Electron microscopy
Comparison between Hexagonal and Tetragonal (or Orthorhombic) Cells
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The silicon rich rare earth compounds with the formula RSi2-x crystallise in three different structure types (as shown in Figure 967):
         i) Tetragonal ThSi2,
         ii) orthorhombic GdSi2,
         iii) hexagonal AlB2.

The orthorhombic GdSi2-type structure is a deformation of the tetragonal ThSi2 type even though their relation with the hexagonal AlB2 type structure is not straightforward. These plates in Figure 967 are stacked along the [001] direction in the tetragonal and orthorhombic structures and along the [1010] direction in the hexagonal one. [1]

Rare earth metal disilicides structures

Figure 967. Rare earth metal disilicides structures. (A) Building element. (B) Building elements stacking and hexagonal cell in the AlB2 structure. (C) Building elements stacking and tetragonal (or orthorhombic) cell in the ThSi2 type (or GdSi2 type) structure. [1]














[1] Leo Miglio & Francois d'Heurle, SUICIDES Fundamentals and Applications: Proceedings of the 16th Course of the International School of Solid State Physics, 2000.