Infrared Spectroscopic Studies on the Surface Chemistry of High‐Surface‐Area Gallia Polymorphs

Abstract

Polymorphs α, β, and γ of Ga 2 O 3 having hexagonal (corundum-type), monoclinic and cubic (spinel-type) structure, respectively, were prepared in a high-surface-area form, and characterized by powder X-ray diffraction. Nitrogen adsorption at 77 K showed these gallia samples to have specific surface areas of 77 (α-Ga 2 O 3 ), 40 (β-Ga 2 O 3 ) and 120 m 2 g Ϫ1 (γ-Ga 2 O 3 ). Fourier transform infrared spectroscopy of adsorbed carbon monoxide (at 77 K) and pyridine (at room temperature) showed that the three gallia polymorphs have a very similar surface Lewis acidity, regardless of their different crystal structures. This Lewis acidity was assigned, mainly, to coordinatively unsaturated tetrahedral Ga 3ϩ ions situated on the surface of the small crystallites which constitute the different metal 2115 oxide varieties. Ga 3ϩ ···CO adducts formed after CO adsorption gave (in all cases) a characteristic CϪO stretching band at 2195Ϫ2200 cm Ϫ1 , while Lewis-type adducts formed with adsorbed pyridine were characterized by IR absorption bands at 1610Ϫ1612 and 1446Ϫ1450 cm Ϫ1 . The three (partially hydroxylated) gallia polymorphs showed also a very weak Brønsted acidity, which they manifested by forming hydrogen-bonded adducts with both CO and pyridine; however no protonation of adsorbed pyridine occurred.

Synthesis and characterization of α-, β-, and γ-Ga2o3 prepared from aqueous solutions by controlled precipitation

“…9b), consistent with its still relatively high surface area ( Table 1). It has been reported [30,33,35,53,54], that the diffractogram of -Ga 2 O 3 closely resembles that of γ-and alumina with the exception of a shift towards lower Bragg angles, as expected for the larger Ga 3+ ions. No strong morphological change due to calcination could be detected by SEM or STEM (Fig.…”

“…Five modifications of Ga 2 O 3 are known: α, β, γ, δ and ε [27] and a thermodynamic description of the system Ga-O has been presented [28]. Among these five modifications, β-Ga 2 O 3 is the most stable modification (mp 1740 °С); it has a monoclinic structure with the oxide ions in distorted ccp arrangement and Ga III in distorted tetrahedral and octahedral sites [29][30][31]. α-Ga 2 O 3 crystallizes in the structure of corundum.…”

“…They attributed the presence of excess water in gallia gels to the formation of a product, probably GaOOH, which is not a precursor of γ-Ga 2 O 3 but rather of the α-Ga 2 O 3 polymorph. Hence, they advised to avoid the use of water as a solvent when preparing γ-Ga 2 O 3 [30,33]. δ-Ga 2 O 3 crystallizes like In 2 O 3 in the C-type structure of rare earths [28].…”

Simultaneous broadband near-infrared emission and magnetic properties of single phase Ni2+-doped β-Ga2O3 nanocrystals via mediated phase-controlled synthesis

“…[17][18][19] It is reported that the Ga 2 O 3 crystal possesses the following five crystalline forms: a-form, b-form, g-form, d-form and e-form, 20 and among these crystalline forms, b-Ga 2 O 3 crystal, the most stable form (melting point at 1740 1C), has a monoclinic structure equipped with O 2À ions in a distorted ccp arrangement and Ga 3+ ions in distorted tetrahedral and octahedral sites. 21,22 While the a-Ga 2 O 3 crystal could be generated prior to the formation of the b-phase Ga 2 O 3 crystal from the as-prepared precursor, crystallizing in a structure of corundum. 23 Moreover, with a wide bandgap of about 4.7 eV, the b-Ga 2 O 3 crystal has extended transparency to the deep ultraviolet wavelength range among transparent semiconducting oxides, and coupling with a strong absorption below 260 nm.…”

Diffusion-driven and size-dependent phase changes of gallium oxide nanocrystals in a glassy host

“…18,19 Among these phases, b-Ga 2 O 3 is the thermodynamically stable one, with monoclinic structure. 20 The a-Ga 2 O 3 phase crystallizes in the structure of corundum, and single crystals can only be grown under specific conditions of temperature and pressure. 21 g-Ga 2 O 3 shows a cubic spinel-type structure, analogous to g-Al 2 O 3 .…”