Ge coordination in $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ glass upon compression to 131 GPa

Structural transformations at high pressure in $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ glass were investigated by means of x-ray absorption spectroscopy at the Ge $K$ edge in combination with a diamond anvil cell. The obtained results provide a detailed picture of the local structural behavior of Ge in a chemically complex glass under compression. First and second shell bond distances ($R_{\text{Ge-O}}$ and $R_{\mathrm{Ge}...\mathrm{Ge}}$) were extracted assuming contributions of two scattering paths (Ge-O and Ge…Ge). We observed a significant extension of the Ge-O distance from 1.73 to 1.82 Å between 3 and $\sim 26\mathrm{GPa}$, accompanied by an increase of the fitted number of nearest neighbors from $\sim 4$ to $\sim 6$. These observations can be attributed to the change from tetrahedral to octahedral Ge coordination. Second shell bond distances Ge…Ge are also consistent with this structural transformation. Between 34 and 131 GPa, the evolution of the fitted Ge-O distance implies a gradual volume reduction of the Ge octahedra. At the highest probed pressure of 131 GPa a Ge-O distance of 1.73 Å was found, which is similar to the one obtained at ambient conditions for Ge in fourfold coordination. The compressibility of the Ge-O octahedron in $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ beyond 34 GPa is considerably higher than the one reported for amorphous $\mathrm{Ge}{\mathrm{O}}_2$ from x-ray diffraction analysis but it is similar to the one reported for the Ge octahedron in crystalline rutile-type $\mathrm{Ge}{\mathrm{O}}_2$. We attribute the high compressibility of the Ge-O bond in $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ glass to the presence of Al and Na that increase the system's complexity and therefore its degrees of freedom. Beyond 110 GPa the data on $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ glass indicate the onset of polyhedral distortion. The performed study provides insights into the structural changes of complex and polymerized germanate glasses or melts at extreme pressure conditions.

Figure 1

Normalized Ge $K$ edge XANES spectra as a function of pressure (1–121 GPa). Left side (a) covers the energy range around the first XANES peak maximum (FXPM). The two vertical lines indicate the position of the maximum of the first XANES peak at 0 and 121 GPa, respectively, which exhibits an energy shift of $\sim 3\mathrm{eV}$ over this pressure range. Right side (b) covers the extended energy range between first XANES peak maximum up to 11 240 eV, in which changes in the beginning of the EXAFS region are apparent upon compression. All spectra are shifted along the $y$ axis for clarity. Spectra in red color indicate pressure range with the strongest energy shift (see also Fig. 2).

Figure 2

Energy shift of the fitted first XANES peak maximum (FXPM in Fig. 1) as a function of pressure from 0 to 121 GPa. The point at 131 GPa was not included in this plot due to uncertainties in the energy calibration of the monochromator.

Figure 3

(a) Evolution of $k^3$ weighted EXAFS spectra at the Ge $K$ edge of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ upon compression to 131 GPa. (b) Magnitude of the Fourier transform ($R$) of the EXAFS function shown in (a) of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ to 131 GPa. Plotted data are not corrected for the phase-shift.

Figure 4

Selected $k^3$ weighted EXAFS spectra of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ at different pressures and corresponding fits.

Figure 5

Evolution of the average Ge-O distance of the first coordination shell ($R_{\text{Ge-O}}$) of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ with pressure, obtained from fitting the experimental spectra acquired in run 1 (circles) and run 2 (squares). The grey shaded area presents the pressure interval in which an asymmetric pair distribution function (PDF) was included in the EXAFS analysis.

Figure 6

Evolution of the fitted number of neighbors of the first coordination shell ($N_{\text{Ge-O}}$) of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ with pressure. Symbols as in Fig. 5.

Figure 7

Evolution of the second coordination shell distance ($R_{\mathrm{Ge}...\mathrm{Ge}}$) of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ with pressure. Symbols as in Fig. 5. The grey shaded area (between 5.3 and 24 GPa) corresponds to the pressure range in which very high structural disorder in the second coordination shell inhibited the EXAFS analysis.

Figure 8

Evolution of mean Ge-O distances as a function of pressure for amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ (red symbols) compared to those reported in literature on amorphous $\mathrm{Ge}{\mathrm{O}}_2$ (blue symbols) using different probe techniques as indicated (squares for XRD, circles for EXAFS, full line for VtC XES). Data of the mean Ge-O bond length in rutile-type crystalline $\mathrm{Ge}{\mathrm{O}}_2$ (sixfold coordinated Ge) are plotted for comparison, including those reported by Haines et al. [39] (brown crosses) and those obtained from extrapolation of the compression data reported in Hazen and Finger [25] using a third order Birch-Murnaghan equation of state with $K_{\mathrm{T}0}=270\mathrm{GPa}$ and $K_0^{\prime }=7$ [24, 25] (grey dashed line).

Figure 9

Evolution of the normalized Ge-O distance expressed by the ratio $R_{\mathrm{HP}}/R_{\mathrm{P}0}$ of amorphous $\mathrm{NaAlG}{\mathrm{e}}_3{\mathrm{O}}_8$ with pressure from this study (symbols as in Fig. 8) compared to literature data on the relative changes of the Si-O distance (green symbols) and Ge-O (blue symbols) distances obtained from XRD (squares), EXAFS (circles), and VtC-XES (full line).