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Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 2 Part 10

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 478 27 Fast Ion Conductors Fig. 27.2. Crystal structure of a-AgI. Large circles I ions filled small circles octahedral sites filled squares tetrahedral sites filled triangles trigonal sites. Octahedral tetrahedral and trigonal sites can be used by Ag ions Fig. 27.3. Probabiliy distribution of Ag in a-AgI at 300 C according to Cava REIDINGER and WUENSCH 42 tetrahedral sites. The data also indicate that the Ag ions are preferentially found in oblong ellipsoidal regions centered at the tetrahedral sites and extending in the directions of the neighbouring octahedral sites. This suggests that the motion of the Ag ions is not completely liquid-like and that the 100 directions can be regarded as channel-like diffusion paths 29 . In other words diffusion of Ag ions occurs mainly by jumps between neighbouring tetrahedral sites. Besides o-AgI the phases o-CuBr o-Ag2S o-Ag2Se and o-Ag3SI have bcc anion structures. The number of cations per bcc unit cell is two for o-AgI and o-CuBr three for o-Ag3SI and four for o-Ag2S and o-Ag2Se. In 27.1 Fast Silver-Ion Conductors 479 Fig. 27.4. Cation pathway in an fcc anion sublattice according to Funke 19 . Filled squares tetrahedral sites small filled circles octahedral sites contradistinction to o-AgI and o-CuBr which have bcc anion lattices the anion lattice of o-CuI is fcc. This is understandable from the ratios of the cationic and anionic radii of these compounds as the cation sites provided by an fcc lattice are smaller in size when compared to those of a bcc lattice. The same systematic variation of the anion structure is observed in the case of the Ag and Cu chalcogenides while o-Ag2S and o-Ag2Se still exhibit the bcc structure o-Ag2Te o-Cu2S and o-Cu2Se have fcc arrangements. Possible cation diffusion paths in fcc and hcp anion lattices have been discussed in 43 . In an fcc unit cell there are 8 tetrahedral and 4 octahedral interstitial sites. The cation diffusion paths consist of alternating octahedral and tetrahedral sites. .