17:15 〜 19:15
[SIT18-P04] New high-pressure phase of Ni2P
キーワード：惑星コア、相転移、高圧実験
Metallic cores of Earth and other terrestrial planets such as Mercury, Venus, and Mars likely contain light elements in addition to iron and nickel [1]. The metallic cores have been initially molten just after planetary formation and then partially or fully solidified to form inner cores through their cooling history. The solid inner cores are also believed to include light elements depending on the cores' composition. Therefore, it is crucial to understand the crystal structures and elastic properties of FeNi-light element alloys under high-pressure and -temperature conditions to comprehend the internal structure and dynamics of the planetary cores. Recently, it was reported that Fe2S and Fe2P both adopt the C37-type structure (Pnma, Orthorhombic) under high-pressure and high-temperature conditions, suggesting the possible presence of C37-(Fe,Ni)2(Si,P,S) phase in the planetary solid inner cores [2-4]. Here, we examined the phase stability and elastic property of Ni2P up to 154 GPa and 2400 K by X-ray diffraction (XRD) measurements.
We have carried out XRD measurements under high-pressure and -temperature conditions using laser-heated diamond anvil cells at the beamline BL10XU of the synchrotron facility SPring-8. Ni2P was confirmed to be stable in the C22-type structure (hex, P-62m) up to 50 GPa. The stability field and lattice parameters of the C22 phase agree with previous measurements [5]. However, we found that the C22-type phase transforms to the ζ’-Ag2Ga-type (hex, P-62m) phase at 50 GPa, with a volume reduction of ~2.2%. Upon the isosymmetric transition, the hexagonal unit cell is expanded by ~8.8% along the a-axis and reduced by ~16.8% along the c-axis. The new Ni2P phase was confirmed stable up to at least 154 GPa and 2400 K.
The coordination number (CN) of phosphorous in the ζ’-Ag2Ga-type Ni2P is expected to be 11, higher than 9 in the C22 structure. The CN of the new phase is also larger than 10 in the C37 structure stabilized in Fe2S and Fe2P at higher pressure [3,4]. This CN comparison implies that the ζ’-Ag2Ga-type structure could have a stability field over the C37 structure at higher pressures. Indeed, first-principle calculations predicted that the ζ’-Ag2Ga-type structure is stabilized relative to the C37-type in Ni2S above 250 GPa [6]. The present results indicate that the (Fe,Ni)2(Si,P,S) phase may form in the ζ’-Ag2Ga-type structure under the relevant P-T conditions of center cores of terrestrial planets such as Earth and Venus, depending on Ni and P contents in these cores.
[1] McDonough., Treatise on Geochemistry, 3, 559–577 (2014).
[2] Tateno et al., Geophys. Res. Lett. 46(21), 11944-11949 (2019)
[3] Nakajima et al., Ame. Mineral. 105(11), 1752-1755 (2020).
[4] Oka et al., Ame. Mineral. 107(7), 1249-1253 (2022).
[5] Dera et al., J. Geophys. Res. 114, B03201 (2009).
[6] Inerbaev et al., ACS Earth and Space Chemistry, 4, 1978-1984 (2020).
We have carried out XRD measurements under high-pressure and -temperature conditions using laser-heated diamond anvil cells at the beamline BL10XU of the synchrotron facility SPring-8. Ni2P was confirmed to be stable in the C22-type structure (hex, P-62m) up to 50 GPa. The stability field and lattice parameters of the C22 phase agree with previous measurements [5]. However, we found that the C22-type phase transforms to the ζ’-Ag2Ga-type (hex, P-62m) phase at 50 GPa, with a volume reduction of ~2.2%. Upon the isosymmetric transition, the hexagonal unit cell is expanded by ~8.8% along the a-axis and reduced by ~16.8% along the c-axis. The new Ni2P phase was confirmed stable up to at least 154 GPa and 2400 K.
The coordination number (CN) of phosphorous in the ζ’-Ag2Ga-type Ni2P is expected to be 11, higher than 9 in the C22 structure. The CN of the new phase is also larger than 10 in the C37 structure stabilized in Fe2S and Fe2P at higher pressure [3,4]. This CN comparison implies that the ζ’-Ag2Ga-type structure could have a stability field over the C37 structure at higher pressures. Indeed, first-principle calculations predicted that the ζ’-Ag2Ga-type structure is stabilized relative to the C37-type in Ni2S above 250 GPa [6]. The present results indicate that the (Fe,Ni)2(Si,P,S) phase may form in the ζ’-Ag2Ga-type structure under the relevant P-T conditions of center cores of terrestrial planets such as Earth and Venus, depending on Ni and P contents in these cores.
[1] McDonough., Treatise on Geochemistry, 3, 559–577 (2014).
[2] Tateno et al., Geophys. Res. Lett. 46(21), 11944-11949 (2019)
[3] Nakajima et al., Ame. Mineral. 105(11), 1752-1755 (2020).
[4] Oka et al., Ame. Mineral. 107(7), 1249-1253 (2022).
[5] Dera et al., J. Geophys. Res. 114, B03201 (2009).
[6] Inerbaev et al., ACS Earth and Space Chemistry, 4, 1978-1984 (2020).