New and Unforeseen Crystal Growth Processes for a Metal Oxide

The synthesis of corundum (α-Al2O3) via a layered Al2O3–MoO3 system was directly observed for the first time. This revealed a new crystal growth process with three key features: (1) the formation of an Al2(MoO4)3 intermediate layer through a solid–solid interaction in the temperature range of ∼705–860 °C; (2) the melting of the Al2(MoO4)3 layer between approximately 870 and 890 °C; and (3) the decomposition of Al2(MoO4)3 to corundum between 950 and 1100 °C. This molten intermediate decomposition (MIND) mechanism produced corundum, which was light bluish-gray in color and was defined in CIE (L* a* b*) color space as L* = 76.65, a* = −1.09, and b* = −6.20. The reagents used in this study were the same as those used in MoO3 flux growth studies on the synthesis of corundum, therefore demonstrating that the previous work only gave a superficial treatment of the mechanism of formation.


■ INTRODUCTION
Flux methods are advantageous to crystal growth as it allows for the production of single crystals at significantly lower temperatures than melt growth methods. 1 A molten oxide or combination of molten oxides, referred to as the flux, can act as a solvent and facilitate the dissolution of a solute, such as aluminum oxide, at a lower temperature than its melting point.The mechanism proceeds to promote crystal growth as a direct result of supersaturation, which can occur via one of three processes: evaporation of the flux, slow cooling of the solution, or by a thermal gradient. 1,2−10 The synthesis of corundum via a MoO 3 flux closely resembles catalytic Al 2 O 3 −MoO 3 systems.Several observations made in Al 2 O 3 −MoO 3 systems contradict the synthesis occurring via a conventional flux method.For example, a solid−solid interaction produces an intermediate phase of aluminum molybdate (Al 2 (MoO 4 ) 3 ), which is known to decompose into corundum above 900 °C, with uncertainty surrounding the phase transitions undergone by MoO 3 and Al 2 (MoO 4 ) 3 . 8,11For these reasons, in this work, we undertook the synthesis of corundum using a layered Al 2 O 3 −MoO 3 system and directly observed the growth mechanism via optical imaging for the first time.This revealed a multistep process that is more complex than was previously thought.Based on the evidence presented in this work, the crystal growth process of corundum via MoO 3 flux methods should be more accurately described as proceeding via a molten intermediate decomposition (MIND) mechanism.■ EXPERIMENTAL SECTION Sample Preparation.Identical syntheses were carried out in different receptacles: a 13 mL quartz vial and a 30 mL platinum crucible to assess the reaction across different temperature ranges.In each receptacle, 0.5 g of Al 2 O 3 (99.7+wt %, extra pure, Acros Organics) was placed at the bottom followed by a 2.1175 g MoO 3 (99+ wt %, Acros Organics) layer on top.Each receptacle was then loosely covered with a platinum lid.The quartz vial was placed into a Carbolite HZS 12/900E three-zone tube furnace and taken from room temperature to 700 °C at 10 °C/min and then to 950 °C at 1 °C/min.The platinum crucible was placed into a Nabertherm L 9/11/SW Weighting Muffle furnace and taken from room temperature to 700 °C at 10 °C/min and then to 1100 °C at 1 °C/min.After a dwell time of 0 min, the furnaces were allowed to cool to room temperature before the receptacles were removed.
Optical Imaging.A Nikon D3200 camera was trained on the quartz vial within a Carbolite HZS 12/900E three-zone tube furnace with an LED light source at the opposite end of the tube.A timelapse program was set on the camera to capture an image every 60 s throughout the experiment.
Powder X-ray Diffraction.Powder X-ray diffraction (pXRD) measurements were carried out using a Bruker D8 advance diffractometer configured with Cu Kα radiation (λ = 1.54 Å).The patterns were collected at 0.011°step intervals over a 2θ range from 10 to 70°or 20 to 80°, both at 1 s per step.
Scanning Electron Microscopy with Energy-Dispersive X-ray Analysis.Scanning electron microscopy (SEM) was carried out on a JEOL IT300 instrument.The samples were mounted onto aluminum stubs with carbon adhesive pads and sputter-coated with silver prior to imaging.Energy-dispersive X-    ray analysis (EDXA) was achieved using an X-Max 80 mm 2 detector and analyzed via the AZtec platform.
Diffuse Reflectance UV−Vis Spectroscopy and CIE 1931 Standard Colorimetric Observer.Diffuse reflectance UV−vis spectroscopy was carried out on a PerkinElmer LAMBDA 650 UV/vis spectrophotometer using an integrating sphere between 360 and 830 nm with a step of 1 nm.The diffuse reflectance spectrum, β(λ), was used in combination with the CIE 1931 standard colorimetric observer to obtain the XYZ tristimulus values.The color-matching functions x(λ), y(λ), and z(λ) along with the spectral radiant power distribution for standard illuminant D 65 , S(λ), were taken from Color Science: Concepts and Methods, Quantitative Data and Formulae. 12The following formula was used to calculate X, X = similarly, Y and Z were calculated. 12These were converted to CIE 1976 (L* a* b*)-space using the appropriate equations.

■ RESULTS AND DISCUSSION
The following crystal growth mechanism reflects the physical and chemical interactions witnessed in the layered Al (3) the decomposition of Al 2 (MoO 4 ) 3 to corundum between 950 and 1100 °C (Figure 1F).
In the temperature range of 20−700 °C, the MoO 3 layer undergoes physical transformations.A color change from gray to yellow is seen upon heating as defects form within the crystal lattice. 13Also, the volume occupied by MoO 3 decreases, which is likely due to sublimation (Figure 1A).At ∼709 °C, the formation of a crack appears below the interface between the layers (Figure 1B).The crack gradually extends over the course of 2 h 35 min as Al 2 (MoO 4 ) 3 is formed through a solid−solid interaction between Al 2 O 3 and MoO 3 (eq 1).It is thought that the MoO 3 layer continuously sublimes throughout the mechanism into the atmosphere surrounding the system.As the underneath of the MoO 3 layer becomes exposed, the atmosphere becomes saturated, and the deposition of α−MoO 3 crystals can be seen on the walls of the vial (Figure 1B).The intermediate layer separates from the MoO 3 layer above, and three distinct regions can be seen (Figure 1C).The deposition of α−MoO 3 on the walls continues until ∼870 °C, and then it begins to melt along with the Al 2 (MoO 4 ) 3 layer (Figure 1D).Over the course of the following 20 min, liquid Al 2 (MoO 4 ) 3 flows down through the solid Al 2 O 3 underneath and the MoO 3 layer above sinks down into the liquid (Figure 1E).It is thought that the resulting mixture continues to form Al 2 (MoO 4 ) 3 through solid−liquid reactions and/or dissolution.This is followed by decomposition of Al 2 (MoO 4 ) 3 to corundum and gaseous MoO 3 between 950 and 1100 °C (Figure 1F) A mixed solid disc with protrusions of thin film-like crystals remained at the bottom of the quartz vial after the 950 °C experiment.Also, thin film-like crystals were seen on the walls of the vial (Figure 2A).Images of the top and bottom of the disc can be seen in Figure 2B,C  (JCPSD card 84-1652) (Figure 5).Additionally, sublimation of MoO 3 from underneath the layer followed by growth of the intermediate layer was captured across a 30 min time frame (Figure S1).This suggests that Al 2 (MoO 4 ) 3 can be formed through gas−solid interactions between MoO 3 and Al 2 O 3 .
SEM and EDXA of the mixed solid disc reveal heterogeneous regions of Al/Mo/O or Mo/O.Elemental mapping from the middle of the top and bottom of the disc is shown in Figures 6  and 7, respectively.Three different morphologies can be seen in the micrographs: an aggregation of blocky crystals of between 8 and 80 μm with depressions similar to hopper crystals, thin filmlike/lamellar structures, and spherical particles.Based on the elemental mapping, the spherical particles are presumed to be silicon dioxide originating from the quartz vial.These spherical particles are not present when a platinum crucible is used.EDXA spectra for each morphology on the top and bottom of the disc can be seen in Figures 6 and 7, respectively.The spectra in Figure 6B show the composition of the blocky crystals to be Al 2.00 Mo 3.00 O 10.44 , and the spectra in Figure 6C show the composition of the thin film-like crystals to be MoO 2.61 .The spectra in Figure 7B show the composition of the blocky crystals to be Al 2.00 Mo 2.56 O 10.00 , and the spectra in Figure 7C show the composition of the thin film-like crystals to be MoO 1.68 .The compounds under the disc were more oxygen-deficient compared to those at the top of the disc.This is confirmed further by the backscattered electron micrographs from the bottom of the disc of regions containing the thin film-like crystals, as various shades of gray can be seen in correlation to different atomic weights (Figure 8).As stoichiometric ratios of Al 2 (MoO 4 ) 3 or MoO 3 were not detected in the analyzed areas, this suggests that after the intermediate Al 2 (MoO 4 ) 3 layer has ] at octahedral sites depending on whether the dispersion threshold has been reached. 14As higher surface concentrations are reached, crystalline orthorhombic MoO 3 will form as [MoO 6 ] layers parallel to the [010] plane. 14The adsorption of [MoO 4 ] polyhedrons on the surface of Al 2 O 3 is therefore promoting the formation of the Al 2 (MoO 4 ) 3 intermediate layer. 15he product collected from the platinum crucible, post 1100 °C experiment, was characterized as corundum via pXRD (JCPSD card 46-1212) (Figure 9).The corundum was light bluish-gray in color and can be defined

Figure 1 .
Figure 1.Schematic showing the molten intermediate decomposition (MIND) mechanism: (A) initial layered system, (B) formation of intermediate, (C) crystallization of α−MoO 3 on the wall of vial, (D) start of melting, (E) end of melting, and (F) final product.

Figure 2 .
Figure 2. Post 950 °C product: (A) preremoval of the mixed solid disc and thin film-like crystals, (B) and (C) mixed solid disc, top and bottom, respectively.

Figure 3 .
Figure 3. pXRD pattern of α−MoO 3 thin film-like crystals removed from the quartz vial post 950 °C experiment.

Figure 4 .
Figure 4. pXRD pattern of α−MoO 3 thin film-like crystals removed from the walls of the quartz vial post 875 °C experiment.

Figure 5 .
Figure 5. pXRD pattern of the intermediate layer from the quartz vial post 875 °C experiment.

2 O 3 −
MoO 3 system between room temperature and 950 °C as captured by the timelapse images (see Video S1 in the Supporting information) and the result of heat treatment to 1100 °C.A schematic of the molten intermediate decomposition (MIND) mechanism is shown in Figure 1.The three key features of the mechanism are (1) the formation of an Al 2 (MoO 4 ) 3 layer via a solid−solid interaction between ∼705 and 860 °C (Figure 1B); (2) the melting of the Al 2 (MoO 4 ) 3 layer between approximately 870 and 890 °C (Figure 1D); and

Figure 6 .
Figure 6.(A) SEM micrograph and EDXA maps from the top of the mixed solid disc highlighting two regions of different morphologies, (B) EDXA spectrum 5 associated with the blocky crystals, and (C) EDXA spectrum 6 associated with the thin film-like crystals.Scale bar = 100 μm.
. The pXRD pattern for the thin filmlike crystals correlates to α−MoO 3 (JCPSD card 05-0508) preferentially orientated in the (0k0) plane where k = 2, 4, 6, or 10 (Figure 3).At approximately 870 °C, three distinct layers could be seen, and deposition of thin crystals on the walls of the vial was at a maximum.The intermediate layer and the thin crystals from the walls were obtained for characterization by repeating the experiment to 875 °C.The thin crystals were α− MoO 3 (JCPSD card 05-0508) preferentially orientated in the (0k0) plane where k = 2, 4, 6, or 10 (Figure 4) and the intermediate layer had a dominant phase of Al 2 (MoO 4 ) 3

Figure 7 .
Figure 7. (A) SEM micrograph and EDXA maps from the bottom of the mixed solid disc highlighting two regions of different morphologies, (B) EDXA spectrum 1 associated with the blocky crystals, and (C) EDXA spectrum 2 associated with the thin film-like crystals.Scale bar = 100 μm.

Figure 8 .
Figure 8. SEM micrographs from the bottom of the mixed solid disc; panels (A, C, E, and G) from backscattered electron detection and panels (B, D, F, and H) from secondary electron detection in the same areas, respectively.Scale bars = 500 μm, except for panels (A) and (B) where scale bars = 100 μm.
in CIE (L* a* b*) color space as L* = 76.65,a* = −1.09,and b* = −6.20.The coloration of the corundum is likely due to Mo impurities.Similar gray− blue Al 2 O 3 −MoO 3 systems have been reported along with their performance as pigments, 14 so the corundum synthesized in this work could potentially find use as a nonhazardous inorganic pigment.■ CONCLUSIONS A new crystal growth process for the synthesis of corundum, the molten intermediate decomposition (MIND) mechanism, was discovered through the direct observation of an Al 2 O 3 −MoO 3 system.The three key features of this mechanism are (1) the formation of an Al 2 (MoO 4 ) 3 intermediate layer through a solid− solid interaction in the temperature range of ∼705−860 °C; (2) the melting of the Al 2 (MoO 4 ) 3 layer between approximately 870 and 890 °C; and (3) the decomposition of Al 2 (MoO 4 ) 3 to corundum between 950 and 1100 °C.This work gives a deeper understanding of the crystal growth processes involved in the synthesis of corundum from Al 2 O 3 −MoO 3 , which, until now, were presumed to occur via the conventional flux method.■ ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c07772.Timelapse video clip of the 950 °C experiment (Video S1) (AVI) Images from an 875 °C experiment displaying the sublimation of MoO 3 followed by the growth of Al 2 (MoO 4 ) 3 (Figure S1) (PDF) ■ AUTHOR INFORMATION Corresponding Author Simon R. Hall − School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.; orcid.org/0000-0002-2816-0191;Email: simon.hall@bristol.ac.uk