I. INTRODUCTION
Determination of the crystal structure of aluminium oxynitride allows better understanding of its physical properties and currently plays an important role because of the increasing use of this group of materials in a few branches of industry, such as refractory and optic materials.
In the AlN–Al2O3 pseudo-binary phase diagram described in the literature, there are three polytypes of aluminium oxynitrides, which differ in the sequence of stacking: Al9O3N7 (27R), Al7O3N5 (21R), and Al6O3N4 (12H) (McCauley, Reference McCauley2002). The possibility for 15R politype to appear in the Al–O–N system by analogy to similar Si–Al–O–N system is postulated (Jack, Reference Jack1976). In literature, only the crystal structure of the three of them is described: Al3O3N (cubic), Al7O3N5 (21R), and Al9O3N7 (27R) (Lejus, Reference Lejus1962; Asaka et al., Reference Asaka, Kudo, Banno, Funahashi, Hirosaki and Fukuda2013a, Reference Asaka, Banno, Funahashi, Hirosaki and Fukudab).
The aim of the work was to solve the crystal structure of the transition phase of Al5O3N3 (15R), co-existing in the composite material with α-Al2O3 (corundum).
II. EXPERIMENTAL
A. Preparation of aluminium oxynitride-bonded composite
The sample was prepared in the process of two-step synthesis, in a graphite furnace in a nitrogen atmosphere. In the first step, the aluminium powder was fired at 1450 °C in a nitrogen atmosphere. The obtained product contained mainly AlN and small amounts of unreacted Al, α-Al2O3, γ-Al2O3 solid solution and several forms of aluminium oxynitrides: Al8O3N6, Al7O3N5, Al6O3N4. In the second step, pure α-Al2O3 (99.9 wt%), homogenised in ethanol, and the AlN powder, obtained in the first step, were mixed in the ratio of 21:79% by weight, palletised and fired at 1650 °C in a nitrogen atmosphere. The goal was to obtain an AlON-bonded alumina ceramic composite. In the sample, the following were identified: α-Al2O3 (corundum) and solid solution isostructural with SiAl4O2N4. Figure 1 presents the result of identification of the phase composition of the corundum material with an oxynitride bonding. Owing to the lack of silicon in the material produced, it was assumed that the identified compound was the 15R polytype, Al5O3N3, which is isostructural with SiAl4O2N4 [PDF 00-042-0160 (ICDD, 2015)].
Figure 1. X-ray pattern of Al2O3–AlON composite material with identified SiAl4O2N4 isostructural with an Al5O3N3 aluminium oxynitride compound [PDF 00-042-0160 (ICDD, 2015)].
B. Experimental methods
One of the reaction products, α-Al2O3 in a synthesised AlON-bonded alumina composite was quantified using the external standard X-ray method. It was found that the corundum content was 73 wt%. From the remaining substrates the stoichiometric composition of the second phase was calculated. The results of calculations revealed that the second phase could be Al5O3N3, whose structure should be solved.
The applied method of crystal structure determination consisted of three stages. In the first stage, the Le Bail method of X-ray pattern decomposition for the extraction of Al5O3N3 (15R) diffraction lines from a two-phase diffractogram was used (Le Bail et al., Reference Le Bail, Duroy and Fourquet1988). For this purpose, the HighScorePlus software (PANalytical, Reference PANalytical2015) was applied. The space group and unit-cell dimensions from the isostructural SiAl4O2N4 phase producing the same X-ray pattern were used as input data [PDF 00-042-0160 (ICDD, 2015)]. After X-ray pattern decomposition only diffraction lines of Al5O3N3 (15R) for structure solving were used and input file for next step was prepared. Diffraction lines of corundum were rejected. Only the strongest (40) reflections below 77.3°2θ and relatively well separated from α-Al2O3 ones were taken for structure solving. The high-angle part of a diffraction pattern of Al5O3N3 (15R) could be subject to errors both in 2θ and intensity values because of low intensity and several overlaps with α-Al2O3 reflections. Next, the direct structure determination in real space was applied for the initial structural model derivation using the Endeavour software (Putz et al., Reference Putz, Schon and Jansen1999; Crystal Impact, 2009). Endeavour is used for solving structures by means of real-space method based on energy minimisation (Lennard–Jones potential). The advantage of the programme is the fact that it allows using a diffraction pattern in the form of discrete values of intensity and position of reflections, which enables testing samples that are not single phase. The last step of solving the crystal structure of Al5O3N3 was Rietveld refinement. In this stage of investigations, HighScorePlus was used.
A PANanalytical X'PERT PRO MPD diffractometer was used for powder X-ray diffraction pattern collection. Data collection within the angular range of 5 to 140°2Θ with CuKα radiation (40 kV, 30 mA), Ni filter, fixed slits, 0.04 rad. Soller slits and X'Celerator detector was carried out.
III. RESULTS
The Al5O3N3 (15R) is trigonal with space group R-3m and refined unit-cell parameters a 0 = 3.0128 Å, c 0 = 41.8544 Å, and volume V = 329.00 Å3. The calculated density is 3.405 g cm−3 and the unit cell contains 33 atoms. The solved crystal structure is presented in Table I and in Figure 2. The model of the structure of Al5O3N3 (15R) polytype has positional disordering in Al2 (6c) site; as a result, one-third of the tetrahedrons from layers L3R and L4R, L8R and L9R, L13R and L14R are rotated by 180°. Those stacking faults between layers L3R–L4R, L8R–L9R and L13R–L14R allow joining oxygen/nitrogen layers in the crystal structure. In consequence, the structure consists of 18 layers and in nine of them the tetrahedral voids are not fully occupied by cations. The empty tetrahedrons in Figure 2 present unoccupied Al2a and Al2b sites. If these both sites Al2a and Al2b were fully occupied, then the unit cell would contain 39 instead of 33 ions. As a result, the stoichiometry corresponding to Al5O3N3 would not be retained, leading to densification in the unit cell. The authors of the work (Asaka et al., Reference Asaka, Kudo, Banno, Funahashi, Hirosaki and Fukuda2013a), who have solved the structure of a similar solid solution of Al7O3N5 (21R), found two separate sites in the structure. They interpreted the obtained result as the existence of five domains differing in their local symmetry: one with R-3m symmetry and four with R-3m symmetry.
Figure 2. Crystal structure of AlON polytype Al5O3N3 (15R): (a) 110 projection, (b) 110 perspective projection, and (c) random projection. The empty tetrahedrons represent unoccupied sites Al2a and Al2b from Table I.
TABLE I. Crystal structure of Al5O3N3 (15R).
Table II presents the average cation–anion distances in the tetrahedrons and octahedrons. In the tetrahedrons, they range from 1.784 to 1.998 Å, whereas in the octahedrons they reach 1.998 Å. According to Shannon, the theoretical values of the cation–anion distance in oxides for Al–O in the tetrahedron are 1.770 Å, and in the octahedron – 1.915 Å (Shannon, Reference Shannon1976). On the other hand, in case of the tetrahedral coordination, the Al–N distances calculated from the data published in ICDD database [PDF 01-080-6097 (ICDD, 2015)] reach 1.883 (×3) and 1.927 (×1) Å, whereas in case of the octahedral coordination the distances calculated from ICDD data are 2.023 Å [PDF 00-046-1200 (ICDD, 2015)]. An analysis of the data contained in Table II allows concluding that in O2/N2 site the oxygen ions may be prevailing, while in the remaining anion sites their prevalence is invisible. In the tetrahedrons composed of Al cations from Al2A and Al2b sites, one of the cation–anion distances is extended to the value of 1.998 and 1.971 Å for Al2a–O1/N1 and Al2b–O2/N2, respectively (Table II). The Al2a and Al2b sites are not fully occupied, giving rise to stacking faults.
TABLE II. Average cation–anion distances in octa- and tetrahedral coordinations in the Al5O3N3 structure.
The obtained result – the solved structure of alumina oxynitride Al5O3N3 (15R) was used to conduct a quantitative analysis of the two-phase mixture consisting of the title compound and corundum by the Rietveld method. The result has been given in Figure 3. The obtained result agrees with the result of a quantitative analysis of corundum in the examined material obtained after its synthesis (73%) within the uncertainty limit.
Figure 3. Results of the Rietveld refinement of Al2O3–AlON composite material (GoF = 1.78; R wp = 3.65; R exp = 2.05).
IV. SUMMARY
The crystal structure of Al5O3N3 was solved by X-ray powder diffraction data using the direct space method. The investigations were conducted on a sample of a two-phase mixture consisting of the title compound and corundum. Reflections of Al5O3N3 were extracted by the Le Bail method. The Al5O3N3 (15R) is trigonal with space group R-3m, unit-cell dimensions a 0 = 3.0128 Å, c 0 = 41.8544 Å, and volume V = 329.00 Å3. It was expected that the crystal structure of Al5O3N3 (15R) was formed from 15R layers, three octahedral and 12 tetrahedral ones. Stacking faults were found between six tetrahedral layers L3R and L4R, L8R and L9R, L13R and L14R. The stacking fault layers are made up of one-third of the tetrahedrons from these layers, rotated by 180°. These stacking faults between layers L3R–L4R, L8R–L9R and L13R–L14R allow joining oxygen/nitrogen layers in the crystal structure. As a result, the structure consists of 18 layers, three octahedral and 15 tetrahedral ones. In nine of these layers 2/3 of the tetrahedral voids are occupied by cations.
SUPPLEMENTARY MATERIAL
The supplementary material for this article can be found at https://doi.org/10.1017/S088571561700001X.
ACKNOWLEDGEMENTS
This paper has been prepared with the financial support of the statutory funds of the Institute of Ceramics and Building Materials (ICiMB).
