Transformation of the corundum structure upon high-temperature reduction

ISSN 1063-7745, Crystallography Reports, 2008, Vol. 53, No. 7, pp. 1112–1118. © Pleiades Publishing, Inc., 2008. STRUCTURE OF INORGANIC COMPOUNDS Transformation of the Corundum Structure upon HighTemperature Reduction A. Ja. Dan’ko, M. A. Rom, N. S. Sidelnikova, S. V. Nizhankovskiy, A. T. Budnikov, L. A. Grin’, and Kh. Sh-o. Kaltaev Scientific and Technological Corporation “Institute for Single Crystals,” National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkov, 61001 Ukraine e-mail: danko@isc.kharkov.ua Received November 1, 2006 Abstract—This paper reports on the results of investigations into the transformation of the corundum structure upon reducing annealing at high temperatures in the range from 1700 to 2050°C. It is established that the reduction results in the transformation of corundum into new phases with a lower oxygen content, including the phase with a spinel structure. These structures are assumed to be stabilized by anion vacancies. A model of the crystal structure of the spinel phase is proposed. This model provides an adequate description of the compound obtained in the experiment. PACS numbers: 61.66.-f DOI: 10.1134/S1063774508070031 At present, the only well-studied oxide that exists in the condensed state in the aluminum–oxygen system is aluminum oxide Al2O3. The data available in the literature on the existence of oxide compounds of aluminum with the lowest valence in the condensed state are very scarce. Filonenko et al. [1] were the first to report on the formation of the AlAl2O4 phase (the alumina spinel) upon the carbothermal reduction of corundum. There are only few publications containing information on the preparation of some compounds (including the aforementioned phase) in the aluminum–oxygen system; however, up to now, special investigations in this direction have not been performed. The present study continues a series of investigations [2–4] concerning the transformation of corundum into phases with a different symmetry upon reducing annealing at high temperatures. and reducing additives (CO, H2, and CO + H2). In the case when the chamber was filled with argon, the formation of the reducing medium in the chamber occurred spontaneously as a result of the interaction of adsorbed oxygen and water vapors with carbon–graphite materials. The concentration of the reducing component in this case did not exceed 5%. The concentration of the reducing component (CO + H2) in the gaseous medium was determined with the use of a Kristall 2000M gas chromatograph. The duration of annealing was varied from 2 to 10 h. The temperature of the samples was controlled using a Marathon MRISCSF integrated infrared pyrometer. The X-ray powder diffraction analysis was carried out on a DRON-1.5 diffractometer (Cu K α1, 2 radiation, pyrolytic graphite(002) monochromator, θ–2θ scan mode). The thickness of the polycrystalline zone formed on a sapphire substrate was determined according to standard techniques [2]. OBJECTS AND METHODS OF INVESTIGATION RESULTS AND DISCUSSION For our investigations, we used plane-parallel sapphire samples and layers of finely dispersed α-Al2O3 powders (with particle sizes of ~15–25 µm) in which the content of majority impurities was less than or equal to 10 ppm. Annealing was performed in a furnace equipped with carbon–graphite heat screens [5] at temperatures in the range from 1700 to 2050°C. After the preliminary vacuum technical preparation was accomplished and the residual pressure (~0.1 Torr) was attained, the chamber was filled up to a pressure of 800 Torr either with argon or with a mixture of argon The X-ray powder diffraction analysis revealed that, upon the reducing annealing, the near-surface layer of the single-crystal samples transforms into a polycrystalline layer with a phase composition different from that of the original material (Fig. 1). The thickness of the polycrystalline zone formed on the sapphire substrate varies in the range from 1 to 80 µm. Figure 2 shows typical X-ray diffraction patterns of the transformed surface polycrystalline layers, which were formed as a result of the reduction of corundum at temperatures ranging from 1700 to 1900°C. The X-ray dif- INTRODUCTION 1112 TRANSFORMATION OF THE CORUNDUM STRUCTURE fraction lines observed for one of the phases (Fig. 2a) correspond to the cubic structure (space group Fd3m). The unit cell parameter of this phase varies from sample to sample in the range 7.933–7.948 Å, which is somewhat different from the data reported by Filonenko et al. [1] and close to the value obtained by Vert et al. [6]. All the diffraction lines observed for the other revealed phase (the H phase) (Fig. 2b) correspond to the hexagonal structure, which can be assigned to either the space group P63/mmc or a space group with similar symmetry elements. As in the case of the spinel phase, the estimated lattice parameters of this structure differ for different samples and have the following values: a ~ 3.107–3.115 Å and c ~ 4.984–4.985 Å. In the surface layer, the phase composition varies from a mixture of phases (Fig. 2b) with a high content of the H phase (>90%) to an almost complete (within the limits of the sensitivity of the X-ray powder diffraction analysis) transformation of the material into the spinel phase (Fig. 2a). Annealing of thin layers (with a thickness of ~0.5 mm) composed of α-Al2O3 powders leads to similar phase transformations. Both phases are stable under normal conditions. The observation of the synthesized compounds during a period of several years did not reveal any variation. The X-ray diffraction pattern of the transformed surface layer remains unchanged after annealing at a temperature of ~700–800°C for 10 h in air. Insignificant changes in the X-ray diffraction pattern of the transformed surface layer are observed after annealing under similar conditions at a temperature of 1000°C, whereas the annealing at a temperature of ~1300°C leads to a complete transformation of both phases into corundum. This transformation is accompanied by an increase in the weight of the sample. Under the assumption that the revealed increase in the weight of the sample occurs as a result of oxygen absorption, we can evaluate the composition of new phases. It has been established that the phase transition of the material of the transformed layer on the surface of the sapphire substrate to corundum is attended by the absorption of ~0.15–0.25 oxygen atoms per molecule of the corundum thus formed. According to these estimates, the compound formed on the sapphire surface has the composition Al2.8–2.9O4. The transformation into corundum upon oxidation of the powders is accompanied by a more considerable absorption of oxygen. In this case, the material absorbs ~0.36–0.58 oxygen atoms per molecule of the newly formed corundum. From the aforesaid, it follows that, prior to oxidation, the initial compound had the composition Al3O3.95–3.63. The observed difference, most likely, can be explained by the fact that the transformed layer on the sapphire surface consists of phases with a low oxygen content and partially oxidizes at the expense of oxygen diffusing into this layer from sapphire. Further investigations included the construction of the models describing the crystal structures of the CRYSTALLOGRAPHY REPORTS Vol. 53 No. 7 2008 1 1113 2 10 mm Fig. 1. Illustration of the transformation of the sapphire surface due to the reducing annealing: (1) the initial polished sapphire sample and (2) the sapphire sample with a transformed near-surface layer ~30 µm thick after annealing. revealed phases, the calculations of the model X-ray diffraction patterns, and comparison of the results obtained from these calculations with the experimental data. The construction and analysis of the models of the crystal structures and the calculation of the model X-ray diffraction patterns were carried out with the Diamond 2.1e [7] and POWDER CELL (Version 2.3) [8] software packages. The solution and simulation of the crystal structures were performed using the X-ray diffraction patterns of the reduced powders. When constructing the model of the spinel structure, we carried out a comparative analysis of the X-ray diffraction patterns calculated for the structure of the compound with a variable composition (space group Fd3m, Fig. 3) for all diffraction lines within the 2θ range from 10° to 150°. The theoretical X-ray diffraction patterns were calculated by varying the coordinate of the oxygen atom at the 32e symmetry position in the range 0.3600–0.3875 for different occupancies of the symmetry positions of aluminum (8a and 16d) and oxygen (32e), which corresponded to the composition of the compound in the Al2O3–Al3O4–Al3O4 – x region. The temperature factor in the calculations was ignored (B = 0). The results of the calculations were compared with the experimental X-ray diffraction pattern of the spinel phase. The X-ray diffraction characteristics of this phase are listed in Table 2. The unit cell parameter of the spinel structure was determined to be 7.9437 Å. According to the amount of absorbed oxygen, the composition of the compound under investigation was estimated as Al3O3.76. The R factor served as a criterion for reliability of the model. In the simulation of the structure, good agreement was achieved for the model with the atomic coordinates presented in Table 1. The dependences of the R factor on the occupancy of the 32e symmetry position according to the results obtained from the calculations of the X-ray diffraction patterns for the 1114 DAN’KO et al. I, counts/s 4000 800 0 800 3200 Sp (664) (931) (840) (660)(822) (751)(555) (800) (533) (620) (531) 400 0 H Sp (844) H 1200 (b) (422) (333)(511) (222) (111) (400) (220) 2000 (444) (711)(551) (642) (553)(731) (440) (311) (844) (a) Sp 400 Sp Sp Sp Sp Sp Sp Sp Sp Sp Sp Sp H Sp H Sp H H 0 10 30 50 Sp H H Sp H Sp H H H H Sp Sp Sp Sp H H Sp H SpH H 70 Sp 90 110 130 H 150 2θ, deg Fig. 2. X-ray diffraction patterns of the polycrystalline layer on the (0001) surface of the sapphire sample: (a) spinel and (b) spinel + H phase with the 〈001〉 texture (Cu K α radiation). 1, 2 c a b 8a—Al 16d—Al 32e—O Fig. 3. A model of the crystal structure of the spinel phase according to the calculations performed with the Diamond 2.1e program [7]. compound of variable composition Al3O4 – x in comparison with the experimental X-ray diffraction pattern are shown in Fig. 4. As can be seen from this figure, the best agreement is observed for the occupancies in the range 0.88–0.96, which corresponds to the compound of the composition Al3O3.52–3.84; i.e., the structure of the compound contains anion vacancies. It should also be noted that there is a good agreement with the experi- mental results for the compound of the composition Al3O3.76. The results of the calculations of the theoretical X-ray diffraction pattern for the occupancy equal to 0.94 (Al3O3.76) (I calcd) in comparison with the experimental data (I exp) and the quantity |∆I | = |I exp – I calcd | are presented in Table 2. According to the calculation with the inclusion of all 22 observed diffraction lines, the R CRYSTALLOGRAPHY REPORTS Vol. 53 No. 7 2008 TRANSFORMATION OF THE CORUNDUM STRUCTURE factor is approximately equal to 0.11, whereas the calculation performed without regard to the diffraction lines observed in the range 110°–150° (where the temperature factor is more pronounced) gives R ≈ 0.07. Therefore, the proposed model provides an adequate description of the compound obtained in the experiment. The revealed H phase cannot be identified as one of the known phases Al2O3 with a hexagonal structure (β, χ, ε). A similar phase with a hexagonal lattice and the unit cell parameters a = 3.11 Å and c = 4.97 Å in the products of the reduction of corundum was observed simultaneously with the spinel phase in the experiments performed by Vert et al. [6]. Upon the condensation of sublimates of reduced α-Al2O3, Beletskiœ and Rapoport [9] also obtained a compound (supposedly of the composition Al2O) with a lattice assigned to one of the 4 4 space groups D3h, C 6v , or D 6h with the unit cell parameters a = 3.1 ± 0.01 Å, c = 4.99 ± 0.01 Å, and c/a = 1.61. A more detailed analysis of the crystal structure and the X-ray diffraction characteristics of the synthesized compound were not reported by those authors. In our investigations, we failed to establish, within the limits of experimental error, any significant dependence of the amount of absorbed oxygen on the ratio between the content of the spinel phase and the content of the H phase in the products of corundum reduction R 0.14 Al3O3.76 Al3O4 0.13 1115 Table 1. Atomic coordinates in the model crystal structure of the spinel phase (space group Fd3m) Atom Position Al Al O 8a 16d 32e Coordinates x y z 0 0.625 0.38 0 0.625 0.38 0 0.625 0.38 Occupancy 1 1 0.88–0.96 which would indicate a noticeable difference in their structural formulas. Therefore, we cannot argue that the H phase has the composition Al2O; however, from a comparison of all the data obtained in our studies with those presented in [6, 9], it can be assumed that, quite possibly, we are dealing here with the same structure. This assumption is also supported by the fact that needlelike formations similar to those observed in [9] (Fig. 5) exist in the transformed surface layer containing the H phase with a clearly pronounced 〈001〉 texture (Fig. 2b). The averaged X-ray diffraction characteristics of the H phase are presented in Table 3. To the best of our knowledge, data for the Al–O compound with the obtained set of interplanar distances are not available in the reference literature. A similar phase Al2O3 with close values of the unit cell parameters (a ~ 3.10 Å and c ~ 4.99 Å) is found in the reference book of X-ray diffraction analysis [10]; however, more detailed data are not reported. In our calculations, the hypothetical structure of the compound with a variable composition (Table 4) was considered as an approximate model of the H phase. The occupancies of the 2a symmetry position, which corresponded to the compositions Al2O3 and Al2O2.667 1 0.12 0.11 0.10 0.09 0.08 2 0.07 0.84 0.88 0.92 40 µm 0.96 1.00 Occupancy Fig. 4. Dependences of the R factor on the occupancy of the 32e symmetry position according to the (1) calculation with the inclusion of all the observed diffraction lines and (2) experimental data for diffraction lines in the range 2θ < 110°. CRYSTALLOGRAPHY REPORTS Vol. 53 No. 7 Fig. 5. Micrograph (in reflected light) of the (0001) surface of the sapphire sample after the reducing annealing at a high temperature of ~1800°C for 5 h. The surface layer with a thickness of ~10 µm contains the spinel phase (~30%) and the H phase (~70%) with the 〈001〉 texture. 2008 1116 DAN’KO et al. Table 2. X-ray diffraction characteristics of the spinel phase No. d, Å hkl I calcd I exp |∆I| No. d, Å hkl I calcd I exp |∆I| 1 2 3 4 5 * 6 7 4.5863 2.8085 2.3951 2.2931 1.9859 1.8224 1.6215 1.5288 0.15 3.07 0 2.7 4.35 * 0.86 3.53 15 * * 16 0.993 0.9705 0.9633 0.9362 0.9173 13.7 2.17 77.5 * 2.2 3.8 12.5 * 6 * 2.49 * 2.2 0.55 1.15 * 1.16 * * 18 19 0.9112 0.8881 0.8719 * 9.6 1.9 * 4.12 1.04 * 20 21 22 * 0.8667 0.8468 0.8327 0.8108 0.7984 11.76 0 0 1.24 2.27 12.71 3.16 0.7 13.72 0.71 0.15 0.01 2.3 19.25 64.83 0.52 0.49 0 0.66 * * 0.99 17 800 733 644 660 822 751 555 662 840 911 753 842 664 931 844 933 755 771 11.1 * * 4.5 1.4043 1.3427 1.324 1.256 1.2114 1.1976 1.1466 1.1123 * 1.5 15 51 * * 0.8 4.25 13.83 * 13 14 1.0615 1.0342 11.55 27.93 100 2 54.35 0.27 10.86 6.29 32.94 75.01 1.59 0 4.35 11.35 0.72 7.16 0.09 0.95 5.6 8.9 11.41 11.7 31 100 4.7 50 * 10 35.7 8 * 9 10 11 * 12 * 111 220 311 222 400 331 422 333 511 440 531 442 620 533 622 444 711 551 642 553 731 6.3 16.5 0.7 3.81 * Peaks with a low intensity were disregarded in the calculations. Table 3. X-ray diffraction characteristics of the H phase No. d, Å hkl I calcd I exp |∆I| No. d, Å hkl I calcd I exp |∆I| 1 2 3 4 5 6 7 8 9 10 11 12 13 2.695 2.494 2.371 1.830 1.556 1.415 1.348 1.320 1.301 1.247 1.186 1.132 1.047 100 002 101 102 110 103 200 112 201 004 202 104 203 100 49 47 13 60 5 12 33 3 8 5 13 1 100 64 44 32 27 11 18 40 20 20 10 26 18 0 15 3 19 33 6 6 7 17 12 5 13 17 14 15 16 * * 17 18 * 19 20 21 * 1.019 0.998 0.973 0.943 0.936 0.915 0.898 0.884 0.869 0.845 0.831 0.802 210 211 114 212 105 204 300 301 213 302 006 205 10 2 24 7 1 9 12 0 2 15 3 2 12 15 9 * * 2 2 * 2 3 9 * 2 13 15 * * 7 10 * 0 12 6 * * Peaks were disregarded in the calculations. (Al3O4), were equal to 0.500 and 0.333, respectively. The temperature factor in these calculations was ignored (B = 0). Table 3 presents the results obtained from the comparative analysis of the experimental (I exp) and calculated (I calcd) X-ray diffraction patterns for this structure with the unit cell parameters a ~ 3.112 Å and c ~ 4.988 Å, when the occupancy of the 2a symmetry position corresponds to the composition α-Al2O3, and CRYSTALLOGRAPHY REPORTS Vol. 53 No. 7 2008 TRANSFORMATION OF THE CORUNDUM STRUCTURE 1300 I, counts/s 1000 δ Sp 500 δ Sp δ Sp δ H Sp X 0 10 H Sp δ X H Sp Mo 30 1117 Sp δ Sp X X 50 δ H X 70 2θ, deg Fig. 6. X-ray diffraction pattern of the crystallized reduced melt. Designations: Sp is the spinel, H is the H phase, δ is the δ phase [11], Mo is molybdenum (the container material), and X denotes additional unknown diffraction lines. the quantities |∆I| = |I exp – I calcd |. All the principal diffraction lines of the model and experimental structures coincide; however, satisfactory quantitative agreement between the calculated and experimental data is not observed (the R factor exceeds 0.3). Although the approximate model under consideration does not provide an adequate description of the obtained structure, it can be used for a qualitative description of the observed corundum–spinel phase transformations [4]. The number of oxygen-deficient structures formed upon reduction of corundum is not exhausted by the two aforementioned structures. After the reduction at temperatures above 1950°C, including the reduction of the melt, the situation is significantly complicated and the products of reduction are multiphase systems of more complex composition. In this case, the X-ray diffraction patters contain not only the lines associated with the corresponding spinel phases and the H phase but also a number of other additional lines. Some of these additional lines can be assigned to the compounds known from the reference literature, whereas the other lines cannot yet be uniquely identified. As an example, Fig. 6 shows the X-ray diffraction pattern of the system formed as a result of the reduction of the α-Al2O3 melt in the medium containing 10% hydrogen. It can be seen from this figure that, upon reduction, corundum completely transforms into other phases. Apart from the lines that can be assigned to the aforementioned phases, Table 4. Atomic coordinates in the model crystal structure of the H phase (space group P63/mmc) Atom Position Al O O 2b 2c 2a Coordinates x y z 0 0.3333 0 0 0.6667 0 0.25 0.25 0 CRYSTALLOGRAPHY REPORTS Vol. 53 Occupancy 1 1 0.5–0.333 No. 7 2008 as well as to the container material (molybdenum), the X-ray diffraction pattern exhibits lines that can be attributed to the δ phase [11] and also additional unknown diffraction lines (denoted as X in the X-ray diffraction pattern). CONCLUSIONS Thus, the results obtained in this study have demonstrated that the high-temperature reduction of α-Al2O3 is accompanied by the transformation of corundum into new phases with a lower oxygen content. It is obvious that these phases, in particular, the spinel phase (Fig. 3, Table 1), with a structure in which aluminum ions are distributed over both octahedral and tetrahedral positions, can exist only in the presence of stabilizers. A vacancy formed in the structure of aluminum oxide owing to the reduction “localizes” two excess electrons. Therefore, the vacancies can decrease the valence of the surrounding cations and play the role of stabilizers of the structures of these new phases, much as water and different impurity ions (for example, Li+, Na+, and K+) are stabilizers of the structures of the metastable modifications of aluminum oxide Al2O3 [12]. REFERENCES 1. N. E. Filonenko, I. V. Lavrov, O. V. Andreeva, and R. L. Pevzner, Dokl. Akad. Nauk SSSR 115 (3), 583 (1957). 2. N. S. Sidelnikova, M. A. Rom, A. Ya. Danko, et al., Funct. Mater. 11 (1), 26 (2004). 3. A. Ya. Dan’ko, M. A. Rom, N. S. Sidel’nikova, et al., Poverkhnost, No. 11, 89 (2005). 4. A. Ya. Dan’ko, M. A. Rom, N. S. Sidelnikova, et al., Funct. Mater. 12 (4), 725 (2005). 5. A. Ya. Dan’ko, N. S. Sidel’nikova, G. T. Adonkin, et al., Funct. Mater. 4 (1), 92 (1997). 1118 DAN’KO et al. 6. Zh. L. Vert, M. V. Kamantsev, V. I. Kudryavtsev, and M. I. Sokhor, Dokl. Akad. Nauk SSSR 116 (5), 834 (1957). 7. G. Bergerhoff, M. Berndt, and K. Brandenburg, J. Res. Natl. Inst. Stand. Technol. 101, 221 (1996). 8. W. Kraus and G. Nolze, J. Appl. Crystallogr. 29, 301 (1996). 9. M. S. Beletskiœ and M. B. Rapoport, Dokl. Akad. Nauk SSSR 80 (5), 751 (1951). 10. L. I. Mirkin, Reference Book on X-Ray Diffraction Analysis of Polycrystals (Fizmatgiz, Moscow, 1961), p. 626 [in Russian]. 11. ASTM 16-0394 (American Society for Testing Materials, West Conshohocken, PA, United States). 12. A. A. Khanamirova, Alumina and Methods for Decreasing the Content of Impurities in It (ArmSSR Academy of Sciences, Yerevan, Soviet Union, 1983), pp. 40–62 [in Russian]. Translated by O. Borovik-Romanova CRYSTALLOGRAPHY REPORTS Vol. 53 No. 7 2008