Synthesis and Polymorphism of Mixed Aluminum-Gallium Oxides

Site-directed cation ordering in chabazite-type AlxGa1-xPO4-34 frameworks revealed by NMR crystallography

We report the first synthesis of the mixed-metal chabazite-type AlxGa1-xPO4-34(mim) solid solution, containing 1-methylimidazolium, mim, as structure directing agent (SDA), from the parent mixed-metal oxide solid solution, γ-(AlxGa1-x)2O3. This hitherto unreported family of materials exhibits complex disorder, arising from the possible distributions of cations over available sites, the orientation of the SDA and the presence of variable amounts of water, which provides a prototype for understanding structural subtleties in nanoporous materials. In the as-made forms of the phosphate frameworks, there are three crystallographically distinct metal sites: two tetrahedral MO4 and one octahedral MO4F2 (M = Al, Ga). A combination of solid-state NMR spectroscopy and periodic DFT calculations reveals that the octahedral site is preferentially occupied by Al and the tetrahedral sites by Ga, leading to a non-random distribution of cations within the framework. Upon calcination to the AlxGa1-xPO4-34 framework, all metal sites are tetrahedral and crystallographically equivalent in the average R3̄ symmetry. The cation distribution was explored by 31P solid-state NMR spectroscopy, and it is shown that the non-random distribution demonstrated to exist in the as-made materials would be expected to give remarkably similar patterns of peak intensities to a random distribution owing to the change in average symmetry in the calcined materials.

In situ X-ray diffraction study of the solvothermal formation mechanism of gallium oxide nanoparticles

Gallium oxides are of broad interest due to their wide band gaps and attractive photoelectric properties. Typically, the synthesis of gallium oxide nanoparticles is based on a combination of solvent-based methods and subsequent calcination, but detailed information about solvent based formation processes is lacking, and this limits the tailoring of materials. Here we have examined the formation mechanisms and crystal structure transformations of gallium oxides during solvothermal synthesis using in situ X-ray diffraction. γ-Ga₂O₃ readily forms over a wide range of conditions. In contrast, β-Ga₂O₃ only forms at high temperatures (T > 300 °C), and it is always preceded by γ-Ga₂O₃, indicating that γ-Ga₂O₃ is a crucial part of the formation mechanism of β-Ga₂O₃. The activation energy for formation of β-Ga₂O₃ from γ-Ga₂O₃ is determined to be 90-100 kJ mol^-1 in ethanol, water and aqueous NaOH based on kinetic modelling of phase fractions obtained from multi-temperature in situ X-ray diffraction data. At low temperatures GaOOH and Ga₅O₇OH form in aqueous solvent, but these phases are also obtained from γ-Ga₂O₃. Systematic exploration of synthesis parameters such as temperature, heating rate, solvent and reaction time reveal that they all affect the resulting product. In general, the solvent based reaction paths are different from reports on solid state calcination studies. This underlines that the solvent is an active part of the solvothermal reactions and to a high degree determines different formation mechanisms.

Regulating the trap distribution of ZnGa2O4:Cr3+ by Li+/Ga3+ doping for upconversion-like trap energy transfer NIR persistent luminescence

Doping Li+/Ga3+ in ZnGa2O4:Cr3+ splits the energy level of electron traps into shallow traps and deep traps, which makes the developed phosphors be successfully excited by a low-energy NIR light and suitable for long-term biological imaging.

Effect of extended defects on photoluminescence of gallium oxide and aluminum gallium oxide epitaxial films

In this work, a systematic photoluminescence (PL) study on three series of gallium oxide/aluminum gallium oxide films and bulk single crystals is performed including comparing doping, epitaxial substrates, and aluminum concentration. It is observed that blue/green emission intensity strongly correlates with extended structural defects rather than the point defects frequently assumed. Bulk crystals or Si-doped films homoepitaxially grown on (010) β-Ga₂O₃ yield an intense dominant UV emission, while samples with extended structural defects, such as gallium oxide films grown on either (-201) β-Ga₂O₃ or sapphire, as well as thick aluminum gallium oxide films grown on either (010) β-Ga₂O₃ or sapphire, all show a very broad PL spectrum with intense dominant blue/green emission. PL differences between samples and the possible causes of these differences are analyzed. This work expands previous reports that have so far attributed blue and green emissions to point defects and shows that in the case of thin films, extended defects might have a prominent role in emission properties.

Defects and impurities in colloidal Ga2O3 nanocrystals: new opportunities for photonics and lighting

Defects, both native and extrinsic, critically determine functional properties of metal oxides. Gallium oxide has recently gained significant attention for its promise in microelectronics, owing to the unique combination of conductivity and high breakdown voltage, and solid-state lighting, owing to the strong photoluminescence in the visible spectral region. These properties are associated with the presence of native defects that can form both donor and acceptor states in Ga2O3. Recently, Ga2O3 nanocrystal synthesis in solution and optical glasses has been developed, allowing for a range of new applications in photonics, lighting, and photocatalysis. This review focuses on the structure and properties of Ga2O3 nanocrystals with a particular emphasis on the electronic structure and interaction of defects in reduced dimensions and their role in the observed photoluminescence properties. In addition to native defects, the effect of selected external impurities, including lanthanide and aliovalent dopants, and alloying on the emission properties of Ga2O3 nanocrystals are also discussed.

Uncovering selective and active Ga surface sites in gallia-alumina mixed-oxide propane dehydrogenation catalysts by dynamic nuclear polarization surface enhanced NMR spectroscopy

Gallia–alumina (Ga,Al)₂O_3(x : y) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as ²⁷Al and ⁷¹Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning ¹⁵N{²⁷Al} and ¹⁵N{⁷¹Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using ¹⁵N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al)₂O₃ oxides. Notably, we demonstrate that (Ga,Al)₂O₃ catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al)₂O₃ catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance.

Coordination geometry and Lewis acidity of Ga and Al (bulk and surface) sites in mixed oxide gallia–alumina nanoparticles is correlated with the performance in propane dehydrogenation.

Gallium Species Incorporated into MOF Structure: Insight into the Formation of a 3D Polycrystalline Gallium-Imidazole Framework

The formation of a polycrystalline 3D gallium-imidazole framework (MOF) was closely studied in three steps using ssNMR, XRPD, and TGA. In all steps, the reaction products show relatively high temperature stability up to 500 °C. The final product was examined by structural analysis using NMR crystallography combined with TG and BET analyses, which enabled a detailed characterization of the polycrystalline MOF system on the atomic-resolution level. ⁷¹Ga ssNMR spectra provided valuable structural information on the coexistence of several distinct gallium species, including a tunable liquid phase. Moreover, using an NMR crystallography approach, two structurally asymmetric units of Ga(Im₆)^6- incorporated into the thermally stable polycrystalline 3D matrix were identified. Prepared polycrystalline MOF material with polymorphic gallium species is promising for use in catalytic processes.

Application of NMR Crystallography to Highly Disordered Templated Materials: Extensive Local Structural Disorder in the Gallophosphate GaPO-34A

We present an NMR crystallographic investigation of two as-made forms of the recently characterized gallophosphate GaPO-34A, which has an unusual framework composition with a Ga:P ratio of 7:6 and contains both hydroxide and fluoride anions and either 1-methylimidazolium or pyridinium as the structure-directing agent. We combine previously reported X-ray crystallographic data with solid-state NMR spectroscopy and periodic density functional theory (DFT) calculations to show that the structure contains at least three distinct types of disorder (occupational, compositional, and dynamic). The occupational disorder arises from the presence of six anion sites per unit cell, but a total occupancy of five of these, leading to full occupancy of four sites and partial occupancy of the fifth and sixth (which are related by symmetry). The mixture of OH and F present leads to compositional disorder on the occupied anion sites, although the occupancy of some sites by F is calculated to be energetically unfavorable and signals relating to F on these sites are not observed by NMR spectroscopy, confirming that the compositional disorder is not random. Finally, a combination of high-field ⁷¹Ga NMR spectroscopy and variable-temperature ¹³C and ³¹P NMR experiments shows that the structure directing agents are dynamic on the microsecond time scale, which can be supported by averaging the ³¹P chemical shifts calculated with the SDA in different orientations. This demonstrates the value of an NMR crystallographic approach, particularly in the case of highly disordered crystalline materials, where the growth of large single crystals for conventional structure determination may not be possible owing to the extent of disorder present.

Synthesis and Polymorphism of Mixed Aluminum-Gallium Oxides

The synthesis of a new solid solution of the oxyhydroxide Ga_5–xAl_xO₇(OH) is investigated via solvothermal reaction between gallium acetylacetonate and aluminum isopropoxide in 1,4-butanediol at 240 °C. A limited compositional range of 0 ≤ x ≤ 1.5 is produced, with the hexagonal unit cell parameters refined from powder X-ray diffraction (XRD) showing a linear contraction in unit cell volume with an increase in Al content. Solid-state ²⁷Al and ⁷¹Ga nuclear magnetic resonance (NMR) spectroscopies show a strong preference for Ga to occupy the tetrahedral sites and Al to occupy the octahedral sites. Using isopropanol as the solvent, γ-Ga_2–xAl_xO₃ defect spinel solid solutions with x ≤ 1.8 can be prepared at 240 °C in 24 h. These materials are nanocrystalline, as evidenced by their broad diffraction profiles; however, the refined cubic lattice parameter shows a linear relationship with the Ga:Al content, and solid-state NMR spectroscopy again shows a preference for Al to occupy the octahedral sites. Thermal decomposition of Ga_5–xAl_xO₇(OH) occurs via poorly ordered materials that resemble ε-Ga_2–xAl_xO₃ and κ-Ga_2–xAl_xO₃, but γ-Ga_2–xAl_xO₃ transforms above 750 °C to monoclinic β-Ga_2–xAl_xO₃ for 0 ≤ x ≤ 1.3 and to hexagonal α-Ga_2–xAl_xO₃ for x = 1.8, with intermediate compositions of 1.3 < x < 1.8 giving mixtures of the α- and β-polymorphs. Solid-state NMR spectroscopy shows only the expected octahedral Al for α-Ga_2–xAl_xO₃, and for β-Ga_2–xAl_xO₃, the ∼1:2 tetrahedral:octahedral Al ratio is in good agreement with the results of Rietveld analysis of the average structures against powder XRD data. Relative energies calculated by periodic density functional theory confirm that there is an ∼5.2 kJ mol^–1 penalty for tetrahedral rather than octahedral Al in Ga_5–xAl_xO₇(OH), whereas this penalty is much smaller (∼2.0 kJ mol^–1) for β-Ga_2–xAl_xO₃, in good qualitative agreement with the experimental NMR spectra.

Solvothermal reactions in 1,4-butanediol or isopropanol yield a new solid solution of the oxyhydroxide Ga_5−xAl_xO₇(OH) or the spinel series γ-Ga_2−xAl_xO₃, respectively. We have explored the possible composition range and the thermal stability of the materials. For γ-Ga_2−xAl_xO₃ (x ≤ 1.3), transformation to the monoclinic β-polymorph occurs above 1000 °C, providing a convenient route to this material of interest for electronic applications.