Phase transitions in hydroxide perovskites: A Raman spectroscopic study of stottite, FeGe(OH) ₆, to 21 GPa

The effect of pressure on the naturally occurring hydroxide-perovskite stottite, FeGe(OH) ₆, has been studied in situ by micro-Raman spectroscopy to 21 GPa at 300 K. The ambient spectrum contains six OH-stretching bands in the range 3064 3352 cm ^-1. The presence of six non-equivalent OH groups is inconsistent with space group P4 ₂/n. In view of this inconsistency a new ambient structure determination of stottite from Tsumeb was carried out, but this did not allow the clear rejection of P4 ₂/n symmetry. However, a successful refinement was also carried out in space group P2/n, a subgroup of P4 ₂/n, which allows for six non-equivalent O atoms. The two refinements are of comparable quality and do not allow a choice to be made based purely on the X-ray data. However, taken with the ambient and 150 K Raman spectra, a good case can be made for stottite having P2/n symmetry at ambient conditions. On this basis, the pressure induced spectroscopic changes are interpreted in terms of a reversible phase transition P2/n ↔ P4 ₂/n.