Mossbauer Spectroscopy

2.1.2 Basic Principles

Mössbauer spectroscopy stands for the recoil-free emission and absorption of gamma rays in solids. Because atoms in a solid can vibrate, recoil-free events occur if the recoil energy is less than the lowest quantized vibrational mode.

A basic characteristic of Mössbauer spectroscopy is that it only ‘sees’ the nuclide under survey. Thus the sole effects arising from the presence of other elements in samples of complex chemical composition are dilution and absorption. The vast majority of Mössbauer spectra are taken in transmission mode. Here, a source (⁵⁷Co for iron spectra) emitting gamma rays of the appropriate energy is periodically moved over a succession of velocities, and the radiation transmitted by the sample (absorber) is registered as a function of the source velocity (i.e., energy).

Comparison of the line half-width of the ⁵⁷Fe Mössbauer lines and the gamma ray energy gives a resolution of better than 1 in 10¹³. This outstanding energy resolution allows the observation of nuclear hyperfine interactions, that is, the interactions between nuclei and their electric and magnetic environments.

The principal hyperfine interactions that can be observed by Mössbauer spectroscopy are (1) the electric monopole interaction, giving rise to the isomer shift (δ); (2) the electric dipole interaction, leading to the quadrupole splitting (Δ); and (3) the magnetic hyperfine interaction when a magnetic hyperfine field (B_hf) acts at the nuclei of the resonant atoms. The isomer shift is the shift of the centroid of the spectrum from zero velocity, and is given relative to either the source or some standard material—in the case of ⁵⁷Fe, usually metallic iron. The quadrupole splitting is the separation of the two lines of an ⁵⁷Fe doublet. Isomer shifts are related to the oxidation state of iron and may provide information on iron coordination, whereas the quadrupole splitting provides a measure for Fe^3 + site distortion. Both isomer shift and quadrupole splitting are customarily given in terms of the source velocity in millimetres per second. Intrinsic (hyperfine) or extrinsic (externally applied) magnetic fields split Fe^3 + Mössbauer spectra into sextets, the spread of which is proportional to the field, and are usually expressed in kilo oersteds or tesla (1 T = 10 kOe). Simulated Mössbauer spectra resulting from the mentioned hyperfine interactions are shown in Fig. 2.1.1.

Figure 2.1.1. Shapes of ⁵⁷Fe Mössbauer spectra. Depending on the local environments of the Fe atoms and the magnetic properties, Mössbauer spectra of iron-bearing minerals can consist of a singlet, a doublet, or a sextet. In randomly oriented powders, the doublet lines have an intensity ratio of 1:1 and the sextet lines of 3:2:1:1:2:3.

In complex spectra, the relative intensities of individual components are often taken as proportional to the corresponding site population. This relationship, however, holds only at a first approximation, for example, Fe^2 + exhibiting a somewhat lower recoil-free fraction than Fe^3 + (De Grave and Van Alboom, 1991). Additional information may be obtained from the widths and shapes of the lines. The ideal Mössbauer line shape is the Lorentzian, and experimental data are often computer-fitted with this. However, deviations from Lorentzian shape may occur because of variations of local environments or fluctuations of parameters, to name just two factors. In such cases, the data may have to be fitted using other functions, for example, the Voigtian (a convolution of Gaussian and Lorentzian functions) or distributions of Lorentzians.

Numerous textbooks on the Mössbauer effect and its applications have been published, for example, Gonser (1975) and Gibb (1976), to name two of the better known ‘classics’, and more recently a book by Murad and Cashion (2004) that focuses on the Mössbauer spectra of materials formed on the Earth's surface. For more information on the Mössbauer effect than can be included here, the reader is referred to these and similar sources.

Depending upon sample structure and composition, ⁵⁷Fe Mössbauer spectra may consist of one or more singlet(s), quadrupole-split doublet(s), and magnetically split sextet(s). Singlets develop only in the case of cubic symmetry around Fe^3 + and are therefore not observed for phyllosilicates. Sextets, which arise from iron in magnetically ordered materials, are observed only for extremely iron-rich phyllosilicates at low temperatures (≤ 10 K), and for iron oxides and oxyhydroxides (here collectively called ‘iron (hydr)oxides)’ of sufficiently good crystallinity and chemical purity (but may also require measurements to be taken at temperatures of 77 K or below). The Mössbauer spectra of phyllosilicates thus generally consist of one doublet for iron of each oxidation state on every structural site.

I.C.1 Isomer Shift (δ)

In order to obtain the total isomer shift (δ_T), the effects of the second-order Doppler shift (SOD) must be included: δ_T = δ + SOD. The SOD is due to the fact that the source and absorber nuclei are in motion. When a moving source emits resonant radiation, there is a frequency shift relative to a stationary observer as a result of the relativistic time dilation. The size of the SOD depends on the nature and temperature of the solids containing the source and absorber nuclei.

IV.C.2 Positron Spectroscopy

Like Mössbauer spectroscopy, positron spectroscopy is a nuclear process for probing the chemical and physical environments of solid materials. It is a more versatile technique than Mössbauer spectroscopy, but it does not provide as much information on the structure surrounding the nucleus or the immediate environment of the atom.

Applications of the technique to heterogeneous catalysts have been few, but they have demonstrated that the method is useful for catalyst characterization. For example, the lifetime of the orthopositronium species is inversely proportional to the number of Brönsted acid sites present in alumina–silica cracking catalysts. This interpretation was derived from a correlation between the activity for the alkylation of cumene and the lifetime of the orthopositronium species.

As the time resolution of the equipment improves and computer programs become available for data reduction, additional applications of the technique to heterogeneous catalysis can be expected.

6.2.5 Mössbauer Spectroscopy

Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. This effect, discovered by Rudolf Mössbauer in 1957 (Mössbauer, 1958; Craig et al., 1959), consists of recoil-free, resonant absorption and emission of gamma rays in solids. Like NMR spectroscopy, Mössbauer spectroscopy probes tiny changes in the energy levels of an atomic nucleus in response to its environment. Typically, three types of nuclear interactions may be observed: an isomeric shift, also known as a chemical shift; quadrupole splitting; and magnetic or hyperfine splitting, also known as the Zeeman effect. Due to the high-energy and extremely narrow line widths of gamma rays, Mössbauer spectroscopy is a very sensitive technique in terms of energy (and hence frequency) resolution, capable of detecting change in just a few parts per 10¹¹. During Mössbauer absorption spectroscopy, the source is accelerated through a range of velocities using a linear motor to produce a Doppler effect and scan the gamma ray energy through a given range. A typical range of velocities for ⁵⁷Fe, for example, may be ± 11 mm/s (1 mm/s = 48.075 neV). In the resulting spectra, the gamma ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, a fraction of the gamma rays are absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. The number, positions and intensities of the dips (also called peaks—dips in transmitted intensity are peaks in absorbance) provide information about the chemical environment of the absorbing nuclei and can be used to characterise the sample.

2.20.1.2 Advantages and Disadvantages of the Effect

Mössbauer spectroscopy is a nondestructive technique which probes a specific element which may occupy one or more crystallographic sites, may have one or more electronic configurations, and may or may not carry a magnetic moment. The absorbers may be in the form of single crystals, powders, foils, surfaces, or frozen solutions; solutions or liquids can not be studied. The influence of temperature, pressure, applied magnetic field, and electromagnetic irradiation is easily studied by Mössbauer spectroscopy.

Unfortunately, the requirements of recoil-free emission and resonant absorption and transmission through the absorber limit the useable energy range of the Mössbauer effect γ-ray to approximately 10–100 keV. Further, in order to obtain rather sharp absorption lines and a reasonable spectral resolution, the mean lifetime of the Mössbauer γ-ray precursor state should be between 1 ns and 100 ns. Further, the Mössbauer nuclide must have a sufficiently high isotopic abundance in the element to yield a usable signal-to-noise ratio over a reasonable acquisition time. Finally, the radioactive source containing the Mössbauer γ-ray precursor state must be easily prepared and have a mean lifetime of several weeks to be practical. These various requirements limit the number of nuclides available for typical Mössbauer spectral studies.

Iron-57 is the most important Mössbauer nuclide even though it has a natural abundance of only 2.2%. The half-life of its 14.41 keV excited state is 98.1 ns and the half-life of its precursor source, cobalt-57, is 270 days. As a result, the resolution of iron-57 Mössbauer spectroscopy, 6.5 × 10⁻¹³—the ratio of the linewidth to the γ-ray energy—is excellent. Because the typical iron-57 hyperfine parameters are of the order of a few millimeters per second and range up to many times the natural linewidth of 0.194 mm s⁻¹, they are easily measured with excellent resolution.

The hyperfine parameters result from shifts in, or the removal of, the degeneracy of the nuclear energy levels^14,15 through the electric and magnetic interactions between the nucleus and its surrounding electronic environment. The expressions for the hyperfine parameters, the isomer shift, the quadrupole interaction, and the magnetic hyperfine field always contain two contributions, a nuclear contribution that is fixed for a given nuclide, and an electronic contribution that varies from compound to compound.

2.13.4.2 Mössbauer Spectroscopy

Mössbauer spectroscopy probes the recoilless emission and resonant absorption of γ-rays by nuclei (Mössbauer, 1958). The energy levels of the nuclei are sensitive to the electrostatic and magnetic fields present at the nuclei and thus to changes in chemical bounding, valence (i.e., the oxidation state of an atom, ferrous versus ferric), and magnetic ordering. Mössbauer spectroscopy reveals information on the hyperfine interactions and the local electronic and magnetic fields at a nucleus. Only a few nuclei exhibit the Mössbauer effect (e.g., ⁵⁷Fe, ¹¹⁹Sn, ¹²¹Sb, ¹⁵³Eu, and ¹⁹⁷Au), and the great majority of applications in Earth science (and in general) are with ⁵⁷Fe. The natural abundance of ⁵⁷Fe is 2.1%, so some studies require enrichment of ⁵⁷Fe to obtain a stronger signal. The technique has been used extensively in the mineralogy of deep Earth materials, including in situ high-pressure measurements. A typical probe for mineral systems is based on the reaction ⁵⁷Co + ⁰β⁵⁷ → Fe + γ. In a conventional Mössbauer experiment, one modulates the energy of the γ-rays by continuously vibrating the parent source to introduce a Doppler shift of the radiation (see Dickson and Berry (1986) for an introduction to Mössbauer spectroscopy and McCammon (1995) for a review of applications to minerals).

The isomer shift is the shift in the nuclear energy level and corresponds to the source velocity at which maximum absorption appears. The electric quadrupole splitting arises from the interaction between the nuclear moment and quadrupole splitting and the local electric field gradient at the nucleus. This occurs for nuclei with spin quantum number I > 1 (i.e., for ⁵⁷Fe, I = 3/2). The hyperfine field is due primarily to the contact interaction between electrons at the nucleus and the nucleus, rather than to the macroscopic magnetic field in a material. A hyperfine field arises from different densities of spin up and down electrons at the nucleus.

Applications include determination of Curie and Neél transition temperatures, valence state, site occupancy, and local order. In studies of deep Earth materials at high pressure, each of these can be probed as a function of pressure (and temperature), including the measurement of changes across various transformations (crystallographic, electronic, and magnetic). Instrumentation consists of a radioactive source, vibrating drive, and detector (scintillator or proportional counter). Recent examples of high-pressure studies include measurements on a series of pyroxenes to 10 GPa (Zhang and Hafner, 1992) and measurements on CaFeSi₂O₆ to 68 GPa, which revealed evidence for a phase transition associated with a change in spin state. The Mössbauer technique has been extended into the megabar pressure range as discussed later in the text for FeO (Pasternak et al., 1997b; Figure 6). The Mössbauer studies on FeO along with acoustic sound velocity measurements show fascinating interplay between magnetic ordering and elasticity (Kantor et al., 2004b).

Figure 6. A sketch depicting the setup for high-pressure Mössbauer studies using perforated diamond anvil cells (see Dadashev et al. (2001) for details).

15.11.3.4.2 Electronic spin and valence states by SMS

MS has proven to be very effective in the past in studying iron-bearing minerals. There is a large body of evidence that points out the relationship between measured hyperfine interaction parameters and valence, spin, and magnetic states of iron and its compounds under ambient and extreme conditions. The use of the synchrotron radiation to excite Mössbauer transitions has further advanced the applicability of this technique to high-pressure studies due to an astonishing six orders of magnitude increase in the x-ray brightness when compared to lab-based radioactive sources.

Because of the partially filled 3d-electron orbitals, the electronic valence and spin states of iron in the host lower-mantle phases can be changed, thus providing a probe to better understand of a wide range of physical and chemical properties of the lower mantle (e.g., Lin and Tsuchiya, 2008). Previous studies have shown that the spin transitions of Fe and variations in the Fe valence states can cause changes in density, elastic properties, electrical conductivity, and radiative thermal conductivity of the lower-mantle minerals. In particular, electronic spin-pairing transitions of iron that were proposed decades ago have been recently observed in analogue lower-mantle minerals including ferropericlase, perovskite, and possibly in post-perovskite at high pressures (e.g., Badro et al., 2003, 2004; Li et al., 2004; Lin et al., 2005c, 2006, 2007a,b). For example, the measured values of QS (~ 0.8 mm s^− 1) and CS (~ 1.2 mm s^− 1) from the Mössbauer spectra of the lower-mantle ferropericlase under ambient conditions are consistent with predominant high-spin Fe^2 + in the octahedral coordination (Lin et al., 2006; Speziale et al., 2005). The simultaneous disappearance of the QS and the drop of the CS at approximately 60 GPa are consistent with a high-spin to low-spin electronic transition of iron in the ferropericlase (Lin et al., 2006; Speziale et al., 2005). The ratio of the high-spin to low-spin states of iron in ferropericlase as a function of pressure can be derived from the modeling of the SMS spectra with the changes in the QS and CS values.

Iron exists in the Fe^2 + and Fe^3 + states in both perovskite and post-perovskite. Both Fe^2 + and Fe^3 + exist in the high-spin state in perovskite under ambient conditions. Most of the recent studies have observed extremely high-QS values of the Fe^2 + as high as ~ 4.4 mm/s in perovskite at high pressures (Lin et al., 2008; McCammon et al., 2008). The relative area of the high-QS doublet increases with pressure at the expense of the low-QS doublet, and this has been assigned as an intermediate-spin Fe^2 + in the A site occurring at approximately 30 GPa (McCammon et al., 2008). However, recent theoretical calculations support the notion that the extremely high-QS site is a result of the atomic-site change rather than a high-spin to an intermediate-spin transition (Bengtson et al., 2009; Hsu et al., 2010). At higher pressures, a new doublet component was assigned to the low-spin Fe^2 + occurring at 120 GPa and high temperatures (McCammon et al., 2010). Recent studies on Fe^3 +-containing perovskite suggest that the Fe^3 + in the octahedral B site undergoes a spin-pairing transition in perovskite, whereas Fe^3 + in the A site remains high spin to at least 136 GPa (Catalli et al., 2010).

Fe^2 + likely exists in the bipolar-prismatic site, with an extremely high QS of 3.8–4.5 mm s^− 1 and relatively high CS, in post-perovskite, which has been assigned to the intermediate-spin Fe^2 + state with a total spin momentum (S) of one (Lin et al., 2008; Mao et al., 2010). However, theoretical calculations have found the intermediate-spin state unstable at lower-mantle pressures. Fe^3 + exists in two different sites, the high-spin Fe^3 + in the bipolar-prismatic site and the low-spin Fe^3 + in the octahedral site. These site assignments indicate that the Fe^3 + in the octahedral site undergoes a high-spin to low-spin transition at high pressures through charge-coupled substitution. The formation of metallic iron and Fe^3 + in post-perovskite is suggested to be achieved by self-reduction of Fe^2 + to form iron metal and Fe^3 +, similar to that in the perovskite (Jackson et al., 2009a,b). Since part of the interpretations of Mössbauer results requires knowledge of the total spin momentum of the 3d electronics in the samples, further integration of the Mössbauer results with other synchrotron x-ray spectroscopic results will be needed to better understand the spin and valence states of Fe in the deep Earth.

2.21.1 Introduction

⁵⁷Fe Mössbauer spectroscopy is a powerful tool for studying the electronic structure of monomeric complexes, iron-containing clusters, and cluster assemblies. The technique measures transitions between the sublevels of the nuclear ground state (I_g = 1/2) and the levels of the 14.4 keV nuclear excited state (I_e = 3/2), see Figure 3.4 of Gütlich et al.¹ Because hyperfine splittings depend on electronic quantities such as charge distribution, symmetry, spin, and orbital angular momentum, the ⁵⁷Fe nucleus is a sensitive probe of the oxidation and spin state of an iron site and a good probe of the coordination environment. ⁵⁷Fe Mössbauer spectroscopy has been applied to many problems in coordination chemistry and magnetochemistry;^2,3 the fundamentals have been covered in basic texts.^1,4 Since the early 1970s, Mössbauer spectroscopy has been developed into an incisive tool for the study of metalloproteins.^5–10 In particular, complementary information obtained from Mössbauer and EPR spectroscopy has been exploited for the characterization of complex cluster systems.^7–9,11 The methodology developed for studies of metalloproteins is directly applicable to iron-containing crystals, polycrystalline samples, and (frozen) molecular solutions. Since the early 1990s, the synthesis and characterization of catalysts based on metal clusters found in biology has moved to the forefront of inorganic chemistry. The present article focuses on the information that can be obtained from a Mössbauer study.

The Mössbauer spectra of compounds with integer electronic spin such as Fe^II and Fe^IV complexes or Fe^IIIFe^III and Fe^IIFe^II clusters generally exhibit simple patterns when the samples are studied in the absence of an applied magnetic field. The spectra consist of doublets from which one can extract the quadrupole splitting, ΔE_Q, and the isomer shift, δ (Figure 1a). The quadrupole splitting, or more exactly the electric field gradient (EFG) tensor (see below) contains a valence part and a ligand contribution. The valence contribution, together with the isomer shift, which measures the s-electron density at the nucleus and indirectly, through shielding, the d-electron population, provides generally sufficient information for assigning oxidation and spin states of the iron.^3,4 For a paramagnetic complex, integer or half-integral spin, additional data concerning electronic structure can be obtained by studying the samples in applied magnetic fields. Many parameters can be extracted from an analysis of the Mössbauer spectra with a spin Hamiltonian such as

Figure 1. Structure of [2Fe–2S] cluster and Mössbauer spectra of the [2Fe–2S]¹⁺ cluster of the reductase of methane monooxygenase from Methylosinus trichosporium OB3b.¹⁵ (a) The 200 K spectrum consists of two quadrupole doublets representing a ferric (inner doublet) and a ferrous site. The triangles mark the isomer shift of each site, obtained by taking the centroid of the spectrum. (b) 4.2 K spectrum, recorded in a field of 6.0 T applied parallel to the observed Mössbauer radiation. The solid lines above the experimental data show a decomposition of the high-field spectrum into contributions from the ferric and ferrous site. Details are described elsewhere.¹⁵

(1)$\text{H}={\text{H}}_{\text{e}}+{\text{H}}_{\text{hf}}$

(2)${\text{H}}_{\text{e}}=D[{\text{S}}_z^2-\frac{1}{3}\text{S}(\text{S}+1)]+E({\text{S}}_x^2-{\text{S}}_y^2)+\beta \mathbf{S}\cdot \mathbf{g}\cdot \mathbf{B}$

(3)${\text{H}}_{\text{hf}}=\mathbf{S}\cdot \mathbf{a}\cdot \mathbf{I}-g_n{\beta }_n\mathbf{B}\cdot \mathbf{I}+\frac{1}{2}eQ{V}_{zz}\left[3{\mathbf{I}}_z^{\text{2}}-\text{I}(\text{I}+1)+\eta \left({\mathbf{I}}_x^2-{\mathbf{I}}_y^2\right)\right].$

In Equation (2), D and E are the axial and rhombic zero-field splitting (ZFS) parameters, respectively, and g is the electronic g tensor. The magnetic hyperfine interactions of the electronic system with the ⁵⁷Fe nucleus are described by $\mathbf{S}\cdot \mathbf{a}\cdot \mathbf{I}$, and $-g_n{\beta }_n\mathbf{B}\cdot \mathbf{I}$ is the nuclear Zeeman term. The quadrupole interaction involves the traceless EFG tensor. The EFG tensor has principal components V_xx, V_yy, and V_zz. The asymmetry parameter η = (V_xx − V_yy)/V_zz can be confined to 0 ≤ η ≤ 1 if the convention $\left|V_{\mathit{zz}}\right|\ge$$\left|V_{\mathit{yy}}\right|\ge$$\left|V_{\mathit{xx}}\right|$ is adopted. A quadrupole doublet yields the magnitude of $\Delta E_{\text{Q}}=\frac{1}{2}eQ{V}_{zz}\sqrt{1+\frac{1}{3}{\eta }^2}$; determination of η and the sign of ΔE_Q requires the presence of magnetic hyperfine interactions (for diamagnetic compounds an applied field >2.0 T is required). Sites with axial symmetry have η = 0, but η = 0 does not imply this symmetry. (A d_xy-orbital, for instance, has tetragonal symmetry, yet this orbital can be an eigenstate in rhombic symmetry.) In Equations (2) and (3), the ZFS and EFG tensors are assumed to have a common principal axis system x, y, z. However, in symmetries lower than rhombic, all tensors in Equations (2) and (3) may have different principal axis systems.

Future research will generate an increasing number of complexes with less common oxidation states (Fe^VI, Fe^V, Fe^IV, and Fe^I) and novel coordination environments. Even “simple” Fe^II compounds still yield interesting spectroscopy. For instance, Holland and co-workers¹² have synthesized a series of three-coordinate, high-spin Fe^II complexes [LFeX]⁰ (L = diketiminate; X = Cl⁻, CH₃⁻, NHTol⁻, NHtBu⁻). The complex with X = CH₃ yields low-temperature Mössbauer spectra with unusually large magnetic hyperfine splittings, corresponding to internal magnetic field B_int =  + 82.0 T, the largest hyperfine field observed for an iron complex (B_int is defined in Equation (7); typical B_int values range from 10 to 50 T.). The positive sign of B_int indicates a large, unquenched orbital angular momentum, which was confirmed by the observation of an integer spin EPR signal corresponding to g_z = 2.85 (in $\text{H}=\beta \mathbf{S}\cdot \mathbf{g}\cdot \mathbf{B}$), the largest value observed for high-spin Fe^II. This large, unquenched orbital angular momentum results from an accidental near degeneracy of two orbitals with $d_{z^{\text{2}}}$ and $d_{yz}$ symmetry.¹²

2.33.2 Nuclear Decay Induced Excited Spin State Trapping (Niesst)

⁵⁷Fe Mössbauer spectroscopy provides an elegant technique to generate excited electronic states in coordination compounds and to simultaneously identify them via the characteristic hyperfine interactions, and even measure their lifetimes within the time window given by the lifetime of the excited nuclear state of ∼140 ns.^37–39 To this end, the compound of interest doped with ⁵⁷Co is used as Mössbauer source towards a single-line absorber. The decay of ⁵⁷Co to ⁵⁷Fe by electron capture (EC) is accompanied by energy release, which leads to electronic excitations of the nucleogenic ⁵⁷Fe ion. Thus, several years prior to the discovery of LIESST, anomalous resonance signals were observed in such Mössbauer emission spectra of ⁵⁷Co-labeled coordination compounds for which the corresponding Fe^II complexes are LS complexes. These signals can be assigned to metastable HS states of the nucleogenic ⁵⁷Fe^II.⁴⁰ The proposed mechanism for this “Nuclear Decay-induced Excited Spin State Trapping (NIESST)” is much the same as for LIESST (see Figure 1), but with an intramolecular light source provided by the ⁵⁷Co(EC)⁵⁷Fe nuclear decay.

NIESST was first observed on [M(phen)₃](ClO₄)₂ (phen = 1,10-phenantroline).⁴⁰ The neat iron compound is a LS compound at all temperatures, and so are [⁵⁷Fe(phen)₃]²⁺-doped systems with ∼0.1% ⁵⁷Fe. Thus, the Mössbauer absorption spectra for M = ⁵⁷Fe_x/Co_1−x, x = 0.001, of Figure 4a simply consist of a typical Fe^II–LS quadrupole doublet. The Mössbauer emission spectra for M = ⁵⁷Co_x/Co_1−x, x = 0.001, of Figure 4b are similar down to 250 K (A), except for a small fraction of Fe^III–LS (B), which arises from the loss of a valence electron after the nuclear decay. At lower temperatures, however, two typical Fe^II–HS doublets (C,D) appear at the expense of the Fe^II–LS doublet. Time-differential Mössbauer emission spectroscopy^41,42 shows that the relaxation from the highly excited states to the HS state occurs rapidly, and it allows the determination of the lifetime of the metastable HS state. For the above system, the lifetime at 80 K is ∼500 ns, and it decreases rapidly at higher temperatures. Time-differential Mössbauer emission spectra on [⁵⁷Co/Mn(bpy)₃](PF₆)₂ and laser pump probe experiments on the corresponding [Fe/Mn(bpy)₃](PF₆)₂ system between 10 K and 130 K gave values for the lifetimes of the metastable HS states in very good agreement with each other.⁴³ Thus, the NIESST phenomenon actually proves that the light-induced transient states in LS complexes are indeed HS states.

Figure 4. (a)⁵⁷Fe Mössbauer absorption spectra of [⁵⁷Fe/Co(phen)₃](ClO₄)₂ as a function of temperature versus ⁵⁷Co/Rh (295 K) as source. (b) Time-integral Mössbauer emission spectra of a [⁵⁷Co/Co(phen)₃] (ClO₄)₂ source as a function of temperature versus K₄[Fe(CN)₆] (295 K) as absorber. A: Fe^II–LS, B: Fe^III–LS, C: Fe^II–HS1, D: Fe^II–HS2. In (a) the source was moved relative to the absorber; in (b) the absorber was moved relative to the source. For direct comparison, the sign of the velocities must be changed either in (a) or in (b).⁴⁰

For complexes with a somewhat lower ligand-field strength such as in ⁵⁷Co-labeled [Fe(phen)₂(NCS)₂], in which the thermal LS ↔ HS transition occurs at a temperature of T_1/2 ≈ 175 K, the Mössbauer emission spectrum consists of the typical Fe^II–HS doublet at all temperatures down to 4.2 K.⁴⁴ A similar behavior was observed in the Mössbauer emission spectra of a whole series of ⁵⁷Co-doped Fe^II spin-crossover compounds and their Co^II analogs.^45–48 In all the spin-crossover systems, metastable Fe^II–HS states originating from the ⁵⁷Co nuclear decay are “trapped” at temperatures below the thermal transition temperature with an efficiency of nearly 100% with respect to the decay events.

6 SMS spectra under high pressures and temperatures

In the SMS experiments, the time spectrum is a collection of events that reveal the time span between the arrival of the synchrotron radiation pulses and the arrival of scattered photons. The time spectra were recorded by an APD in the forward direction (Figs 1 and 2). Time spectra of Fe₂O₃ have been collected up to 24 GPa and 1400 K in steps of ~100 K (Fig. 5a–b). Time spectra at 10 and 24 GPa upon laser heating reveal that Fe₂O₃ undergoes a magnetic to nonmagnetic transition at 900 (±100) K and 1000 (±100) K, respectively, from a room-temperature magnetic state to a high-temperature nonmagnetic state (Fig. 5a, b). This magnetic transition is found to be reversible with temperature. Since the Néel temperature of hematite is 956 K at ambient pressure, our results show that the Néel temperature remains unchanged with the present accuracy up to 24 GPa. A magnetic-to-nonmagnetic transition in hematite has also been observed under high pressures at ~50 GPa and 300 K; although, the magnetic breakdown is reported to be connected with a first-order structural transition (Pasternak et al., 1999; Badro et al., 2002). The time spectra are evaluated with the CONUSS programs to permit derivation of magnetic hyperfine parameters (Sturhahn, 2000) (Fig. 6). The magnetic hyperfine field is 52.20 T (±0.02) at 24 GPa and 300 K, a typical value of the hyperfine field for ionic ferric oxide bonding, consistent with previous Mössbauer studies (Pasternak et al., 1999). At approximately 1100 K, a magnetic-to-nonmagnetic transition occurs; the magnetic component is highly reduced to approximately 5% at 1100 K. A pure quadrupole spectrum is observed at 1400 K. We note that the absolute temperature measurement is also very useful in the SMS study at temperatures below 1000 K, where the spectroradiometric method is limited due to the weak thermal emission. The sample temperature can be directly determined from the energy spectra, whereas the time spectra reveal magnetic ordering within the sample.

Figure 5. Representative time spectra of the SMS of Fe₂O₃ at 10 GPa (a) and 24 GPa (b) upon laser heating. Temperatures were determined from the energy spectra based on the detailed balance principle. The decay time is dictated by the nuclear lifetime of the ⁵⁷Fe nuclei in Fe₂O₃ Oscillations in the time spectra are observed that originate from the nuclear-level splitting (Sturhahn, 2004), whereas the flat feature indicates a nonmagnetic state. The spectra were collected at a temperature step of ~100 K from 300 to 1400 K.

Figure 6. The SMS spectra at 300 K (a), 1100 K (b), and 1400 K (c) are evaluated by CONUSS programs to permit derivation of magnetic hyperfine parameters (Sturhahn, 2000). The magnetic hyperfine field is 52.20 T (±0.02) at 24 GPa and 300 K, a typical value of the hyperfine field for ionic ferric oxide bonding, which is also consistent with previous Mössbauer studies (Pasternak et al., 1999). At approximately 1100 K, a magnetic-to-nonmagnetic transition occurs; the magnetic component is highly reduced to approximately 5% at 1100 K. A pure quadrupole spectrum is observed at 1400 K. Solid curves: experimental time spectra; dashed curves: time spectra calculated from the CONUSS programs.