High pressure single crystal X-ray diffraction study on ruby up to 31 GPa

The high pressure behavior of ruby (alpha-Al2O3 doped with Cr3+) was investigated to pressures up to 31 GPa. Whereas data on compressibility has been obtained at pressures up to 175 GPa, no structure determination exists above 9 GPa. Results show that the highly incompressible material has a bulk modulus of K = 242(8) GPa and its pressure derivative K' is calculated to be 6(1). In order to compensate for the applied forces, the MO6 octahedra (M = Al, Cr) volume decreases with increasing pressure. At pressures above 20 GPa, a distinct change in the oxygen positional parameter is observed. The reason might be a short M-O distance of 1.814(2) Angstrom which stays constant throughout the pressure range investigated. A comparable effect of a considerable change of the internal structure under high pressure despite a very low compressibility was recently observed for the mineral spinel [1].