Visualizing Crystal Structures with the Naked Eye:The Role of X-ray and Neutron Scattering

What are X-rays? X-rays, also known as Röntgen rays, Roentgen radiation, or X-radiation, are a type of electromagnetic radiation with a very short wavelength and high frequency. They are produced when electrons undergo transitions between energy levels in atoms with a large energy difference. X-rays have wavelengths ranging from 0.01 nanometers to 10 nanometers (corresponding to a frequency range of 30 petahertz to 30 exahertz), placing them between ultraviolet light and gamma rays. While optical spectra are emitted during electron transitions in the outer layers of atoms, X-ray spectra are emitted during transitions in the innermost layers of atoms. X-rays do not undergo deflection in electric or magnetic fields, making them electrically neutral.

These invisible rays have high penetrating power, capable of passing through many materials that are opaque to visible light, such as ink, paper, books, and wood. X-rays can also cause visible fluorescence in many solid materials, induce air ionization, and affect photographic film sensitivity.

Discovery of X-rays On the evening of November 8, 1895, Wilhelm Conrad Roentgen was conducting an experiment on cathode rays. To prevent external light from affecting the discharge tube and to ensure the experiment's accuracy, he wrapped various pieces of equipment tightly with tin foil and cardboard. He allowed the cathode rays to pass through a discharge tube without an aluminum window installed. To his surprise, a screen coated with barium platinocyanide (which was used for another experiment) emitted light when exposed to the cathode rays. Additionally, a stack of tightly sealed photographic plates placed next to the tube turned gray-black, indicating that the plates had been exposed.

This seemingly insignificant phenomenon caught Wilhelm Conrad Roentgen's attention and curiosity. It sparked a keen interest in him, and he repeated the experiment several times, with the same result each time—the screen emitted light. He pondered over this, realizing that the exposure of the plates indicated that the discharge tube emitted a type of radiation with extremely high penetrating power. This radiation was not the cathode rays, but rather a new type of ray that could even penetrate the bag containing the photographic plates. However, the nature of this radiation was still unknown, so he named it "X-ray."

Wilhelm Conrad Roentgen began to study this mysterious X-ray. He first placed a screen coated with phosphorescent material between the discharge tube and the screen and repeated the previous experiment, finding that the screen immediately emitted light. Next, he tried to block the invisible mysterious rays with usually opaque materials such as books, rubber, and wood, but they could not block it. The screen showed almost no shadows, and it could easily penetrate even a 15-millimeter thick aluminum plate! Finally, he placed a thick metal plate between the discharge tube and the screen, and the screen finally showed the shadow of the metal plate. It seemed that this kind of radiation still could not penetrate too thick materials. Through experiments, he also found that lead and platinum plates could block this radiation and prevent the screen from emitting light. When the cathode tube was turned on, even a thick black paper wrapped around the photographic plate next to it would become photosensitive.

Furthermore, Wilhelm Conrad Roentgen discovered an even more amazing phenomenon. One evening, his wife visited him in the laboratory. He asked her to cover the photographic plate with her hand and then expose it to X-rays for 15 minutes. After developing the plate, they were astonished to find clear images of her finger bones and the ring on her finger.

On January 5, 1896, many X-ray photographs were exhibited at a meeting of the Physical Society in Berlin. Simultaneously, the Vienna "Neue Freie Presse" also published news of the discovery of X-rays. This great discovery quickly spread worldwide and attracted great attention. In the following months, hundreds of scientists conducted research on X-rays, and within a year, thousands of papers on X-rays were published.

Applications of X-rays

Despite Wilhelm Conrad Roentgen's discovery of X-rays, at the time, no one knew what these rays were. In 1906, experiments proved that X-rays are actually electromagnetic waves with very short wavelengths, shorter than those of light waves, which allows them to produce interference and diffraction phenomena. Their discovery provided important evidence for a major change in physics. Initially used for medical imaging diagnosis and X-ray crystallography, X-rays are now widely used not only in medical diagnostics and treatment, becoming a powerful weapon for humans to overcome diseases, but also in industrial non-destructive testing of materials. Because of this, Wilhelm Conrad Roentgen was awarded the Nobel Prize in 1901, becoming the first person in the world to receive the Nobel Prize in Physics. In honor of Wilhelm Conrad Roentgen, X-rays were named Roentgen rays.

It is important to note that X-rays are a type of ionizing radiation and can be harmful to the human body.

Neutron radiation

Neutron radiation, also known as neutron rays, is a form of ionizing radiation that consists of free neutrons. Neutrons are one of the basic particles of atomic nuclei. When atomic nuclei are bombarded by external particles, nuclear reactions occur, resulting in the emission of neutrons from the nucleus, forming neutron radiation. Therefore, the sources of neutron radiation are nuclear reactors, accelerators, or neutron generators.

Neutrons are classified by energy into fast neutrons, slow neutrons, and thermal neutrons. Neutrons have a high ionization density and can often cause significant mutations. In radiation breeding, thermal neutrons and fast neutrons are more commonly used.

A Valuable Tool for Detecting Crystal Structures

The crystal structure is the microscopic structure of a crystal, referring to the specific arrangement of actual particles—atoms, ions, or molecules—in the crystal.

Solid materials in nature can be divided into crystals and non-crystals. Crystals are mostly solid metals and alloys. The most essential difference between crystals and non-crystals is that the arrangement of particles—atoms, ions, or molecules—in crystals is regular, while the particles in non-crystals are generally stacked together irregularly. In most cases, metals and alloys are used in their crystalline state. The crystal structure is one of the fundamental determinants of the physical, chemical, and mechanical properties of solid metals. So how can we see the arrangement of atoms and other particles in materials?

The diameter of an atom is about 10^-10 meters, but the smallest size visible to the human eye is generally around 0.1 millimeters, which is only as thick as a strand of hair, and the highest magnification of the best optical microscope is only 1000 times. So, how can we see the distribution and arrangement of atoms and other particles in materials? Scientists have found some special methods for this.

X-ray Diffraction

As mentioned earlier, the wavelength of X-rays ranges from 1 nanometer (10^-9 meters) to 0.01 nanometers, which is close to the scale of atoms, and X-rays also have very strong penetrating power. Therefore, scientists use X-ray diffraction to construct more detailed microscopes.

X-rays are electromagnetic waves with wavelengths exactly at the atomic scale. When they are incident on a crystal material at a specific angle, they are reflected by the regularly arranged atomic layers. The reflection process follows Bragg's law, which states that only when the atomic layer spacing and the incident wavelength satisfy a fixed equation will diffracted waves be produced. Therefore, by detecting X-rays at different incident or exit angles, one can obtain various possible atomic layer spacings within the material, and thus infer the arrangement of atoms. X-ray diffraction is like putting on a sophisticated "glasses" for observers, allowing them to "perceive" the arrangement of atoms in crystalline materials through "perspective."

Neutron Scattering In addition to X-ray diffraction, we can also use neutron scattering to investigate the arrangement of atoms. Neutrons are electrically neutral (no net charge) and will mainly be reflected by atomic nuclei, allowing for a very precise determination of atomic positions. Neutrons also carry a magnetic moment, so they have another unique function—to detect the arrangement of internal magnetic moments in materials and study the origin of solid-state magnetism.

The scattering of X-rays and neutrons can also be used to study the dynamic properties of atoms or electrons in materials. For example, the thermal vibration of atoms, the mode of electron motion, the interaction between electrons and atomic nuclei, the interaction process between electrons, and many other related issues. These dynamic processes are the microscopic manifestations of the macroscopic properties of materials such as heat, electricity, and magnetism. By studying them, we can help and promote our understanding of the properties of materials and guide us in finding more suitable materials for applications.

X-ray diffraction and neutron scattering are important means for modern condensed matter physics research, and their implementation relies on large-scale scientific instruments (such as synchrotrons and nuclear reactors) to provide X-ray and neutron sources. The Shanghai Synchrotron Radiation Facility (SSRF) in China and the China Spallation Neutron Source (CSNS) under construction are examples of such facilities.

Quasicrystals

Quasicrystals are solid materials that combine two or more different crystal structures. They can be obtained by rapidly cooling a metal alloy liquid at high temperatures and are a kind of solid state material between crystals and non-crystals. Different crystals can combine to produce these materials with unique symmetries. Like crystals, quasicrystals also have regular shapes, but their shapes are very strange.

The discovery of quasicrystals reveals the wonders of nature. Therefore, in 2011, the discoverer of quasicrystals, Israeli scientist Shechtman, was awarded the Nobel Prize in Chemistry.