Sialon X-Phase

Abstract

Sialon X-phase has a triclinic unit cell with a, 9.69; b, 8.56; c, 11.21Åα, 90.0; β, 124.4;γ, 98.5° and essentially a point composition close to Si₃Al₆O₁₂N₂. When liquid of this composition is cooled, well-crystalline “low”-X is formed with the above unit-cell dimensions. Its crystal structure is similar to mullite and consists of alternate chains of octahedra and tetrahedra linked to form sheets in the (100) plane; these sheets are joined together by a complex network of tetrahedra, some units of which resemble the Si₆N₈ units in the unit cell of β-Si₃N₄. Whereas the repeat distance along the chains is 2.85Å. the tetrahedral network repeats every three tetrahedra (8.56Å).

At more silica-rich starting compositions, X-phase crystallises as very fine needles (“high”-X) which give diffuse and often very weak X-ray reflections for k ≠ 3n. This is due to frequent faulting of the structure by shifts of b/3 in the (100) plane which results in occupation of all available tetrahedral sites in the network and gives an apparent b repeat distance of 2.85Å.