地函深處可能有超離子相物質形成的質子河流

 原文網址：http://hpstar.ac.cn/contents/27/11981.html

Pierfranco Demontis在1988年說：「冰在高溫高壓之下會變成快離子導體(fast-ion conductor)。」但他的預測不久之前才脫離假說的層次。2018年，經過30年的研究之後才經由實驗證明超離子水冰的存在。而超離子性或許最後還能解釋巨型行星內部的強力磁場如何產生。

那麼同樣處於極端溫度壓力條件之下的地球內部呢？雖然地球表面有四分之水都被水覆蓋，但是地球內部卻很少有單獨存在的水或冰。在此「水」最常見的的單元為氫氧根，它會跟礦物結合而形成含水礦物。北京高壓科學研究中心的三位博士胡清揚、DuckYoung Kim、劉錦領導的研究團隊，發現就像巨型行星內部的水冰一樣，地球內部的一種含水礦物也會變成這種奇異的超離子相。研究結果發表於《自然―地球科學》(Nature Geosciences)。

「在超離子水當中，氫會脫離氧而像流體一樣，在氧所形成的固體晶格中自由移動。同樣地，在我們研究的含水礦物羥基氧化鐵(iron oxide-hydroxide ,FeOOH)之中，氫原子也能在固體的FeO₂晶格當中自由移動，」負責用電腦進行模擬的何博士表示。

「它在溫度大約1700℃以上且壓力超過80萬大氣壓力時會轉變成超離子相。這樣的溫度壓力條件使得下部地函的大部分區域都允許此種含水礦物進入超離子相，因此地球深處可能會有在固體當中四處流動的質子河川，」Kim博士補充。

在有了理論預測的基礎之後，團隊嘗試證實羥基氧化鐵內部在高溫之下會一如預期地進入超離子相。他們利用鑽石砧與雷射加熱技術來進行高溫高壓試驗。

「從技術上來說，要在實驗中辨認出氫原子的運動是相當困難的。不過拉曼光譜法對於O-H鍵結的演變相當敏銳，」研究主要作者之一的胡博士說。「因此，我們可以追蹤O-H鍵的演變過程，然後把這種特殊的狀態從平常的形式中捕捉出來。」

他們發現壓力大於73000大氣壓時O-H鍵會急遽變軟，同時在拉曼光譜中O-H的強度峰值也降低了大約55%。這些結果象徵某些H⁺可以不受氧的限制而移動，使得O-H鍵弱化，與模擬結果一致。「由於氫原子要在高溫之下才能具有足夠的移動能力來脫離晶格，因此在高壓常溫條件之下O-H鍵的軟化與弱化只能視為超離子態的前身，」侯明强博士解釋。

導電度在超離子物質中會有明顯的改變，因此可以作為超離子化的確切證據。團隊測量樣本在高高壓條件下的導電度變化，觀察到在大約1500-1700℃與121,000大氣壓力時導電度會遽增，顯示擴散出去的氫已經包覆住整個固體樣本，也就是進入了超離子態。

「這種黃鐵礦型的FeO₂H_x是第一個礦物會在下部地函深處進入超離子態的例子，」共同主持這項研究的劉博士強調。「最近發現有些高密度的含氫氧化物，比方說高密度的含水相礦物，在下部地函深處的高溫高壓條件之下仍然可以保持穩定，它們內部的氫相當有可能也會展現出超離子行為。」

這項成果對於地球科學來說具有十分重要的啟發，因為超離子相可以劇烈改變導電度、磁性和物質傳輸等地球物理性質。由於物質在固體之間及內部的交換相當沒有效率，因此過往認為地函對流的速度相當緩慢，通常是以數千年到數百萬年的時間尺度來描述。雖然我們無法直接觀察地表之下數千公里深的物質如何流動，但是超離子相的存在暗示地函對流的速率或許比過去認為的還高出幾個數量級。這些快速流動的氫原子就跟河流一樣，可以運輸熱和質量而把相隔甚遠的地方連結起來。因此固體地球也許比我們想的還要更加活躍。

 

Earth's deep mantle may have proton rivers made of superionic phases

Pierfranco Demontis said in 1988, “Ice becomes a fast-ion conductor at high pressure and high temperatures,” but his prediction was only hypothetical until recently. After 30 years of study, superionic water ice was verified experimentally in 2018. Superionicity may eventually explain the strong magnetic field in giant planetary interiors.

What about Earth, whose interiors are also under extreme pressure and temperature conditions? Although three-quarters of Earth’s surface is covered by water, standalone water or ice rarely exists in Earth’s interiors. The most common unit of “water” is hydroxyl, which is associated with host minerals to make them hydrous minerals. Here, a research group led by Dr. Qingyang Hu, Dr. Duckyoung Kim, and Dr. Jin Liu from the Center for High Pressure Science and Technology Advanced Research discovered that one such hydrous mineral also enters an exotic superionic phase, similar to water ice in giant planets. The results are published in Nature Geosciences.

"In superionic water, hydrogen will get released from oxygen and become liquid-like, and move freely within the solid oxygen lattice. Similarly, we studied a hydrous mineral iron oxide-hydroxide (FeOOH), and the hydrogen atoms move freely in the solid oxygen lattice of FeO₂,” said Dr. He, who conducted the computational simulation.

"It developed into the superionic phase above about 1700°C and 800,000 times normal atmospheric pressure. Such pressure and temperature conditions ensure a large portion of Earth’s lower mantle can host the superionic hydrous mineral. These deep regions may have rivers made of protons, which flow through the solids.” added Dr. Kim.

Guided by their theoretical predictions, the team then tried to verify this predicted superionic phase in hot FeOOH by carrying out high-temperature and high-pressure experiments using a laser-heating technique in a diamond anvil cell.

"It is technically challenging to recognize the motion of H atoms experimentally; however, the evolution of O-H bonding is sensitive to Raman spectroscopy,” said Dr. Hu, one of the lead-authors. “So, we tracked the evolution of the O-H bond and captured this exotic state in its ordinary form.”

They found that the O-H bonding softens abruptly above 73,000 times normal atmospheric pressure, along with ~ 55% weakening of the O-H Raman peak intensity. These results indicate that some H⁺ may be delocalized from oxygen and become mobile, thus, weakening the O-H bonding, consistent with simulations. “The softening and weakening of the O-H bonding at high-pressure and room-temperature conditions can only be regarded as a precursor of the superionic state because high temperature is required to increase the mobility beyond the unit cell,” explained Dr. Hou.

In superionic materials, there will be an obvious conductivity change, which is robust evidence of superionization. The team measured the electrical-conductivity evolution of the sample at high-temperature and pressure conditions. They observed an abrupt increase in electrical conductivity around 1500-1700°C and 121,000 times normal atmospheric pressure, indicating the diffusive hydrogen had covered the entire solid sample and thus, entered a superionic state.

"The pyrite-type FeO₂H_x is just the first example of superionic phases in the deep lower mantle,” remarked Dr. Liu, a co-lead author of the work. “It is very likely that hydrogen in the recently-discovered dense hydrogen-bearing oxides that are stable under the deep lower mantle’s high P-T conditions, such as dense hydrous phases, may also exhibit superionic behavior.”

This work has important implications for Earth science because a superionic phase will dramatically change the geophysical picture of electrical conductivity, magnetism, and materials transportation. Because materials exchange in-between solids are extremely inefficient, the mantle convection was previously thought to be slow, often described by thousands to millions of years. There is no direct observation of how materials are cycling thousands of kilometers below the surface. However, the existence of a superionic phase suggests the rate of mantle convection could be magnitudes higher. Similar to rivers, fast-moving hydrogen connects remote regions by transporting heat and mass. The solid Earth could be more dynamic than previously thought.

原始論文：Mingqiang Hou, Yu He, Bo Gyu Jang, Shichuan Sun, Yukai Zhuang, Liwei Deng, Ruilian Tang, Jiuhua Chen, Feng Ke, Yue Meng, Vitali B. Prakapenka, Bin Chen, Ji Hoon Shim, Jin Liu, Duck Young Kim, Qingyang Hu, Chris J. Pickard, Richard J. Needs & Ho-Kwang Mao. Superionic iron oxide–hydroxide in Earth’s deep mantle. Nature Geoscience, 2021; DOI:10.1038/s41561-021-00696-2

引用自：Center for High Pressure Science & Technology Advanced Research. “Earth's deep mantle may have proton rivers made of superionic phases.”