High-pressure Raman study of the ferroelastic rutile-to-$\mathrm{Ca}{\mathrm{Cl}}_2$ phase transition in $\mathrm{Zn}{\mathrm{F}}_2$

The high-pressure Raman spectra of $\mathrm{Zn}{\mathrm{F}}_2$ have been measured up to $6.8\mathrm{GPa}$ and provided unambiguous evidence of a phase transition from its ambient-pressure rutile $\left(D_{4h}^{14}\right)$ to the $\mathrm{Ca}{\mathrm{Cl}}_2$-type orthorhombic $\left(D_{2h}^{12}\right)$ structure at a pressure $P_c\approx 4.5\mathrm{GPa}$. Both macro- and micro-Raman spectra have been independently obtained at high pressures leading to the same conclusion about the high-pressure phase and the $P_c$. The transition appears to be closely associated with the $B_{1g}$ $\left(A_g\right)$ Raman-active soft mode of the rutile $\left(\mathrm{Ca}{\mathrm{Cl}}_2\right)$ structure, implying that $\mathrm{Zn}{\mathrm{F}}_2$ undergoes a second-order ferroelastic phase transition.

Figure 1

Unpolarized macro-Raman spectra of $\mathrm{Zn}{\mathrm{F}}_2$ at various pressures. Two Raman peaks are observed in the low-pressure (rutile-type) phase and five in the high-pressure ($\mathrm{Ca}{\mathrm{Cl}}_2$-type) one. ${}^{*}$, Argon lamp calibration lines; +, Raman lines of the sapphire window of the high-pressure cell; ×, pressure monitoring Raman line of a Si chip (placed inside the high-pressure cell).

Figure 2

Micro-Raman spectra of $\mathrm{Zn}{\mathrm{F}}_2$ at various pressures. The spectra at ambient pressure and at pressures 0.9, 5.7, and $6.8\mathrm{GPa}$ are unpolarized (without an analyzer in the path of the scattered beam); the spectrum at $2.5\mathrm{GPa}$ is polarized with the scattering configuration $z\left(xx\right)\overline{z}$, giving a relatively enhanced $A_{1g}$ phonon, which corresponds to diagonal elements of the Raman tensor; the spectra at 3.8 and $4.5\mathrm{GPa}$ are cross-polarized corresponding to the scattering configuration $z\left(yx\right)\overline{z}$, which relatively elevates the nondiagonal $E_g$ phonon (see text for details). In the ambient-pressure spectrum, the sample was outside the high-pressure cell; the weak $B_{1g}$ and $B_{2g}$ phonon lines of this spectrum have been vertically enlarged and shown as insets. Two Raman peaks are observed in the low-pressure (rutile) phase and four in the high-pressure ($\mathrm{Ca}{\mathrm{Cl}}_2$-type) structure, with one of the latter peaks being double corresponding to the unresolved $B_{2g}$ and $B_{3g}$ modes.

Figure 3

Raman mode frequencies of $\mathrm{Zn}{\mathrm{F}}_2$ plotted against pressure. Solid circles correspond to macro-Raman measurements and open circles to micro-Raman ones. All modes display linear dependence, except the $A_g$ symmetry one of the high-pressure phase which shows nonlinear, soft mode behavior. Note the change of slope of the $A_{1g}(\to A_g)$ and the splitting of the $E_g(\to B_{2g}+B_{3g})$ modes of the low-frequency phase at $\sim 4.5\mathrm{GPa}$.

Figure 4

Cross-polarized $z\left(xx\right)\overline{z}$ micro-Raman spectra of $\mathrm{Zn}{\mathrm{F}}_2$ showing that the doubly degenerate $E_g$ mode of the low-pressure phase evolves into a double peak at high pressures. The dual spectra at each of the pressures 5.7 and $6.2\mathrm{GPa}$ correspond to two orientations of the $\mathrm{Zn}{\mathrm{F}}_2$ crystal sample differing by 90° with respect to the direction of the incident laser beam (see text for details), confirming that this line is a double peak spectral feature; the single spectra at 4.5 and $6.8\mathrm{GPa}$ correspond to the mid-position orientation of the sample, i.e. rotation of the DAC and sample at 45° relatively to either of the two extreme orientations. The inset shows plots for the full width at half-maximum (FWHM) of the $A_{1g}\left(A_g\right)$ and $E_g(B_{2g}+B_{3g})$ modes of the low- (high-) pressure phase; a sharp increase of the $E_g$ mode width is observed at $\sim 5.0\mathrm{GPa}$, indicating the onset of splitting of this mode.