GROUP B MODEL ANSWER Howard et al. , Temperature-induced structural changes in Cacao, Caber and critic A synchrotron x-ray powder diffraction Pays. Rev.

B 72, 214114 (2005). DO NOT PLAGIARISM THIS MODEL ANSWER paragraph 1 The polymorph involved are the halides in tetragonal retile (Tie) structure and the halides in so-called calcium chloride structure. As the name implies, the unit cell of halides in tetragonal retile structure is tetragonal, with the lattice parameters and It belongs to the space group POP/ MN.

On the other hand, the unit cell of halides in calcium chloride structure s orthorhombic, I. E. And The space group of this Cacao structured polymorph is Penn. To compare symmetry operators in the two polymorph, we refer to the space group diagrams of POP/MN and Penn space groups.

(Figure 1). The positions of the symmetry operators in the unit cell are denoted by the fractional co-ordinate system used in the space group diagrams. Both polymorph have two a-glide planes at x = 0. 25 and x =0.

75 respectively and two beside planes tat = 0. 25 and y = 0. 5 respectively. Moreover, the two polymorph also share twofold root-inversion operators on (0.

5, O), (O, 0. ), (1, 0. 5) and (0. 5, 1). However, the root-inversion operators on (0,0), (O, 1 (0. 5, ). 5), (1 ,O), (1, 1) in the Cacao-structured polymorph were replaced by operators of higher symmetry, I. E.

42 screw axis with centre of symmetry in the tetragonal retile structured polymorph. Besides, two diagonal glide lines and four mirror lines which are absent in unit cell of Cacao-structured polymorph are present in that of tetragonal retile structured polymorph.Cacao and Caber start as a Cacao-structured polymorph at room temperature. As temperature approaches the transition temperature, the MIX (M is metal action while X is the halogen anion) octahedron in the centre of the unit cell rotates about the c-axis, causing the rectangular base of the unit cell to approach square, which results in the conversion of some twofold root-inversion operators to 42 screw axis with centre of symmetry.

As a result, the orthorhombic Cacao-structured polymorph undergoes changes in crystal structure and converts into the tetragonal retile-structured polymorph.Fig. 1 : (a) POP/MN: The pace group diagram which tetragonal retile structured polymorph belongs to. B) Penn: The space group which Cacao- structured polymorph belongs to. Paragraph 2 Polymorph of Cacao and Caber has very similar structure, while it is significantly different with Occur that stays as orthorhombic as it does not undergo any phase transformation. In Occur, each Cry atoms are connected with co-planar 4 CLC atoms, while above and below it, it is connected with each 1 CLC atoms, summing up the connectivity to 6 atoms, forming Occur octahedron in crystal structure.Upon heating only stretching of lattice parameters and bond lengths occur due to thermal strain, in which the preference of cell edge lengths are calculated in paragraph 3. However, the increase in unit cell edge lengths do not differ as much as the other 2 Structures, as there is no symmetric distortion, which I can guess, along with other effects mentioned in paragraph 3.

In addition, the tilt angle also remain the same due to the coupling effect in paragraph 3.Furthermore, comparing Cacao and Caber, obviously Caber has higher bond length and cell edges as BRB has greater in atomic radius than CIA, resulting in higher unit cell edge and bond length. Using Cacao as illustrated in Figure 2, initially they have orthorhombic structure (Penn) at ambient temperature. In [001] direction, we can see 2 CLC atoms (green) and 1 Ca atom (blue) are slightly bent along the same axis. However, upon heating, there is thermal strain that distorts the structure.Then, it transforms into tetragonal (POP/MN) structure, in which the atoms are more aligned or have lower octahedral tilting especially in [001] direction.

As tetragonal has higher symmetry than orthorhombic, it is expected that symmetry will increase by reducing tilt angle. Interestingly, comparing Cacao and Caber for either same orthorhombic or tetragonal Truckee, octahedral tilting of Cacao is higher than Caber, especially along [001] direction, which may be attributed to the lower transition temperature, higher electronegative of CLC and/or lower length of shared octahedral edge or longer apical bond.To sum up, for Cacao and Caber, the cell edge length and bond length are increased upon transformation of orthorhombic to tetragonal, while the octahedral tilt angle is reduced in order to achieve and maintain higher symmetry. Fig. 2: (a) Cacao Penn and (b) Cacao POP/MN Paragraph 3 Cacao exists as the calcium chloride structured polymorph at room imperative.

In addition, the length of cell edges a, b and c is dependent on the temperature. The length cell edge a decreases from 6. 45? to 6. K while both length of edge b and c increases when the temperature is close to the transition temperature of 235 degree Celsius. On top of that, the maximum distortion of Cacao octahedron occurs at just below the phase transition temperature due to the increment of the length of the apical bond and the decrement of the length of the equatorial bond. The lengths of cell edge and c of the unit cell of the tetragonal retile structured polymorph increase with the imperative when it is higher than 235 degree Celsius.

The degree of difference between apical and equatorial bond lengths is lower than that at 200 degree Celsius. Similarly, this trend could also be observed in Caber. However, the transition temperature is 553 degree Celsius and the lengths of cell edge a and b differ as well. Compared to Cacao, Caber, the cell edges a, b and c of unit cell Occur increases steadily within the range of temperature of O to 800 degree Celsius. On top of that, the length difference between apical and equatorial bond increases with temperature due to the increment of the apical bond length.

There is no noticeable difference in the tilting angle of Occur octahedral as well. From the observation above, we could conclude that the transition temperature of Occur is not within O to 800 degree Celsius. With the plot of the square of the orthorhombic strain against temperature, the transition temperature could be higher 2000 degree Celsius which is much higher than the melting point.

The elongation of the octahedral allows greater amount of orthorhombic strain for the same degree of octahedral tilting (coupling effect).