A Comparative Study of a High-Pressure Polymorph of IO and its Ambient-Pressure Modification.

A hitherto unknown modification of IO was obtained from high-pressure/high-temperature syntheses in a Walker-type multianvil device at 8 GPa and 250 °C. HP-IO crystallizes in the monoclinic crystal system with space group P2/c (no. 14) with the unit cell parameters a=12.0612(3) Å, b=4.8613(2) Å, c=6.9585(2) Å, β=100.10(1)° (at 173 K), and four formula units per cell. The single-crystal structure data are accompanied by powder X-ray diffraction data at ambient and elevated temperatures. Furthermore, DFT calculations were carried out to investigate the phase transition between the ambient-pressure polymorph NP-IO to the newly synthesized high-pressure phase.