January 2016 Model Answer Written Assignment Group A
GROUP A
MODEL ANSWER
Brafman et al., Five New Zinc Sulphide Polytypes: 10L (8 2); 14L (5 4 2 3); 24 L (5 3)3; 26L (17 4 2 3) and 28L (9 5 5 0). Acta Cryst. 22, 347 - 352 (1967).
DO NOT PLAGIARISE THIS MODEL ANSWER
Paragraph 1
ZnS exists in two polymorphs, namely sphalerite and wurzite. Analysis of its symmetry and crystal structure using ATOMS will be used to deduce its possible polytypic stacking sequences as well as the orientation relationship between these two structures. Sphalerite is a face centered cubic structure with each sulphur bonded with four zinc atoms is observed. The four interior sulphur atoms are located at 0.25, 0.25, 0.25 away from the nearest unit cell corners. There are 4 formula units of ZnS per unit cell.
Fig 1 : Sphalerite Purple : Zinc & Yellow: Sulphur Fig 2 : Polyhedras of sphalerite
The arrangement of atoms can further be visualized by connecting the Sulphur bonds to form tetragonal polyhedras with Sulphur being enclosed in each of them. The normal of one of the faces can be calculated by finding the cross product of the two edge directions. Given that the directions of the cell edges corresponds to <100>, normal of the tetragonal face => [1,-1,0] x [1,0,-1] = <111>. Using ATOMS, by expanding the number of unit cells, rotating the structure and observe down the [111] axis, stacking of tetragonal polyhedral are observed. As the Zinc atoms are coordinated within its polyhedral, every polyhedral layer is a close packed structure with one atom being in contact with 6 other atoms. By separating it into individual layers, it can also observed that there are three different close packed layers that differ in spatial orientation 600 rotation apart. The three layers are given the fundamental ABC notation. Hence, a FCC structure follows an ABC stacking sequence.
Fig 3: Upper right hand corner: Sphalerite polyhedral. Lower right hand corner: Polyhedras viewed from [111] direction
Wurzite has a hexagonal structure with a basis of
MODEL ANSWER
Brafman et al., Five New Zinc Sulphide Polytypes: 10L (8 2); 14L (5 4 2 3); 24 L (5 3)3; 26L (17 4 2 3) and 28L (9 5 5 0). Acta Cryst. 22, 347 - 352 (1967).
DO NOT PLAGIARISE THIS MODEL ANSWER
Paragraph 1
ZnS exists in two polymorphs, namely sphalerite and wurzite. Analysis of its symmetry and crystal structure using ATOMS will be used to deduce its possible polytypic stacking sequences as well as the orientation relationship between these two structures. Sphalerite is a face centered cubic structure with each sulphur bonded with four zinc atoms is observed. The four interior sulphur atoms are located at 0.25, 0.25, 0.25 away from the nearest unit cell corners. There are 4 formula units of ZnS per unit cell.
Fig 1 : Sphalerite Purple : Zinc & Yellow: Sulphur Fig 2 : Polyhedras of sphalerite
The arrangement of atoms can further be visualized by connecting the Sulphur bonds to form tetragonal polyhedras with Sulphur being enclosed in each of them. The normal of one of the faces can be calculated by finding the cross product of the two edge directions. Given that the directions of the cell edges corresponds to <100>, normal of the tetragonal face => [1,-1,0] x [1,0,-1] = <111>. Using ATOMS, by expanding the number of unit cells, rotating the structure and observe down the [111] axis, stacking of tetragonal polyhedral are observed. As the Zinc atoms are coordinated within its polyhedral, every polyhedral layer is a close packed structure with one atom being in contact with 6 other atoms. By separating it into individual layers, it can also observed that there are three different close packed layers that differ in spatial orientation 600 rotation apart. The three layers are given the fundamental ABC notation. Hence, a FCC structure follows an ABC stacking sequence.
Fig 3: Upper right hand corner: Sphalerite polyhedral. Lower right hand corner: Polyhedras viewed from [111] direction
Wurzite has a hexagonal structure with a basis of