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2 Synchrotron Mössbauer Spectroscopy at High Pressures and Temperatures

2 Synchrotron Mössbauer Spectroscopy at High Pressures and Temperatures

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2.2 SYNCHROTRON MOSSBAUER

SPECTROSCOPY AT HIGH PRESSURES AND TEMPERATURES



45



FIGURE 2.1

Schematics of the double-sided laser heating system combined with the SMS technique at sector 3 of the Advanced

Photon Source [24]. Two infrared laser beams are used to laser heat the sample in a DAC, whereas temperatures of

the heated sample are measured by thermal radiation spectra fitted to Planck’s function. The SMS signal is recorded

by an avalanche photodiode detector in the forward direction. A stainless steel (SS) foil is used as a reference for

deriving the CS of the iron sites.



of the high-spin Fe2ỵ in the sample (Fig. 2.2) [23]. Absence of these time beats (the QS) at $60 GPa thus indicates a highspin to low-spin electronic transition of iron, allowing one to evaluate the ratio of the high-spin to low-spin states of iron

in the sample. The SMS spectra collected from the laser-heated DAC experiments, however, show no observable time

beats within the observable time window of 153 ns at the APS, even though Fe2ỵ should remain in the high-spin state at

such P–T conditions [24]. For 57 Fe, the 153 ns time window limits observable QS values at $0.3 mm sÀ1 (Fig. 2.3) . It is

conceivable that SMS spectra without time beats manifest a high-temperature effect on the QS of the high-spin Fe2ỵ in

ferropericlase. Thus, for accurate determination of the lower QS values and the spin transition, there is a need to extend

the observation time window. Recent development of a mechanical bunch chopper to select certain bunches of the



FIGURE 2.2

Representative SMS spectra of

ferropericlase ((Mg0.75,Fe0.25)O)

as a function of pressure at

room temperature (a) [23] and

along with a stainless steel (SS)

for CS measurements (b). Dots:

Experimental measurements;

black lines: modeled spectra

with the MOTIF program. The

quantum bits at 0, 13, and

45 GPa are generated from the

QS of the high-spin Fe2ỵ in the

sample, whereas the flat feature of the spectra at 70, 79, and

92 GPa indicates disappearance

of the QS and the occurrence of

the low-spin Fe2ỵ.



46





2 MOSSBAUER

SPECTROSCOPY IN STUDYING ELECTRONIC SPIN AND VALENCE STATES OF IRON



FIGURE 2.3

The simulated time spectra

showing the location of the time

beats as a function of different

quadrupole splitting values in

SMS measurements. The vertical

line at 153 ns indicates the time

window available at the

Advanced Photon Source during

the normal operations.



synchrotron X-ray pulses may provide a solution, whereas some synchrotron radiation facilities offer a larger time

window between the synchrotron bunches.

2.3.1 Crystal Field Theory on the 3d Electronic States

Crystal field theory provides a simple picture on understanding the electronic structures of the 3d electrons in iron

including the split energy levels of the 3d electrons and their associated spin states in the mantle minerals [28]. In the

simple case of the sixfold, octahedrally coordinated Fe2ỵ in ferropericlase, the 3d electrons of iron can occupy different

degenerate sets of 3d orbitals, namely the triplet t2g-like and doublet eg-like orbitals (Fig. 2.4) [28]. The occupation of the



FIGURE 2.4

Energy level diagram of the high-spin Fe2ỵ in the octahedral site in ferropericlase based on the crystal field theory

[23,28]: (a) free ion; (b) undistorted field; (c) distorted octahedral site. Dc: crystal-field splitting energy; D1, D2: energy

separation between the lowest dxy and upper dxz and dzy levels of the t2g states in a distorted octahedral Fe2ỵ site,

respectively. D1 and D2 are assumed to be the same and are denoted as D3 for simplicity. The energy separations in

the diagram are not drawn to scale. Arrows indicate electron spins. The sixth electron is shown in gray to be in the

spin down and paired up with another electron in the lowest dxy orbital. In the low-spin state, all six electrons are

paired in the three lower t2g states. The crystal-field energies between t2g and eg under ambient conditions are taken

from the literature [28].





2.2 SYNCHROTRON MOSSBAUER

SPECTROSCOPY AT HIGH PRESSURES AND TEMPERATURES



47



3d orbitals is defined by the surrounding environment of the iron ion, such as the bond length between ligands and the

central atom, crystallographic site symmetry, pressure, and temperature (some of these parameters can be interconnected). Under ambient conditions in silicates and oxides, it is energetically favorable for the 3d electrons to occupy

different orbitals with the same electronic spin, that is, the high-spin state with four unpaired electrons and two paired

electrons in Fe2ỵ (spin quantum number (S) ẳ 2) is more stable. In this case, the hybridized t2g-like and eg-like orbitals of

the octahedrally coordinated Fe2ỵ are separated by the crystal field splitting energy (CFSE, DC), which is lower than the

electronic spin-pairing energy (P) [28]. The crystal field splitting energy with respect to the spin-pairing energy, however,

can be significantly influenced by the energy change associated with pressure, temperature, and/or composition. The

increase of the crystal field splitting energy with respect to the spin-pairing energy under high pressures can eventually

lead to the pairing of the 3d electrons of the opposite spin, that is, the low-spin state with all six 3d electrons paired in

Fe2ỵ (S ¼ 0) [28].

The scheme of the crystal field splitting energy relative to the spin-pairing energy, however, becomes rather complex

through crystallographic site distortion and electronic band overlap in perovskite and postperovskite where occupancy

degeneracy may be lifted, making modeling of the experimental data and theoretical treatment of the spin transition

rather complicated [29,30]. In particular, the degeneracy of the electronic energy levels can be lifted through the Jahn–

Teller distortion effect. For the electronic structures of the 3d orbitals of Fe2ỵ in a simplified, non-distorted environment

of the large dodecahedral site (the A site to the 8–12 site) of perovskite, the five 3d energy levels are split into three upper

t2g and two lower eg levels, separated by DC. Depending on the relative energies DC, P, and the splitting energies between

the individual t2g and eg orbitals, the six 3d electrons could occupy three different states, the high-spin Fe2ỵ with four

unpaired electrons (S ẳ 2), the intermediate-spin Fe2ỵ with two unpaired electrons (S ẳ 1), or the low-spin Fe2ỵ with all

six paired electrons (S ẳ 0) [29,30]. For example, under ambient conditions, the Fe2ỵ is more stable in the high-spin state

in the A site.

2.3.2 Electronic Spin Transition of Fe2ỵ in Ferropericlase

Ferropericlase [(Mg,Fe)O], with a cubic rock-salt (B1) structure, forms a solid solution between periclase (MgO) and

w€

ustite (FeO). Periclase is a wide bandgap insulator and prototype monoxide, whereas w€ustite is an important member

of the highly correlated transition metal monoxide (TMO) group [31]. In TMOs, the oxygen 2p states are occupied, the

TM 4s states are empty, and the TM 3d states are partially occupied. According to the Mott–Hubbard theory, the strong

intra-atomic d–d Coulomb interaction is greater than the 3d bandwidth. As a result, a gap forms between the occupied

and unoccupied 3d states. Strong correlation effects are the main reason for the gap formation. In TMO compounds, a

high-spin to low-spin (spin-pairing) transition occurs when the crystal field splitting exceeds the Hund’s-rule exchange

energy and the material becomes diamagnetic; this results in the collapse of the magnetic state. (Mg,Fe)O contains

partially occupied Fe2ỵ in 3d orbitals situated in the bandgap. Hence, the local environment of iron, the 3d bandwidth, and

iron–iron interaction play an important role in the electronic, structural, and physical properties of ferropericlase under

high pressures. Since lower-mantle ferropericlase contains $20% Fe2ỵ, our discussion here focuses on the MgO-rich

ferropericlase.

ossbauer spectra of ferropericlase

The measured values of QS, $0.8 mm sÀ1, and CS, $1.2 mm sÀ1, from the M€

under ambient conditions are consistent with predominant high-spin Fe2ỵ in the octahedral coordination (Fig. 2.5)

[15,23,27,32,33]. The presence of the high-spin Fe3ỵ, however, is typically very low (e.g., under the detection limit of the

SMS technique). At room temperature, the QS of the high-spin Fe2ỵ increases with increasing pressure up to about

30 GPa, plateaus at further pressure increase up to about 60 GPa, and then disappears with further increasing pressure

(Fig. 2.5) [15,23,27,32,33]. The CS of the high-spin state decreases with increasing pressure; a noticeable drop of the CS

occurs at $60 GPa. The simultaneous disappearance of the QS and the drop of the CS at $60 GPa are consistent with a

high-spin to low-spin electronic transition of iron in the ferropericlase [23]. 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. Depending on the experimental conditions, a narrower to a broader width of the spin transition

has been reported (Fig. 2.5) [15,23,27,32,33].

The QS arises from the interaction between the nuclear quadrupole moment and the nonspherical component of

the electronic charge distribution described by its effective electric field gradient (EFG) in a simplified model [19,28]. The

effective electric field gradient is dramatically reduced due to the spin-pairing transition. The electron shell of the highspin Fe2ỵ ions in the octahedral coordination is spherically asymmetric with S ¼ 2 and t32g" e2g" t12g# , whereas Fe2ỵ ions in the

low-spin state are more spherically symmetric with S ¼ 0 and t32g" t32g# (the arrows indicate spin up or spin down whereas

the superscripted numbers represent the number of the occupied electrons) [19,28]. On the other hand, the negative



48





2 MOSSBAUER

SPECTROSCOPY IN STUDYING ELECTRONIC SPIN AND VALENCE STATES OF IRON



FIGURE 2.5

QS and CS of ferropericlase as a

function of pressure at room

temperature. Circles: (Mg0.75,

Fe0.25)O [23]; diamonds: (Mg0.8,

Fe0.2)O [32].



slope in the CS as a function of pressure reflects the increase in electron density in the high-spin state and low-spin state,

respectively. The spin-pairing process in the 3d electrons causes a jump in the total s-electron density at the nucleus and

hence a drop in the CS (Fig. 2.5) [19]. This observation is evidenced by the observed volume decrease (or density

increase) of a few percent across the spin transition in X-ray diffraction measurements [11,14,18]. The negative slope in

the CS for the low-spin state is lower than that for the high-spin state, suggesting an increase in the incompressibility

across the transition (Fig. 2.5) [11,14].

The spin-pairing transition pressure can be affected by the concentration of FeO in the (Mg,Fe)O solid solution [23].

When iron concentration in the (Mg,Fe)O system is above the percolation threshold, Fe2ỵ atoms form an interconnected

percolation path through the whole structure, where each Fe2ỵ atom has at least two Fe2ỵ neighbors. That is, the 3d

outer electrons of an Fe2ỵ ion interact with the neighboring 3d electrons of the dxy, dxz, and dzy orbitals, which

correspond to the t2g states (split down from the e2g orbitals by the octahedral crystal field). This iron–iron interaction

would increase the effective crystal field by further splitting mostly t2g orbitals and lowering them with respect to the eg

orbitals and thus would stabilize the high-spin state in the system. In particular, the FeO end member of the system is well

known for its strong electron correlations. As such, the FeO-rich (Mg,Fe)O is expected to exhibit a higher spin-pairing

transition pressure as compared to the MgO-rich counterpart [23]. Although (Mg,Fe)O has always been regarded as a

fully disordered solid solution system without noticeable short-range order at ambient conditions, recent M€

ossbauer

analyses have demonstrated broadening of the high-spin Fe2ỵ quantum beats associated with significant changes in the

short-range order that deviates from the behavior of the homogeneous solid solution in ferropericlase [32,33]. These

studies have indicated that the Fe2ỵ ions can form localized clusters in nearest-neighboring environments under high

pressures. The short-range ordering of Fe2ỵ ions can cause an extension of the width of the spin crossover in (Mg,Fe)O at

high pressures (Fig. 2.5) [32,33].

The spin crossover arises from the condition where the thermal energy at high temperatures and pressures is

sufficient to overcome the energy difference between the high-spin state and low-spin state [16,34,35]. Therefore, the

local 3d electronic structures of the iron ions in ferropericlase are strongly temperature dependent [16,34,35]. Most

importantly, recent results show that the electronic spin-pairing transition of iron in ferropericlase occurs over a very

narrow range of pressure at room temperature but turns to a spin crossover with an extended transition pressure at the

lower-mantle temperatures [16,34–37]. In particular, high P–T SMS studies show that the QS values of the dominant Fe2ỵ





2.2 SYNCHROTRON MOSSBAUER

SPECTROSCOPY AT HIGH PRESSURES AND TEMPERATURES



49



site in ferropericlase drop significantly with increasing temperature, indicating a strong temperature effect on the QS

[24]. The high-temperature contribution on the QS and the splitting of the energy levels leads to a broader spin crossover

region at high P–T because the degeneracy of the energy levels is partially lifted.

2.3.3 Spin and Valence States of Iron in Silicate Perovskite

Magnesium silicate perovskite, with 5–10 mol% of Fe and Al, is the most abundant silicate mineral in the lower mantle

existing at 660 km depth to several hundred kilometers above the core–mantle boundary. Iron in perovskite exists in

both Fe2ỵ and Fe3ỵ states and can possibly occupy one of two crystallographic sites, the large dodecahedral Mg site

(the A site) and the small octahedral Si site (the B site) [4–6]. Current consensus on the site occupancy is that Fe2ỵ

mainly occupies the A site whereas Fe3ỵ can occupy both the A and B sites [4–6]. Several studies have further shown

that Fe2ỵ may self-disproportionate into Fe metal and Fe3ỵ, creating Fe3ỵ-enriched perovskite [3840]. That is, the

significant amount of Fe3ỵ in perovskite is not a result of the oxidation state of the lower mantle, which is expected to

be relatively reducing, but due to the crystal chemistry of perovskite at high pressure. The Fe3ỵ content in perovskite

is also found to vary with the amount of the Al3ỵ likely through charge-coupled substitution with the Si4ỵ in the

octahedral site [46]. The Fe3ỵ content in perovskite is $20% without the presence of Al and 5075% with the

presence of Al3ỵ [46,4143].

ossbauer

Electronic spin states of Fe2ỵ and Fe3ỵ in perovskite have been extensively studied using synchrotron M€

spectroscopy, X-ray emission spectroscopy (XES), X-ray absorption near-edge spectroscopy (XANES), and theoretical calculations [4–6,13,29,41–55]. The reported spin transitions are much more complex than those in

ferropericlase likely due to the low-symmetry oxygen ligand field and multiple site occupancies. These studies

have shown that both Fe2ỵ and Fe3ỵ exist in the high-spin state in both of the A and B sites under ambient conditions

[46]. M

ossbauer analyses show that the high-spin Fe2ỵ exhibits a QS value of $1.6–2.2 mm sÀ1 and a CS of

À1

ossbauer analyses have been quite

$1 mm s in the A site (Table 2.1) [4–6]. Interpretations of the high-pressure M€

€ ssbauer Spectroscopic Results on the Spin and Valence States of Iron

TABLE 2.1 List of Experimental Mo

in Perovskite [30,41–45,48–51,53–55]



Composition, Pressure Range

(Mg0.9Fe0.1)SiO3, 42–75 GPa



(Mg0.95Fe0.05)SiO3, 0–120 GPa



(Mg0.88Fe0.09)(Si0.94Al0.10)O3, 12–100 GPa



(Mg0.88Fe0.12)SiO3, 0–120 GPa



(Mg0.6Fe0.4)SiO3, 110 GPa

(Mg0.82Fe0.18)SiO3, 120–130 GPa

MgSiO3 with 10 mol%

Fe2O3, 40–136 GPa

(Mg0.88Fe0.12)SiO3, 30–80 GPa



(Mg0.75Fe0.25)SiO3, 135 GPa



Suggested Spin State



QS (mm s1)



CS (mm s1)



Reference



Fe

Fe2ỵ

Fe3ỵ

Fe2ỵ

Fe2ỵ

Fe3ỵ

Fe2ỵ

Fe2ỵ

Fe3ỵ

Fe2ỵ

Fe2ỵ

Fe3ỵ

Fe2ỵ

Fe2ỵ

Fe2ỵ

Fe2ỵ

Fe3ỵ



HS

IS

HS

IS

IS

IS

LS

HS



3.373.39

2.742.79

1.151.47

1.893.51

1.592.75

0.571.63

2.43.45

2.112.75

0.540.74

1.83.5

3.24.1

0.61.5

4.18

3.98

4.4

00.7

0.51





















0.91.1

1

00.3

0.97

0.84

1.0

0





[44]

[44]

[44]

[44]

[44]

[44]

[46]

[46]

[46]

[48]

[48]

[48]

[30]

[30]

[53]

[53]

[9,42]



Fe3ỵ

Fe2ỵ

Fe2ỵ

Fe3ỵ

Fe2ỵ

Fe3ỵ

Fe3ỵ



LS

IS

HS

HS

IS

LS

LS



2.83.6

3.94.2

22.7

0.51.5

4.1

2.99

1.84





11.1

0.91

00.4

1.02

1.21

1.0



[9,42]

[53]

[53]

[53]

[55]

[55]

[55]



Valence State

2ỵ



When a pressure range is used, the range of the hyperfine parameters also shows the effects of pressure. The CS values are reported as relative to a-Fe.



50





2 MOSSBAUER

SPECTROSCOPY IN STUDYING ELECTRONIC SPIN AND VALENCE STATES OF IRON



FIGURE 2.6

€ ssbauer spectrum of silicate perovskite (Mg0.6,Fe0.4)SiO3 (pv40, a) at 110 GPa [30] and (Mg0.75,

Representative Mo

Fe0.25)SiO3 (pv25, b) at 135 GPa and 300 K [55]. Figures at the top: the measured SMS spectrum (filled circles)

compared with the modeled spectrum by the CONUSS program (solid lines). Figures at the bottom: modeled energy

spectrum from the evaluation of the SMS data.



different, although their main results on the QS values were similar—both experimental and theoretical studies have

reported extremely high QS values of the Fe2ỵ (as high as $4.4 mm s1) at high pressures (Figs. 2.6 and 2.7)

[43,48,50,53,55]. The relative area of the high-QS doublet (3.4–4.4 mm sÀ1) increases with pressure at the expense of

the low-QS doublet (1.6–2.2 mm sÀ1), indicating that the transition involves Fe2ỵ in the A site (Fig. 2.7; Table 2.1)

[48,53]. The high-QS doublet becomes the dominant spectral feature in the high-pressure SMS spectra at above

$30 GPa. Particularly, the new quadrupole doublet with the extremely high QS appears to have a very narrow

linewidth and high CS ($1.1 mm sÀ1) [48]. Together with XES analyses for the total spin momentum of iron in

perovskite, the occurrence of the Fe2ỵ site with the extremely high QS, very narrow linewidth, and relatively high CS

has been assigned to be an intermediate-spin Fe2ỵ with a total spin momentum of 1 (S ¼ 1) in the A site [48,53,55].

That is, a high-spin to an intermediate-spin crossover occurs in perovskite at around 30 GPa, and Fe2ỵ remains stable

in the intermediate-spin state at above 30 GPa in the lower mantle [48,53,55]. At higher pressures, the intensity of the

high QS component in the SMS spectra of perovskite decreases, whereas the intensity of a new component with very

low QS of less than 0.5 mm sÀ1 and a CS of $0 mm s1 increases. This new component was assigned to the low-spin

Fe2ỵ occurring at 120 GPa and high temperatures (Fig. 2.7) [53].

It has been pointed out, however, that the occurrence of the intermediate-spin state is very rare in geological

materials and that high QS in iron does not necessarily imply the occurrence of the intermediate-spin state

[43,50,52,56]. For example, iron in almandine garnet is in the high-spin state with a QS of $3.5 mm sÀ1 under

ambient conditions [56]. Theoretical calculations have been performed to interpret the spin states and the extremely

high QS values of Fe2ỵ in perovskite. Although they are not always in agreement with each other, these calculations

show that the intermediate-spin state is not stable at all lower-mantle pressures, irrespective of the exchange–

correlation functional used in the calculations [39,43,45,47,49,50,52,54]. The high-spin Fe2ỵ with QS of





2.2 SYNCHROTRON MOSSBAUER

SPECTROSCOPY AT HIGH PRESSURES AND TEMPERATURES



51



FIGURE 2.7

Quadrupole splitting of Fe in

perovskite from recent experimental (a) and theoretical

(b) results (Table 2.1) [30,41–

45,48–51,53–55]. Light gray:

high-spin state; dark gray:

intermediate-spin state; black:

low-spin state. The spin and

valence state assignments are

taken from the literatures. The

large range given for the QS in a

specific spin state reflects the

effect of pressure on the QS and

uncertainties in various studies.

Black arrows show the effect of

increasing pressure on the QS

values. A and B noted in (c) and

(d) represent two different sites

of Fe in perovskite.



2.3–2.5 mm sÀ1 is more stable at relatively low pressures, while the high-spin Fe2ỵ with QS of 3.3–3.6 mm sÀ1 is more

favorable at higher pressures [43,52,54]. Based on these calculations, the extremely high QS site is a result of the iron

atomic-site change, in which iron ions move away from the central positions in the A site, rather than a spin-pairing

transition [43,52,54].

For Fe3ỵ in perovskite, it has been shown both experimentally and theoretically that Fe3ỵ enters into both A and B

sites, suggesting a charge-coupled substitution mechanism [4–6,41,42,45,46]. High-spin Fe3ỵ is known to exhibit very low

QS compared to Fe2ỵ because all five 3d electrons are unpaired and form relatively spherical orbitals. Theoretical

calculations further suggest that the Fe3ỵ in the A site has an even lower QS value than the B site Fe3ỵ (Fig. 2.7) [54].

Combined SMS and XES results for Fe3ỵ-containing perovskite suggest that the low-spin population in the B site gradually

increases with pressure up to 50–60 GPa; eventually all Fe3ỵ in the B site becomes the low-spin state, whereas Fe3ỵ in the

A site remains high spin to at least 136 GPa, consistent with recent theoretical calculations [41,42,44,54]. SMS analyses

further suggest that Fe3ỵ in the A site remains in the high-spin state in Al-bearing perovskite in which Fe3ỵ is expected to

predominantly exist in the A site [41,42]. Therefore, these results indicate that the Fe3ỵ in the octahedral B site

undergoes a high-spin to low-spin transition in perovskite. The spin transition in the B site is predicted to be accompanied

by a noticeable volume reduction and an increase in the QS [54]. It has recently been shown that perovskite can also

accommodate a very high amount of iron at high P–T (as high as 40% reported in the literatures) [38–40], although this

amount does not appear to significantly affect the QS values of perovskite (Fig. 2.8).

The QS of the Fe2ỵ in the A site and the Fe3ỵ in the B site exhibits exactly the opposite trends across the high-spin to

the low-spin transition, and can be understood through their orbital occupancies. The low-spin Fe2ỵ in the A site located

near the center of the site forms a 3d charge density with a cubic-like shape that barely contributes to the effective

electric field gradient, leading to a very small QS [28,54]. The high-spin Fe3ỵ also has a small QS (a small effective electric

field gradient) in the A and B sites because of an almost spherically shaped electron charge distribution [28,54]. However,

the 3d orbitals of the low-spin Fe3ỵ contribute more significantly to the effective electric field gradients and lead to a

larger QS.



52





2 MOSSBAUER

SPECTROSCOPY IN STUDYING ELECTRONIC SPIN AND VALENCE STATES OF IRON



FIGURE 2.8

Comparison of the quadrupole

splitting of Fe in perovskite as a

function of composition [30,41–

45,48–51,53–55]. Open columns:

experimental results; solid columns: theoretical results. Gray:

high-spin state; red: intermediatespin state; blue: low-spin state.

Arrows represent the effect of

increasing pressure on the QS values.



2.3.4 Spin and Valence States of Iron in Silicate Postperovskite

Postperovskite is expected to be the most abundant phase in the Earth’s core–mantle boundary region [2,3]. Deciphering

the spin and valence states of iron in postperovskite at P–T conditions thus provides new insights into the properties of

the region, including seismic discontinuities, rheology and plasticity, dynamic evolution and formation of superplumes,

thermal gradients, core–mantle heat flux, and chemistry. The postperovskite phase is found to be stable in the CaIrO3type structure (Cmcm). However, a number of kinked structures, formed by sliding the {010} planes of the perovskite

structure with variation in the stacking sequence of SiO6 octahedral layers, have also been reported [2,3,57–60]. The

corresponding plane slips in perovskite might introduce more crystallographic defects favorable for the substitution of

the Fe3ỵ. Thus, the presence of the kinked postperovskite phase has been found to increase the amount of Fe3ỵ in the

sample. Experimental SMS results show Fe2ỵ exhibits extremely high QS of 3.8–4.5 mm sÀ1 and relatively high CS in

postperovskite (Figs. 2.9 and 2.10; Table 2.2) [30,60]. The Fe2ỵ likely exists in the bipolar-prismatic site, similar to the A

site in perovskite. Together with a complementary high P–T XES study that is sensitive to the total spin momentum of

iron, it has been suggested that iron predominantly exists in the intermediate-spin Fe2ỵ state with S ¼ 1 in the CaIrO3typed postperovskite at relevant P–T conditions of the lowermost mantle [30,60]. Since the intermediate-spin Fe2ỵ in

postperovskite is found stable with 25 and 40 mol% Fe, it is conceivable that the intermediate-spin Fe2ỵ is stable over a

wide range of Fe content in postperovskite relevant to the D00 region, where Fe-enrichment may be expected [30,60].

Based on the Rietveld refinement of the X-ray diffraction patterns, the postperovskite phase also exhibits an increase in

the octahedral tilt angles and a shortening of the bond length that could stabilize the intermediate-spin Fe2ỵ by Jahn

Teller distortions [60]. Similar to the discussion for Fe2ỵ in perovskite, however, theoretical calculations suggested that

Fe2ỵ in postperovskite is in the high-spin state at all mantle pressures [61], although theoretical QS values of the Fe2ỵ and

Fe3ỵ remain to be computed.

SMS results on Fe3ỵ-rich postperovskite show that Fe3ỵ exists in two different sites, one site with a high QS of

$2 mm sÀ1 and another with a low QS of 0.3 mm s1, similar to the Fe3ỵ in perovskite (Figs. 2.9 and 2.10; Table 2.2) [9].

The site with the low QS is assigned to be the high-spin Fe3ỵ in the bipolar-prismatic site, whereas the high QS site with

QS of $2 mm sÀ1 is assigned to be the low-spin Fe3ỵ in the octahedral site [9]. Based on the hyperfine parameters

associated with the site assignments, the Fe3ỵ in the octahedral site undergoes a high-spin to low-spin transition at high

pressures, causing the QS to increase from the high-spin state to the low-spin state [9]. In this mixed spin-state scenario,





2.2 SYNCHROTRON MOSSBAUER

SPECTROSCOPY AT HIGH PRESSURES AND TEMPERATURES



53



FIGURE 2.9

€ ssbauer

Representative Mo

spectrum of postperovskite

(Mg0.75,Fe0.25)SiO3 (ppv40, a) at

134 GPa [30] and (Mg0.75,Fe0.25)

SiO3 (ppv25, b) at 142 GPa and

300 K [60]. Figures at the top:

the measured SMS spectrum

(filled circles) compared with

the modeled spectrum by the

CONUSS program (solid lines);

figures at the bottom: modeled

energy spectrum from the

evaluation of the SMS data.



Fe3ỵ enters the sites through charge-coupled substitution, which is also suggested by theoretical calculations. Because

the ionic radius of the low-spin Fe3ỵ is smaller than that of the high-spin Fe3ỵ, the low-spin Fe3ỵ is thus more stable in the

smaller octahedral site in postperovskite. SMS studies also show an increase in the Fe3ỵ abundance with the presence of

the kinked structures in postperovskite [59,60]. Based on a small negative CS with respect to a-Fe and no QS in the SMS

spectrum, a metallic iron phase is proposed to coexist with the Fe3ỵ-rich postperovskite. The formation of metallic iron

and Fe3ỵ in postperovskite is suggested to be achieved by self-reduction of Fe2ỵ to form iron metal and Fe3ỵ, similar to

that in the perovskite [59].

The notable difference between the M€

ossbauer parameters of the perovskite and postperovskite phases is that the

QS of the low-spin Fe3ỵ site is much smaller in postperovskite (Tables 2.1 and 2.2; Figs. 2.7 and 2.9). The smaller QS of the

low-spin site in postperovskite can be explained by a less distorted octahedral site, since distortion from a cubic

environment around the iron nucleus results in an increased QS value [62]. Since postperovskite is the high-pressure

polymorph of perovskite, the QS of Fe2ỵ in postperovskite could be similar or related to that of Fe2ỵ in perovskite



ssbauer Spectroscopic Results on the Spin and Valence States of Iron

TABLE 2.2 List of Experimental Mo

in Postperovskite



Composition, Pressure Range



Valence State



Suggested Spin State



QS (mm s1)



CS (mm s1)



Reference



1

0.95

0.67

0.45

0.16

1.07

1.33

1.11





[30]







(Mg0.6Fe0.4)SiO3, 134 GPa



Fe2ỵ

Fe2ỵ



IS

IS



(Mg0.87Fe0.12)SiO3, 112119 GPa



Fe3ỵ

Fe0

Fe2ỵ

Fe3ỵ

Fe3ỵ

Fe3ỵ



HS

IS

LS

HS/LS

HS



3.76

4.5

3.27

0.74

0

3.77

2.53

1.55

0.318



Fe3ỵ



LS



1.99



(Mg0.75Fe0.25)SiO3, 142 GPa



MgSiO3 with 8.5 mol%

Fe2O3, 128–138 GPa



The CS values are reported as relative to a-Fe.



[59]

[60]



[9,42]



54





2 MOSSBAUER

SPECTROSCOPY IN STUDYING ELECTRONIC SPIN AND VALENCE STATES OF IRON



FIGURE 2.10

Experimentally observed QS and

CS of iron and their assigned spin

and valence states in postperovskite (Table 2.1) [9,30,59,60]. The

CS values are reported relative to

a-Fe. Dark gray: intermediatespin Fe2ỵ; light gray: high-spin

Fe3ỵ [9,59]; black: low-spin Fe3ỵ

[9,60].



FIGURE 2.11

Comparison of the hyperfine QS

and CS of Fe in postperovskite as

a function of composition

(Table 2.2) [9,30,59,60]. Squares:

intermediate-spin Fe2ỵ [30,60];

circles: low-spin Fe3ỵ [9,60]);

down triangles: high-spin Fe3ỵ

[9,59]; upper triangles: metallic

Fe [59]; diamonds: unassigned

state of Fe [30,60].



as both phases exhibit extremely high QS values of around 4 mm s1. The calculated QS of Fe2ỵ in perovskite is

3.3–3.6 mm sÀ1 at lower-mantle pressures, consistent with the QS values for Fe2ỵ from SMS analyses in postperovskite

[30,54,60]. However, the occurrence of the Fe2ỵ site with extremely high QS is explained as a result of atomic-site

change in the high-spin state. Iron content in mantle minerals is known to affect the stability of Fe spin states [49].

Increasing Fe content in postperovskite does not appear to affect the hyperfine fields of Fe2ỵ in the A sites, whereas the

QS of Fe3ỵ appears to increase with increasing Fe content (Fig. 2.11) [9,30,59,60].



2.4 CONCLUSIONS

Here, we have evaluated recent high P–T M€

ossbauer results to address the spin and valence states of iron in the lowermantle ferropericlase, perovskite, and postperovskite using site-specific hyperfine parameters, QS and CS values. The



REFERENCES



55



disappearance of the QS and the drop in the CS of the Fe2ỵ in the octahedral sites of ferropericlase indicate the high-spin

to low-spin transition at high pressures. The spin transition of iron in ferropericlase turns into a wide spin crossover

under lower-mantle temperatures. As the spin crossover of iron occurs in the lower-mantle ferropericlase at high P–T, its

properties, such as sound velocities and densities, will be continuously influenced by the ratio of the high-spin and lowspin states along the lower-mantle geotherm.

Iron exists in the Fe2ỵ and Fe3ỵ states in both perovskite and postperovskite. Both Fe2ỵ and Fe3ỵ exist in the high-spin

state in perovskite under ambient conditions. Recent studies have reported extremely high QS values of the Fe2ỵ as high

as $4.4 mm sÀ1 in perovskite at high pressures. The relative area of the high-QS doublet increases with pressure at the

expense of the low-QS doublet, and has been assigned as an intermediate-spin Fe2ỵ in the A site occurring at $30 GPa.

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. At higher pressures, a new doublet component was

assigned to the low-spin Fe2ỵ occurring at 120 GPa and high temperatures. Recent studies on Fe3ỵ-containing perovskite

suggest that the Fe3ỵ in the octahedral B site undergoes a spin-pairing transition in perovskite, whereas Fe3ỵ in the A site

remains high spin up to at least 136 GPa.

Fe2ỵ likely exists in the bipolar-prismatic site with an extremely high QS of 3.8–4.5 mm sÀ1 and relatively high CS in

postperovskite, which has been assigned to the intermediate-spin Fe2ỵ state with S ¼ 1. However, theoretical

calculations have found the intermediate-spin state unstable at lower-mantle pressures. Fe3ỵ exists in two different

sites, the high-spin Fe3ỵ in the bipolar-prismatic site and the low-spin Fe3ỵ in the octahedral site. These site assignments

indicate that the Fe3ỵ in the octahedral site undergoes a high-spin to low-spin transition at high pressures through chargecoupled substitution. The formation of metallic iron and Fe3ỵ in postperovskite is suggested to be achieved by selfreduction of Fe2ỵ to form iron metal and Fe3ỵ, similar to that in the perovskite.

Since part of the interpretations for the intermediate Fe2ỵ in perovskite and postperovskite is based on the XES

analyses for the total spin momentum of the 3d electronics in the samples, further understanding of the XES spectra

involving multiple electronic transitions as well as theoretical calculations incorporating lattice distortion effects is

needed to resolve the discrepancy between current experimental and theoretical results and interpretations.

Although SMS spectra can now be collected from the laser-heated DAC experiments at relevant P–T conditions

of the lower mantle, extended time windows are needed to extract more meaningful information to decipher the

spin and valence states of iron in the lower-mantle minerals at relevant P–T conditions. Knowing the exact spin and

valence states of iron in the lower-mantle minerals would then help geophysicists to address properties of the

deep Earth.



ACKNOWLEDGMENTS

We acknowledge XOR-3 and HPCAT, APS, ANL for the use of the synchrotron M€

ossbauer facilities. Use of the

Advanced Photon Source was supported by U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences

(BES), under contract No. DE-AC02-06CH11357. We thank A. Wheat and C. Jacobs for their constructive comments. J.

F.L. and Z.M. acknowledge financial support from NSF Earth Sciences (EAR-0838221), Carnegie/DOE Alliance Center

(CDAC), and Energy Frontier Research in Extreme Environments (EFree) Center.



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