Ionothermal synthesis of β-NH4AlF4 and the determination by single crystal X-ray diffraction of its room temperature and low temperature phases

β-NH4AlF4 has been synthesised ionothermally using 1-ethyl-3-methylimidazolium hexafluorophosphate as solvent and template provider. β-NH4AlF4 crystals were produced which were suitable for single crystal X-ray diffraction analysis. A phase transition occurs between room temperature (298 K) and low temperature (93 K) data collections. At 298 K the space group=I4/mcm (no. 140), α=11.642(5), c=12.661(5) Å, Z=2 (10NH4AlF4), wR(F2)=0.1278, R(F)=0.0453. At 93 K the space group=P42/ncm (no. 138), α=11.616(3), c=12.677(3) Å, Z=2 (10NH4AlF4), wR(F2)=0.1387, R(F)=0.0443. The single crystal X-ray diffraction study of β-NH4AlF4 shows the presence of two different polymorphs at low and room temperature, indicative of a phase transition. The [AlF4/2F2]− layers are undisturbed except for a small tilting of the AlF6 octahedra in the c-axis direction.