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Reduced Radiative Conductivity of Low-Spin (Mg,Fe)O in the Lower Mantle

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Science  26 May 2006:
Vol. 312, Issue 5777, pp. 1205-1208
DOI: 10.1126/science.1125622

Abstract

Optical absorption spectra have been measured at pressures up to 80 gigapascals (GPa) for the lower-mantle oxide magnesiowüstite (Mg,Fe)O. Upon reaching the high-spin to low-spin transition of Fe2+ at about 60 GPa, we observed enhanced absorption in the mid- and near-infrared spectral range, whereas absorption in the visible-ultraviolet was reduced. The observed changes in absorption are in contrast to prediction and are attributed to d-d orbital charge transfer in the Fe2+ ion. The results indicate that low-spin (Mg,Fe)O will exhibit lower radiative thermal conductivity than high-spin (Mg,Fe)O, which needs to be considered in future geodynamic models of convection and plume stabilization in the lower mantle.

Silicate perovskite (Mg,Fe)SiO3 and magnesiowüstite (Mg,Fe)O are the major constituents of Earth's lower mantle. Because of partially filled d-electron orbitals, the presence of Fe in these minerals strongly influences the radiative component of conduction and thus their ability to transfer heat effectively (1). Pressure- and temperature-dependent thermal conductivity of minerals (2, 3) is now thought to control some aspects of mantle convection and plume stability (47). In addition to the presence of large so-called superplumes (8, 9), the complex seismic structure of Earth's lower mantle also reveals lateral heterogeneities that have been explained by compositional and thermal variation, partial melting, and phase transformations (1013). The recent discovery of a spin-pairing high-spin (HS) to low-spin (LS) electronic transition of iron in silicate perovskite (14) and magnesiowüstite (15) have also been invoked to explain these anomalies. In (Mg,Fe)O, the HS-LS transition between 50 and 70 GPa strongly influences its bulk elastic properties (16, 17) and is expected to blue-shift iron absorption bands in the infrared (IR) (1, 18), although the transport properties of low-spin (Mg,Fe)O at high pressure have remained only speculative. Here, we provide experimental evidence for reduced radiative conductivity in LS-(Mg,Fe)O from optical absorption spectra through the spin-pairing transition in magnesiowüstite single crystals with varying iron contents spanning possible lower-mantle compositions.

Absorption spectra were obtained at pressures up to 80 GPa for high-quality single-crystal samples of [Mg(1-x)Fex]O with x = 0.06, 0.15, and 0.25 by a proper referencing of the transmission spectra measured through the sample (19) (fig. S1). The measurements were carried out in a diamond anvil cell with Ne or Ar pressure media (19). At low pressure, the (Mg0.94Fe0.06)O sample shows a broad absorption maximum ∼8000 to 10,000 cm–1, resulting from electronic transitions between Fe2+ d-orbitals of T2g and Eg symmetry split by the crystal field, and an absorption tail in the visible-ultraviolet (UV) range that was assigned to electron charge transfer between Fe2+ and O2– ions (1, 18). We assume Fe3+-O2– charge transfer also contributes to the absorption edge but note that the ferric iron content of the samples is very low (19).

In agreement with earlier studies up to 30 GPa (2022), applying pressure red-shifts the absorption edge in the UV range, which causes the overall absorption in the near-IR region to increase with pressure. The relatively low iron concentration in the (Mg0.94Fe0.06)O sample (Fig. 1) allows us to observe a modification of the crystal-field levels through the spin-pairing transition (Fig. 2). Two new absorption bands gradually appear above 40 GPa. At the same time, the T2g-Eg transition of the high-spin phase becomes less prominent, although at higher pressure a band close to this energy persists. We assign the newly observed higher energy bands to the A1g-T2g and A1g-A2g transitions, while the one similar to the T2g-Eg band is assigned to the A1g-T1g transition. Transformation of the crystal-field energy levels (Fig. 2) is related to the change of the electronic ground state (T2g to A1g), i.e., the HS to LS transition. We relate the pressure point at which the new crystal field bands appear to the onset of transition pressure and note that the spectra continue to change at higher pressure as the LS state is more populated. Accordingly, we observe a decrease of the absorption coefficient in the visible-UV range. On decompression, the spectral changes are reversible without hysteresis.

Fig. 1.

Optical absorption spectra of (Mg,Fe)O containing 6% Fe at various pressures and 300 K. The absorption coefficient has been calculated using k = A*ln10/d, where A = log10(I0/I), I0 and I are the transmission signals for the reference and sample configuration, respectively, and d is the sample thickness measured at ambient conditions from the white-light interferometry and calculated at high pressure using an isothermal equation of state over this pressure range (16). Ripples in the spectra at low pressures are interference fringes resulting from multiple reflections between the parallel polished sample surfaces. The beats that can be seen in interference fringes are due to a multiple interference with the beams reflected from the culet surfaces. This complex interference pattern is smoothed (thick gray line) for the 11-GPa spectrum for clarity. Vertical arrows designate new crystal-field transitions, which are characteristic of the low-spin phase.

Fig. 2.

The Tanabe-Sugano (29) diagram for Fe2+ (d6). E is the energy, B is the interelectronic repulsion parameter, Dq is a crystal field strength parameter. The vertical line at Dq/B = 2 corresponds the spin-pairing transition. Only the terms relevant for spin-allowed optical transitions are shown [adapted from Figgis (30)]. The crystal field is assumed to be of cubic Oh octahedral symmetry. The thick gray square and thick black diamonds correspond to the positions of the absorption bands measured in the high- and low-spin phases, respectively. The energies of these bands are scaled (B = 500 cm–1) to match the T2g-Eg transition in the high-spin phase near the spin-pairing transition.

With increasing pressure, the high-energy absorption edge of (Mg0.75Fe0.25)O (Fig. 3) red-shifts, as previously noted in experiments below 30 GPa (20, 21). The T2g-Eg crystal-field band shows a moderate blue shift and a gradual decrease of intensity, which is difficult to quantify accurately because of the steep background. However, contrary to prediction, the broad absorption in the near-IR range increases with pressure due to the red-shift of the absorption edge up to ∼55 to 65 GPa, where above that, pressure shift is no longer observed (see inset to Fig. 3). The large overall absorption makes it more difficult to observe the spin-crossover modifications than in the 6% Fe samples, but the spin transition is detected by following the decrease of T2g-Eg band intensity and subsequent decrease of the absorption in the visible range. Similar behavior is observed for the sample with 15% Fe, except that the A1g-T2g spin transition near 15,000 cm–1 is observed above 60 GPa. In general, the absorption edge is less steep in HS (Mg,FeO) than in LS (Mg,Fe)O.

Fig. 3.

Optical absorption spectra of magnesiowüstite (x = 0.25) at elevated pressure and 300 K. The absorption coefficient has been calculated in the same manner as it has been described for Fig. 1. The inset shows the pressure dependence of the points with equal absorption.

The pressure shift of the absorption edge for (Mg,Fe)O with the 15% and 25% Fe is similar (–0.03 eV/GPa), but the absorption edge shift is only about (–0.01 eV/GPa) in the low-Fe content (6%) sample. Further support for nonlinear behavior at low-Fe concentrations is observed by directly comparing the absorption spectra for the various compositions at high pressure (Fig. 4). The spectra, normalized to the iron concentration, do not scale particularly well, especially for the low iron concentration (23). Nevertheless, at lower frequencies (<12,000 cm–1), the absorption coefficients of materials with high iron content scale almost perfectly. The strong UV through near-IR absorption in Fe bearing (Mg,Fe)O originates from the UV band, which red-shifts under pressure (20, 21, 24). This absorption changes dramatically at the spin-pairing transition. Given a nonlinear compositional dependence of the pressure shifts, it is difficult to accept that it originates from the Fe-O charge transfer bands, as has been suggested (1). Indeed, one would expect the same pressure shift for samples with different Fe content and a linear scaling with the Fe content in this case. Moreover, the charge-transfer gap in transition metal oxides is typically wide and almost does not depend on pressure (25).

Fig. 4.

Molar absorptivity (ϵ) for the high- and low-spin magnesiowüstite with different iron content (x). The values of ϵ are calculated from Beer's law A = ϵxd, where A is absorbance and d is the sample thickness. Spectra are shown for the sample with x = 0.06 in the high-spin state at 43 GPa (black) and in the low-spin state at 69 GPa (gray). Spectra are shown for x = 0.15 in the high-spin state at 56 GPa (black) and in the low-spin state at 78 GPa (black dotted curve). The spectra are blue for x = 0.25 in the high-spin state at 56 GPa and pink for x = 0.25 in the low-spin state at 74 GPa. The dashed (red) line represents the calculated blackbody radiation (arbitrary units) at 2400 K, close to lower mantle conditions.

We propose that the bulk of the observed UV-visible absorption in HS-(Mg,Fe)O at iron content above 10 to 12% (fig. S2) originates from charge transfer d-d transitions between adjacent Fe2+ sites (26). The importance of this absorption mechanism is often neglected (18). The strong pressure, spin state, and Fe-concentration dependence of the corresponding excitations, which is governed by the strength of the d-d Coulomb interaction (figs. S3 and S4), can be explained if Fe in magnesiowüstite is randomly distributed in the lattice so that coupling between different iron sites strongly depends on the local environment. In particular, the anomalous behavior of the x = 0.06 sample can be reconciled because this composition is well below the percolation limit (∼12% Fe) for the face-centered cubic lattice (27). Below this limit, the number of adjacent Fe sites is relatively small because of random distribution of Fe2+ ions. When the iron concentration in (Mg,Fe)O is above the percolation threshold, an interconnected network of overlapping (adjacent) 3d orbitals occurs through the structure. Thus, the absorption coefficient and its pressure dependence are greatly enhanced above this limit. Application of pressure broadens the zone formed by the d-d charge-transfer transitions, resulting in a decrease of the observed optical gap. Naturally, this coupling changes at the spin-pairing transition when the ground state changes its spin, which is revealed in our spectra by a change in intensity, shape, and spectral positions (Figs. 1, 3, and 4). A more detailed study on thinner samples (<8-μm thickness) is necessary to reveal fine structure of the pressure and state dependence of the d-d charge-transfer gap. Finally, we note that because the spin-pairing transition is observed from optical absorption spectra (Figs. 1, 2, 3, 4), this study also provides an important cross-check of the HS to LS transition pressure previously inferred from x-ray emission spectroscopy (14, 16) and Mössbauer spectroscopy (17). A plot of the HS to LS transition pressure as a function of composition inferred from the optical spectra is shown in fig. S5.

In contrast to previous notions, the radiative component of thermal conductivity in low-spin (Mg,Fe)O is remarkably low. We invoke a previously overlooked absorption mechanism in low-spin iron-bearing materials, which appears to become dominant at high pressure and results from charge fluctuations between d-orbitals of adjacent Fe sites. Increasing Fe concentration and the spin-pairing transition results in greater absorption in the near-IR range, indicative of reduced radiative conductivity, and therefore the radiative component of thermal conductivity may be blocked. The absorption spectra will modify at high temperatures corresponding to the conditions of Earth's mantle (28), but we do not expect the change of relative intensity in the HS and LS phases because they are determined by the same physical mechanisms.

The notion of reduced thermal conductivity in the lower mantle may challenge existing theories on the stability of superplumes, which appear to require enhanced thermal conductivity to mitigate excessive temperature gradients. It is possible that the reduced radiative component of heat transfer in low-spin phases is compensated by elevated lattice conduction (low-spin phase lattice is stiffer), both of which contribute to the overall thermal conductivity of the lower mantle.

Supporting Online Material

www.sciencemag.org/cgi/content/full/312/5777/1205/DC1

Materials and Methods

Figs. S1 to S5

References

References and Notes

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