Differences Between Crystal Structures And Crystal Chemistry Of Quartz

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There are both similarities and differences between the crystal structures and crystal chemistry of quartz, α-FePO4 and β-FePO4. One of the observed differences is that when the quartz is present at different temperatures. The α-FePO4 phase when present at low temperatures, tend towards the values obtained for β-FePO4 at high temperature. In addition, this case is true for all α quartz when at low temperature, in that it will tend to the values obtained for its respective β quartz experienced at high temperature. When the temperature of a system approach 980 Kelvin, the Fe-O-P bridging angles subsequently rise and the tetrahedral tilt angles decreases drastically during the α-β phase transition. Another difference lies in the case of thermal …show more content…
Soft constraints were applied to the P-O distances for the α-FePO4 phase in the temperature range between 860 Kelvin and 970 Kelvin and this is essential due to the greatly decreased intensity of a large number of reflections with low d spacing due to the high degree of thermal motion at these temperatures. Crystals, which are grown from the melt, are small and thus twinning is almost inevitable. β-FePO4 has significant change in bond distance and angle, thus resulting in the high degree of dynamic disorder. As shown in the picture below is the mechanism of the symmetry change for the FePO4 polymorphs. The cell parameters and volume of the alpha phase increase markedly and non-linearly as a function of temperature. The volume data were fitted to a cubic polynomial as for the alpha quartz. The expression was obtained for the thermal expansion coefficient: alpha (K inverse) = 2.924 x 10^-5 +2.920x10^-10 (T-300)^2. Thermal expansion is lower than alpha quartz and beta quartz. The principal contribution to the thermal expansion arises from the angular variations manifested by the changes in the two symmetrically independent inter-tetrahedral Fe-O-P bridging angles and the correlated tilt angles. The temperature dependence of the volume strongly follows the behavior of the average θ and δ angles as a function of temperature and there is a preferential expansion along alpha. The c/a ratio decreases from 2.2356 at 294 Kelvin to 2.2069 at 969 Kelvin. The initial decrease of the c/a ratio is in fact greater than that observed for any alpha quartz isotype due to the greater average angular variations in

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