Micro-Raman and micro-infrared spectroscopic studies of Pb- and Au-irradiated $\mathrm{Zr}\mathrm{Si}{\mathrm{O}}_4$: Optical properties, structural damage, and amorphization

The optical properties of damaged periodic and aperiodic domains created by ${\mathrm{Pb}}^{+}$ $\left(280\mathrm{keV}\right)$ and ${\mathrm{Au}}^{4+}$ $\left(10\mathrm{MeV}\right)$ implantation of zircon were studied using micro-infrared (IR) and micro-Raman spectroscopy. The ${\mathrm{Pb}}^{+}$ and ${\mathrm{Au}}^{4+}$ irradiations caused a dramatic decrease in the IR reflectivity similar to that observed for metamict natural zircon. The irradiation with $10\mathrm{MeV}$ ${\mathrm{Au}}^{4+}$ ions (to fluences of $1\times {10}^{15}{\mathrm{Au}}^{4+}\mathrm{ions}∕{\mathrm{cm}}^2$) also results in the formation of an amorphized phase similar to that observed in metamict zircon. These results show that high-energy, heavy-ion irradiations provide a good simulation of the ballistic effects of the recoil nucleus of an alpha-decay event and, in both cases, the result is the creation of aperiodic domains. Additional IR and Raman features were recorded in samples irradiated with $280\mathrm{keV}$ ${\mathrm{Pb}}^{+}$ ions (to fluences of $1\times {10}^{14}$ and $1\times {10}^{15}{\mathrm{Pb}}^{+}\mathrm{ions}∕{\mathrm{cm}}^2$), indicating the formation of an irradiation-induced additional phase(s). The frequencies of the features are consistent with lead silicates, $\mathrm{Zr}{\mathrm{O}}_2$, and $\mathrm{Si}{\mathrm{O}}_2$. The results show that spectral features of the ${\mathrm{Au}}^{4+}$- and ${\mathrm{Pb}}^{+}$-irradiated zircon are different from those of quenched $\mathrm{Zr}\mathrm{Si}{\mathrm{O}}_4$ melts, and the finding further confirms that the amorphous state produced by high-energy ion irradiations is structurally different from the glassy state that results from quenching a high temperature melt. In contrast to significant changes in the frequency and width of the Raman ${\nu }_3$ band observed in metamict zircon, the ${\mathrm{Pb}}^{+}$ and ${\mathrm{Au}}^{4+}$ irradiations do not cause similar variations, indicating that the remaining zircon crystalline domains in irradiated samples have a crystalline structure with fewer defects than those of metamict zircon.

Figure 1

Micro-infrared reflectance spectra of ${\mathrm{Pb}}^{+}$-irradiated zircon with doses of $1\times {10}^{13}$, $1\times {10}^{14}$, and $1\times {10}^{15}{\mathrm{Pb}}^{+}\text{ions}∕{\mathrm{cm}}^2$. For the dose of $1\times {10}^{15}{\mathrm{Pb}}^{+}\text{ions}∕{\mathrm{cm}}^2$, two more spectra taken in different spots are also included to illustrate variations in the local structure.

Figure 2

Grazing angle IR reflectance spectrum (recorded with an 80° grazing angle objective) of ${\mathrm{Pb}}^{+}$-irradiated zircon irradiated at $1\times {10}^{15}{\mathrm{Pb}}^{+}\text{ions}∕{\mathrm{cm}}^2$.

Figure 3

Comparison of IR reflectance spectra of ${\mathrm{Pb}}^{+}$- and ${\mathrm{Au}}^{4+}$-irradiated zircon with that of natural metamict zircon (with $f$ of $\sim 1.0$), whose crystal structure is damaged and amorphized by natural alpha-decay event processes over geological time. The dashed line indicates the feature of the aperiodic phase or domains of metamict zircon, which could coexist in the ${\mathrm{Pb}}^{+}$-irradiated samples.

Figure 4

Effect of ${\mathrm{Pb}}^{+}$ irradiation on the Raman spectrum of zircon.

Figure 5

Lead silicates are found in ${\mathrm{Pb}}^{+}$-irradiated zircon (at $1\times {10}^{15}{\mathrm{Pb}}^{+}\text{ions}∕{\mathrm{cm}}^2$). (a) Raman spectrum of lead silicate (modified from Ref. 89). (b) Raman spectrum of ${\mathrm{Pb}}^{+}$-irradiated zircon (bands with given frequencies are all extra features). (c) Spectrum of tetragonal zirconium oxide $\left(t\text{−}\mathrm{Zr}{\mathrm{O}}_2\right)$. (d) Raman spectrum of reidite (high-pressure polymorph of zircon with a scheelite-type structure).

Figure 6

Comparison of Raman spectrum of Pb-irradiated zircon with those of PbO, $\mathrm{Pb}\mathrm{Zr}{\mathrm{O}}_3$, and ${\mathrm{Pb}}_3{\mathrm{O}}_4$.

Figure 7

Raman measurements with depth changes from the Pb-irradiated surface to the bulk of a zircon crystal: (a) data between 160 and $700{\mathrm{cm}}^{-1}$, (b) between 800 and $1150{\mathrm{cm}}^{-1}$, and (c) detailed spectra showing the change in the irradiation-induced additional bands. Note that for clarity, the top spectrum in (c) corresponds to that measured on the surface, whereas in (a) and (b), the spectra recorded on the surface are shown at the bottom. In (c), the band positions given above the spectrum recorded at the surface represent those of irradiation-induced additional features, and those appearing below those spectra measured in the bulk correspond to the band position of zircon.

Figure 8

Raman band profiles from the damaged surface to the bulk of irradiated zircon (at $1\times {10}^{15}{\mathrm{Pb}}^{+}\text{ions}∕{\mathrm{cm}}^2$): (a) depth dependence of reduced Raman intensities for the ${\nu }_3$ band of zircon and the irradiation-induced band near $841{\mathrm{cm}}^{-1}$; (b) depth dependence of the measured bandwidth (full width at half maximum) of the ${\nu }_3$ mode (near $1008{\mathrm{cm}}^{-1}$) of zircon. The dramatic changes near the surface are due to ${\mathrm{Pb}}^{+}$-irradiation damage.

Figure 9

The effect of different physical processes on the frequency and width of the ${\nu }_3$ Raman mode of zircon: (a) laser irradiation and melting [modified from Ref. 86; the spectra are recorded along a length of $12\mu \mathrm{m}$ crossing the boundary between the unmolten (with sharp features) and molten (with broad features) regions]; (b) $280\mathrm{keV}$ ${\mathrm{Pb}}^{+}$ and $10\mathrm{MeV}$ ${\mathrm{Au}}^{4+}$ irradiations; (c) natural metamictization ($f$ is the fraction of amorphous phase in natural zircon produced by metamictization); and (d) high-pressure shocking (with shocking pressures of 20 and $38\mathrm{GPa}$). The data show that different physical processes may result in different changes in the phonon modes of zircon. Quenched “glass” $\mathrm{Zr}\mathrm{Si}{\mathrm{O}}_4$ from high-temperature melting shows that the ${\nu }_3$ Raman mode shifts to a higher frequency and becomes broad significantly, while the ion irradiations lead to no or little change in frequency and bandwidth. In contrast to (a) and (b), metamictization and high-pressure shocking both cause a significant decrease in the band frequency and a band broadening.