What kinds of crystals are there?

This question is a bit troublesome to answer because there are different classification methods.

According to the function, there are as many as 20 kinds of crystals, such as semiconductor crystals, magneto-optical crystals, laser crystals, electro-optical crystals, acousto-optic crystals, nonlinear optical crystals, piezoelectric crystals, pyroelectric crystals, ferroelectric crystals, scintillation crystals, insulating crystals, sensitive crystals, photochromic crystals, superconducting crystals, multifunctional crystals and so on.

The above comes from the following (reading is very interesting, I sincerely hope to help you! ):

Advances in Crystallography and Crystal Materials 2006-09-1312: 51With the development of computer technology and laser technology, mankind has entered a brand-new photoelectron era; The material basis for realizing this great change is nothing more than silicon single crystal and laser crystal. It can be asserted that the further development of crystal materials will surely write a new chapter of human scientific and technological civilization.

First, the process of human understanding of crystals and crystal-related concepts.

1. The process of human understanding of crystals

What is a crystal? Since ancient times, human beings have been tirelessly exploring this problem. As early as the Stone Age, people discovered all kinds of stones with regular shapes and made them into tools, thus opening the prelude to exploring the essence of crystals. Later, after long-term observation, it was found that the most remarkable feature of this crystal was its regular shape. 1669, Italian scientist nicolaus steno discovered the law of conservation of crystal plane angles, and pointed out that in the crystal of the same substance, the angles between corresponding crystal planes are constant. Then, Rene Just Haü y, a French scientist, put forward the famous unit cell theory in 1784, which made a big step for human understanding of crystals. According to this theory, the unit cell is the smallest unit that constitutes a crystal, and the crystal is composed of a large number of unit cells. 1885, this theory was developed into a space lattice theory by A.Bravais, a scientist in this country. He believes that the atoms, molecules or ions that make up a crystal are arranged according to certain rules, and this arrangement forms a certain form of spatial lattice structure. 19 12 years, German scientist Max van Laue carried out X-ray diffraction experiments on crystals, which confirmed the correctness of this theory for the first time and won the Nobel Prize in physics.

2. The concept of crystal

Objects with spatial lattice structure are crystals, and there are 14 spatial lattice structures. For example, sodium chloride (NaCl), the main component of salt, has a face-centered cubic structure and is a common crystal. In addition, many metals (such as tungsten, molybdenum, sodium, iron, etc. at room temperature. ) has a body-centered cubic structure, so they all belong to crystals. It is worth noting that many crystals are crystal clear, but not all transparent solids are crystals, such as glass. This is because the particles that make up the glass are only arranged regularly near an atom, and do not form a spatial lattice structure in the whole glass.

3. rock crystal and artificial crystal

Crystals are divided into rock crystal and artificial crystals. For thousands of years, many beautiful crystals have been formed in nature, such as rubies, sapphires and emeralds. These crystals are called natural crystals. However, due to the scarcity and high price of natural crystals, people began to explore various methods to grow crystals from the end of 19. This kind of artificially grown crystal is called artificial crystal. So far, people have invented dozens of crystal growth methods, such as Czochralski method, floating zone method, flame melting method, crucible descending method, flux method, hydrothermal method, cooling method, recrystallization method and so on. Using these methods, people can not only grow the existing crystals in nature, but also make crystals that are not found in nature. From red, orange, yellow, green, blue, indigo and purple to various mixed colors, these artificial crystals are colorful, and some are even more beautiful than rock crystal.

4. Crystal * * *

Because of its periodic spatial lattice structure, the crystal has the following properties: homogeneity, that is, the macroscopic properties of different parts of the crystal are the same; Anisotropy, that is, crystals have different physical properties in different directions; Self-limiting, that is, crystals can spontaneously form regular geometric shapes; Symmetry, that is, the physical and chemical properties of crystals are exactly the same in some specific directions; Has a fixed melting point; The internal energy is the smallest.

5. Crystallography

In addition to studying the structure, growth and general properties of crystals, people have also explored other problems about crystals, thus forming the discipline of crystallography. Its main research contents include crystal growth, crystal geometry, crystal structure analysis, crystal chemistry and crystal physics. Among them, crystal growth is the method and law of studying artificial cultivation of crystals, and it is an important basis of crystallography research; The geometric structure of crystal is a geometric theory to study the crystal shape and the internal particle arrangement, which belongs to the classical theory of crystallography. However, the discovery of equal rotational symmetry five times in recent years has challenged this classical theory. Crystal structure analysis is an analysis method to collect a large number of diffraction data related to crystal structure and find out the specific crystal structure and X-ray structure. Crystal chemistry mainly studies the relationship between chemical composition and crystal structure and properties; Crystal physics studies physical properties of crystals, such as optical properties, electrical properties, magnetic properties, mechanical properties, acoustic properties and thermal properties.

Second, the performance, application and progress of crystals

A physicist once said: "Crystal is the best gift given to physicists by crystal growth workers." This is because when a substance exists in a crystal state, it will show excellent physical properties that other substances do not have, thus becoming an important basis for human beings to study the structure and properties of solid substances. In addition, because it can realize the interaction and transformation of electricity, magnetism, light, sound and force, crystals are also important materials in electronic devices, semiconductor devices, solid-state laser devices and various optical instruments, and are widely used in communication, photography, aerospace, medicine, geology, meteorology, architecture, military technology and other fields.

According to the function, there are as many as 20 kinds of crystals, such as semiconductor crystals, magneto-optical crystals, laser crystals, electro-optical crystals, acousto-optical crystals and nonlinear crystals.

Optical crystals, piezoelectric crystals, thermoelectric crystals, ferroelectric crystals, scintillation crystals, insulating crystals, sensitive crystals, photochromic crystals, superconducting crystals and multifunctional crystals. Here are a few important ones.

1 semiconductor crystal

Semiconductor crystal is the main basic material in the semiconductor industry, and it occupies a first-class position in the crystal from the perspective of universality and importance of application. Semiconductor crystals began to develop in the 1950s. The first generation semiconductor crystals are germanium (Ge) single crystal and silicon single crystal.

(Silicon). Devices made of them, such as diodes, triodes, field effect transistors, silicon controlled diodes and high-power transistors, are widely used in the wireless electronics industry. Their development has made the integrated circuit rapidly develop from a dozen unit circuits to a very large-scale integrated circuit with thousands of components, which greatly promoted the miniaturization of electronic products, greatly improved the reliability of work and reduced the cost, thus promoting the wide application of integrated circuits in aerospace research, nuclear weapons, missiles, radars, electronic computers, military communication equipment and civil use.

At present, in addition to developing large diameter, high purity, high uniformity and defect-free silicon single crystals, people have also studied the second generation semiconductor crystals-III-V compounds, such as (CaAs), gallium phosphide (GaP) and other single crystals. Recently, in order to meet the demand of higher performance, semiconductor crystals, such as ternary or multicomponent compounds, have been developed. Among the semiconductor crystal materials, gallium nitride (GaN) crystal is particularly worth mentioning. Because of its wide band gap (3. 4eV at room temperature), which is an ideal material for blue-green light emitting diodes (led), laser diodes (LD) and high-power integrated circuits. In recent years, there has been a research upsurge in the world, which has become a research hotspot. At present, the Institute of Physics, Chinese Academy of Sciences has developed a new method for the growth of this crystal, and a 3mm×4mm flaky crystal has been grown by molten salt method for the first time. Once the crystal quality is further improved, it will be widely used in light-emitting devices, optical communication systems, CD players, full-color printing, high-resolution laser printing, large-screen full-color display systems, ultra-thin TVs and so on.

2. Laser crystal

Laser crystal is the working substance of laser, which can emit laser after pumping, so it is called laser crystal. 1960, American scientist Maiman successfully developed the world's first laser with ruby crystal as its working material, and made great scientific achievements that attracted worldwide attention.

Achievement. Hundreds of laser crystals have been developed. Among them, the most commonly used are ruby (Cr :Al 2O3), titanium sapphire (Ti :Al2O3), neodymium-doped gadolinium aluminum garnet (Nd: Y3Al 5O 12), dysprosium-doped calcium fluoride (Dy: CaF2), neodymium-doped yttrium vanadate (Nd: YVO4), neodymium aluminum tetraborate (Nd

In recent years, due to the continuous emergence of new laser crystals and the development of nonlinear frequency doubling, difference frequency and parametric oscillation, lasers have been applied.

The laser obtained by crystal has been involved in ultraviolet, visible and infrared spectra, and has been successfully applied in military technology, space exploration, medicine and chemistry.

And many other fields. For example, in the processing of various materials, the laser generated by crystals plays an important role, especially in the processing of superhard materials, which has unparalleled advantages. For example, it takes more than two hours to drill holes in diamonds by traditional methods, and the laser generated by crystals is less than 0. 1 sec. In addition, laser welding can assemble many electronic components with high density, which greatly improves the working reliability of the circuit and thus greatly reduces the volume of electronic equipment. Laser crystals can also be made into laser rangefinders and laser altimeters for high-precision measurement. What is exciting is that the French Observatory realized the tracking observation experiment of the same artificial satellite for the first time by using the device with ruby crystal, and accurately measured the distance from this satellite to the ground. In medicine, laser crystals have been skillfully applied. The laser it emits is transmitted through a freely bendable light tube, and a lens and a surgeon's handle are installed at the exit end. Through the lens, the laser is focused into a tiny spot with a diameter of only a few angstroms, becoming an invisible but very dexterous scalpel, which can not only be completely

Sterilization, and can quickly cut off tissue, or even cut off a cell. Erbium-doped laser crystals are most suitable for very fine ophthalmic surgery. This crystal can generate laser with a wavelength close to 3 μ m. Because the laser is strongly absorbed by water, it only has a short penetration depth of several microns after entering biological tissues. Therefore, this kind of laser is very safe and will not bring any pain to patients. Because this kind of laser can cut quickly and accurately, the operation time is very short, thus avoiding the interference of involuntary eye movement to the operation and ensuring the smooth operation. In addition, laser tv, laser color stereoscopic film, laser photography and laser computer will all be exciting new uses of laser crystals.

3. Nonlinear optical crystals

When light propagates in the crystal, it will cause the polarization of the crystal. When the light intensity is not too large, the polarization intensity of the crystal has a linear relationship with the optical frequency electric field, and its nonlinear relationship can be ignored. However, when the light intensity is high, such as when the laser propagates in the crystal, the nonlinear relationship between polarization intensity and optical frequency electric field becomes very significant and can not be ignored. This optical effect related to light intensity is called nonlinear optical effect, and crystals with this effect are called nonlinear optical crystals.

Nonlinear optical crystal is closely related to laser and is the key material to realize laser frequency conversion, modulation, deflection and Q-switching. At present, the laser band directly obtained by laser crystal is limited, and there is still a blank laser band from ultraviolet to infrared spectrum. Using nonlinear optical crystal, the laser directly output by laser crystal can be converted into laser with new wave band, thus opening up a new laser light source and expanding the application range of laser crystal. Commonly used nonlinear optical crystals include lithium iodate (α-Li IO3), barium sodium niobate (Ba2NaNb5O 15), potassium deuterium phosphate (KD2PO4), barium metaborate (β- BaB2O4), lithium triborate (LiB3O5) and so on. Among them, barium metaborate and lithium triborate crystals were successfully developed for the first time in China in 1980s, which have outstanding advantages of large nonlinear optical coefficient and high laser damage threshold, and are excellent laser frequencies.

Rate switch crystal materials have caused great repercussions in the world. Another famous crystal is potassium titanyl phosphate crystal (KTiOPO4), which is the nonlinear optical crystal with the best comprehensive performance so far. It is recognized as the first choice for laser frequency doubling of 1. 064μm and 1. It can convert 1. 32 μm infrared laser. 064μm turns into 0.53μ m green laser .. Because green light can be used not only for medical treatment and laser ranging, but also for underwater photography and underwater communication, potassium titanyl phosphate crystal has been widely used.

4.piezoelectric crystal

When the crystal is subjected to external force, it will be polarized and form surface charge, which is called positive piezoelectric effect. On the contrary, when the crystal is subjected to an external electric field, the crystal will be deformed, which is called the inverse piezoelectric effect. Crystals with piezoelectric effect are called piezoelectric crystals, which only exist in crystals without symmetrical centers. The earliest piezoelectric crystal was crystal (α- SiO2). It is an ideal piezoelectric material with stable frequency, which can be used to manufacture resonators, filters, transducers, optical deflectors, surface acoustic wave devices and various thermal, gas, photosensitive and chemical sensitive devices. It is also widely used in people's daily life, such as quartz watches, electronic clocks, color TV sets, stereo radios and tape recorders.

In recent years, many new piezoelectric crystals have been developed, such as lithium niobate (LiNbO3) and potassium tantalate (KTaO3) with perovskite structure.

And so on, Ba2NaNb5O 15 with tungsten bronze structure, K 1-xLiNbO3 with layered structure and bismuth germanate (Bi 12GeO20). Using the piezoelectric effect of these crystals, various devices can be made, which are widely used in military and civil industries, such as sphygmomanometer, respiratory heart sound tester, piezoelectric keyboard, delay line, oscillator, amplifier, piezoelectric pump, ultrasonic transducer, piezoelectric transformer and so on.

5. Scintillation crystal

This kind of crystal will produce fluorescence under the excitation of X-rays, forming a flicker phenomenon. Thallium-doped sodium iodide (Tl :NaI) crystal is the earliest scintillation crystal used. The luminescent wavelength of crystals is in the visible region, which has high scintillation efficiency and is easy to grow large-size single crystals, and is widely used in nuclear science and engineering.

Widely used in the industry. In the early 1980s, China Academy of Sciences Shanghai Institute of Silicate successfully grew large-size bismuth germanate (Bi 4Ge 3O 12) single crystal by crucible descent method. This crystal is especially suitable for the detection of high-energy particles and high-energy rays because of its strong ability to block high-energy rays and high resolution. It is widely used in the research fields of elementary particles, space physics and high-energy physics, and has been successfully applied to the electromagnetic calorimeter of L3 electron-positron collider at the European Institute of Particle Physics. Since then, BaF2 crystal has become another new scintillation material. In addition to its application in high-energy physics, the crystal is also used in positron annihilation spectrometer in low-energy physics, which improves the resolution and counting efficiency of the spectrometer.

Have been greatly improved. In addition, it can also be used to check hidden explosives, oil detection, radioactive mineral detection and positron emission layer.

Analytical photography (PET for short) has a good application prospect.

6. Acoustooptic crystal

When light wave and sound wave hit the crystal at the same time, there will be interaction between sound wave and light wave, which can be used to control the light beam, such as deflecting the light beam and changing the light intensity and frequency. This kind of crystal is called acousto-optic crystal, such as lead molybdate (PbMoO4), tellurium dioxide (TeO2) and tellurium thioarsenate (Tl 3AsS4). Using these crystals, people can make various acousto-optic devices, such as acousto-optic deflector, acousto-optic Q-switch, surface acoustic wave devices and so on. Therefore, these crystals are widely used in laser radar, television and large screen display scanning and photon meters.

Optical storage and laser communication of computer.

7. Photorefractive crystals

Photorefractive crystal is the most wonderful crystal among many crystals. When the external weak laser irradiates this crystal, the carriers in the crystal are excited, migrate in the crystal and are recaptured, thus generating a space charge field inside the crystal. Then, through the electro-optical effect, the space charge field changes the spatial distribution of refractive index in the crystal, forming a refractive index grating, thus producing the optical devitrification effect. The characteristic of photorefractive effect is that it can show obvious effect under the action of weak light. For example, in the self-pumped phase * * yoke experiment, a milliwatt laser beam can interact with photorefractive crystals to generate phase * * yoke wave, which makes the distorted image clear as before. Because the refractive index grating is non-local in space and has a certain spatial phase shift relative to the interference fringe in the wave vector direction, the energy conversion between beams can be realized. For example, in the two-wave coupling experiment, when weak signal light and strong light interact in photorefractive crystals, the weak signal light can be enhanced by 1000 times. In addition, due to photorefractive effect, photorefractive crystals have the following special properties: 5 000 different images can be stored in a volume of 3cm3, and any one of them can be displayed quickly; It can accurately detect the distance change as small as 10-7 meters; You can filter out static images and track the image changes that have just happened; It can even simulate the associative thinking ability of the human brain. Therefore, once this crystal was discovered, it aroused great interest.

At present, the photorefractive crystals with application value include barium titanate (BaTiO3), potassium niobate (KNbO3), lithium niobate (LiNbO3) and strontium niobate.

Barium (Sr 1-xBaxNb2O6) series, bismuth silicate (Bi 12SiO20) and other crystals. Among them, cerium-doped barium titanate (Ce :BaTiO3) was first successfully developed by Institute of Physics, Chinese Academy of Sciences in 1990s. Its excellent properties make a breakthrough in theoretical research and practical application of photorefractive crystals. At present, photorefractive crystal has developed into a new type of functional crystal, which shows good application prospects in the fields of optical image and information processing, phase yoke, holographic storage, optical communication and optical computer neural network.

Third, the development trend of crystal research

With the deepening of people's understanding of crystals, the direction of crystal research has gradually changed, and its general development trend is: from crystalline to amorphous; From bulk single crystal to thin film crystal; From ordinary lattice to superlattice; From single function to multi-function; From bulk properties to surface properties; From inorganic to organic, and so on. In addition, in view of fully realizing the importance of the relationship between crystal structure and properties, people have begun to explore various new crystals through molecular design. Moreover, with the emergence and development of photonic crystals and nanocrystals, human understanding of crystals has made a new leap. It is believed that in the near future, there will be more varieties of crystals, better performance and wider application scope.

In a word, crystals are not only beautiful, but also useful. It contains rich contents and is a precious wealth of mankind. But so far,

People's understanding of it is only the tip of the iceberg, and there are still many unknown fields waiting for us to explore.

(Wang Wanyan, Institute of Physics, Chinese Academy of Sciences, Ph.D., Beijing 100080)