in the range 109 − 1011 Hz. 9 Relaxation (French “trainage”, lagging behind) is a generic term for the time-dependent response of a system to external stimuli. In the case of superparamagnetism, the minimum time required for the nucleus to detect a hyperfine field is the nuclear (“Larmor”) precession time; for 57Fe this is 30 ns (34 MHz) in a 50 T hyperfine field. Fluctuations of the electron spin must remain constant for at least one Larmor precession period for the hyperfine field to be observed. 10 Caveats: Mössbauer spectroscopy is a deceptively simple technique which, however, is riddled with pitfalls. Fits to Mössbauer data are not always equivocal (“non-unique solutions”), and the application of other techniques may be more often necessary than is generally realized to arrive at sound interpretations. Some examples of possible pitfalls and nonsense are given below. An apparently trivial error that can nevertheless be observed from time to time is the overinterpretation of spectral data. As mentioned above, fitting is a purely mathematical procedure in which deviations between the experimental and fitted spectra are minimized by a least-squares procedure. Spectral fitting thus provides no information on whether a model is physically appropriate or not, and no more components should be fitted to a spectrum than is reasonable on 12 the basis of the spectral quality and physical relevance. The “goodness of fit” parameter, χ², is furthermore known to improve when fitting poorer quality data, where the discrepancy between the true and modeled function is less apparent (which should, of course, not be taken as a recommendation for collecting spectra of poor quality; in fact fewer spectra of better quality are often preferable to more spectra of mediocre quality). The magnetic field (~ 55 T) resulting from slow paramagnetic relaxation in Fe-poor materials at low temperatures has occasionally been mistaken as resulting from magnetically-ordered hematite (Bhf(0) ≈ 54 T). Particular caution is necessary when studying samples that have been doped with small amounts of 57Fe. Careful analysis, however, shows hematite to have a distinctive quadrupole shift (either +0.41 or −0.20 mm/s, depending on whether the hematite has or has not passed though a Morin transition) and significantly narrower resonant lines than resonances ensuing from paramagnetic relaxation. In early Mössbauer work, relatively high quadrupole splittings and low hyperfine fields of soils that resembled those of paramagnetic and magnetically-ordered akaganéite, respectively, lured some research groups into deeming akaganéite to be a common constituent of soils. Later work, however, indicated that the paramagnetic spectra more probably result from ferrihydrite and the reduced hyperfine fields observed in the magnetically ordered material to be rather due to Al substitution in goethite. Although the akaganéite “problem” thus has long been settled, it does show how tenuous assignments based solely on selected Mössbauer parameters can be. A more recent debate relates to Mössbauer data collected by the spectrometers on the Mars Exploration Rovers: while Fe2+ doublets with high quadrupole splitting were initially taken as resulting from olivine, subsequent work including detailed analyses of infrared spectra indicated that these doublets might originate from Fe2+ sulfates with similar Mössbauer parameters that have also been observed in terrestrial acid drainage environments 14. Finally, there have been some abortive attempts at dating of laterites and archaeological artifacts by Mössbauer spectroscopy. The laterite “dating” papers were based on the well-known fact that the quadrupole splittings of Fe oxides, notably hematite, increase as particle sizes decrease. Reduced Fe3+ quadrupole splittings have therefore been implied to indicate the hematites to have matured, and thus to have higher ages. Besides the fact that these studies were not accompanied by detailed investigations of the Fe mineralogy, there is no reason to assume that hematites would recrystallize in the course of time (unless they underwent a dissolution/ recrystallization stage, but that would make them younger anyway). A later careful study of laterites from a single profile in Australia indeed did not reveal any dependence of Fe mineralogy on age 15. “Dating” of archaeological artifacts, in contrast, has been based on apparently varying asymmetries of paramagnetic Fe3+ doublets. Inspection of the data provides another explanation: in samples that contain both Fe3+ and Fe2+, the low-velocity peaks of paramagnetic Fe3+ and Fe2+ are nearly coincident and therefore have a higher dip than the separate high-velocity Fe3+ and Fe2+ peaks. When spectra are taken in a relatively narrow velocity range (≤ 2 mm/s) the latter peak may be missed, and different Fe2+/Fe3+ ratios could then be mistaken for varying degrees of asymmetry of an Fe3+ doublet. 13 De Grave E & Van Alboom A (1991) Evaluation of ferrous and ferric Mössbauer fractions. Physics and Chemistry of Minerals 18, 337-342. 14 Bishop JL, Dyar MD, Lane MD & Banfield JF (2004) Spectral identification of hydrated sulfates on Mars and comparison with acidic environments on Earth. International Journal of Astrobiology 3, 275-285. 15 St. Pierre TG, Webb J & Butt CRM (1990) Laterite mineralization near Kalgoorlie, Western Australia: dating by Mössbauer spectroscopy? Hyperfine Interactions 57, 2279-2294. Acknowledgments: These notes have benefited significantly from helpful reviews by Janice J. Bishop and M. Darby Dyar. 13 11 12 Fe oxides and oxyhydroxides that have been observed in nature Mineral Occurrence Hematite Composition Structure Space Group Unit-Cell Dimensions (Å) a b c very common α-Fe2O3 corundum R3c 5.034 a Magnetite common Fe3O4 inverse spinel Fd3m 8.396 Maghemite common γ-Fe2O3 disordered spinel Fd3m or P422 8.3474 8.347 diaspore Pnma 9.956 3.0215 4.608 β-FeOOH b hollandite I2/m 10.600 3.034 10.513 Lepidocrocite common γ-FeOOH boehmite Bbmm 3.071 Feroxyhyte very rare δ’-FeOOH disordered CdI2 P3m1 2.93 4.56 Ferrihydrite common Fe5HO8 4H2O disordered corundum 2.955 9.37 Bernalite extremely rare Fe(OH)3 disordered ReO3 Immm 7.544 Goethite very common α-FeOOH Akaganéite very rare 13.752 a 25.01 12.52 3.873 7.560 7.558 Mössbauer parameters of Fe oxides and oxyhydroxides that have been observed in nature Mineral TN, TC MAG a Bhf (K) δ/Fe Δ Δ Bhf Room Temperature 4.2 K Hematite 955 wfm 51.8 0.37 -0.20 53.5 or 54.2 b -0.20 0.41 Magnetite 850 fim 49.2 46.1 0.26 0.67 0.02 0.02 50.6 36 - 52 c 0.00 1.18-(-0.79) Maghemite 950 fim 50.0 50.0 0.23 0.35 0.02 0.02 52.0 53.0 0.02 0.02 Goethite 400 afm 38.0 0.37 -0.26 50.6 -0.25 Akaganéite 299 afm – 0.38 0.37 0.55 0.95 47.3 47.8 48.9 -0.81 -0.24 -0.02 77 afm – 0.37 0.53 45.8 0.02 Feroxyhyte 450 fim 41 0.37 -0.06 53 52 0.0 0.0 Ferrihydrite 115 d 25 d spm – – 0.35 0.35 0.62 e 0.78 e 50 e 47 e -0.07 -0.02 Bernalite 427 wfm 41.5 0.38 0.01 56.2 0.01 Lepidocrocite Bhf in Tesla; δ and Δ in mm/s. a Magnetic character: weakly ferromagnetic (wfm), ferrimagnetic (fim), antiferromagnetic (afm), speromagnetic (spm). b For afm hematites (that have passed through a Morin transition). c Several magnetic B-site subspectra below the Verwey transition at ~ 120 K. d Range of superparamagnetic blocking temperatures which vary as a function of crystallinity. e Maximum probabilities of quadrupole-splitting and hyperfine-field distributions.