Synthetic Crystal

(also, man-made crystal), any of the crystals grown artificially either in the laboratory or in industry. Of the total number of synthetic crystals, approximately 10,000 are inorganic substances, and some of these do not occur in nature. Organic synthetic crystals, however, are far more numerous, numbering in the hundreds of thousands; these crystals are of various compositions and, in general, do not occur in nature. On the other hand, of the 3,000 crystals that make up the variety of natural minerals, it has been possible to grow only a few hundred, of which only 20–30 are of practical importance (see Table 1). This low number is explained by the complexity of the crystallization processes and the technical difficulties involved in precise maintenance of the conditions necessary for the growth of single crystals.

The first attempts to synthesize crystals were made in the 16th and 17th centuries; they consisted of the recrystallization of water-soluble crystalline substances encountered in crystalline form in nature (sulfates, halides). Later, knowledge of the composition of natural minerals brought attempts to synthesize minerals from powders using roasting techniques. The first small synthetic crystals were obtained by this method. In the early 20th century, E. S. Fedorov and G. V. Vul’f studied the synthesis of crystals, investigated the conditions of crystallization of water-soluble compounds, and improved the crystallization apparatus. Subsequently, A. V. Shubnikov worked out general principles for the formation of crystals from aqueous solutions (Seignette salt, potassium dihydrogen phosphate) and melts (single-component and multicomponent systems). Under his direction, the first factory for the synthesis of crystals was set up.

Synthetic crystals of quartz are obtained under hydrothermal conditions. Small crystals (seeds) of various crystallographic directions are cut from natural quartz crystals. Although quartz is very common in nature, supply does not meet the demands of technology. Furthermore, natural quartz contains many impurities. Synthetic crystals of quartz of up to 15 kg are grown in autoclaves over a period of many months; the growth period for high-purity crystals (large flawless quartz crystals used in optical devices) can reach several years.

Table 1. The most common synthetic crystals
Name	Chemical formula	Method of growing	Average crystal size	Use
Quartz	SiO₂	Hydrothermal	1–15 kg, 300 × 200 × 150 mm	Piezoelectric transducers, jewelry, optical instruments
Corundum	Al₂ O₃	Verneuil and Czochralski methods, zone melting	Rods of diameter 20–40 mm, length up to 2 m; plates 200 × 300 × 30 mm	Instrument-making and watchmaking industries, jewelry
Germanium	Ge	Czochralski method	100 g to 10 kg, cylinders 200 mm × 500 mm	Semiconductor devices
Silicon	Si	Czochralski method	100 g to 10 kg, cylinders 200 mm × 500 mm	Semiconductor devices
Halides	KCI, NaCI	Czochralski method	1 to 25 kg, 100 × 100 × 600 mm	Scintillators
Seignettesalt	KNaC₄ H₄ O₆–4H₂ O	Crystallization from solution	1–40 kg, 500 × 500 × 300 mm	Piezoelectric elements
Potassium dihydrogen phosphate	KH₂ PO₄	Crystallization from solution	1–40 kg, 500 × 500 × 300 mm	Piezoelectric elements
Yttrium aluminum garnet	Y₃ AI₅ O₁₂	Czochralski method, zone melting	40 × 40 × 150 mm 30 × 200 × 150 mm	Lasers, jewelry
Yttrium iron garnet	Y₃ Fe₅ O₁₂	Crystallization from solutions of melts	30 × 30 × 30 mm	Radioacoustics, electronics
Gadolinium gallium garnet	Gd₃ Ga₅ O₁₂	Czochralski method	20 × 30 × 100 mm	Substrates for magnetic films
Diamond	C	Crystallization at ultrahigh pressures	0.1–3 mm	Abrasives
Lithium niobate	LiNbO₃	Czochralski method	10 × 10 × 100 mm	Piezoelectric and ferroelectric elements
Naphthalene	C₁₀H₈	Kyropoulos method	Blocks of several kilograms	Scintillation instruments
Potassium biphthalate	C₈ H₅ O₄ K	Crystallization from aqueous solutions	40 × 100 × 100 mm	X-ray analyzers, nonlinear optics
Calcite	CaCO₃	Hydrothermal	10 × 30 × 30 mm	Optical devices
Cadmium sulfide	CdS	Growth from gaseous phase	Rods 20 × 20 × 100 mm	Semiconductor devices
Zinc sulfide	ZnS	Growth from gaseous phase	Rods 20 × 20 × 100mm	Semiconductor devices
Gallium arsenide	GaAs	Gas transport reactions	Rods 20 × 20 × 100 mm	Semiconductor devices
Gallium phosphide	GaP	Gas transport reactions	Rods 20 × 20 × 100 mm	Semiconductor devices
Molybdates of rareearth elements	Y₂(MoO₄)₃	Combined Czochralski method	10 × 10 × 100 mm	Lasers
Zirconium dioxide	ZrO₂	Induction heating in a cold container	Blocks of approximately 2 kg, columnar crystals 100 × 10 × 50 mm	Jewelry
Hafnium dioxide	HfO₂	Induction heating in a cold container	Blocks of approximately 2 kg; columnar crystals 100 × 10 × 50 mm	Jewelry
Calcium tungstate	CaWO₄	Induction heating in a cold container	10 × 10 × 100 mm	Lasers
Yttrium aluminate	YAIO₃	Czochralski method	10 × 10 × 100 mm	Lasers
Aluminum (tubes of various cross section)	AI	Stepanov method	length 10³mm, diameter 3–200 mm	Metallurgy

Because geometrically regular crystals are often associated with gems, the efforts of many scientists have been directed toward the synthesis of, for example, diamonds, rubies, aquamarines and sapphires. Synthetic ruby crystals (minute dark crimson crystals) were obtained in the early 19th century from solutions in melts of potash and sodium carbonate. Later, at the end of the century, the French scientist Verneuil invented a special apparatus, subsequently improved, for producing synthetic crystals of ruby. Here, Al₂ O₃ powder with an additive of a few percent Cr₂ O₃ is fed continuously into the zone of a furnace where hydrogen is burning in oxygen. Drops of the molten mass then fall onto a cooler segment of the seed crystal and immediately crystallize. In the USSR, apparatus operate according to the system of S. K. Popov, which makes possible the production of synthetic crystals of ruby in the shape of rods with a diameter from 20 to 40 mm and a length of up to 2 m. These crystals are used in lasers and yarn carriers and in the glass used in space instruments. A large fraction of the synthetic crystals of ruby is used in the watchmaking industry, although the major consumer is the jewelry industry. The addition of impurities in the form of the salts of Ti, Co, and Ni to Al₂ O₃ permits the production of synthetic crystals having colors similar to those of such natural gems as sapphires, topazes, and aquamarines.

Synthetic diamond crystals were obtained in the 1950’s from graphite powder mixed with Ni. The mixture was pressed into small (2–3 cm) disks, which were then heated to 2000°-3000°C at pressures of 100,000–200,000 atmospheres. Under these conditions, graphite is converted into diamond. The size of synthetic crystals of diamond is of the order of tenths of a millimeter, although under special conditions it is possible to obtain crystals of up to 2–3 mm. The diamond industry in the USSR was established mainly to meet the requirements of drilling technology. Synthetic crystals of diamond able to compete with the natural diamonds used in jewelry have so far been obtained in only small quantities.

The 1950’s saw the development of the industry for the synthesis of organic crystals. These crystals, which include those of naphthalene, stilbene, tolan, and anthracene, are used in scintillation counters. The synthesis is carried out mainly by the Czochralski method. In size, the crystals are comparable to large inorganic (water-soluble) crystals. The most widely used semiconductor crystals (Ge, Si, Ga, As) do not occur in nature. All are grown from melts; they are cylindrically shaped, with a diameter ranging from 10 to 20 cm and a length ranging from 30 to 50 cm.

Synthetic crystals of iron garnets and emeralds are grown under laboratory conditions from solutions of the melts. However, these methods have not yet been developed industrially. Studies are currently under way on the industrial production of synthetic gemstones from yttrium aluminum garnets (garnetites) and zirconium and hafnium dioxides (flanites). These synthetic crystals have a wide range of colors and resemble emeralds, topazes, and diamonds because of their high light refraction.

REFERENCES

Fedorov, E. S. “Protsess kristallizatsii.” Priroda, December 1915.
Vul’f, G. V. Kristally, ikh obrazovanie, vidistroenie. Moscow, 1917.
Shubnikov, A. V. Kak rastut kristally. Moscow-Leningrad, 1935.
Ansheles, O. M., V. B. Tatarskii, and A. A. Shternberg. Skorostnoe vyrashchivanie odnorodnykh kristallov iz rastvorov. [Leningrad] 1945.
Popov, S. K. “Novyi proizvodstvennyi metod vyrashchivaniia kristallov korunda.” Izv. AN SSSR: Seriia fizicheskaia, 1946, vol. 10, nos. 5–6.
Shternberg, A. A. Kristally v prirode i tekhnike. Moscow, 1961.
“Usloviia rosta i real’naia struktura kvartsa.” In IV Vsesoiuznoe soveshchanie po rostu kristallov. Yerevan, 1972. Part 2, p. 186.
Mil’vidskii, M. G., and V. B. Osvenskii. “Poluchenie sovershennykh monokristallov poluprovodnikov pri kristallizatsii iz rasplava.” Ibid. Part 2, p. 50.
Bagdasarov, Kh. S. “Problemy sinteza krupnykh tugoplavkikh opticheskikh monokristallov.” Ibid. Part 2, p. 6.
Timofeeva, V. A., and I. B. Dokhnovskii. “Vyrashchivanie ittrievo-zhelezistykh granatov iz rastvorov-rasplavov na tochechnykh zatravkakh v dinamicheskom rezhime.” Krislallografiia, 1971, vol. 16, issue 3, p. 616.
Iakovlev, Iu. M., and S. Sh. Gendelev. Monokristally ferritov v radioelektronike. Moscow, 1975.

V. A. TIMOFEEVA