Fe-O-Op3 Experiment

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In the paper, FePO4 was studied in through neutron powder diffraction across a wide range of temperatures, and was found to have 2 states in general, the α (alpha) phase and β ( beta ) phase. The alpha phase exhibits a tetrahedral structure, which is similar to alpha quartz, and this occurs during generally low temperatures. In the case of this paper, the temperatures experimented on were between 294K and 1073K. The beta phase, however, is denser and exhibits a octahedral structure. The transition between these 2 phases was found to have occurred at a temperature of 980K. When temperatures start to rise from 294K, cell-parameters and volume of the alpha phase FePO4 was seen to increase non-linearly, and this is due to changes in the Fe-O-P …show more content…
This results from increasing disorder when temperature increases. One slight difference, however, was that the alpha phase of FePO4 had greater angular vibrations as compared to its quartz counterparts, as well as the fact that FePO4 is in itself a transition metal, due to Fe being present. Also, the tilt angle drops at a quicker rate when compared to other quartz variations. When FePO4 hits the beta phase, however, there is no more expansion due to increasing temperatures. This probably shows that there is a vast difference in the mechanisms between the alpha and beta phases, indicating either a structural limitation or a chemical property at high temperatures, in which the beta phase FePO4 is unable to continue to expand with higher temperatures. An equation governing the alpha to beta transition can be modelled with respect to the averaged tilt angle from all the individual tilt angles. In general, behaviours of FePO4 in both the alpha and beta phase is different from …show more content…
This tetrahedral distortion happens due to changes in the tetrahedral tilt angle, as well as changes in the tetrahedral bridging angle, and this two when combined will lead to the distortion. As temperature increases, the O1-Fe-O2 angle increases steadily, and the O2-Fe-O2 angle decreses steadily, until it becomes hexagonal, where both the values return back to near their original values at K=294. The O-P-O angles, however, largely remain constant regardless of the temperature rise, until the hexagonal structure comes in, where there is a noticeable change in all O-P-O bonds in relation to the angle. With regards to the bond length, Fe-O2 is observed to have a decreasing bond length as temperature increases, and coupled with the decreasing bond angle, we can deduce that the volume changes and thus the unit cell size undergoes a change as well. This is a bit different from the length of the P-O bonds, as the generally do not exhibit much change in bond length regardless of temperature, and therefore we can deduce that the P-O bonds do not undergo a major volume change or a very big change in terms of size of the unit cell. Thermal expansion in this case is actually dependant on both the tetrahedral tilt angle, as well as the bridging angles. From the alpha state to beta state transition, there is a noticeable change in all O-P-O bond angles, with all O-Fe-O angles

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