Feo4, Fepo4: Phase In Different Phases At Different Temperatures

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Quartz-type iron phosphate, FePO4, exists in different phases at different temperatures. At low temperatures, it exists in the α-quartz-type phase, while at high temperatures, it exists in the β-quartz-type phase. The α-β phase transition occurs at 980K.
In terms of lattice symmetry, α-FePO4 exhibits a trigonal lattice system, whereas β-FePO4 exhibits a hexagonal lattice system.
In terms of space symmetry, α-FePO4 belongs to the P3121 space group, while β-FePO4 belongs to the P6422 space group. The reason why α-FePO4 and β-FePO4 are able to adopt the α-quartz (SiO2) and β-quartz structures respectively is that chemical substitution takes place to enable the same symmetry to be maintained.
2 Si4+ --> Fe3+ + P5+
By replacing Si4+
3 and 4 become steeper with increasing temperature). The thermal expansion coefficient is given by α (K-1) = 2.924 x 10-5 + 2.920 x 10-10 (T-300)2
This thermal expansion is possible because of increases in the two intertetrahedral Fe-O-P bridging angles, 1 and 2, and decreases in the tilt angles, δFeO4 and δPO4. The cell parameters and fractional atomic coordinates of α-FePO4 tend towards those of β-FePO4 up until the transition temperature of 980K, during which they exhibit discontinuous shifts.
In the β phase, there is essentially no thermal expansion even as temperature increases (the line charts in Fig. 3 and 4 plateau past 980K). This is because the structure has expanded to its maximum size, and the intertetrahedral bridging angles and tilt angles can no longer be varied to accommodate further
The increase in volume corresponding to increase in temperature and α-β phase transition can be clearly seen by comparing ATOMS 7&8 with ATOMS 9&10.
In the β phase, the atoms are more spread out. While time-averaged Fe-O distances decrease with increasing temperature due to the increasing disorder at high temperature arising from excited low-energy, high-amplitude vibrations, instantaneous Fe-O distances increase with increasing temperature.
In addition to that, the intertetrahedral Fe-O-P bridging angles are larger, and the tilt angles are smaller compared to those of the α phase, and these give rise to the fully expanded structure as seen in ATOMS 9&10.
Once the β phase has been reached, there can be no more thermal expansion with increase in temperature as the intertetrahedral bridging angles can no longer be increased, and the tilt angles are fixed to zero. Any further expansion will lead to the breaking of