Subcrystalline Diamond

Date of publication:2022-01-13

The solid matter that surrounds us, whether it is dust, gravel or metal gems, is essentially formed by the accumulation of atoms in space. According to the orderly arrangement of atoms, solid matter can be divided into crystalline and amorphous. We generally believe that the arrangement of atoms in crystalline materials is uniform and regular, while the arrangement of atoms in non-crystalline materials exhibits general disorder.


Recently, Jin Huiyang, a researcher at the Beijing High-Pressure Science Research Center, and others synthesized a new form of diamond under high temperature and high pressure conditions—subcrystalline diamond. The advent of this achievement links the amorphous and crystalline states in the structural topology, which has far-reaching significance for revealing the complex structural nature of amorphous materials.


The solid matter that surrounds us, whether it is dust, gravel or metal gems, is essentially formed by the accumulation of atoms in space. According to the orderly arrangement of atoms, solid matter can be divided into crystalline and amorphous. In a crystal, atoms have a specific packing order in three-dimensional space, and their crystal structure can be expressed periodically by a small structural unit. And from a macroscopic perspective, we cannot distinguish discontinuities, so we generally think that the arrangement of atoms in crystalline materials is uniform and regular. At the same time, this also makes all parts of the crystalline material have the same physical and chemical properties.


In contrast, atoms in amorphous materials lack long-range periodic arrangement, and only have short-range order, that is, each atom only exhibits certain regularity in arrangement with its neighboring atoms in a small range. Therefore, from a macroscopic observation, its atomic arrangement shows a general disorder. The difference in the structure of this amorphous material also directly leads to its great difference in the material properties of force, sound, light, electricity, magnetism, heat and so on. The glass that we see everywhere in our daily life is one of the most typical amorphous materials.


Huiyang Hui said that in the traditional sense, the order presented by atoms in the diameter range of 0-0.5 nanometers is generally called short-range order, and the order presented in the range of 0.5-2.0 nanometers is called medium-range order. Those larger than 2.0 nanometers are called long-range order. But he also mentioned that in practical work, the more commonly used method is to define the spatial order by the number of ordered coordination shells, which takes into account the space between different materials due to differences in bond lengths, etc. Size difference.


However, the material world is ever-changing. The researchers found that as the temperature increased, the long-range order in the crystals decreased significantly, gradually transitioning to the short-range order, at which point understanding the difference between the two states became extremely difficult.


So for solids, especially strong covalent and covalent-like solids, is there an intermediate state between long-range order and short-range order? To explore this structural mystery, theoretical scientists have proposed a "subcrystalline" structural model. "Since 1930, the concept of subcrystalline state has occasionally appeared in the scientific community. In 1950, based on the discovery of some soft matter, Professor Hossmann of Germany proposed subcrystalline state as a state independent of crystal and amorphous state." Huiyang Jin Said that the concept was gradually extended to polymers, colloids, biological materials, and even some molten metals and alloys, and glasses around 1980. However, in covalently bonded and quasi-covalently bonded materials, scientists have not been able to find in nature or in the laboratory such subcrystals that are entirely composed of medium-range order without long-range order. sexual state of matter. Although it has been proposed in the semiconductor material silicon, the content is only less than 18%. For the diamond of the same family, there has been no relevant research, let alone experimental phenomena and evidence.


But the scientific community is not without attempts. Since the concept of subcrystalline state was proposed, scientists have been trying to broaden this state from a theoretical concept to a variety of substances.


According to Huiyang Hui, in 2017, Zeng Zhengdan, a researcher at the Beijing High Voltage Science Research Center, and others used a diamond counter press combined with laser heating technology to successfully synthesize amorphous diamond under the pressure and temperature of 40-50 gigapascals and 1800 Kelvin. , however the extremely high pressure limits the size of the synthesized samples. The work successfully determined the real existence of sp3-bonded amorphous diamond and was able to preserve it.


Moreover, the scientific and industrial circles have mastered the technology of preparing nano-scale diamond, and nano-diamond has been widely used in various fields and has wide practical value. Based on this research background, the Huiyang team decided to use the most advanced large-cavity high-temperature and high-pressure technology to break through the pressure range of traditional large-volume presses and conduct research on millimeter-scale samples with pressures above 30 gigapascals.


Huiyang Hui and his team selected precursors with different characteristics, namely fullerenes, glassy carbon and onion carbon, to explore the structure and microstructure transformation process and paths of different precursors under high pressure. As expected, the research team observed the formation of nanodiamonds in the high temperature range above 1800 Kelvin at a pressure of 30 gigapascals. However, only fullerenes under the pressure and temperature conditions of 30 gigapascals and 1500-1600 Kelvin have amorphous diamonds with medium-range order that can be retained to atmospheric pressure, which has never been found before.


But discovery is not enough. To conduct in-depth and meticulous research on it, researchers are also required to carry out detailed structural characterization and model construction of the trapped amorphous diamond with medium-range order. Therefore, Huiyang Hui and his collaborators characterized its structure and microstructure through X-ray, correlation function, spectroscopy, transmission electron microscopy and other methods, and used advanced large-scale molecular dynamics simulations to compare and model them in detail. construction, which was eventually identified as subcrystalline diamond. The diamond of this structure is essentially the introduction of nanometer-sized intermediate-range ordered structures in an amorphous matrix. Its discovery not only enables researchers to deeply understand this special diamond and master its uniqueness, but also fills in the missing link in the atomic arrangement scale between the amorphous structure and the crystalline structure, and provides a better understanding of the complexities of amorphous materials. The structure provides the key.


In addition to filling the theoretical gap, the synthesis of subcrystalline diamond has a wider application value. In addition to the mechanical properties equivalent to ordinary crystalline diamond, subcrystalline diamond also has very unique and adjustable optical properties. "This means that subcrystalline diamond may be a very good window material under extreme conditions." Huiyang pointed out that due to the very broad fluorescence peak and high thermal stability of subcrystalline diamond, it is expected to be used in the future in the future. Wider applications in many fields, including biomedicine.

From Science and Technology Daily