Figure 1

Transitional diagram for metastable $\alpha \mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ plotted on the equilibrium phase diagram of germanium dioxide. The equilibrium phase diagram was reconstructed from Refs. 3, 6, 7, 8. The symbols correspond to the points of pressure-temperature treatment of $\alpha \mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ samples. They display the results of the scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction studies of the samples quenched to ambient conditions, which allowed us to identify the diffusive $\alpha \to r$ (squares), displacivelike $\alpha \to r$ (diamonds), and displacive $\alpha \to r^{\prime }$ (circles) transitions. The degree of symbol filling corresponds to the share of the high-pressure $r$ or $r^{\prime }$ phase, estimated on the basis of x-ray diffraction. The dashed lines approximate the region of pressure-temperature–induced transitions from metastable $\alpha \mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$, serving as its kinetic stability boundary, and, in turn, the vertical bars separate the pressure intervals with different transition modes. The dash-dotted line approximates the region of $r^{\prime }\to r$ recrystallization, obtained in the separate quenching experiments.Reuse & Permissions

Figure 2

SEM photographs of the $r\mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ crystallite grown by diffusion in the $\alpha \mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ surroundings upon room-pressure heating to 1100 K (a) and of the $r^{\prime }\mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ phase obtained via the displacive mechanism upon room-temperature compression of $\alpha \mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ to 11.5 GPa (b).Reuse & Permissions

Figure 3

Elementary cell of the distorted rutilelike monoclinic $r^{\prime }$ phase (a) and layer packing of germanium atoms over oxygen layers in the $r^{\prime }$ phase which is $3\times 2$ octahedrally chained (b).Reuse & Permissions

Figure 4

Electron diffraction pattern (a) from the single crystallite of the monoclinic phase of ${\mathrm{G}\mathrm{e}\mathrm{O}}_2$ for the 010 zone (reciprocal lattice plane), (b) from rutile phase (010 zone of rutile structure) obtained upon annealing of the same crystallite as in (a), and (c) from the partly transformed area revealing both initial $\alpha$ quartz (001 zone, index “$q$” for reflections) and new monoclinic modification [the reflections (index “$m$”) for the $\overline{1}07$ and 001 zones can be observed] related transitionally and crystallographically to one another.Reuse & Permissions

Figure 5

SEM photographs of samples quenched upon pressure-temperature treatment. Image (a) illustrates diffusive-grown $r\mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ grains (the sample treated at 2.6 GPa and 1170 K); (b) nucleated layers of the $r$ phase in the $\alpha \mathrm{\text{−}}{\mathrm{G}\mathrm{e}\mathrm{O}}_2$ surroundings (5.5 GPa and 570 K); (c) the final stage of growth for the layered $r$ phase (4.8 GPa and 800 K).Reuse & Permissions

Figure 6

The x-ray diffraction patterns obtained from ${\mathrm{G}\mathrm{e}\mathrm{O}}_2$ samples upon pressure-temperature treatment at 4.8 GPa and 580 K (curve 1), 2.6 GPa and 1050 K (curve 2), and 11.5 GPa and 290 K (curve 3). The patterns reveal the $\alpha$ phase (reflections of which are marked by asterisks), the $r$ phase (circles) with admixture of the $\alpha$ phase, and the $r^{\prime }$ phase (triangles) with admixture of the $r$ phase, respectively. The indexes correspond to the dominant phases.Reuse & Permissions