New State of Matter at Earth’s Centre

 Seismic mystery solved by finding a new state of matter        

Earth’s inner core may not be a conventional solid at all, but a super ionic material where light elements drift like liquid through a rigid iron lattice. New experiments show that this unusual state dramatically softens the core, matching seismic clues which have puzzled scientists for decades. Beneath Earth s surface, nearly 3,000 km's down, lies a mysterious layer where seismic waves speed up inexplicably. For decades, scientists puzzled over this D' layer. Now, ground breaking experiments have finally revealed that solid rock flows at extreme depths, acting like liquid in motion. This horizontal mantle flow aligns mineral crystals called post-perovskite in a single direction, explaining the seismic behaviour. It s a stunning leap in understanding Earth s deep inner mechanics, transforming a long-standing mystery into a vivid map of subterranean currents which power volcanoes, earthquakes and even the magnetic field.

Chinese researchers have discovered that interstitial carbon in iron-carbon alloys behaves in a superionic, liquid-like state under Earth’s core pressure and temperature conditions. Beneath Earth’s molten outer core lies a solid central region, the inner core, a compact sphere made of an iron light-element alloy pressed by more than 3.3 million atmospheres and heated to temperatures comparable to the Sun’s surface. For many years, researchers have struggled to explain its unusual behaviour, although it is solid, it behaves like an unexpectedly soft metal, slowing seismic shear waves and displaying a Poisson’s ratio closer to butter than to steel. This has raised a long-standing question about how the planet’s solid centre can appear both firm and surprisingly pliable. Researchers found that solid rock flows horizontally, aligning minerals and causing a shift in earthquake wave speeds. This changes our entire view of Earth’s deep interior. Earthquakes, volcanic eruptions, shifting tectonic plates, these are all signs that our planet is alive. But what is revealed deep inside the Earth surprises laymen and scientists alike: Almost 3000 km's below the Earth's surface, solid rock is flowing that is neither liquid, like lava, nor brittle like solid rock. 

A major study provides a strong explanation. Scientists have found that Earth’s inner core is not behaving like an ordinary solid at all; instead, it occupies a superionic state, where light elements move through a rigid iron lattice with liquid-like mobility. This finding reshapes scientific views of what is happening deep within the planet. The research team, led by Prof. Youjun Zhang and Dr. Yuqian Huang of Sichuan University and Prof. Yu He of the Institute of Geochemistry, Chinese Academy of Sciences, demonstrated that iron-carbon alloys shift into a superionic phase when subjected to intense pressure and heat. In this form, carbon atoms travel quickly through the crystal framework of solid iron, greatly reducing its stiffness. “For the first time, we’ve experimentally shown that iron–carbon alloy under inner core conditions exhibits a remarkedly low shear velocity.” said Prof. Zhang. “In this state, carbon atoms become highly mobile, diffusing through the crystalline iron framework like children weaving through a square dance, while the iron itself remains solid and ordered. This so-called “superionic phase” dramatically reduces alloy’s rigidity.

For over 50 years, researchers have been puzzling over a strange zone deep inside the Earth, the so-called D" layer. Earthquake waves suddenly behave differently there: their speed jumps as if they were traveling through a different material. What exactly happens at that layer of the mantle has been unclear for a long time, until now. In 2004, Murakami, who has been a professor at ETH Zurich since 2017, discovered that perovskite, the main mineral of the Earth's lower mantle, transforms into a new mineral near the D" layer under extreme pressure and very high temperatures, so-called "post-perovskite." The researchers assumed that this change explained the strange acceleration of the seismic waves. But that was not the full story. In 2007, Murakami and colleagues found new evidence that the phase change of perovskite alone is not enough to accelerate earthquake waves. Now the superionic model not only explains the core’s puzzling seismic properties but also opens new perspectives on Earth’s internal dynamics. The movement of light elements could help account for seismic anisotropy, variations in wave speeds depending on direction, and may even influence Earth’s magnetic field. “Atomic diffusion within the inner core represents a previously overlooked energy source for the geodynamo,” said Dr. Huang. “In addition to heat and compositional convection, the fluid-like motion of light elements may help power Earth’s magnetic engine.”

While computer models had hinted at such a state in 2022, direct experimental proof remained elusive, until now. Using a dynamic shock compression platform, the team accelerated iron–carbon samples to speeds of 7 km's / second, creating pressures up to 140 gigapascals and temperatures near 2600 kelvin, conditions similar to those in the inner core. Iron atoms form a rigid hexagonal close-packed (hcp) structure, with a subset of these atoms exhibiting collective motion along the [100] and [010] directions. Within this hcp iron lattice, interstitial light elements diffuse freely in a liquid-like manner, while substitutional light elements remain confined to their respective substitutional lattice sites. Consequently, the Earth’s inner core exists in a hybrid state of solid and liquid-like behaviour. By combining in-situ sound velocity measurements with advanced molecular dynamics simulations, the scientists observed a sharp drop in shear wave velocity and a spike in Poisson’s ratio, matching the “soft” seismic signals detected deep within Earth. On the atomic scale, the results revealed carbon atoms slipping freely through the iron lattice, weakening its rigidity without destroying its structure. The findings also settle long-standing debates about how light elements behave under extreme pressures. Previous studies focused on compounds or substitutional alloys, but this research highlights the importance of interstitial solid solutions, especially those involving carbon, in determining the core’s properties.

It is not only a milestone, but also a turning point. The assumption that solid rock flows has been transformed from a theory into a certainty. This discovery shows that the Earth is not only active on the surface, but is also in motion deep inside. With this knowledge, researchers can now begin to map the currents in the Earth's deepest interior and thus visualize the invisible motor which drives volcanoes, tectonic plates and perhaps even the Earth's magnetic field. According to Prof. Zhang, the discovery signals a shift in how scientists view Earth’s centre. “We’re moving away from a static, rigid model of the inner core toward a dynamic one,” he explained. Beyond Earth, the discovery of a superionic phase could also shed light on the magnetic and thermal evolution of other rocky planets and exoplanets. As Zhang notes, “Understanding this hidden state of matter brings us one step closer to unlocking the secrets of Earth-like planetary interiors.” This work was funded by the National Natural Science Foundation of China, the Sichuan Science and Technology Program, and the CAS Youth Interdisciplinary Team.