ISSN 1063-7745, Crystallography Reports, 2007, Vol. 52, No. 6, pp. 945–952. © Pleiades Publishing, Inc., 2007. Original Russian Text © D.S. Shaœtura, A.A. Enaleeva, 2007, published in Kristallografiya, 2007, Vol. 52, No. 6, pp. 981–988.

QUASICRYSTALS

Fabrication of Quasicrystalline Coatings: A Review D. S. Shaœtura and A. A. Enaleeva Russian Research Centre Kurchatov Institute, pl. Akademika Kurchatova 1, Moscow, 123182 Russia e-mail: [email protected] Received March 29, 2007

Abstract—Unusual properties of quasicrystals—a low friction coefficient; high hardness, wear resistance, and oxidation; and low electrical and thermal conductivities—make quasicrystalline materials promising for practical applications. Methods of fabrication of quasicrystalline coatings and examples of their commercial application are considered. PACS numbers: 61.44.Br, 81.15.-z DOI: 10.1134/S1063774507060041

CONTENTS Introduction 1. Properties of Quasicrystals 1.1. Thermal Conductivity 1.2. Optical Properties 1.3. Tribological Properties 1.4. Oxidation Durability 2. Methods for Obtaining Quasicrystalline Coatings 2.1. Thermal Spraying 2.2. Electrochemical Deposition 2.3. Methods of Physical Deposition 3. Examples of Practical Applications of Quasicrystalline Coatings Conclusions

1. PROPERTIES OF QUASICRYSTALS Quasicrystals belong to a peculiar type of solids: with respect to the degree and character of ordering, they form an intermediate class between amorphous and crystalline materials. The quasicrystalline materials that were discovered in 1984 [2] were thermodynamically unstable. Various thermodynamically stable quasicrystals, such as Al–(Fe, Ru, Os)–Cu and Al–(Mn, Re)–Pd [3–7], were found later. For example, the quasicrystalline phase in the Al–Cu–Fe system is stable up to the melting temperature 1135 K (862°ë) [3]. The structural quality of thermodynamically stable quasicrystals with a face-centered icosahedral structure,

INTRODUCTION To date, more than a hundred of different quasicrystalline alloys based, for example, on aluminum, magnesium, zinc, zirconium, cadmium, and titanium have been obtained and investigated [1]. Quasicrystals have properties that are of great interest for practical applications. These properties include a low friction coefficient; high hardness, wear resistance, and oxidation; and low electrical and thermal conductivities. Application quasicrystals are limited by their high brittleness and low deformability at room temperature. These drawbacks can be overcome by using quasicrystals in the form of multiphase and composite materials or as coatings. Quasicrystalline aluminum-based alloys, such as Al–Cu–Fe, are most attractive for commercial use. The components entering the composition of this alloy are not toxic, a circumstance that expands their range of application. 945

Fig. 1. Electron diffraction pattern of a grain of a quasicrystalline 300-nm-thick Al–Cu–Fe film, recorded along the fivefold axis [8].

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Table 1. Thermal conductivity, W m–1 K–1 Copper Aluminum alloys Stainless steel i-Al65Cu20Fe15 i-Al–Mn–Pd i-Al–Pd–Re Fused silica

400 92–220 14–88 2 1.6 0.8 1.36

Table 2. Microhardness and friction coefficient Material Aluminum alloy Steel Copper Al65Cu20Fe15 quasicrystal Al64Cu18Fe8Cr8 quasicrystal Al67Cu9Fe10.5Cr10.5Si3 quasicrystal Al65Cu20Fe15 quasicrystalline coating Al64Cu18Fe8Cr8 quasicrystalline coating Al67Cu9Fe10.5Cr10.5Si3 quasicrystalline coating

Micro- Friction coefficient hardness HV, GPa on steel on diamond 0.87 1.2 0.48 5.2 5.5 7

0.55 0.22 0.24 0.14 0.17 0.13

0.23–0.37 0.11–0.32 0.12–0.42 0.15–0.19 0.1–0.17 0.09–0.17

5.4

0.19

0.08–0.2

5.5

0.22

0.07–0.22

5.8

0.19

0.07–0.23

which was revealed in X-ray and neutron diffraction experiments, is the same as in conventional crystals of highest quality. Figure 1 shows the electron diffraction pattern of a quasicrystalline Al66Cu22Fe12 film, which was recorded at the Russian Research Centre Kurchatov Institute [8, 9]. Quasicrystals have a number of properties that are not characteristic of conventional metal alloys. In particular, they are characterized by low thermal conductivity and high resistivity. The resistivities ρ of quasicrystalline Al–Pd–Re and Al–Cu–Fe systems at liquid-helium temperatures (4.2 K) are 1 Ω cm and 3–10 mΩ cm, respectively. The temperature dependence of the resistance is of the nonmetallic type; i.e., the resistance decreases with an increase in temperature and the ratio R = ρ (4.2 ä)/ρ (295 K) reaches approximately 100 for Al–Pd–Re and 2 for Al–Cu–Fe [7]. With an increase in the structural quality, the resistance of quasicrystalline aluminum-based alloys increases. These peculiar electronic properties of quasicrystals are related to the existence of a pronounced pseudogap in the electron density of states at the Fermi level [10, 11], as was revealed in photoemission and tunnel experiments [12–14].

1.1. Thermal Conductivity Experiments on heat transport led to an intriguing result: the vibrational modes with energies exceeding 100 K are strongly coupled in quasicrystals. This effect leads to a very low thermal conductivity: about 1 W/mK, which very weakly depends on temperature above 100 K. Such values of thermal conductivity are typical of dielectric glasses (Table 1) [15–18]. The thermal conductivity of quasicrystals is lower than that of copper by a factor of 200, several times smaller than that of stainless steel, and is comparable with that of fused silica. The very low thermal conductivity can be a valuable property, especially in combination with a low friction coefficient and plasticity at high temperatures. 1.2. Optical Properties The results of optical experiments on icosahedral quasicrystals demonstrate a significant decrease in magnitude or even absence of the conventional Drude peak (which is maximum at zero frequency) in the optical conductivity, whereas at higher frequencies this peak is observed [19], an effect indicating very intense interband electronic transitions. The reflectance of all stable and highly ordered icosahedral quasicrystals is about 0.6 and is almost independent of wavelength in the range from 20 µm to 300 nm [19–22]. 1.3. Tribological Properties Investigation of bulk samples and coatings based on quasicrystalline alloys of different compositions (Al65Cu20Fe15, Al64Cu18Fe8Cr8, Al67Cu9Fe10.5Cr10.5Si3, Al70.9Cu9Fe10Cr10B0.1, and Al75.2Ni11.5Co10.6Si2.7; the subscripts indicate at %) showed that they possess a good combination of microhardness, friction coefficient, and wear resistance [15, 23–26]. Table 2 contains the values of tribological properties of the aluminum alloy AU4G (analog of AK4-I), low-carbon steel and copper, bulk quasicrystalline samples of the abovementioned materials, and quasicrystalline coatings from these materials [15]. The table demonstrates that the microhardness of quasicrystalline coatings ranges from 5.4 to 5.8 GPa (540–580 HV) and is very close to that of bulk quasicrystals: 5.2–7.0 GPa (520–700 HV); however, it exceeds the microhardness of copper and the aluminum alloy AU4G by a factor of 6–10 and the microhardness of low-carbon steel by a factor of 4–5. The friction coefficient varies with the indenters used; the average friction coefficient for bulk quasicrystals is 0.15, a value that is smaller than the friction coefficient of the aluminum alloy AU4G by a factor of 2–3. The friction coefficients of quasicrystalline coatings can be somewhat smaller or larger in comparison with bulk quasicrystals, depending on the indenters used, but not larger than the friction coefficient of steel and always smaller than that of the aluminum alloy AU4G. To date, it is unclear if the low friction coefficient in quasicrysCRYSTALLOGRAPHY REPORTS

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tals is related to the peculiar structure of quasicrystals and the absence of periodicity in their structure [27] or to something else. Investigations of the friction of quasicrystals in high vacuum showed that a clean surface of quasicrystals has a higher friction coefficient in comparison with an oxidized surface [28]. However, quasicrystals have a lower friction coefficient than their closest crystalline analogs. At room and lower temperatures, quasicrystals demonstrate enhanced hardness and are brittle, whereas at high temperatures (above ~1000 K) they become plastically deformable [29–31]. 1.4. Oxidation Durability Oxidation of aluminum quasicrystalline alloys leads predominantly to the formation of aluminum oxide [32]. Investigations of the oxidation of quasicrystals showed that this process is significantly hindered in comparison with the case of the crystalline phase. Aluminum oxide grown on quasicrystals is more resistant than that obtained on the closest crystalline phases [33], although the oxide film formed on a quasicrystal is thinner than that on conventional aluminum. 2. METHODS FOR OBTAINING QUASICRYSTALLINE COATINGS Currently, a large number of methods for fabricating quasicrystalline coatings have been developed. A large part of quasicrystalline alloys are thermodynamically stable; therefore, conventional methods can be used to form quasicrystalline coatings. The methods for obtaining quasicrystalline coatings can be divided into two groups with respect to the coating type (powder or film). To fabricate powder coatings, gas-thermal and electrochemical methods are used. Various physical methods of chemical vapor deposition are used to obtain film coatings. 2.1. Thermal Spraying The thermal spraying, such as plasma spraying [34– 38] and high-velocity oxy-fuel spraying [39, 40], are generally used to improve the surface properties of products: to increase hardness, reduce the friction coefficient, and enhance wear resistance. The initial material is a preliminarily prepared quasicrystalline powder. Methods of gas-thermal deposition allow fabrication of coatings with a thickness up to 1 mm; they can be applied to objects of any sizes: bridges, vessel bottoms, walls of pipelines, crankshafts, etc. Metals, ceramics, composites, glass, and even plastics can be used as substrates. In the methods of plasma spraying, jet plasma formed by plasmatrons of different constructions is CRYSTALLOGRAPHY REPORTS

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Powder in the flow of a carrier gas

Isolating housing

Water-cooled copper anode

Cathode Water-cooling system

Inlet for working gas

Fig. 2. Schematic of the plasma-deposition system [41].

Oxygen Powder Fuel

Coating

Cooling

Fig. 3. Schematic of the high-velocity oxy-fuel spraying system [42].

used to heat and sputter materials. Figure 2 shows a schematic of a plasma spraying system [41]. An arc discharge is formed between electrodes, the gas (argon or nitrogen) is ionized, and plasma is formed. The plasma is extracted at a high velocity through a hole in the anode into a tube, into which the initial powder, entrained by a jet of carrier gas, is simultaneously supplied. Powder particles are accelerated and melted, completely or partially. At the substrate surface, the flux is cooled and drops solidify and deposit on the substrate to form a coating. The formation of the quasicrystalline phase depends strongly on the particle size, temperature, and cooling rate [37]. The method of highvelocity oxy-fuel spraying is used also to obtain coatings (Fig. 3); during this process, the powder particles are accelerated by a supersonic flow of fuel combustion products [42]. Figures 4–6 show the comparative characteristics of different quasicrystalline coatings obtained by these two methods [43, 44]. The coatings fabricated by thermal spraying are characterized by a number of drawbacks, primarily, high porosity. Owing to rapid cooling, high internal stress arises on the substrate surface and in the coating, thus leading to the formation of cracks.

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200 µm

(a)

(b)

200 µm

Fig. 4. Photographs of Al–Cu–Fe–Si coatings obtained by (a) plasma and (b) high-speed deposition [44].

The coatings obtained by high-velocity oxy-fuel spraying are less porous than those formed by plasma spraying, a circumstance that affects the hardness, friction coefficient, and wear resistance of coatings. Another peculiarity of thermal spraying is that, during sputtering, the powder undergoes phase transitions and is partially oxidized; therefore, the coatings obtained include other phases. Vickers microhardness 680 PD 660 HGD 640 620 600 580 560 540 520 500 Al–Cu–Fe–Cr–B Al–Cu–Fe–Si Al–Ni–Co–Si Fig. 5. Microhardness of quasicrystalline coatings obtained by plasma spraying (PD) and high-velocity oxy-fuel spraying (HGD) [44].

K 0.6 0.5 0.4 0.3 0.2 0.1 0

(a) PD HGD

Al–Cu–Fe– Cr–B

Al–Ni– Co–Si

Wear, g 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0 Al–Cu– Fe–Cr–B

(b) PD HGD

The Cold Spray technology, which is used to form coatings from plastic metals, is free of the above-mentioned drawbacks. The essence of this method is that particles of an initial metal powder are accelerated by a cold (below 300°ë) jet of compressed gas to ultrasonic velocities. The coatings formed by this method have almost zero porosity and their phase composition corresponds to the initial one. Unfortunately, this method can hardly be used for quasicrystalline powders because they have high hardness and low plasticity at room temperature. However, one can choose deposition conditions under which sputtered powders would be heated to temperatures somewhat below the melting point, at which quasicrystals become plastically deformed. In this case, the coating obtained will be less porous and almost single-phase. The initial powder can be obtained in two ways: grinding of large particles of a quasicrystalline material in a mill and gaseous atomization. According to the experimental data [35], the second method is advantageous because it yields coatings of higher quality. The gas-atomization method (Fig. 7) is widely used in industry. A metal alloy with a necessary ratio of elements is melted in a furnace to form a homogeneous liquid phase. The melt is sputtered at a constant rate in intense counter fluxes of an inert gas. Small drops are cooled and solidify. The formation of the quasicrystalline phase depends on the cooling rate. The higher the gas pressure, the smaller the particle size. To obtain small (<1 µm) particles of a regular spherical shape, a centrifuge is used. 2.2. Electrochemical Deposition

Al–Ni– Co–Si

Fig. 6. Tribological properties of the Al–Cu–Fe–Cr–B and Al–Ni–Co–Si coatings obtained by plasma spraying (PD) and high-velocity oxy-fuel spraying (HGD) [44]: (a) friction coefficient and (b) wear.

The recently developed method for deposing quasicrystalline coatings—electrodeposition of a quasicrystalline powder on metal substrates [46]—can be promising for practical applications. Quasicrystalline powder of the Al65Cu23Fe12 alloy is poured into a nickelplating electrolyte to form a suspension. Deposition is performed in the dc mode. The coating consists of quasicrystalline particles surrounded with a thin nickel layer. X-ray diffraction analysis showed that the coating structure corresponds to that of the initial quasicrysCRYSTALLOGRAPHY REPORTS

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talline powder. Friction and wear tests of the coating demonstrated its high quality. A low friction coefficient (much lower than in aluminum) and high wear resistance of the coating were observed. The efficiency of this method is 100 times greater than that of the abovedescribed thermal spraying owing to the equipment cost reduction.

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Melt Pump

Gas

2.3. Methods of Physical Deposition Let us consider the methods for obtaining film coatings. They are based on the principle of transforming a material from the solid state to the vapor phase with subsequent deposition on a substrate. Sputtering of the initial material can be performed using two techniques: thermal evaporation and ion bombardment. In the case of thermal evaporation, a material is heated either resistively or by electron or laser beams. Figure 8 shows a schematic of the system of deposition by electron-beam evaporation. This process is implemented at a residual pressure in the chamber no higher than 10–2–10–3 Pa. A metallic alloy with a stoichiometry corresponding to the quasicrystalline phase is evaporated by a high-power electron beam (electrons are accelerated to energies from 5 to 30 keV). Evaporated atoms deposit on the substrate to form a coating. During deposition, the substrate is heated by the electron beam. Formation of the quasicrystalline phase depends on the substrate temperature. As was experimentally shown, to obtain a single-phase quasicrystalline coating, the substrate should be heated to a temperature above 500°C but no higher than 800°C [47]. If deposition is performed at room temperature, the quasicrystalline phase is formed upon subsequent annealing [48]. The process of ion-plasma deposition is implemented as follows. Positive inert-gas ions, accelerated by an electric field, bombard the cathode, thus causing its sputtering. Sputtered atoms deposit on the substrate to form a thin film. A particular case of ion-plasma deposition is magnetron sputtering. Figure 9 shows a schematic of the magnetron sputtering system. The main elements are a cathode (made of the deposited material), an anode, a system of magnets, and a water-cooling system. An electric field is applied between the target and the substrate holder to excite an abnormal glow discharge. A closed magnetic field near the target surface localizes the discharge near this surface. Positive ions from the abnormal glow-discharge plasma are accelerated by the electric field and bombard the target (cathode), thus causing its sputtering. The electrons emitted from the cathode under ion bombardment arrive at the region of crossed electric and magnetic fields and become trapped. The ionization efficiency and plasma density significantly increase in this region. As a result, the ion concentration near the target surface increases, the ion CRYSTALLOGRAPHY REPORTS

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Nozzle

Chamber

Fine powder

Powder Fig. 7. Schematic of the gas-atomization system [45].

Electron gun for substrate heating

Electron gun for sample heating Substrate

Shutter

Vacuum chamber

Metallic alloy

Fig. 8. Schematic of the electron-beam deposition system.

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Magnetic system Target Sputtering zone Magnetic field lines Sputtered material flow Substrate Substrate holder

S N

N S

S N B

Fig. 9. Schematic of the magnetron sputtering system (S and N are the magnet poles and B is the magnetic field vector).

bombardment of the target becomes more intense, and the target sputtering rate greatly increases. Advantages of this method are a high sputtering rate at low operating voltages (600–800 V) and low working gas pressures (5 × 10–1–10 Pa), a low degree of film contamination, and the possibility of obtaining films of uniform thickness on larger substrate areas. The Al–Cu–Fe–Cr alloy was used in [49] as a material for targets in magnetron sputtering, and an Al–Cu–Fe target made of pressed initial powders was used in [50]. There is a version of ion-plasma sputtering in which the working gas pressure (4 × 10–3 Pa) is much lower than in magnetron sputtering. In this case, sputtering of a material from a pair of cathodes by working-gas ions occurs in a dc magnetic field directed perpendicularly to the cathodes; a ring anode is located between the cathodes. Characteristic features of this method are a low sputtering rate and ballistic flight of sputtered atoms. It was used to obtain quasicrystalline Al–Cu–Fe films at successive deposition of the components with subsequent vacuum annealing [8, 9]. In deposition of quasicrystalline coatings, the main problems are reproduction of the stoichiometry of films with an error no larger than several percent and obtainment of highly homogeneous coatings. Either one [47– 50] or several [8, 9, 51–53] targets can be sputtered. When a composite target is used [54] or several targets from initial elements are simultaneously sputtered, it is necessary to carefully monitor the sputtering rates of the materials to obtain the required stoichiometry of the film. Sputtering from one target, either alloyed or pressed from initial powders, has significant difficulties with transferring the target composition and requires highly homogeneous targets. Application of most thermal methods is hindered in this case because of the change that occurs in the target composition owing to the selective evaporation of elements [48]. 3. EXAMPLES OF PRACTICAL APPLICATIONS OF QUASICRYSTALLINE COATINGS The first commercial application of quasicrystalline materials was their use as cookware coatings (trademark Cybernox, Sitram, France). Powders of the Al71Cu10Fe8.5Cr10.5 composition, obtained by gas atom-

ization, were deposited by the gas-thermal method on steel pans with aluminum bases [55]. It is known that the Ames Laboratory, in cooperation with General Motors and Deere & Co, has investigated the properties of quasicrystalline Al–Cu–Fe coatings intended for decreasing friction, increasing hardness, protecting against corrosion, and decreasing surface energy [56]. These investigations could result in the use of quasicrystals for coating engine components; this suggestion was confirmed only in private communications. Nevertheless, the possibility of using antifriction and heat-protection properties of quasicrystalline coatings in internal combustion engines of trucks is being investigated by Caterpillar [57]. The Al71Co13Cr8Fe8 coating is formed by high-speed deposition on the surfaces of pistons and sealing rings. A quasicrystalline coating deposited on a piston of an internal combustion engine serves a temperature barrier reducing the piston heating. Deposition of a quasicrystalline coating on sealing rings should decrease friction. It is believed that application of quasicrystalline coatings will make it possible to decrease the engine piston temperature and fuel consumption. Developed by Lynntech, Inc., the method of joint electrodeposition of a quasicrystalline powder in a nickel electrolyte is promising for depositing coatings on aluminum cookware, bearings, undercarriages, and engine parts [46]. The strong suppression of the Drude peak in optical conductivity, i.e., low absorption and emission in the IR region, and the high absorption coefficient in the visible spectral range (where solar radiation has a maximum intensity) are combined with the high thermal stability and anticorrosion properties of icosahedral quasicrystals. Owing to the combination of these properties, quasicrystalline films are promising for practical application in selective solar radiation absorbers with a multilayer coating containing a quasicrystalline film as one of the layers [58, 59]. CONCLUSIONS Methods of physical deposition are highly appropriate for obtainng high-quality laboratory samples for scientific investigations. The coatings obtained by these methods have low porosity and homogeneous phase composition. It should be noted that the physical methods of chemical vapor deposition require special vacuum equipment and that the size of coated articles is generally limited by the size of the vacuum system. These methods can be used to deposit coatings on bearings and friction pairs. Methods of gas-thermal sputtering allow formation of coatings of any size and are applicable for products of all types. Electrochemical deposition is the simplest and most inexpensive method from all considered above. This CRYSTALLOGRAPHY REPORTS

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method is used to obtain wear-resistant and nonstick coatings. ACKNOWLEDGMENTS We are grateful to N.A. Chernoplekov for his interest in this study and to M.N. Mikheeva and A.A. Teplov for fruitful discussions. REFERENCES 1. H. S. Jeevan and S. Ranganathan, J. Non-Cryst. Solids 334–335, 184 (2004). 2. D. Schechtman, I. Blech, D. Gratias, et al., Phys. Rev. Lett. 53, 1951 (1984). 3. A. P. Tsai, A. Inoue, and T. Masumoto, Jpn. J. Appl. Phys. 26, L1505 (1987). 4. A. P. Tsai, A. Inoue, Y. Yokoyama, et al., Philos. Mag. Lett. 61, 9 (1990). 5. A. P. Tsai, A. Inoue, and T. Masumoto, Philos. Mag. Lett. 62, 95 (1990). 6. H. Akiyama, Y. Honda, T. Hashimoto, et al., Jpn. J. Appl. Phys. 32, L1003 (1993). 7. Ö. Rapp, in Physical Properties of Quasicrystals, Ed. by Z. M. Stadnik (Springer-Verlag, Berlin, 1999), p. 131. 8. D. S. Shaœtura, M. N. Mikheeva, and A. A. Teplov, Poverkhnost, No. 10, 105 (2001). 9. D. S. Shaœtura, A. G. Domantovskiœ, A. A. Teplov, and E. D. Ol’shanskiœ, Poverkhnost, No. 6, 79 (2002). 10. J. C. Phillips, Phys. Rev. B: Condens. Matter Mater. Phys. 47, 2522 (1993). 11. E. Belin-Ferre, J. Non-Cryst. Solids 334–335, 323 (2004). 12. D. N. Davydov, D. Mayou, and C. Berger, Phys. Rev. Lett. 77 (15), 3173 (1996). 13. T. Schaub, J. Delahaye, C. Gignoux, et al., J. Non-Cryst. Solids 250–252, 874 (1999). 14. Z. M. Stadnik, D. Purdie, M. Garnier, et al., Phys. Rev. B: Condens. Matter Mater. Phys. 55 (16), 10 938 (1997). 15. J. M. Dubois, S. S. Kang, and A. Perrot, Mater. Sci. Eng. A 179–180, 122 (1994). 16. A. L. Pope, T. M. Tritt, M. A. Chernikov, and M. Feuerbacher, Appl. Phys. Lett. 75, 1854 (1999). 17. K. Kirihara, T. Nagata, and K. Kimura, J. Alloys Compd. 342, 469 (2002). 18. Physical Values: A Handbook, Ed. by I. S. Grigor’ev and E. Z. Meœlikhov (Energoatomizdat, Moscow, 1991). 19. C. C. Homes, T. Timusk, X. Wu, et al., Phys. Rev. Lett. 67, 2694 (1991). 20. L. Degiorgi, M. A. Chernikov, C. Beeli, et al., Solid State Commun. 87, 721 (1993). 21. D. Macko and M. Kasparkova, Philos. Mag. Lett. 67, 307 (1993). 22. D. N. Basov, F. S. Pierce, P. Volkov, et al., Phys. Rev. Lett. 73, 1865 (1994). CRYSTALLOGRAPHY REPORTS

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Translated by Yu. Sin’kov

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