1 research outputs found
Тепловой баланс помещения с электрической кабельной системой отопления
Solvothermal
oxidation of metallic gallium in monoethanolamine for 72 h at 240
°C yields a crystalline sample of γ-Ga<sub>2</sub>O<sub>3</sub> (∼30 nm crystallites). While Rietveld refinement (cubic
spinel structure, <i>Fd</i>3̅<i>m</i>; <i>a</i> = 8.23760(9) Å) reveals that Ga occupies two pairs
of octahedral and tetrahedral sites (ideal spinel and nonspinel),
it provides no information about their local distribution, which cannot
be statistical owing to the short Ga–Ga contacts produced if
neighboring ideal spinel and nonspinel sites are simultaneously occupied.
To create an atomistic model to reconcile this situation, a 6 ×
6 × 6 supercell of the crystal structure is constructed and refined
against neutron total scattering data using a reverse Monte Carlo
(RMC) approach. This accounts well for the local as well as long-range
structure and reveals significant local distortion in the octahedral
sites that resembles the structure of thermodynamically stable β-Ga<sub>2</sub>O<sub>3</sub>. <sup>71</sup>Ga solid-state NMR results reveal
a octahedral:tetrahedral Ga ratio that is consistent with the model
obtained from RMC. Nanocrystalline samples of γ-Ga<sub>2</sub>O<sub>3</sub> are produced by either a short solvothermal reaction
(240 °C for 11 h in diethanolamine; ∼15 nm crystallites)
or by precipitation from an ethanolic solution of gallium nitrate
(∼5 nm crystallites). For these samples, the Bragg scattering
profile is broadened by their smaller crystallite size, consistent
with transmission electron microscopy results, and analysis of the
relative Bragg peak intensities provides evidence that a greater proportion
of tetrahedral versus octahedral sites are filled. In contrast, neutron
total scattering shows the same average Ga–O distance with
decreasing particle size, consistent with <sup>71</sup>Ga solid-state
NMR results that indicate that all samples contain the same overall
proportion of octahedral:tetrahedral Ga. It is postulated that increased
occupation of tetrahedral sites within the smaller crystallites is
balanced by an increased proportion of octahedral surface Ga sites,
owing to termination by bound solvent or hydroxide