The Formation of Cr-Al Spinel under a Reductive Atmosphere

In the present work, for the first time, the possibility of formation of CrAl2O4 was shown from the equimolar mixture of co-precipitated Al2O3 and Cr2O3 oxides under a reductive environment. The crystallographic properties of the formed compound were calculated using the DICVOL procedure. It was determined that it has a cubic crystal structure with space group Fd-3m and a unit cell parameter equal to 8.22(3) Å. The formed CrAl2O4 is not stable under ambient conditions and easily undergoes oxidation to α-Al2O3 and α-Cr2O3. The overall sequence of the phase transformations of co-precipitated oxides leading to the formation of spinel structure is proposed.


Introduction
Aluminum oxide is an important inorganic compound that is extensively used in many industrial applications. It is also an important mineral (e.g., corundum) and a precious gemstone (e.g., ruby and sapphire). It has many crystalline polymorphic phases such as α-Al 2 O 3 , γ-Al 2 O 3 , δ-Al 2 O 3 , θ-Al 2 O 3 , η-Al 2 O 3 , κ-Al 2 O 3 , χ-Al 2 O 3 , and -Al 2 O 3 [1]. Among them, α-Al 2 O 3 is the most stable alumina phase. The corundum structure consists essentially of a dense arrangement of oxygen ions in hexagonal closest packing with Al 3+ ions occupying two-thirds of the available octahedrally coordinated sites.
Due to its structure, α-Al 2 O 3 has some valuable physical and chemical properties such as good acid, alkali, and heat resistances; and high hardness and strength. It is widely used in different fields such as ceramics, surface protective layer materials, refractory materials, and optical materials [2].
Another commonly used form of alumina is γ-Al 2 O 3 . Its structure can be regarded as spinel type in which oxygen atoms are arranged in a cubic close packing and Al atoms occupy the octahedrally and tetrahedrally coordinated sites. γ-Al 2 O 3 , which is also called activated alumina, has a large surface area and strong adsorption capacity, and therefore is widely employed as absorbents, catalyst supports, chromatographic media, and ion exchanges [3][4][5].
Aluminum oxides are usually obtained via the heat treatment of aluminum hydroxides, e.g., boehmite or aluminum salts. The formation of one or another form of alumina depends mainly on the type and properties of the starting material and conditions of the process. In general, all of the transition aluminas transfer into the other modifications at about 600 • C and all are transformed into the thermally stable α-Al 2 O 3 at temperatures above 1000 • C [6].
The physicochemical properties of Al 2 O 3 can be considerably altered by incorporation of second phase particles such as ZrO 2 , SiC, SnO 2 , B 4 C, TiC, Cr 3 C 2 , and Cr 2 O 3 [7][8][9][10]. Such modifications were reported to be beneficial for improving the mechanical properties of Al 2 O 3 -based ceramics [11,12]. Moreover, it was reported that the catalysts supported on Al 2 O 3 -based binary systems are superior in comparison with those supported on Al 2 O 3 alone [13]. An interesting approach to modifying the properties of Al 2 O 3 is the incorporation of Cr 2 O 3 into its structure. Corundum (Al 2 O 3 ) and eskolaite (Cr 2 O 3 ) are isostructural, and Al 3+ and Cr 3+ ions have similar ionic radii. For this reason, they have similar unit cell parameters with a mismatch of~4% [14]. Therefore, at high temperatures, Cr 2 O 3 may form a substitutional solid solution in an Al 2 O 3 lattice by exchange with Al 3+ ions over the full range of compositions without formation of any eutectic. Depending on the content of Cr 2 O 3 or Al 2 O 3 , these solutions can be divided into corundum and eskolaite by the dominant-constituent rule [15]. These materials are used as refractory materials as well as in several other applications [16].
The process of formation of substitutional solid solutions is energy-intensive and requires high temperatures at which the formation of spinel structure compounds can also occur. For instance, Sako [17] and Ping [18] reported the formation of MgAl 2 O 3 from the corresponding oxides at temperatures as high as 1000 • C. The ordinary spinel structure, with a general formula of AB 2 O 4 (where A 2+ and B 3+ are usually metal ions), is related to the arrangement of the octahedra that are the main framework of the spinel structure. This can explain relatively high hardness and high density usually observed for this group of compounds. In the normal spinel structure, the O 2ions form a face-centered cubic array with A 2+ and B 3+ cations occupying one-eighth of the tetrahedrally and one-half of the octahedrally coordinated sites, respectively. In an inverse spinel B(AB)O 4 , alternatively, one-eighth of the tetrahedrally coordinated sites are occupied by cation B 3+ whereas A 2+ and B 3+ each occupy one-quarter of the octahedrally coordinated sites. Usually, spinel compounds crystallize in the high symmetry space group type Fd-3m, but lower symmetries also occur [19]. The mechanism of formation of spinel compounds was investigated by many researchers, particularly Rossi [20], Carter [21], and Navias [22], who performed mechanistic studies related to the formation of magnesium aluminate spinel. Their results showed that the reaction proceeds through interdiffusion of Al 3+ and Mg 2+ ions through the oxygen array. Particularly, Al 3+ ions migrate from the alumina particles to the magnesia ones and Mg 2+ diffuses in the opposite direction, leading to the spinel layer formation at the interface between the alumina and magnesia particles.
The aim of the current research work was to investigate the possibility of the formation of a spinel structure from the aluminum and chromium oxides. It was assumed that under conditions of high temperature and a reductive atmosphere the formation of Cr 2+ ions may occur, which would further diffuse into an Al 2 O 3 matrix.
Moreover, we attempted to investigate the phase transformation of both aluminum and chromium oxides and the interaction between them as a function of temperature.

Preparation of Samples
The mixture of aluminum and chromium oxides was prepared by a co-precipitation method using water solutions of corresponding nitrates as precursors and ammonia water as the precipitator. A mixed metal nitrate solution was prepared with a total concentration of metals equal to 1 M. The nominal Cr/Al molar ratio was 1:1. The ammonia was added into the metal solution until pH = 7.5 under vigorous stirring and slight heating (50 • C). The reaction was carried out for 24 h. After reaction, precipitates were filtered, washed with distilled water, dried in a vacuum dryer below 0.10 atm at 200 • C for 16 h, and then calcined in air at 400 • C for 4 h. Prior to the XRD measurements, the samples were reduced in a stream of pure hydrogen at temperatures ranging from 400 to 1450 • C overnight. Then, the obtained samples were cooled in the flow of an inert gas to ambient temperature and sealed to avoid contact with the air.

Physicochemical Characterization
The phase transformation of the prepared mixture as a function of temperature was investigated by X-ray diffraction analysis. The diffraction patterns were collected using a PANalytical X'Pert Pro MPD diffractometer in Bragg Brentano reflection geometry (Malvern Panalytical Ltd., Royston, UK). The diffractometer was equipped with a CuK α radiation source (λ = 1.5418 Å). Data were collected in the 2θ range of 5 • -80 • with a step size of 0.0167 • and exposure per step of 27 s.
For qualitative and Rietveld quantitative phase analyses, the PANalytical High Score software package (ver. 4.9) was used, combined with the International Centre for Diffraction Data's (ICDD) powder diffraction file (PDF-2 ver. 2020) database of standard reference materials. The structural details for Rietveld quantitative analyses were obtained from the Crystallography Open Database [23]. To estimate the full width at half maximum (FWHM) of the diffraction profiles, the pseudo-Voigt function was applied. Then, after taking into account the broadening caused by the diffractometer, the FWHM was used to calculate the size of the crystallites using the Scherrer equation.
where K = 0.9, a dimensionless shape factor; λ is the wavelength of scattered X-ray radiation; β D is the full width at half maximum of the hkl diffraction peak; and θ max is the scattering angle at the maximum of the hkl diffraction peak. The unit cell parameters for CrAl 2 O 4 were determined by the DICVOL04 algorithm [24].

Theoretical Calculations
In order to confirm the conjecture formulated on the basis of the experimental data, the proposed spinel structure of CrAl 2 O 4 was optimized by computing the analytical gradients of the energy with respect to both unit cell parameters and atom coordinates, treating CrAl 2 O 4 as a 3-dimensional crystal system. The calculations were performed using CRYSTAL17 [25,26] and we employed B3LYP density functional with D3 version of Grimme's dispersion [27]. The used basis set was the POB double-ζ polarization basis set (POB-DZVP) [28]. The initial structural parameters, which were taken as an input, were those found for the spinel structure of MgAl 2 O 4 [29]. The data from the calculations were then compared with the experimental ones.

Results and Discussion
The formation of either separate aluminum and chromium oxides or their substitutional solution during the dehydration of corresponding hydroxides is governed mainly by the temperature of the process and the molar ratio of aluminum to chromium. According to the literature, at a temperature as high as 1250 • C, these oxides can form Cr x Al 2−x O 3 solid solution over a broad range of compositions (0 ≤ x ≤ 2). However, at a lower temperature, a miscibility gap is present, which originates from the different mechanisms of dehydration of aluminum and chromium hydroxides [30,31]. For instance, the dehydration of chromium hydroxide leads directly to the formation of α-Cr 2 O 3 at a temperature above 400 • C, whereas the dehydration of aluminum hydroxide proceeds through the formation of aluminum oxyhydroxides, and then via different metastable alumina polymorphs, such as γ, δ, and θ, whose structure differs significantly from α-Cr 2 O 3 . The formation of α-Al 2 O 3 , which is isostructural with eskolaite, occurs only at a temperature above 1000 • C [32].
The XRD measurements of the sample after calcination at 400 • C revealed that the formed oxides are amorphous, as indicated by the absence of any diffraction peaks (Figure 1). At a higher reduction temperature (600 • C), the weak broad diffraction maxima appeared at 2θ ca. 24 This indicates that at this temperature, only chromium oxide started to crystallize, which is consistent with data reported in the literature [14]. The further increase in temperature up to 800 • C resulted in an intensification and sharpening of those maxima as well as the appearance of new small diffraction peaks related to α-Al 2 O 3 phase (JCPDS Card No. 01-088-0826). Moreover, diffraction peaks of α-Cr 2 O 3 were slightly shifted toward a higher angle, indicating the formation of (Al,Cr) 2 O 3 substitution solution. The unit cell parameters of the formed solid solution calculated from the peak (012) offset is presented in Table 1. This indicates that at this temperature, only chromium oxide started to crystallize, which is consistent with data reported in the literature [14]. The further increase in temperature up to 800 °C resulted in an intensification and sharpening of those maxima as well as the appearance of new small diffraction peaks related to α-Al2O3 phase (JCPDS Card No 01-088-0826). Moreover, diffraction peaks of α-Cr2O3 were slightly shifted toward a higher angle, indicating the formation of (Al,Cr)2O3 substitution solution. The unit cell parameters of the formed solid solution calculated from the peak (012) offset is presented in Table 1. Based on a comparison with literature data [31], the content of Cr2O3 in solid solution was found to be ca. 80%. The formation of (Al,Cr)2O3 solid solution did not exclude the presence of isolated α-Cr2O3 phase, which is not detectable due to the overlap of diffraction peaks.
Notably, no other crystallographic phases of alumina were observed at this temperature. On the other hand, when the reduction process was performed at 1000 °C, the diffraction maxima of metastable alumina polymorphs appeared. Particularly, the characteristic diffraction peaks of γ-Al2O3 (JCPDS Card No 01-074-4629) and χ-Al2O3 (JCPDS Card No 00-051-0769) phases were observed at 31.66°, 36.91°, and 45.12°; and at 7.76°, 15.52°, and 33.42°, respectively (Figure 2). Such results imply that the formation of α-Al2O3 grains proceeds via two different pathways, depending on the initial location of their formerly amorphous form (Figure 3). The Al2O3 located in close proximity to chromium oxide tends  Based on a comparison with literature data [31], the content of Cr 2 O 3 in solid solution was found to be ca. 80%. The formation of (Al,Cr) 2 O 3 solid solution did not exclude the presence of isolated α-Cr 2 O 3 phase, which is not detectable due to the overlap of diffraction peaks.
Notably, no other crystallographic phases of alumina were observed at this temperature. On the other hand, when the reduction process was performed at 1000 • C, the diffraction maxima of metastable alumina polymorphs appeared. Particularly, the characteristic diffraction peaks of γ-  (Figure 2). Such results imply that the formation of α-Al 2 O 3 grains proceeds via two different pathways, depending on the initial location of their formerly amorphous form (Figure 3). The Al 2 O 3 located in close proximity to chromium oxide tends to nucleate directly on its surface and then grow as α-Al 2 O 3 (Figure 3, Route 2) . The process can proceed at temperatures as low as 450 • C. On the contrary, the isolated grains of aluminum oxide crystallize as metastable forms, which then undergo structural transformation to thermodynamically stable α-Al 2 O 3 (Figure 3, Route 1). The process is usually accomplished at temperatures above 1000 • C. The presence of both χand γmay indicate that the dehydration process proceeds from different alumina precursors.
Particularly, it was reported that the γ is the first Al 2 O 3 polymorph that is formed during the calcination/dehydration of boehmite, whereas χ-Al 2 O 3 is initially formed from the gibbsite. Both hydroxides can be originally formed during the precipitation stage and/or as a result of decomposition of gibbsite to boehmite during calcination process. The formed (Al,Cr) 2 O 3 solution was not stable and decomposed under the investigated conditions, as evidenced by the disappearance of the corresponding diffraction maxima.
to nucleate directly on its surface and then grow as α-Al2O3 (Figure 3, Route 2). The process can proceed at temperatures as low as 450 °C. On the contrary, the isolated grains of aluminum oxide crystallize as metastable forms, which then undergo structural transformation to thermodynamically stable α-Al2O3 (Figure 3, Route 1). The process is usually accomplished at temperatures above 1000 °C. The presence of both χ-and γ-may indicate that the dehydration process proceeds from different alumina precursors. Particularly, it was reported that the γ is the first Al2O3 polymorph that is formed during the calcination/dehydration of boehmite, whereas χ-Al2O3 is initially formed from the gibbsite. Both hydroxides can be originally formed during the precipitation stage and/or as a result of decomposition of gibbsite to boehmite during calcination process. The formed (Al,Cr)2O3 solution was not stable and decomposed under the investigated conditions, as evidenced by the disappearance of the corresponding diffraction maxima.   No 01-075-0713). In order to confirm the experimental data, theoretical calculations were performed. Assuming isostructurality with MgAl2O4, good agreement between the experimental data and theoretical calculations was noted. On this basis, we think that a CrAl2O4 compound with a spinel structure can be At 1150 • C, the XRD measurements revealed the presence of diffraction peaks at 2θ ca. 18 O 32 , in which 32 oxygen anions (occupying 48f positions), are face-centered cubic close packed with a space group Fd-3m, with eight CrAl 2 O 4 units per cubic cell. The Cr 2+ ions occupy tetrahedral 8a symmetry position between O 2− ions and the Al 3+ ions are sited in octahedral 16d sites. To confirm the CrAl 2 O 4 structure, we calculated the bond-valence parameters, as proposed by Gagné and Hawthorne [33], at the tetrahedrally and octahedrally coordinated sites. The bond valence sum at the four-fold site is 2.26 v.u., which is a little larger than the +2 expected for the Cr 2+ cation; the bond valence sum at the six-fold site is 2.92 v.u., which is a little smaller than the +3 expected for the Al 3+ cation. These bond valence values not only confirm that Cr 2+ and Al 3+ are the dominant cations at the tetrahedrally and octahedrally coordinated sites, respectively, but they also indicate the occurrence of a partial Cr-Al disorder (likely less than 10% in terms of Cr 2+ content) over the tetrahedrally and octahedrally coordinated sites. This type of disorder is always present in Cr-bearing MgAl 2 O 4 spinels [34]. Cámara et al. [35] reported occurrence of the mineral dellagiustaite, with ideal formula V 2+ Al 2 O 4 , formed under super-reduced geological conditions from high-temperature melts trapped in corundum aggregates. Since dellagiustaite is an inverse spinel, the V 2+ cations prefer six-fold coordination, unlike Cr 2+ cations, which prefer four-fold coordination in our spinel structure. In our opinion, the spinel CrAl 2 O 4 may be found in nature under very reducing conditions typical of basaltic systems [36]. However, no minerals containing Cr 2+ have been reported so far [37]. In conclusion, to the best of our knowledge, the CrAl 2 O 4 compound has not been described in the literature yet. Table 2 presents the comparison of the unit cell parameters and crystallographic data for the newly formed compound, calculated using the DICVOL04 procedure and the CRYSTAL17 package. On the basis of the obtained results, it can be postulated that at temperatures above 1150 • C, the α-Cr 2 O 3 undergoes reduction to CrO, the chromium ions of which further diffuse into an α-Al 2 O 3 matrix, forming CrAl 2 O 4 . At further increasing temperatures, the agglomeration of the formed compounds was observed as evidenced by the increase in crystallite size of both spinel compound and alumina. Particularly, the size of the α-Al 2 O 3 particle increased from 29 nm at 1150 • C to 54 nm at 1450 • C, whereas the size of the crystallites of CrAl 2 O 4 increased from 27 nm at 1150 • C to 125 nm at 1300 • C. In general, the increase in temperature beyond 1000 • C resulted in a gradual decrease in chromium-containing compounds due to their sublimation (Table 3). At 1450 • C, small residual amounts of metallic chromium were observed, which formed as a result of a reduction of the spinel compound. All attempts to obtain pure CrAl 2 O 4 spinel in order to study its physical properties more closely have failed. When exposed to ambient conditions, the Cr-Al spinel structure is metastable and readily undergoes oxidation to individual oxides. The general scheme of phase transformation of co-precipitated oxides as a function of temperature is depicted in Figure 3.

Summary
The obtained results showed that the crystallization of atomically mixed amorphous Al-Cr oxides proceeds first via the formation of α-Cr 2 O 3 , followed by the nucleation and growth of α-Al 2 O 3 on its surface. Isolated grains of Al 2 O 3 are also present as indicated by the crystallization of metastable alumina phases, which are subsequently converted into α-Al 2 O 3 at high temperature. Under harsh reductive conditions, the investigated oxides are partially transformed to Cr-Al spinel compound. It is proposed that the formation of this structure is due to the formation of Cr 2+ ions (because of reduction of α-Cr 2 O 3 ) and then interdiffusion of Al 3+ and Cr 2+ ions through the oxygen array.