Static compression of Fe4N to 77 GPa and its implications for nitrogen storage in the deep Earth
Static compression of Fe4N to 77 GPa and its implications for nitrogen storage in the deep Earth
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DOI：
10.2138/am-2019-7065
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发表时间：
2019-12-01
影响因子：
3.1
通讯作者：
Ohishi, Yasuo
中科院分区：
文献类型：
--
作者：
Breton, Helene;Komabayashi, Tetsuya;Ohishi, Yasuo
在金刚石对顶砧装置上对面心立方（fcc）γ ′-Fe_4 N进行了室温压缩和减压实验，并结合原位X射线衍射（XRD）研究了其在高压下的稳定性。在所研究的压力范围内，γ ′-Fe_4 N没有表现出任何结构转变。然而，在60 GPa以上的XRD图谱中观察到峰加宽。将得到的60 GPa下的压力-体积数据拟合到三阶Birch-Murnaghan状态方程（EoS），其产生以下弹性参数：K-0 = 169（6）GPa，K' = 4.1（4），在1巴下固定V-0 = 54.95埃。在15-60 GPa和300-1600 K下，通过将γ '-Fe 4 N的EoS与所报道的低压下的相稳定性数据相结合，获得定量的Schreinemakers网。该网络表明在41 GPa和1000 K下存在一个不变点，其中γ '-Fe 4 N，六方密堆积（hcp）ε-Fe 7 N3，双六方密堆积β-Fe 7 N3和hcp Fe相是稳定的。从不变量点看，反应γ ′-Fe_4 N = β-Fe_7 N_3 + hcp Fe向高压侧发生，决定了γ ′-Fe_4 N在56 GPa和300 K下的高压稳定性。因此，在超过该压力的实验中观察到的γ ′-Fe 4 N相必须是亚稳态的。所得结果支持了现有的观点，即β-Fe 7 N3将是堆芯条件下最富氮的铁化合物。铁碳氮化物Fe-7（C，N）（3）是一种幔源金刚石包裹体，表明β-Fe_7 N_3和Fe_7 C_3可能在1000 km深度以上的地幔中形成连续的固溶体。金刚石的形成可能与地幔中流体的存在有关，高压含水相D的脱水反应可能在深度大于1000 km的地幔中提供了自由流体。因此，金刚石中Fe-7（C，N）（3）的存在可能是水向深部地幔输送的指示。
Compression and decompression experiments on face-centered cubic (fcc) gamma'-Fe4N to 77 GPa at room temperature were conducted in a diamond-anvil cell with in situ X-ray diffraction (XRD) to examine its stability under high pressure. In the investigated pressure range, gamma'-Fe4N did not show any structural transitions. However, a peak broadening was observed in the XRD patterns above 60 GPa. The obtained pressure-volume data to 60 GPa were fitted to the third-order Birch-Murnaghan equation of state (EoS), which yielded the following elastic parameters: K-0 = 169 (6) GPa, K' = 4.1 (4), with a fixed V-0 = 54.95 angstrom at 1 bar. A quantitative Schreinemakers web was obtained at 15-60 GPa and 300-1600 K by combining the EoS for gamma'-Fe4N with reported phase stability data at low pressures. The web indicates the existence of an invariant point at 41 GPa and 1000 K where gamma'-Fe4N, hexagonal closed-packed (hcp) epsilon-Fe7N3, double hexagonal closed-packed beta-Fe7N3, and hcp Fe phases are stable. From the invariant point, a reaction gamma'-Fe4N = beta-Fe7N3 + hcp Fe originates toward the high-pressure side, which determines the high-pressure stability of gamma'-Fe4N at 56 GPa and 300 K. Therefore, the gamma'-Fe4N phase observed in the experiments beyond this pressure must be metastable. The obtained results support the existing idea that beta-Fe7N3 would be the most nitrogen-rich iron compound under core conditions. An iron carbonitride Fe-7(C,N)(3) found as a mantle-derived diamond inclusion implies that beta-Fe7N3 and Fe7C3 may form a continuous solid solution in the mantle deeper than 1000 km depth. Diamond formation may be related to the presence of fluids in the mantle, and dehydration reactions of high-pressure hydrous phase D might have supplied free fluids in the mantle at depths greater than 1000 km. As such, the existence of Fe-7(C,N)(3) in diamond can be an indicator of water transportation to the deep mantle.