Structural evolution of rutile-type and CaCl2-type germanium dioxide at high pressure

Germanium dioxide was found to undergo a transition from the tetragonal rut ile-type to the orthorhombic CaCl2-type phase above 25 GPa. The detailed st ructural evolution of both phases at high pressure in a diamond anvil cell has been investigated by Rietveld refinement using angle-dispersive. X-ray powder-diffraction data. The square of the spontaneous strain (a - b)/(a b) in the orthorhombic phase was found to be a linear function of pressure and no discontinuities in the cell constants and volume were observed, indi cating that the transition is second-order and proper ferroelastic. Compres sion of the GeO6 octahedra was found to be anisotropic, with the apical Ge- O distances decreasing to a greater extent than the equatorial distances an d becoming shorter than the latter above 7 GPa. Above this pressure, the Ge O6 octahedron exhibits the common type of tetragonal distortion predicted b y a simple ionic model and observed for most rutile-type structures such as those of the heavier group-14 dioxides and the metal difluorides. Above th e phase transition, the columns of edge-sharing octahedra tilt about their two fold axes parallel to c and the rotation angle reaches 10.2(5)degrees b y 36(1) GPa so as to yield a hexagonal close-packed oxygen sublattice. The compressibility increases at the phase change as is expected for a second-o rder transition at which all additional compression mechanism becomes avail able.