Research 

As a noble gas, helium (He) is chemically inert, rarely forming compounds with other elements. Via experiments and calculations, however, we demonstrated for the first time that helium can react with molten iron (Fe) under high pressure (P) and temperature (T) to form several stable compounds. At the University of Tokyo, a team led by Professor Kei Hirose compressed a sample containing Fe and He using a laser-heated diamond anvil cell, in which high P–T conditions was produced (up to 54 GPa and 2820 K). Through synchrotron X-ray diffraction, they observed that the unit-cell volume of the Fe lattice in the sample was significantly larger than that of pure Fe under the same P–T conditions. Such volume expansion was attributed to the incorporation of He atoms into the Fe lattice, a hypothesis later supported by secondary-ion mass spectrometry analysis conducted at Hokkaido University. At NCU, we investigated the Fe-He compounds via first-principles calculations, confirming their dynamical stability and revealing their atomic structures, magnetic properties, and bonding characteristics. Our results provide evidence to support the long-standing hypothesis that the Earth’s core contains a reservoir of primordial 3He. For further details, see Phys. Rev. Lett. 134, 084101 (2025) or its reprint on the Publications page. 

Iron-bearing magnesium oxide with the B1 (NaCl-type) structure, also known as ferropericlase (Mg1–xFex)O (with 0.1 < x < 0.2), is the second most abundant mineral in the Earth's lower mantle (depth 660–2890 km, pressure range 23–135 GPa), constituting ~20 vol% of this region. Experiments and computations have shown that B1 MgO remains stable up to ~0.5 TPa and transforms into the B2 (CsCl-type) structure upon further compression. First-principles calculations also predict that B2 MgO remains dynamically stable up to at least ~4 TPa. Based on these findings and the abundance of iron, (Mg1–xFex)O has been considered a major constituent of terrestrial super-Earths (exoplanets with up to ~10 times of the Earth's mass), where the interior pressure can reach to the tera-Pascal regime. In the Earth's lower mantle, B1 (Mg1–xFex)O undergoes a high-spin (HS, S = 2) to low-spin (LS, S = 0) transition at ~45 GPa, accompanied by anomalous changes of this mineral's physical properties, while the intermediate-spin (IS, S = 1) state has not been observed. In this work, we investigate (Mg1–xFex)O (x ≤ 0.25) up to 1.8 TPa via first-principles calculations. Our calculations indicate that (Mg1–xFex)O undergoes a simultaneous structural and spin transition at ~0.6 TPa, from the B1 phase LS state to the B2 phase IS state. Remarkably, Fe's total electron spin (S) re-emerges from 0 to 1 at ultrahigh pressure, against the tenet that spin/magnetization is suppressed by pressure. Upon further compression, an IS–LS spin transition occurs in the B2 phase. Depending on the Fe concentration (x), metal–insulator transition and rhombohedral distortions can also occur in the B2 phase. These results suggest that Fe and spin transition may affect the transport, thermodynamic, and elastic properties planetary interiors over a vast pressure range. For further details, see Nat. Commun. 13, 2780 (2022) or its reprint on the Publications page. 

Ferromagnesite [(Mg1–xFex)CO3] is believed to enter the Earth’s lower mantle via subduction and is considered a major carbon carrier in the Earth’s lower mantle, playing a key role in the Earth’s deep carbon cycle. Both experiments and theory have shown that ferromagnesite undergoes a pressure-induced spin crossover, from the high-spin (HS, S = 2) to the low-spin (LS, S = 0) state, in the lower-mantle pressure range. Here we investigate thermal properties of (Mg1–xFex)CO3 (0 < x ≤ 1) via first-principles calculations. Our calculations indicate that nearly all thermal properties of (Mg1–xFex)CO3 are drastically altered by iron spin crossover, including anomalous reduction of volume, anomalous softening of bulk modulus, and anomalous increases of thermal expansion, heat capacity, and the Grüneisen parameter. Remarkably, the anomaly of heat capacity remains prominent at high temperature without smearing out, which suggests that iron spin crossover may significantly affect the thermal properties of subducting slabs and the Earth’s deep carbon cycle. For further details, see Phys. Rev. B 103, 054401 (2021) or its reprint on the Publications page. 

In experiments, strontium cobaltite (SrCoO3) has been confirmed to be a ferromagnetic metal (Curie temperature TC ≈ 305 K) at ambient conditions and remains in cubic perovskite structure up to ~60 GPa. Using local density approximation + self-consistent Hubbard U (LDA+Usc) calculations, we show that ferromagnetic metallic (FMM) SrCoO3 at low pressure is in an intermediate-spin (IS) state with d6L character: nearly trivalent (Co3+) instead of tetravalent cobalt (Co4+) accompanied by spin-down O-2p electron holes (ligand holes L). Our calculations further predict that upon compression (~7 GPa), SrCoO3 undergoes a transition to a low-spin (LS) ferromagnetic half-metal (FMHM) with an energy gap opened in the spin-up channel. Compared to the FMM (IS) state, the FMHM (LS) state exhibits even more prominent d6L character, including nearly nonmagnetic Co3+ and exceptionally large oxygen magnetic moments, which contribute most of the magnetization. By analyzing x-ray diffraction data of compressed single-crystal SrCoO3, we point out an anomalous volume reduction of ~1%. This previously unnoticed volume anomaly is in great agreement with our predictive calculations, providing quantitative evidence for the simultaneous metal–half-metal and spin transition in SrCoO3. In the above figure, panels (a)-(c) show the projected density of states (PDOS) of the FMM (IS), FMHM (LS), and antiferromagnetic (AFM) high-spin (HS) states, respectively; (d)-(f) show the electron spin density of these states, where yellow and cyan lobes denote for the isovalue surfaces of 0.03 and –0.03 a.u.-3. For further details, see Phys. Rev. Materials 2, 111401(R) (2018) or its reprint on the Publications page. 

The new hexagonal aluminous (NAL) phase is considered as a major component (~20 vol%) of mid-ocean ridge basalt (MORB) under the lower-mantle condition. As MORB can be transported back into the Earth’s lower mantle via subduction, a thorough knowledge of the NAL phase is essential to fully understand the fate of subducted MORB and its role in mantle dynamics and heterogeneity. In this work, iron spin crossover in the NaMg2(Al5Si)O12 NAL phase is revealed by a series of local density approximation + self-consistent Hubbard U (LDA+Usc) calculations. Only the ferric iron (Fe3+) substituting Al/Si in the octahedral site undergoes a crossover from the high-spin (HS) to the low-spin (LS) state at ~40 GPa, while iron substituting Mg in the trigonal-prismatic site remains in the HS state, regardless of its oxidation state (Fe2+ or Fe3+). The volume/elastic anomalies and the iron nuclear quadrupole splittings (QSs) determined by calculations are in great agreement with room-temperature experiments. The calculations further predict that the HS-LS transition pressure of the NAL phase barely increases with temperature due to the three nearly degenerate LS states of Fe3+ (mLS ≈ 3), suggesting that the elastic anomalies of this mineral can occur at the top lower mantle. For further details, see Phys. Rev. B 95, 020406(R) (2017) or its reprint on the Publications page.

(Mg1–xFex)CO3 ferromagnesite, an iron-bearing carbonate stable up to 100–115 GPa, is believed to be the major carbon carrier in the earth’s lower mantle and play a key role in the earth’s deep carbon cycle. Using the local density approximation + self-consistent Hubbard U (LDA+Usc) method, we study iron spin crossover in ferromagnesite with a wide range of iron concentration (12.5–100%). Our calculation shows that this mineral undergoes a crossover from the high-spin (HS) (S = 2) to the low-spin (LS) (S = 0) state at around 45–50 GPa, regardless of the iron concentration (x). The intermediate-spin (S = 1) state is energetically unfavorable and not involved in spin crossover. The anomalous changes of volume, density, and bulk modulus accompanying the spin crossover obtained in our calculation are in great agreement with experiments. For further details, see Phys. Rev. B 94, 060404(R) (2016) or its reprint on the Publications page.