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Final writing exercise Essay

There are three phases whereby each has a different crystal structure at three different temperatures. At room temperature (298K), Phase III is present whereby Cs3H(SeO4)2 has a crystal structure of a monoclinic with a space group of C2/m. At 400K, Phase II is present whereby Cs3H(SeO4)2 has a crystal structure of a monoclinic-A2/a symmetry. At 470K, Phase I is present whereby Cs3H(SeO4)2 has a crystal structure of a trigonal with a space group of R3-m. In Phase III, as we can see in Figure 2(a), the positioning of the tetrahedrons is parallel to the a-axis, and in between these SeO4 tetrahedrons are the hydrogen bonds. Looking at a 2dimensional perspective, we can also see that there is a translation movement of the SeO4 tetrahedrons along the a-axis; hence the symmetry operator would be a glide line parallel to a-axis. In a 3-dimensional perspective, we can see that Phase III has a 2-fold rotation axis and contains glide planes.

In Phase II, from Figure 2(b), we can see that the positioning of the SeO4 tetrahedrons are along the approximate direction [310]. Observing the schematic of the crystal structure in Phase II, we can see that there is a vertical mirror line in between the SeO4 tetrahedrons. There is also an a-glide reflection vertically. In Phase I, from Figure 2(c), the positioning of SeO4 tetrahedron is similar to that of Phase II, however the difference is the crystal structure and the hydrogen bonding. Comparing both Phase II and Phase III crystal structures of the compound, Phase II contains two-fold screw axis, inversion center and a two-fold rotation axis, which is the sole reason for Phase II to be twice of that of Phase III in terms of geometrical arrangement of hydrogen bonds.

From the above analysis of the symmetry of the crystals structures in different phases, we can tell that Phase III has the most symmetry operators and hence achieving the highest crystal symmetry generating a low geometrical arrangement of hydrogen bonds. Due to the low geometrical arrangement of hydrogen bonds, the mobility of protons decreases giving the result of ferroelasticiy. The drastic change from superprotonic conductivity to ferroelasticty happens when there is a change from Phase II to Phase III. The major difference between theses 2 phases is the hydrogen bond arrangement.

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Under the optical microscope, we can observe that the polymorphic domains will alter at each phase transition to a different extent. We can see in phase III that the domains in the Cs3H(SeO4)2 crystal are made up of polydomains separated by two kinds of domain boundaries. The two kinds of domain boundaries are categorized as the planes of {311} and {11n}, where n is determined by the strain compatibility condition. The domains at the sides of each domain boundary are related to the reflective symmetry or the rotational symmetry on that boundary itself. Furthermore, we can observe that the angle between any domain and its neighboring domains is approximately 120°, which is very close to the theoretical values calculated using the lattice parameters.

As we move on from phase III to phase II, we can observe that the domain structure alters slightly by the phase transition of TII–III. Similarly, the reflective symmetry and rotational symmetry also changes at the same phase transition. However, the kinds of domain and domain boundary remain the same as those in phase III despite a change in domain pattern. This could be due to the slight change in alignment of hydrogen bonding between the SeO4 tetrahedrons when the existing hydrogen bonds were broken to form new weaker ones. This might explains why their lattice parameters a and b do not really change appreciably. Compared to phase III previously, the angle between any domain and its neighboring domains in phase II is also approximately 120° and is justified by the theoretical values determined from the same equation we used for phase III.

Hence, this suggest a slight change in the Cs3H(SeO4)2 crystal structure at the phase transition of TII–III. From phase II to phase I, the domain boundaries is observed to have disappear just before the curie temperature of the phase transition of TI–II and the crystal structure changes from optically biaxial to optically uniaxial. This could be due to an external stress caused by the atomic rearrangement of the SeO4 tetrahedrons in the Cs3H(SeO4)2 crystal as a result of breaking the hydrogen bonds between them.

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Higher temperatures for most material will enable atoms to move to low energy sites, fitting into a perfect crystal symmetry. Cs3H(SeO4)2 however behaves differently. As the temperature increases (above 396K), its crystal symmetry decreases when it changes phase from III to II. The orientation of the hydrogen bond for phase II and III differs. For phase II, the orientation is along [310] and [3-10] direction whereas for phase III, it is parallel to the aaxis. As the transition from phase III to II occurs, the precursor of the superprotonic conductivity is observed. In order for movement of proton to occur, the breaking and then recombination of hydrogen bonds are required.

For phase III, in order for the movement of one proton, the breaking of 2 hydrogen bonds is needed. The reason as to why 2 hydrogen bond is needed to be broken and recombined again is because for the movement of one proton to occur, it must break the hydrogen bond it resides in and then change its orientation, recombining at another site; the mirroring effect of opposite hydrogen bond is required to maintain the crystal symmetry i.e. to say that the another hydrogen bond parallel to the previous hydrogen bond site needs to be broken and recombined at other site parallel to the newly recombined hydrogen bond.

In this way, in phase III, the recombination of two hydrogen bonds is simultaneously needed for one proton transport. Phase II however, behaves differently. The movement of the proton is independent of the other protons at other hydrogen site. The crystal structure allows for this flexibility of the proton motion, which the superprotonic conduction takes place. The mechanism in which proton transportation occurs in the polymorphs is by the diffusion of protons through a hydrogen bond network, by the cleaving and formation of the hydrogen bonds. However, in certain phases, the cleavage and formation of the hydrogen bond might differ. The fuel cell works on the basis of the movement of protons. The movement of electrons should be disallowed as it would short circuit the fuel cell. Hence, a membrane is used to allow only the movement of protons across and not electrons and gases. On top of that, in order for a superprotonic effect to occur, the flexibility for proton motion must be allowed. Hence, the lesser symmetrically patterned the phases the protons reside in, the higher this flexibility.


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