August 2016 Model Answer Written Assignment Group A
GROUP A
MODEL ANSWER
Howard et al., Temperature-induced structural changes in CaCl2, CaBr2 and CrCl2: A synchrotron X-ray powder diffraction Phys. Rev. B 72, 214114 (2005).
DO NOT PLAGIARISE THIS MODEL ANSWER
Paragraph 1
This article concerns about 2 polymorph structures that is associated with temperature-induced structural transitions. One of the structure being studied is the orthorhombic structure, or sometimes referred to as the calcium chloride structure, that is observed before the transition. It has a space group of Pnnm and it is observed in halides – CaCl2, CaBr2, CrCl2, under ambient conditions. The space group diagram is shown in Figure 1 where we can see that it is a primitive cell with 4 mirror lines within the unit cell in 2-D (2 horizontal and 2 vertical). There is 1 unique mirror plane. Also, there are 4 double glide planes (dashed/dotted lines: 2 horizontal and 2 vertical) by which there are a total of 2 unique glide planes. In a double glide plane, there is vertical and horizontal translation and a periodic repeat of the basis to produce a chiral pair. In addition, it has 2 fold rotation points (diad) at the corners, centre and at the middle along the sides. The 2 fold rotation points are also centres of inversion. The second structure being studied is the tetragonal rutile structure that is commonly found in metal dioxides and dihalides. With analysis of the tetragonal rutile, there are 2 possibilities that the basis atoms (octahedrons) can be arranged in 3-D space. One of the P42/mnm occurs with the origin at mmm and 42/m at 1/4,0,0. In this case, there are 2 unique mirror lines that runs diagonally across the plane cell. The dashed lines represents a glide line in the unit cell where there is reflection and translation of objects. Roto-inversion twofold screw axis with centre of symmetry is found at the centre and corners of the unit cell. Fourfold screw axis with inversion centres are observed to be at the middle of the cell
MODEL ANSWER
Howard et al., Temperature-induced structural changes in CaCl2, CaBr2 and CrCl2: A synchrotron X-ray powder diffraction Phys. Rev. B 72, 214114 (2005).
DO NOT PLAGIARISE THIS MODEL ANSWER
Paragraph 1
This article concerns about 2 polymorph structures that is associated with temperature-induced structural transitions. One of the structure being studied is the orthorhombic structure, or sometimes referred to as the calcium chloride structure, that is observed before the transition. It has a space group of Pnnm and it is observed in halides – CaCl2, CaBr2, CrCl2, under ambient conditions. The space group diagram is shown in Figure 1 where we can see that it is a primitive cell with 4 mirror lines within the unit cell in 2-D (2 horizontal and 2 vertical). There is 1 unique mirror plane. Also, there are 4 double glide planes (dashed/dotted lines: 2 horizontal and 2 vertical) by which there are a total of 2 unique glide planes. In a double glide plane, there is vertical and horizontal translation and a periodic repeat of the basis to produce a chiral pair. In addition, it has 2 fold rotation points (diad) at the corners, centre and at the middle along the sides. The 2 fold rotation points are also centres of inversion. The second structure being studied is the tetragonal rutile structure that is commonly found in metal dioxides and dihalides. With analysis of the tetragonal rutile, there are 2 possibilities that the basis atoms (octahedrons) can be arranged in 3-D space. One of the P42/mnm occurs with the origin at mmm and 42/m at 1/4,0,0. In this case, there are 2 unique mirror lines that runs diagonally across the plane cell. The dashed lines represents a glide line in the unit cell where there is reflection and translation of objects. Roto-inversion twofold screw axis with centre of symmetry is found at the centre and corners of the unit cell. Fourfold screw axis with inversion centres are observed to be at the middle of the cell