Phase Stability and Mechanisms of Transformation of La-Doped γ

ABSTRACT: We report the phase stability of cubic γ-Al2O3 with respect to .... Low Speed Diamond Saw (MTI Corporation) was used for cutting the sinter...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Phase Stability and Mechanisms of Transformation of La-Doped γ‑Alumina Tianqi Ren,† Lum-Ngwegia N. Nforbi,‡ Raghunath Kanakala,‡ and Olivia A. Graeve*,†,‡ †

Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive - MC 0411, La Jolla, California 92093-0411, United States ‡ Department of Chemical and Materials Engineering, University of Nevada, Reno, 1664 N. Virginia Street - MS 388, Reno, Nevada 89557, United States ABSTRACT: We report the phase stability of cubic γ-Al2O3 with respect to lanthanum dopant amount and describe a complete phase transition sequence up to a temperature of 1800 °C, which proceeds from La-doped γ-Al2O3 to LaAlO3/γAl2O3 to LaAl11O18. For this purpose, lanthanum contents from 0.81 to 10.0 atom % were incorporated into Al2O3 powders. X-ray diffraction analyses show that only γ-Al2O3 phase was present after heat treatment at 1000 °C for 2 h with 0.81, 1.68, 2.24, and 2.62 atom % lanthanum concentrations. The phase stabilization can be mainly attributed to the combined effects of small crystallite size of the Al2O3 powders and the presence of the lanthanum dopant, which occupies the Al2O3 octahedral sites. At compositions of 3.63, 5.00, 7.49, and 10.0 atom %, the amount of LaAlO3 phase formed by the solid phase reaction between Al2O3 and La3+ ions becomes detectable under X-ray diffraction. For example, Zaharescu et al.12 reported that the γ-Al2O3 phase can be stabilized with lanthanum dopant additions at temperatures between approximately 500 and 900 °C. Ozawa et al.15 suggested that 1.5 mol % La-doped γ-Al2O3 powders of high-surface area can retain the cubic phase after a heat treatment at 1200 °C for 3 h in air. Loong et al.9 showed that γAl2O3 is dominant at temperatures below 800 °C with 1 mol % lanthanum doping. They have also shown that transformation of γ-Al2O3 to another intermediate phase (θ-Al2O3) and finally to the corundum α-Al2O3 phase occurs above 1000 °C. Wang et al.8 obtained Er3+-doped γ-Al2O3 that contained a stable γ(Al,Er)2O3 phase even up to 1100 °C. More recently, Sun et al.16 reported the synthesis of La-doped mesoporous Al2O3 with high thermal stability up to 1000 °C using 3.8 mol % La doping. Ab initio modeling efforts have also been carried out. Jiang et al.17 calculated the total free energy of γ-Al2O3 and αAl2O3 doped with 2.5 mol % silicon, chromium, titanium, scandium, or yttrium and determined that silicon stabilizes γAl2O3, while chromium stabilizes α-Al2O3. Although many previous studies have shown that large ions, in particular several rare-earth ions, can effectively hinder the phase transformation of Al2O3 at high temperatures, detailed mechanisms for the phase transformation are still unclear. Kwak et al.18 suggested that the phase transformation from γ- to θAl2O3 is initiated at oxide particle surfaces and that the presence

1. INTRODUCTION Aluminum oxide (Al2O3) is one of the most widely studied and utilized ceramic materials, having found applications in electronics, catalysis, and as a structural ceramic due to its high mechanical strength and hardness, good alkaline and acidic corrosion resistance at high temperatures, excellent wear resistance, and outstanding dielectric properties.1−3 Apart from the most thermodynamically stable form of Al2O3 (corundum or α-Al2O3), other phases of Al2O3 (γ-Al2O3, ηAl2O3, and θ-Al2O3), especially the cubic γ-Al2O3 phase, have drawn interest for catalytic applications because of their relatively high thermal stability up to about 600 °C.4 However, when exposed to higher temperatures, γ-Al2O3 transforms irreversibly to α-Al2O3. This may occur when used as a threeway catalyst or as support for catalytic noble metals during the combustion of hydrocarbons, where the ambient temperature is generally above 1000 °C. To prevent or delay the hightemperature phase transformation in γ-Al2O3, doping of rareearth elements is commonly employed. Doped Al2O3 has shown promising enhancement in mechanical properties, including improved toughness and high-temperature tensile and compressive creep resistance.5−7 With regard to the phase stabilization, large ions such as erbium (Er3+) and lanthanum (La3+) have shown to exhibit positive effects on stabilizing the γ-Al2O3 phase at high temperatures.8−10 Lanthanum, in particular, is the most commonly employed dopant for thermal stabilization of Al2O3 in the temperature range between 800 and 1200 °C.11−15 © XXXX American Chemical Society

Received: October 16, 2017

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DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of penta-coordinated Al3+ sites formed on the (100) facets of the Al2O3 surfaces have a strong correlation with the thermal stability of γ-Al2O3. Understanding the Al2O3 phase transformation mechanism, and the reason why dopant ions can enhance the phase stability of certain Al2O3 phases, is of particular importance for the development of doped Al2O3 ceramics with controlled properties. Thus, in this contribution, we present the phase stability of γ-Al2O3 doped with lanthanum in the range of 0.81− 10.0 atom % for powders and sintered pellets. We determine that there is a maximum lanthanum dopant amount, which lies between 2.62 and 3.63 atom %. A comprehensive study of the phase evolution based on X-ray diffraction is presented, providing an understanding of how different phases develop during heat treatment of the material at high temperatures. A phase transition sequence describing the formation of two types of lanthanum aluminate phases is reported.

2. EXPERIMENTAL PROCEDURE Aluminum nitrate nonahydrate [Al(NO3)3·9H2O, Alfa Aesar no. 43145, 99.999%], lanthanum nitrate hexahydrate [La(NO3)3·6H2O, Alfa Aesar no. 11267, 99.99%], and carbohydrazide [1,3-diaminourea, CO(NHNH2)2, Alfa Aesar no. 497-18-7, 97%], were used as precursors for powder synthesis. Appropriate amounts of starting materials were mixed to obtain La-doped Al2O3 powder with different lanthanum contents between 0.81 and 10.0 atom %. For example, to produce 0.81 atom % doped powders, 10.7566 g of Al(NO3)3·9H2O, 0.1015 g of La(NO3)3·6H2O, and 13.0213 g of CO(NHNH2)2 were mixed together and dissolved in deionized water in a Pyrex crystallization dish. The solutions were then placed in a furnace preheated to 500 °C. A combustion reaction19−24 occurred in less than 3 min after placing the mixture into the furnace. After synthesis, the reaction products were gently ground in a mortar and pestle and calcined at 1000 °C in air for 2 h. A mass of 2.5 g of powders with different lanthanum contents were pressed into pellets using a Carver Auto Series press. All La-doped as-pressed Al2O3 specimens were sintered using a Nabertherm GmbH high temperature furnace. The sintering cycle was programmed to begin with linear heating from room temperature to 1800 °C at 300 °C/hour. The temperature was then held at 1800 °C isothermally for 4 h. The furnace was subsequently cooled to 1200 °C in 2 h, followed by a final cooling to room temperature. As-synthesized and calcined powders as well as sintered specimens were characterized by X-ray diffraction (XRD) on a Philips 3100 diffractometer (Philips Electronic Instruments, Inc., Mahwah, NJ) at room temperature using Cu Kα radiation. A Nanotrac Ultra (Microtrac, Inc., Montgomeryville, PA) dynamic light scattering (DLS) system was used to determine number-weighted particle sizes of the as-synthesized and calcined powders. The solutions used for DLS measurements were prepared by dissolving 0.01 g of powder in 25 mL of sodium pyrophosphate (NaPPh) solution, prepared by dissolving 0.10 g of NaPPh in 200 mL of deionized water and stirred at room temperature for 48 h. A field-emission scanning electron microscope (S-4700, Hitachi High Technologies America, Inc., Pleasanton, CA) was used to observe the morphologies of both powder samples and sintered specimens. Micrographs of both surface and cross sections were obtained for the sintered specimens. A 150 Low Speed Diamond Saw (MTI Corporation) was used for cutting the sintered specimens.

Figure 1. Number-weighted particle size for (a) the as-synthesized and (b) calcined powders doped with 0.81 atom % and (c) the assynthesized and (d) calcined powders doped with 10.0 atom % La. Three individual measurements for each sample are indicated with red solid line, blue dotted line, and black dashed line. Average particle sizes are uniformly distributed and in the submicrometer range.

sizes in the submicrometer range with some level of >1 μm agglomeration. No significant differences in particle sizes were noticed between the as-synthesized and calcined powders, as has been seen in other studies of powders that undergo calcination.25 Scanning electron micrographs (Figure 2) of the calcined powders and sintered specimens of the Al2O3:0.81 atom % La (Figures 2a and b) and Al2O3:10.0 atom % La (Figures 2c and d) samples, the two extremes of dopant concentration, were obtained. The microstructure of sintered specimens clearly indicates that Al2O3 with higher lanthanum content has much smaller grain sizes (compare Figures 2b and c). It is also observed that Al2O3:10.0 atom % La (Figure 2d) has a greater amount of porosity compared to Al2O3:0.81 atom % La (Figure 2b). This can be attributed to lanthanum segregation to the grain boundaries, decreasing the grain boundary diffusivity and leading to a decrease in the densification rate and a decrease in grain growth. Our findings are in line with earlier research conducted by Thompson et al.26 and Fang et al.27 XRD Analysis and Phase Transformation Mechanisms. The crystal structure of γ-Al2O3 has been extensively studied by

3. RESULTS AND DISCUSSION Particle Size and Morphology. Particle size of the assynthesized and calcined powders was determined from dynamic light scattering. DLS plots for the powders with 0.81 and 10.0 atom % lanthanum are illustrated in Figure 1 and are representative of all samples. The majority of particles have B

DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. X-ray diffraction patterns for as-synthesized La-doped alumina powders with (a) 0.81, (b) 1.68, (c) 2.24, and (d) 2.62 atom % lanthanum doping (γ-Al2O3 phase: PDF no. 50-0741, LaAlO3 phase: PDF no. 31-0022).

Figure 2. Scanning electron micrographs of (a) calcined 0.81 atom % La powders, (b) sintered 0.81 atom % La specimen, (c) calcined 10.0 atom % La powders, and (d) sintered 10.0 atom % La specimen. Note the significant refinement of grain size seen in the 10 atom % La-doped specimen.

Figure 4. X-ray diffraction patterns for as-synthesized La-doped alumina powders with (a) 3.63, (b) 5.00, (c) 7.49, and (d) 10.0 atom % lanthanum doping (γ-Al2O3 phase: PDF no. 50-0741, LaAlO3 phase: PDF no. 31-0022).

previous researchers. It is widely recognized that γ-Al2O3 possesses a defective spinel structure with lattice parameter a = 7.911 Å and Fd3̅m space group. All oxygen ions are ordered in a cubic close-packed structure, while aluminum sublattices are both octahedrally and tetrahedrally coordinated, with 43% of aluminum ions in octahedral sites and 32% in tetrahedral sites, leaving the remaining 25% in quasi-octahedral sites (Al3+ ions sitting in displaced octahedral sites on the surface layer).28 In contrast, the α-Al2O3 phase contains aluminum ions that favor occupying the octahedral sites and all aluminum sublattices are octahedrally coordinated. Figures 3 and 4 illustrate the XRD patterns of the as-synthesized (before the 1000 °C calcination) La-doped Al2O3 powders. The patterns demonstrate that only the samples doped with 0.81, 1.68, 2.24, 2.62, and 3.63 atom % lanthanum contain nearly pure phases of

cubic γ-Al2O3, with minor amounts of the LaAlO3 phase (peaks labeled with a circle). The 5.00, 7.49, and 10.0 atom % samples contain cubic γ-Al2O3 phase, but the peaks are minor compared to the peaks of the LaAlO3 phase, in particular for the powder samples with 7.49 and 10.0 atom % lanthanum. The formation of this compound is a result of the solid phase reaction between Al2O3 and La3+ ions.29 Figures 5 and 6 illustrate the XRD patterns of the calcined powders with various lanthanum contents. The patterns are similar to those for the as-synthesized powders. The rhombohedral LaAlO3 phase is still present in the samples with lanthanum content greater than 3.63 atom % (i.e., 5.00, 7.49, and 10.0 atom %). The nearly pure cubic γ-Al2O3 phase was retained with the addition of lanthanum up to 2.24 atom %. This is consistent with results from Ozawa and Nishio,15 who C

DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX

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Our results are in line with these previous studies, i.e. highpurity γ-Al2O3 was produced, and no transformation to α-Al2O3 was observed after calcination at 1000 °C. Chen et al.29 also suggested that the formation of LaAlO3 at low lanthanum concentrations (i.e., 0 to 0.02 La/Al atomic ratios) is considered to be very unlikely, which is also supported by our XRD results. Further increase in the La concentration leads to the coexistence of crystalline La2O3 and LaAlO3. However, we did not observe this coexistence behavior in our experiments, even with samples containing 10.0 atom % lanthanum, indicating that all of the lanthanum atoms participate in the formation of crystalline LaAlO3. It is known that there is a critical crystallite size below which γ-Al2O3 is more stable and above which α-Al2O3 is more stable. Researchers have demonstrated that this critical crystallite size (rc) is around 10−20 nm.36,37,40 As the phase transition of Al2O3 is essentially a nucleation and growth process,38,39 there is another primary crystallite size (rp) of around 45−55 nm,36,40 beyond which α-Al2O3 starts to grow in a thermodynamically stable manner. The crystallite sizes of our alumina powders (calculated using Scherrer’s equation) are listed in Table 1.

Figure 5. X-ray diffraction patterns for calcined La-doped alumina powders with (a) 0.81, (b) 1.68, (c) 2.24, and (d) 2.62 atom % lanthanum doping (γ-Al2O3 phase: PDF no. 50-0741).

Table 1. Crystallite Sizes Obtained from XRD Patterns on Calcined Powders (at 1000 °C for 2 h) with Varying Lanthanum Concentrations lanthanum concentration (atom %)

phases present after heat treatment

0.81 1.68 2.24 2.62

γ-Al2O3 γ-Al2O3 γ-Al2O3 γ-Al2O3 LaAlO3 γ-Al2O3 LaAlO3 γ-Al2O3 LaAlO3 γ-Al2O3 LaAlO3 γ-Al2O3 LaAlO3

3.63 5.00 7.49 10.0

Figure 6. X-ray diffraction patterns for calcined La-doped alumina powders with (a) 3.63, (b) 5.00, (c) 7.49, and (d) 10.0 atom % lanthanum doping (γ-Al2O3 phase: PDF no. 50-0741, LaAlO3 phase: PDF no. 31-0022).

crystallite size (nm) 10.0 10.4 10.4 17.2

± ± ± ±

0.8 1.3 1.3 2.9

13.3 ± 3.0 15.4 15.4 16.7 16.2 17.0 24.8

± ± ± ± ± ±

3.6 3.2 4.7 2.8 0.3 3.8

These values lie within the critical crystallite size range but are well below rp. Thus, even though there may be α-Al2O3 nuclei formed, the free energy of the system is not favorable for the nuclei to grow, and the presence of the α-Al2O3 phase should be attributed both to the presence of the La dopant and the crystallite size. In addition, it has been both computationally and experimentally shown that for nanocrystalline alumina, the phase stability is dependent on the specific surface area (SSA). The γ-Al2O3 phase has a surface energy lower than that of αAl2O3 and becomes energetically more stable at a surface area greater than 125 cm2/g (i.e., this equals to an average crystallite size of ∼12 nm assuming nonporous and spherical particles).41,42 Thus, small crystallite size is an important factor retarding the γ- to α-Al2O3 phase transformation. In our case, the only powders with crystallite sizes lower than 12 nm are those of composition 0.81, 1.68, and 2.24 atom %, with a crystallite size of approximately 10 nm for all three samples. Our as-synthesized and calcined powders are a mixture of γAl2O3 and LaAlO3, with higher amounts of the LaAlO3 phase as the lanthanum concentration is increased, but in spite of the

reported that low lanthanum contents between 0.5 and 1.5 mol % inhibited the transformation of cubic γ-Al2O3 to corundum α-Al2O3 at temperatures up to 1100 °C, although we show that the maximum dopant for stabilization is greater (i.e., 2.24 atom %). Ozawa and Nishio15 obtained crystals that were less than 20 nm, so their results are confounded by both the addition of the dopant and the small crystallite sizes. Some researchers have also reported the coexistence of θ- and γ-Al2O3 phases when samples were calcined at 1000 °C.15,29,30 For example, Ersoy and Gunay30 observed that 2 wt % La-doped Al2O3 contained θ- and γ-Al2O3 phases at 1000 °C, but the γ-Al2O3 phase was more prevalent compared to undoped samples. Chen et al.31 prepared γ-Al2O3 by pseudo boehmite gelation and calcination at 1000 and 1150 °C for 2 h, reporting that only LaAlO3 was present in samples with La/Al atomic ratio of 0.1. The cubic γ-Al2O3 phase was stabilized at 1000 °C for samples with La/Al ratio less than 0.1 according to their XRD results. D

DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry presence of the LaAlO3 and the increasing crystallite size to values beyond 12 nm, the γ-Al2O3 phase is still present. Thus, the lanthanum has a stabilizing effect connected perhaps to the presence of the LaAlO3. Previous research reported by Béguin et al.10 suggested that the effectiveness of γ-Al2O3 stabilization provided by lanthanum up to 1220 °C was due to the formation of microdomains of LaAlO3 on the alumina surfaces. In the perovskite structure of LaAlO3, Al3+ ions are octahedrally coordinated, as is the case in the α-Al2O3 phase. This arrangement is more stable than the tetrahedral coordination seen in other transition Al2O3 phases. Therefore, for the γ-Al2O3 to transform into α-Al2O3, Al3+ ions must go through a series of diffusion processes to occupy the octahedral sites in the lattice. Because La3+ ions (1.03 Å) are almost two times larger than Al3+ ions (0.54 Å),32 the bulk solubility of lanthanum in Al2O3 is extremely low and is generally believed to be below the detection limit of most experimental methods. Thompson et al.26,35 reported a solubility value of ∼80 ppm, even though the exact number is still unknown and could be much lower. Therefore, it is likely that the large La3+ ions precipitate from the solid solution and segregate into the energetic Al2O3/LaAlO3 interface, where the nucleation of α-Al2O3 most probably initiates. As a result, the diffusion of surface Al3+ ions into octahedral sites is hindered, and the nucleation of α-Al2O3 in bulk lattices is retarded. Therefore, the presence of the LaAlO3 phase, regardless of its amount, is another crucial factor for the phase stabilization of γAl2O3. XRD plots for the same batches of calcined powders sintered at 1800 °C for 4 h are illustrated in Figures 7, 8, and 9. As

Figure 8. X-ray diffraction patterns for La-doped alumina sintered specimens with (a) 0.81, (b) 1.68, (c) 2.24, and (d) 2.62 atom % lanthanum doping (α-Al2O3 phase: PDF no. 46-1212, LaAl11O18 phase: PDF no. 34-0467).

Figure 9. X-ray diffraction patterns for La-doped alumina sintered specimens with (a) 3.63, (b) 5.00, (c) 7.49, and (d) 10.0 atom % lanthanum doping (α-Al2O3 phase: PDF no. 46-1212, LaAl11O18 phase: PDF no. 34-0467).

There is no evidence of perovskite LaAlO3 phases, implying that even if it formed, it transformed into LaAl11O18 at 1800 °C. Zaharescu et al.12 as well as Ropp and Libowitz33 reported that the formation of LaAl11O18 is accomplished by the decomposition of LaAlO3 and its reaction with Al2O3 above 1500 °C. The peaks of LaAl11O18 phase are very small because the precipitated La content is not sufficient for the nucleation of greater amounts of LaAl11O18. As the lanthanum content is increased to 1.68 and 2.62 atom %, the LaAl11O18 peaks become more pronounced, while the relative intensities of the main corundum α-Al2O3 peaks are slightly shifted, e.g. the (1 0 1̅ 4) peak at about 35 degrees 2θ is the 100% peak in 0.81 atom % La specimen, while the (1 1 2̅ 6) peak at about 57 degrees 2θ becomes the 100% peak in specimens with higher La content (Figure 8). The formation of the LaAl11O18 phase is expected

Figure 7. X-ray diffraction pattern for 0.81 atom % La-doped alumina sintered specimen at 1800 °C for 4 h (α-Al2O3 phase: PDF no. 461212, LaAl11O18 phase: PDF no. 34-0467).

indicated by Figure 5, powders containing 0.81−2.62 atom % lanthanum contain nearly pure cubic γ-Al2O3 phase after the calcination step. This cubic phase is stabilized during annealing at 1000 °C but transforms mostly to the more thermodynamically stable corundum α-Al2O3 phase when subjected to sintering at 1800 °C for 4 h, demonstrated in Figures 7 and 8. Another La-containing compound, the hexagonal lanthanum aluminum oxide (LaAl11O18) phase, was observed in these sintered specimens. E

DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry according to the La2O3−Al2O3 phase diagram (Figure 10).34 When alumina content is greater than ∼76 mol %, a mixture of

4. CONCLUSIONS We successfully synthesized La-doped γ-Al2O3 powders with various lanthanum contents using combustion synthesis at a processing temperature of 500 °C. The particle sizes of both assynthesized and calcined powders are in the submicrometer range with good morphological homogeneity. We observed suppressed grain growth in sintered specimens with higher lanthanum compositions due to the pinning effect of La3+ ions at the grain boundaries. While all of the as-synthesized Ladoped samples are of the cubic Al2O3 phase, we found that the maximum lanthanum composition to yield nearly pure cubic γAl2O3 is between 2.62 and 3.63 atom %. We attributed the phase stabilization to the combined effects between the small crystallite sizes and the occupation of the octahedral sites necessary for Al2O3 phase transformation at the Al2O3/LaAlO3 interface by large La3+ ions, which have very low solubility in the alumina matrix and precipitate out during calcination. The perovskite LaAlO3 phase is more clearly formed with higher lanthanum contents (3.63−10.0 atom %) due to the solid phase interaction between La3+ ions and Al2O3. The presence of the LaAlO3 phase is essential for stabilizing γ-Al2O3 with lanthanum. With the sintering and annealing step at 1800 °C to form dense pellets, we confirmed the formation of LaAl11O18 phase at high temperatures and report a high-temperature phase transformation sequence of γ-Al2O3:La to LaAlO3/γAl2O3 to LaAl11O18. This provides guidance toward evaluating the phase evolution of rare-earth doped Al2O3 phases at high temperatures.

Figure 10. Phase diagram of the La2O3−Al2O3 system at elevated temperature.33

La2O3·11Al2O3 (LaAl11O18) and α-Al2O3 is formed. Almost all of the α-Al2O3 phases were eliminated for the 7.49 and 10.0 atom % lanthanum samples after sintering at 1800 °C, replaced by the LaAl11O18 phase (Figure 9). At lower lanthanum amount, the coexistence of corundum α-Al2O3 and LaAl11O18 at 1800 °C is observed, connected to an insufficient content of lanthanum necessary to transform all Al2O3 to LaAl11O18. Several other researchers reported the formation of LaAl11O18 at calcining temperatures of 1300 °C and above.12,15,33,34 In summary, we clarified that the phase transformation in Ladoped alumina takes place in a series of reactions starting from the precipitation of La3+ to the grain boundaries, where LaAlO3 begins to nucleate. La3+ then continues to precipitate and occupy the γ-Al2O3 octahedral sites. When La3+ is depleted, an irreversible transformation from γ- to α-Al2O3 occurs. As the temperature further increases to 1800 °C, LaAlO3 reacts with Al2O3 to form LaAl11O18. In addition, our results have shown that γ-Al2O3 can be stabilized up to 1000 °C for 2 h, at a maximum composition of lanthanum dopant between 2.62 and 3.63 atom %. We confirmed that there is a phase transformation sequence of γ-Al2O3 to LaAlO3/γ-Al2O3 to LaAl11O18 from 1000 to 1800 °C with various lanthanum contents. The crystallite sizes of all samples are within the range of critical crystallite size for phase transformation to α-Al2O3 but much smaller than the primary crystallite size required for thermodynamically stable phase transformation. The formation of La-containing phases initiates when the La3+ ions start to precipitate out of the solid solution and react with the γ-Al2O3. This mechanism proceeds until all the La3+ ions are depleted to form LaAlO3. The γ-Al2O3 phase transforms irreversibly to αAl2O3 above 1000 °C, when the stabilization effect of La3+ is lost due to the depletion of La3+ ions. Grain growth during high temperature sintering is better suppressed with increasing amount of lanthanum (due to grain boundary pinning). Therefore, we attribute the major mechanism responsible for the γ-Al2O3 phase stabilization to the segregation of large La3+ ions into the LaAlO3/Al2O3 interface, which hinders the diffusion of surface Al3+ ions required for Al2O3 phase transformation. The LaAlO3 phase is at the same time acting as a key indicator of the effectiveness of La-doping for Al2O3 phase stabilization.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 858-246-0146; E-mail: [email protected]. ORCID

Olivia A. Graeve: 0000-0003-3599-0502 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant 1334160 from the National Science Foundation.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02635 Inorg. Chem. XXXX, XXX, XXX−XXX