Abstract
The inner core, extending to 1,221 km above the Earth’s center at pressures between 329 and 364 GPa, is primarily composed of solid iron. Its rheological properties influence both the Earth’s rotation and deformation of the inner core which is a potential source of the observed seismic anisotropy. However, the rheology of the inner core is poorly understood. We propose a mineral physics approach based on the density functional theory to infer the viscosity of hexagonal close packed (hcp) iron at the inner core pressure (P) and temperature (T). As plastic deformation is ratelimited by atomic diffusion under the extreme conditions of the Earth’s center, we quantify selfdiffusion in iron nonempirically. The results are applied to model steadystate creep of hcp iron. Here, we show that dislocation creep is a key mechanism driving deformation of hcp iron at inner core conditions. The associated viscosity agrees well with the estimates from geophysical observations supporting that the inner core is significantly less viscous than the Earth’s mantle. Such low viscosity rules out inner core translation, with melting on one side and solidification on the opposite, but allows for the occurrence of the seismically observed fluctuations in inner core differential rotation.
Introduction
The dynamical processes of the inner core rely significantly on the viscous strength of iron^{1,2}. Since plastic deformation of iron may produce crystallographic preferred orientations^{3} (CPO), creep is commonly considered to be a potential source contributing to the seismic anisotropy observed in the inner core^{4}. The viscosity of the inner core also influences the rotational dynamics of the Earth^{5}. In spite of its relevance to the Earth’s dynamics, the viscosity of the inner core is poorly constrained. Estimates from geophysical observations of Earth’s nutation^{6} predict inner core viscosities around ~2–7 × 10^{14} Pa s, while values of ~10^{17} Pa s are suggested from observations of polar wander^{5}. An upper bound value of 3 × 10^{17} Pa s was inferred from geodynamic modeling of the gravitational coupling between the core and mantle^{7} in accordance with seismic observations of fluctuations in the rate of the inner core differential rotation. Viscoelastic relaxation experiments^{8} of iron report a viscosity of ~10^{13} Pa s at ambient pressure, but higher pressure and larger grains in the inner core^{9} tend to increase viscous strength. Experimental approaches however require significant extrapolation of transport and flow properties in metals to the inner core condition due to technical difficulties, leading to a wide uncertainty (~10^{11–22} Pa s)^{10,11,12}. On the other hand, recent simulations^{13} predict a viscosity comparable to that of liquid iron in the outer core close to ~10 mPa s, based however on results questioned by Schultz et al.^{14} and inconsistent with observations of normal modes involving the inner core (e.g. Mäkinen & Deuss^{15}), suggesting it to behave as a solid.
Although the stable phase of iron in the inner core is still matter of debate, most experimental studies suggest the hcp phase to be the likely candidate (e.g. Tateno et al.^{16}; Anzellini et al.^{17}). Its viscosity depends on the dominant creep mechanism governing its deformation. Creep rates in metals at T > 0.4T_{m} are strongly controlled by bulk selfdiffusion accommodated by the migration of atomic vacancies^{18}. Here, we quantify the selfdiffusion in hcp iron explicitly, computing all required parameters by the firstprinciples density functional approach, which is a powerful tool to derive lattice defect properties of Earth materials (e.g. Ritterbex et al.^{19}) because of its nonempirical description of atomic bonding. Particularly at the relevant inner core conditions, there is currently no other way to obtain diffusional and rheological properties of iron.
Results
Iron selfdiffusion
Iron in the solid inner core is stable at pressures between ~329 and 364 GPa and temperatures of 5,000–7,000 K^{16,20}. We predict atomic diffusivity of iron at those P,T in the framework of transition state theory^{21} (TST) as previously developed and applied to metals^{22}. Selfdiffusion in metals occurs typically in the intrinsic regime by vacancy migration^{23}. The associated selfdiffusion coefficient D_{sd} can be represented as^{22,23}
where f is the correlation factor to account for possible unsuccessful or backward atomic jumps^{23}, Z_{f} the number of equivalent ways to form a vacancy, Z_{m} the number of equivalent migration paths, l the jump distance, Γ the atomic jump frequency and X the equilibrium point defect concentration given by
where k_{b} is the Boltzmann constant, and ΔH_{f} and ΔS_{f} are the enthalpy and entropy of vacancy formation, respectively. According to TST, the jump frequency \(\varGamma ={\nu }^{\ast }\exp \left(\frac{\Delta {H}_{m}}{{k}_{b}T}\right)\) is represented in terms of the attempt frequency v* and the migration enthalpy ΔH_{m}. All these parameters required are evaluated by firstprinciples total energy calculations, lattice dynamics and electronic structure theory calculations (see Methods).
The formation (ΔH_{f}) and migration (ΔH_{m}) enthalpies of hcp, face centered cubic (fcc) and body centered cubic (bcc) iron are computed as a function of pressure (Fig. 1a,b). The results for bcc Fe at ambient pressure are in good agreement with previous computational studies^{24,25}. Formation enthalpies of hcp and fcc Fe increase monotonously with increasing pressure until ~400 GPa, whereas that of bcc Fe starts decreasing at ~120 GPa (Fig. 1a). Similarly, migration enthalpies of hcp and fcc Fe increase monotonously with pressure, whereas that of bcc Fe starts decreasing over ~20 GPa (Fig. 1b). The anomalous pressure dependency found in bcc Fe results from the tetragonal shear instability at high pressure^{26}. Recent molecular dynamics simulations^{14,20} (MD) demonstrate that pure bcc Fe at inner core pressures remains mechanically unstable up to ~7,000 K and predict that the closepacked structure of pure iron is more stable at inner core conditions. Moreover, our results suggest that the presence of vacancies enhances the mechanical instability of bcc Fe at high pressure. Therefore, we focus on the closepacked structures as the likely polymorph of iron stable in the inner core. Interstitial defects in the closepacked phases of Fe are found to be energetically unfavorable with larger formation enthalpies of ~3.5 eV than those of vacancies at inner core pressure, implying that vacancies are more abundant (Eq. 2) and that selfdiffusion is mainly controlled by the diffusion of vacancies.
Vacancy migration enthalpies are determined by structure relaxation of equilibrium and transition states (see Methods). Results of transition states in hcp Fe are additionally verified by the climbing image nudged elastic band approach^{27} (CINEB) (see Methods and Supplementary Information). Atomic migrations in bcc and fcc Fe are unique and occur along the <111> and <110> directions, respectively, whereas inbasal (parallel to a) and outbasal plane (along c) diffusion are possible in hcp Fe. The energy barrier of outbasal plane diffusion at 320 GPa from structure relaxation (3.17 eV) is in good agreement with the one obtained by the CINEB approach (3.29 eV) (Supplementary Fig. 1). Figure 1b shows that atomic diffusion in hcp Fe becomes slightly anisotropic at higher pressures with a difference in ΔH_{m} up to ~0.2 eV at 360 GPa in favor of outbasal plane diffusion, reaching ~6% of the total migration enthalpy at 360 GPa. Since the lowest energy diffusion path is most favorable, selfdiffusion in hcp Fe is considered to occur more easily through the outbasal plane path.
The activation volumes for selfdiffusion \({V}_{A}=\partial H/\partial P\) are found to decrease significantly at larger compressions in the closepacked phases of iron (Fig. 1c). Previous experiments^{11} report a constant V_{A} of 2.6 cm^{3} mol^{−1} for FeNi interdiffusion in an fcc iron alloy up to 60 GPa, in fair agreement with our results. At inner core pressures however, the V_{A} is significantly smaller and only ~60% of V_{A} at P < 120 GPa. The nearly constant V_{A} in closepacked iron up to ~120 GPa followed by a significant decrease at larger compression suggests that the selfdiffusivities derived at low pressures cannot be extrapolated to the inner core condition by using a constant V_{A}. It is worth mentioning that the magnitude and pressure dependencies of defect energetics in hcp and fcc Fe are comparable (Fig. 1), implying that their selfdiffusivities (Eq. 1) might be similar even up to the pressures of the inner core.
A combination of lattice dynamics (LD) theory and electronic structure theory are adopted to determine the entropic and vibrational contributions to the diffusion coefficient (Eq. 1) of closepacked Fe in the quasiharmonic approximation (QHA). These thermodynamic properties are derived from the Helmholtz free energy F(V, T) as
where E is the static energy as a function of volume V, F_{vib} and F_{el} the vibrational and electronic contributions to the free energy, and S_{conf} and S_{mag} the configurational and magnetic entropy, with the latter being only considered for fcc Fe at 0 GPa since nonmagnetic states become energetically favorable with increasing pressure (see Methods). The temperature effects on all diffusion parameters are determined based on the calculated equations of state (EoS). Free energies of defectfree closepacked Fe are computed at five volumes with lattice parameters varying by 1.5% to determine the EoS (Supplementary Fig. 2). Phonon frequencies of all systems are obtained by the direct force constant approach^{28} to determine the contribution of F_{vib} and the attempt frequencies v* which are estimated from the maximum frequencies of the phonon spectra^{29} (see Supplementary Information). Migration enthalpies ΔH_{m} and the Gibbs free energies of vacancy formation ΔG_{f} are calculated corresponding to the equilibrium volumes of closepacked Fe at the P,T conditions of interest, with ΔG_{f} defined as
where G(N − 1) and G(N) correspond to the Gibbs free energy of a defective and a defectfree system with N atoms, respectively and ΔS_{f} the total entropy of vacancy formation. The Gibbs free energy of vacancy formation ΔG_{f} in hcp Fe is found to be only ~80% of the formation enthalpy ΔH_{f} at the inner core temperature (Table 1). This emphasizes on the importance of considering correctly the contribution of ΔS_{f} to the total Gibbs free energy of vacancy formation at the inner core temperature. To benchmark our computational approach, selfdiffusion of fcc Fe is calculated at ambient pressure close to the melting temperature T_{m} to compare with experimental results. The diffusion coefficients (Eq. 1) of closepacked Fe are obtained using the computed diffusion parameters (Table 1) after applying a thermal pressure correction at each temperature according to the appropriate EoS. Results for fcc Fe at ambient pressure near T_{m} are in excellent agreement with experimental results^{30,31,32} (Fig. 2). The latter shows that atomic diffusivity of fcc Fe is well predicted within the QHA even close to T_{m}, indicating negligible higherorder anharmonic effects on the diffusion coefficients other than the QHA level anharmonicity. This was also shown in other metals^{22} and provide support that atomic diffusivity might be well predicted within the QHA at inner core conditions. The melting temperature of hcp Fe at the inner core pressure is still not well constrained and commonly considered between 6,000–7,000 K^{33}. Although the temperature at the inner core boundary (ICB) is expected to be lower than the T_{m} of pure iron due to its depression by alloying with light elements, the T_{m}/T ratio of the inner core is commonly considered to be ~1.15–1.05, corresponding to a typical diffusion coefficient of hcp Fe of ~10^{−16}–10^{−17} m^{2}s^{−1} (Fig. 2).
Iron creep
Since bulk diffusion is dominant close to T_{m}, we inferred the contribution of diffusion creep to the plasticity of hcp Fe by NabarroHerring creep^{34,35} (see Supplementary Information). The present diffusion parameters of hcp Fe combined with an inner core grain size in the order of meters, estimated by previous work^{9}, leads to a high viscosity of ~10^{26} Pa s, ruling out diffusion creep as an efficient strain producing mechanism (Supplementary Fig. 5). Moreover, this mechanism is not able to produce CPO, being incompatible with the presence of a strong seismic anisotropy observed in the inner core^{4}. CPO is commonly produced during dislocation creep of iron at high pressure^{3}. Near T_{m}, dislocation creep is expected to be climbcontrolled since diffusion is strongly facilitated^{18}. This together with considerations of large grains^{9} has led to the suggestion that HarperDorn creep controls deformation of the inner core^{10}, but its mechanism cannot be fully understood from experiments and its existence has been subject to debate^{36}. Yet, the ratelimiting creep behavior of metals at T > 0.4T_{m} can be predicted with climbcontrolled dislocation creep models proposed by Weertman^{37} and Nabarro^{38}. Weertman’s model assumes that the glide velocity (v_{g}) of dislocations is much larger than that of climb (v_{c}) at high homologous temperature close to melting, due to low lattice friction. Its constitutive equation describing viscous flow in the limit of low stress can be derived as (see Methods)
where \(\dot{\varepsilon }\) is the strain rate, σ the flow stress, A(σ) a stress dependent dimensionless parameter depending on the climb distance d between glide planes, b the modulus of the Burgers vector and μ the shear modulus. If, however glide would be slower than climb (v_{g} < v_{c}) in hcp Fe at the inner core P,T, plastic strain may be produced exclusively by pure climb as proposed by Nabarro^{38}, in contrast to Weertman creep where strain is mainly produced by glide. This mechanism, known as pure climb creep, is described by the following constitutive equation^{38}
The computed diffusion parameters of hcp Fe are used to parametrize the constitutive Eqs. 5 and 6 at the inner core P,T and a range of relevant steadystate strain rates between 10^{−14}–10^{−18} s^{−1}, typical for potential inner core convection processes^{1}. We employ a shear modulus μ = 212 GPa of hcp Fe^{39} and assume basal slip to dominate glide in hcp Fe^{3}, i.e. b = a and d = c/2. Results are presented in a deformation mechanism map (Fig. 3). At the inner core temperature ~5,500 K, Weertman creep is the most efficient mechanism operating at typical stresses ~1–100 Pa compared to ~0.01–0.1 MPa required to activate pure climb creep. The associated viscosities η are determined as \(\sigma /2\dot{\varepsilon }\) and correspond to ~10^{16}–10^{18} and ~10^{19}–10^{22} Pa s for Weertman and pure climb creep, respectively. The key unknown is the lattice friction opposed to dislocation glide in hcp Fe at inner core conditions. Commonly, lattice friction in metals at T_{m}/T ~ 1.1 is low so that \({v}_{g}\gg {v}_{c}\), activating Weertman creep^{37}. Also, in absence of lattice friction, mobile dislocations can glide freely under the action of seismic stress and induce seismic attenuation^{8}. Indeed, normal mode studies provide evidence of substantial seismic attenuation in the inner core^{15}, arguing for low lattice friction of Fe close to T_{m}. Moreover, recent deformation experiments^{12} of hcp Fe inferred that stresses of ~10 Pa are required to activate glide at low strain rates (\(\dot{\varepsilon } \sim {10}^{18}\) s^{−1}) and inner core P,T. This is comparable to the stress needed for Weertman creep to operate (Fig. 3) and provide support for sufficiently low lattice friction in hcp Fe to activate dislocation creep. It is therefore likely that Weertman creep governs plastic flow of hcp Fe in the inner core, unless glide would be hampered by a limited availability of slip systems (i.e. not satisfying the von Mises’ criterion)^{40} and deformation is forced to occur by pure climb creep, leading to a significantly more viscous inner core (Fig. 3). Alternatively, twinning or grain boundary sliding (GBS) may ensure plastic flow in hcp Fe if the von Mises’ criterion is not satisfied^{40,41}. Those mechanisms rely on intracrystalline plasticity as dislocation creep to maintain macroscopic continuity. This also supports that Weertman creep might play a ratecontrolling role in the plasticity of hcp Fe at inner core conditions leading to a viscosity of ~10^{17±1} Pa s (Fig. 3).
Discussion on the dynamics of Earth’s inner core
Our findings support geophysical estimates^{5,6} of an inner core which is significantly less viscous than the mantle (~10^{21}–10^{24} Pa s)^{42}, but substantially more viscous than the outer core (~10 mPa s)^{13}. The relatively low viscosity of ~10^{17±1} Pa s of hcp iron at inner core conditions inferred from our mineral physics approach is in line with the recent seismic observations of Jwaves which also point towards a readily deforming inner core^{43}. However, the results presented correspond to the viscosity of pure hcp Fe, neglecting the discrepancy between the observed inner core density and that of hcp Fe at the appropriate conditions^{44}. This discrepancy can be explained by the presence of a small amount of melt^{44} or by the stabilization of bcc Fe due to alloying with some light elements such as Si^{26}. The bcc phase of Fe is expected to be less viscous than the hcp phase because of tetragonal shear weakening at inner core pressure^{26}. Thus, the potential presence of melt or bcc Fe might lead to a decrease in viscous strength with respect to pure hcp Fe. This implies that the inner core could be even less viscous than ~10^{17±1} Pa s. In addition, alloying Fe with light elements might influence its mechanical strength by affecting dislocation multiplication and annihilation processes through changes in the glide and climb mobilities, although this is not well understood yet under the relevant P,T and extremely low strain rate conditions of the inner core. Nevertheless, the inferred inner core viscosity fairly agrees with estimates from geodetic observations^{5}. Furthermore, the inner core viscosity is a crucial parameter determining the rotational dynamics of the inner core. Although it has been shown that a steady rate of inner core superrotation should be negligibly small^{45}, the inner core is expected to undergo fluctuations in its rotation rate with amplitudes of 0.1–1° yr^{−1} at timescales of decades to a century^{46}. To ensure that the gravitational torque exerted on the mantle by an oscillating inner core does not exceed the observed lengthofday changes, it is required that \(\Gamma \,{\tau }\lesssim 2\times {10}^{20}\) N m yr, where Γ is a measure of the gravitational strength between the mantle and the inner core and τ the viscous relaxation time of the inner core^{47}. An upper bound value of τ between 1–6 yr is found, based on the latest estimates of Γ from geodynamic modeling^{7}, corresponding to an inner core viscosity of 0.5 − 3 × 10^{17} Pa s^{48}, which falls in the range of values derived from our mineral physics approach. An inner core, gravitationally coupled with the mantle, which is much stiffer or weaker than ~10^{17} Pa s would generate larger fluctuations in the rate of inner core rotation than those observed. Our inferred viscosities are thus consistent with findings of the seismically observed small fluctuations in the inner core rotation rate.
Previous geodynamic modeling^{2,49} show that the viscosity derived from our approach might be too low to account for inner core translation, which is one of the hypotheses to explain the hemispherical patterns of seismic anisotropy in the inner core^{50}. Instead, if the viscosity of the inner core is lower than ~3 × 10^{18} Pa s, these modeling predict that its dynamics is rather governed by large scale convection. Indeed, our modeling predicts that stresses of tens of Pa are required to deform hcp Fe by Weertman creep at low steadystate strain rates (~10^{−16} s^{−1}) which are comparable to the potential driving forces required to initiate various candidates of inner core convection^{1,51,52} supporting that dislocation creep might be a dominant deformation mechanism governing the dynamics of the Earth’s inner core. Since dislocation creep leads to the formation of CPO in hcp metals^{3}, it can be expected that plastic deformation of hcp Fe contributes to the observed seismic anisotropy in the inner core. It is finally worth mentioning that dislocation creep exhibits a nonNewtonian rheology which might be important to consider in future geodynamic modeling of the inner core dynamics.
Methods
Firstprinciples electronic structure calculations
Our computation method relies on firstprinciples density functional techniques with the generalized gradient approximation (GGA) applied for the exchangecorrelation energy^{53,54}. Static relaxations of all structure models were performed based on the PlaneWave SelfConsistent Field code with the planewave and pseudopotential methods implemented in the Quantum ESPRESSO package^{55}. Ultrasoft pseudopotentials^{56} are used to describe the effective core potential of Fe with electronic configurations of 3s^{2}3p^{6}3d^{6.5}4s^{1}4p^{0}. Pseudowavefunction and valence electron density are represented by the planewave basis set with a cutoff energy of 50 Ry. We further apply the FermiDirac smearing with an energy width of 0.002 Ry to enhance selfconsistent convergence. All simulations are conducted using periodic boundary conditions. We employ a supercell approach to minimize the elastic interactions between defects in periodic replica, with defective supercells containing one point defect at a time. The size of supercells is sufficiently large to impose a convergence of the vacancy formation enthalpies better than 0.02 eV to avoid the need of elastic energy corrections. We use defectfree supercells containing 3 × 3 × 3 conventional cells of fcc (108 atoms) and 4 × 4 × 4 of bcc (128) iron. An orthorhombic supercell (108 atoms) was constructed out of the primitive cell of hcp iron. Structure relaxation of perfect and defective supercells were performed at constant volume (V) with a large Brillouin zone kpoint sampling on a 4 × 4 × 4 MonkhorstPack grid^{57} for fcc and bcc Fe and a 6 × 4 × 4 MonkhorstPack grid for hcp Fe to obtain full convergence of the electronic configurations until residual forces were minimized below 1.0 × 10^{−5} Ry/au. Further increase in supercell size did not significantly affect vacancy formation energies. Spin polarization is only considered for bcc iron (all pressures) and fcc iron at 0 GPa, since nonmagnetic states become energetically favorable with increasing pressure^{26}. We find that the effect of spin polarization on the defect energetics in closepacked iron becomes insignificant above ~30 GPa.
Defect energetics
Total energy calculations are conducted based on the constant pressure approach, so that total enthalpy is of concern. The point defect formation enthalpy is generally derived as
where the negative and positive sign corresponds to vacancy and interstitial formation, respectively, H(N) is the enthalpy of a defectfree supercell containing N atoms and H(N ± 1) is the enthalpy of a supercell containing a single point defect.
The energy barrier of vacancy migration ΔH_{m} is defined as the enthalpy difference between its equilibrium (H_{eq}) and transition state (H_{sp}), when the migrating atom is located at its saddle point as
In simple metallic systems such as fcc, bcc and hcp iron, saddle point configurations are constrained by the crystal symmetry to the middle between two nearest neighbor halfvacancies. Because of the lattice symmetry in bcc and fcc Fe, H_{sp} can be obtained by unconstrained structure relaxation of transition states. In the hcp phase, at least two atoms far from the vacancy need to be constrained during structure optimization. To verify the outcome of this approach, we performed CINEB calculations^{27} to find the minimum energy path (MEP) and the corresponding energy barrier of migration. The MEP is sampled using 7 images bonded by springs relying on the variable elastic constants scheme implemented in the Quantum ESPRESSO package. The initial and final configurations correspond to fully relaxed defective supercells with a vacancy at its equilibrium lattice site. Force minimization relies on linear interpolation between the initial and final configurations until the modulus of the force orthogonal to the path becomes less than 0.02 eV/Å. CINEB calculations are performed using a constant volume approach with the MEP obtained in terms of internal energy.
Thermodynamic properties
Thermodynamic properties of the Fe systems are determined in the framework of lattice dynamics (LD) and electronic structure theory combined with the quasiharmonic approximation (QHA). The LD calculations are performed based on the direct force constant approach^{28}. Phonon frequencies v_{i} of supercells are computed by diagonalization of dynamical matrices using the PHONOPY code^{58} where force constants are generated using the finite displacement method. Atomic forces are determined via electronic structure calculations of relaxed supercells with displacements of 0.01 Å applied to all atoms around their equilibrium positions. Since vacancies break the original lattice symmetry, defective supercells of hcp and fcc Fe (107 atoms) require up to 642 displacements to build a single force constant matrix.
The EoS (Supplementary Fig. 2) and other thermodynamic properties (Supplementary Fig. 3) of hcp and fcc Fe are derived from the Helmholtz free energy (Eq. 3) using standard thermodynamic relationships (e.g. Tsuchiya^{59}). The vibrational contribution to the Helmholtz free energy F is computed as
The contribution of Eq. 9 was evaluated on a 10 × 10 × 10 and a 12 × 10 × 10 qgrid for fcc and hcp Fe, respectively. For defective systems, ΔS_{conf} is approximated by the configurational entropy S_{conf} of an ideal solution with vacancy concentration X as
The electronic contributions to the free energy are evaluated as
with the electronic entropy given by
where γ is 1 or 2 for spin polarized or unpolarized systems, respectively. The FermiDirac distributions f_{i} are computed as function of the energies ε_{i} from the electronic density of states (eDoS). The magnetic contribution S_{mag} to the total entropy is evaluated as
with total spin quantum number S = 2 and orbital degeneracy m = 3.
Dislocation creep: the Weertman model
Weertman creep assumes that the glide velocity v_{g} of dislocations exceeds the velocity of dislocation climb v_{c} at high homologous temperature (v_{g} > v_{c}) such as close to melting^{37}. The average dislocation velocity v can then be approximated by v = L/t_{c}, where L is the dislocation line length and t_{c} = d/v_{c} the time needed to climb a distance d between the glide planes. Assuming that the rate of strain \(\dot{\varepsilon }\) produced by creep can be described in terms of Orowan’s equation \(\dot{\varepsilon }={\rho }_{m}bv\), where ρ_{m} is the density of mobile dislocations and b the modulus of the Burgers vector, Weertman’s constitutive law can be easily derived as
The dislocation length L typically scales with the total dislocation density ρ_{t} as \(1/\sqrt{{\rho }_{t}}\). We assume that all dislocations are partially mobile close to T_{m}, i.e. ρ_{t} = ρ_{m}. The climb velocity v_{c} can be represented by^{60}
where A_{c} is a geometrical factor and X and X_{∞} are the equilibrium vacancy concentrations in the bulk and far from the dislocation lines, respectively. The vacancy concentration far from the dislocation is supposed to be equal to that of the bulk (X_{∞} = X), given a cylindrical dislocation geometry \({A}_{c}=2\pi /\,\mathrm{ln}(1/2\sqrt{{\rho }_{t}}{r}_{c})\) and a dislocation core radius r_{c}, typically ~10 Å. Based on the line tension, we use the Taylor relationship \({\rho }_{m}={(\sigma /\alpha \mu b)}^{2}\) to relate the mobile dislocation density to the applied stress^{61} and obtain the constitutive Eq. 5 in the limit of low stress, where \(A={A}_{c}L{V}_{A}/{\alpha }^{2}{b}^{3}d\), and α a dimensionless parameter equal to ~0.1 under the assumption that the obstacle strength is predominantly governed by dipole interactions^{62}. We like to note that the steadystate dislocation creep behavior derived here applies to the limiting case of high homologous temperature and low stress corresponding to the conditions of the inner core. Different temperature and stress conditions might affect the dislocation multiplication and annihilation processes, leading to the development of other microstructures, giving rise to different stress exponents.
References
Sumita, I. & Bergman, M. I. Inner Core Dynamics. In: Treatise on Geophysics (ed. Schubert, G.), 299–318 (Elsevier, 2007).
Deguen, R., Alboussière, T. & Cardin, P. Thermal convection in Earth’s inner core with phase change at its boundary. Geophys. J. Int. 194, 1310–1334 (2013).
Nishihara, Y. et al. Deformationinduced crystallographicpreferred orientation of hcpiron: An experimental study using a deformationDIA apparatus. Earth Planet. Sci. Lett. 490, 151–160 (2018).
Creager, K. C. Anisotropy of the inner core from differential travel times of the phases PKP and PKIKP. Science 356, 309–314 (1992).
Dumberry, M. & Bloxham, J. Inner core tilt and polar motion. Geophys. J. Int. 151, 377–392 (2002).
Koot, L. & Dumberry, M. Viscosity of the Earth’s inner core: Constraints from nutation observations. Earth Planet. Sci. Lett. 308, 343–349 (2011).
Davies, C. J., Stegman, D. R. & Dumberry, M. The strength of gravitational coremantle coupling. Geophys. Res. Lett. 41, 3786–3792 (2014).
Jackson, I., Fitz Gerald, J. D. & Kokkonen, H. Hightemperature viscoelastic relaxation in iron and its implications for the shear modulus and attenuation of the Earth’s inner core. J. Geophys. Res. 105, 23605–23634 (2000).
Bergman, M. Estimates of the Earth’s inner core grain size. Geophys. Res. Lett. 25, 1593–1596 (1998).
Van Orman, J. A. On the viscosity and creep mechanism of Earth’s inner core. Geophys. Res. Lett. 31, L20606 (2004).
Reaman, D. M., Colijn, H. O., Yang, F., Hauser, A. J. & Panero, W. R. Interdiffusion of Earth’s core materials to 65 GPa and 2200 K. Earth Planet. Sci. Lett. 349350, 8–14 (2012).
Gleason, A. E. & Mao, W. L. Strength of iron at core pressures and evidence for a weak Earth’s inner core. Nature Geoscience 6, 571–574 (2013).
Belonoshko, A. B., Fu, J., Bryk, T., Simak, S. I. & Mattesini, M. Low viscosity of the Earth’s inner core. Nature Communications 10, 2483 (2019).
Schultz, A. J., Moustafa, S. G. & Kofke, D. A. No systemsize anomalies in entropy of bcc iron at Earth’s innercore conditions. Sci. Rep. 8, 7295 (2018).
Mäkinen, A. M. & Deuss, A. Normal mode splitting function measurements of anelasticity and attenuation in the Earth’s inner core. Geophys. J. Int. 194, 401–416 (2013).
Tateno, S., Hirose, K., Ohishi, Y. & Tatsumi, Y. The Structure of Iron in the Earth’s Inner Core. Science 330, 359–361 (2010).
Anzellini, S., Dewaele, A., Mezouar, M., Loubeyre, P. & Morard, G. Melting of Iron at Earth’s Inner Core Boundary Based on Fast Xray Diffraction. Science 340, 464–466 (2013).
Martin, J. L. & Caillard, D. Thermally Activated Mechanisms in Crystal Plasticity (Pergamon, New York, 2003).
Ritterbex, S., Harada, T. & Tsuchiya, T. Vacancies in MgO at ultrahigh pressure: About mantle rheology of superEarths. Icarus 305, 350–357 (2018).
Godwal, B. K., GonzálezCataldo, F., Verman, A. K., Stixrude, L. & Jeanloz, R. Stability of iron crystal structures at 0.31.5 TPa. Earth Planet. Sci. Lett. 409, 299–306 (2015).
Eyring, H. The Activated complex in Chemical Reactions. J. Chem. Phys. 3, 107–115 (1935).
Mantina, M., Wang, Y., Arroyave, R., Chen, L. Q. & Liu, Z. K. FirstPrinciples Calculations of SelfDiffusion Coefficients. Phys. Rev. Lett. 100, 215901 (2008).
Tilley, R. J. D. Understanding Solids: The Science of Materials (John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, pp. 215, 2004).
Mendelev, M. & Mishin, Y. Molecular dynamics study of selfdiffusion in bcc Fe. Phys. Rev. B 80, 144111 (2009).
Sandberg, N., Chang, Z., Messina, L., Olsson, P. & Korzhavyi, P. Modeling of the magnetic free energy of selfdiffusion in bcc Fe. Phys. Rev. B 92, 184102 (2015).
Tsuchiya, T. & Fujibuchi, M. Effects of Si on the elastic property of Fe at Earth’s inner core pressures: First principles study. Phys. Earth Planet. Int. 174, 212–219 (2009).
Henkelman, G., Uberuaga, B. P. & Jónsson, H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 113, 9901–9904 (2000).
Wei, S. & Chou, M. Y. Ab Initio Calculation of Force Constants and Full Phonon Dispersion. Phys. Rev. Lett. 69, 2799–2802 (1992).
Runevall., O. & Sandberg, N. Selfdiffusion in MgO – a density functional study. J. Phys. Condens. Matter 23, 345402 (2011).
Buffington, F. S., Hirano, K. & Cohen, M. Self diffusion in iron. Acta Metallurgica 9, 434–439 (1961).
Goldstein, J. J., Hanneman, R. E. & Ogilvie, R. G. Diffusion in the FeNi system at 1 atm and 40 kbar pressure. Trans. AIME 233, 812–829 (1965).
Brown, A. M. & Ashby, M. F. Correlations for diffusion constants. Acta Metallurgica 28, 1085–1101 (1980).
Alfè, D. Temperature of the innercore boundary of the Earth: Melting of iron at high pressure from firstprinciples coexistence simulations. Phys. Rev. B 79, 060101 (2009).
Nabarro, F. R. N. Report of a Conference on Strength of Solids. Phys. Soc. London, 75–90 (1948).
Herring, C. Diffusional Viscosity of a Polycrystalline Solid. J. Appl. Phys. 21, 437–455 (1950).
Blum, W., Eisenlohr, P. & Breutinger, F. Understanding creep – A review. Metal. Mater. Trans. A 33A, 291–303 (2002).
Weertman, J. Steadystate creep through dislocation climb. J. Appl. Phys. 28, 362–364 (1957).
Nabarro, F. R. N., Steadystate Diffusional Creep. Philos. Mag. A 16, 231–237 (1967).
Vočadlo, L., Dobson, D. P. & Wood, I. G. Ab initio calculations of the elasticity of hcpiron as a function of temperature at innercore pressure. Earth Planet. Sci. Lett. 288, 534–538 (2009).
Tegart, W. J. M. Independent slip systems and ductility of hexagonal polycrystals. Phil. Mag. 9, 339–341 (1964).
Bergman, M. I., Yu, J., Lewis, D. J. & Parker, G. K. Grain Boundary Sliding in HighTemperature Deformation of Directionally Solidified hcp Zn Alloys and Implications for the Deformation Mechanism of Earth’s inner Core. J. Geophys. Res. 123, 189–203 (2018).
Mitrovica, J. X. & Forte, A. M. A new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data. Earth Planet. Sci. Lett. 225, 177–189 (2004).
Tkalčić, H. & Pham, T.S. Shear properties of Earth’s inner core constrained by a detection of J waves in global correlation wavefield. Science 362, 329–332 (2018).
Vočadlo, L. Ab initio calculations of the elasticity of iron and iron alloys at inner core conditions: Evidence for a partially molten inner core? Earth Planet. Sci. Lett. 254, 227–232 (2007).
Aubert, J. & Dumberry, M. Steady and fluctuating inner core rotation in numerical geodynamo models. Geophys. J. Int. 184, 162–170 (2011).
Tkalčić, H., Young, M., Bodin, T., Ngo, S. & Sambridge, M. The shuffling rotation of the Earth’s inner core revealed by earthquake doublets. Nature Geoscience 6, 497–502 (2013).
Dumberry, M. & Mound, J. Inner coremantle gravitational locking and the superrotation of the inner core. Geophys. J. Int. 181, 806–817 (2010).
Buffett, B. A. Geodynamic estimates of the viscosity of the Earth’s inner core. Nature 388, 571–573 (1997).
Lasbleis, M. & Deguen, R. Building a regime diagram for the Earth’s inner core. Phys. Earth Planet. Int. 247, 80–93 (2015).
Deuss, A. Heterogeneity and anisotropy of the Earth’s inner core. Annual Review of Earth and Planetary Sciences 42, 103–126 (2014).
Karato, S.I. Seismic anisotropy of the Earth’s inner core resulting from flow induced by Maxwell stresses. Nature 402, 871–873 (1999).
Yoshida, S., Sumita, I. & Kumazawa, M. Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy. J. Geophys. Res. 101, 28085–28103 (1996).
Hohenberg, P. & Kohn, W. Inhomogeneous electron gas. Phys. Rev. 136, B864–B871 (1964).
Kohn, W. & Sham, L. J. Quantum density oscillations in an inhomogeneous electron gas. Phys. Rev. 137, A1697–A1705 (1965).
Giannozzi, P. et al. QUANTUM ESPRESSO: a modular and opensource software project for quantum simulations of materials. J. Phys. Condens Matter. 21, 395502 (2009).
Vanderbilt, D. Soft selfconsistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 41, 7892–7895 (1990).
Monkhorst, H. J. & Pack, J. D. Special points for Brillouinzone integrations. Phys. Rev. B 13, 5188–5192 (1976).
Togo, A. & Tanaka, I. First principles phonon calculations in materials science. Scr. Mater. 108, 1–5 (2015).
Tsuchiya, T. Firstprinciples prediction of the PVT equation of state of gold and the 660km discontinuity in Earth’s mantle. J. Geophys. Res. 108, 246219 (2003).
Boioli, F., Carrez, P., Cordier, P., Devincre, B. & Marquille, M. Modeling the creep properties of olivine by 2.5dimensional dislocation dynamics simulations. Phys. Rev. B 92, 014115 (2015).
Sauzay, M. & Kubin, L. Scaling laws for dislocation microstructures in monotonic and cyclic deformation of fcc metals. Progress Mater. Sci. 56, 725–784 (2011).
Haasen, P. Dislocation Dynamics in the Diamond Structure (McGraw Hill, New York, pp. 701 and 718, 1968).
Dorogokupets, P. I., Dymshits, A. M., Litasov, K. D. & Sokolova, T. S. Thermodynamics and Equations of State of Iron to 350 GPa and 6000 K. Sci. Rep. 7, 41863 (2017).
Acknowledgements
This work was supported by MEXT KAKENHI Grant Number JP15H05834 and JP15K21712. Calculations are performed on the parallel computation system at Geodynamics Research Center, Ehime University, Japan. We would like to thank M. Dumberry for his constructive comments on the inner core dynamics during the final writing of the manuscript.
Author information
Authors and Affiliations
Contributions
S. Ritterbex designed the study and performed the calculations in collaboration with T. Tsuchiya. All authors discussed and interpreted the results and contributed to the writing of the manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary information
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Ritterbex, S., Tsuchiya, T. Viscosity of hcp iron at Earth’s inner core conditions from density functional theory. Sci Rep 10, 6311 (2020). https://doi.org/10.1038/s41598020631666
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41598020631666
Further reading

Gravity Variations and Ground Deformations Resulting from Core Dynamics
Surveys in Geophysics (2022)

Dynamic history of the inner core constrained by seismic anisotropy
Nature Geoscience (2021)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.