Pyrochlore, as a fast-ion conductor for electrolytes in solid oxide fuel cells,^[1] frustrated magnetism,^[2] ferroelectricity,^[3] photocatalys,^[4] photoluminescence,^[5] has aroused the great interest of all. Besides, A₂B₂O₇ pyrochlore has great attraction in present nuclear technology as a material for disposal of nuclear wastes, due to its ability to accommodate minor actinides and an excellent resistance to radiation in the nuclear stopping regime. Pyrochlore has a superstructure of the fluorite structure-type with the A-site and B-site cations ordered and one-eighth of the anions missing.^[6,7] Two different oxygen sites occur in pyrochlore: the 48f sites (0.75-x, 1/8, 1/8) that are occupied by six oxygen atoms (O), and the 8a site (1/8, 1/8, 1/8) where the seventh oxygen atom (O′) is located.^[6–8] Fantastic properties of pyrochlore originate particularly from the presence of one vacant site per eight oxygen sites that could provide fast channels for oxygen diffusion.^[8]

The response of pyrochlore to the extreme conditions has received a great deal of attention. Heavy ion irradiation is usually used to simulate harsh irradiation environment of materials.^[9–11] Heavy ion irradiation has been applied to pyrochlore for precision micromachining, forming nanostructures, processing thin films, and inducing phase transitions. Interestingly, it is found that the phase transition and the microstructure during heavy ion irradiation are varied with the chemical composition of pyrochlore.^[12–16] Quite a lot of researches have been conducted on the binary Gd₂Zr_2–xTi_xO₇ pyrochlore.^[17–20] It is indicated that the resistance to irradiation-induced amorphization significantly increases with the Zr-content. Rare earth titanate Gd₂Ti₂O₇ undergoes a pyrochlore-to-amorphous transition. Whereas Gd₂Zr₂O₇ is transformed into an anion-defect fluorite structure. Actually, the cationic radius ratio r_A/r_B of pyrochlore governs the stability of the pyrochlore structure.^[16,21–23] It is reported that in the ambient conditions, the pyrochlore is stable as its r_A/r_B ranges from 1.46 to 1.78. A disordered fluorite structure can be formed as the r_A/r_B drops below 1.46. While the structure will exhibit a monoclinic, layered-perovskite structure when its r_A/r_B is above 1.78.^[21] The cation ionic ratio r_A/r_B of La₂Zr₂O₇ pyrochlore is about 1.61. Chartier et al. have shown that La₂Zr₂O₇ has a greater tendency toward cation disorder.^[24] It is thought that La₂Zr₂O₇ should be “resistant” to radiation damage by simply disordering to the defect fluorite structure rather than becoming amorphous structure.^[14,24] Moreover, physical properties of the materials are strongly dependent on the structural stability. Sattonnay et al. found that the evolution of the mechanical properties of swift heavy ion irradiated pyrochlore depends on the residual stress introduced into the sample surface layer.^[25,26] Sellami et al. observed that the thermal conductivity decreases in irradiated Gd₂Ti₂O₇ and Y₂Ti₂O₇.^[27] In this work, La₂Zr₂O₇ pyrochlore is exposed to energetic heavy ion irradiation with different ion fluences and (dE/dx)_e varying from 5.2 keV/nm to 39.6 keV/nm. The vibrational modes of irradiated La₂Zr₂O₇ pyrochlore are investigated to find out the dependence of structural transition on irradiation parameters.

Raman spectroscopy is highly sensitive to metal-oxygen vibrational modes. In Raman spectrum, La and Zr atoms in La₂Zr₂O₇ pyrochlore do not contribute to the Raman active vibrations because they are located at the inversion center.^[6,29] Raman activity of pyrochlore results from oxygen vibrations. The Raman spectrum of La₂Zr₂O₇ pyrochlore is similar to that of other A₂B₂O₇ pyrochlore with six Raman active vibrational modes. They are A_1g + E_g+4F_2g. Five obvious bands are observed in the Raman spectra of pristine La₂Zr₂O₇ as shown in Fig. 1. They are the A_1g mode at ∼ 493 cm⁻¹, the E_g mode at ∼ 300 cm⁻¹, and three F_2g bands at ∼ 394 cm⁻¹, 515 cm⁻¹, and a weak peak at about 596 cm⁻¹. The assignment of different modes was reported elsewhere.^[30–33] The most prominent E_g mode is contributed by O–Zr–O bending vibration with ∼ 34% of the potential energy distribution (PED), while ∼ 24% of PED is due to La–O stretching vibration.^[6,29,33,34] This band is complex containing a strong E_g mode and a weak F_2g mode, which are mainly due to the vibration of oxygen in ZrO₆ octahedron. The A_1g mode is associated with the O–Zr–O bending vibrations of ZrO₆ octahedron.^[6,29,33] Three F_2g modes at ∼ 394, 515 and 596 cm⁻¹ are mostly contributed by the distortions of ZrO₆ octahedron.^[6,29,33]

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. Raman spectra of La2Zr2O7 pyrochlore samples irradiated by 260-MeV Ar and 1980-MeV Kr ions.

In this work, heavy ions with different energy are used to irradiate La₂Zr₂O₇ pyrochlore to investigate the influences of irradiation parameters on vibrational modes’ variations of La₂Zr₂O₇. It is observed that no substantial changes appear on the main vibrational modes when La₂Zr₂O₇ is irradiated by 260 MeV Ar and 1980-MeV Kr ions with (dE/dx)_e of 5.2 keV/nm and 9.1 keV/nm, respectively, and the results are shown in Fig. 1.

Figure 2 exhibits the normalized Raman spectra of La₂Zr₂O₇ pyrochlore irradiated by 200-MeV Kr ions with (dE/dx)_e of 16.0 keV/nm at fluences ranging from 6 × 10¹⁴ ions/cm² to 6 × 10¹⁵ ions/cm². Lorenz-shaped peaks are fitted to the Raman spectrum. Full width at half maximum (FWHM) and normalized intensity of the fitted bands are depicted in Figs. 2(b) and 2(c). It is interesting that new peaks are generated in the Raman spectrum of La₂Zr₂O₇ irradiated with Kr ions. Modes at wavenumbers 192 cm⁻¹ and 265 cm⁻¹ rise up in the Raman spectra of La₂Zr₂O₇ pyrochlore irradiated by Kr ions. Besides, it is found that the mode at about 308 cm⁻¹ rises up in the fitted spectrum of irradiated pyrochlore. The band at wavenumber 265 cm⁻¹ is assigned to the remaining F_2g mode due to O–Zr–O vibration mixed with La–O′ bending vibration. It is not possible to unambiguously assign the low wavenumber Raman mode at 192 cm⁻¹ in Kr ion irradiated pyrochlore. Possibilities may include one of the defect-activated Raman forbidden F_1u modes. Infrared mode F_1u at 192 cm⁻¹ is caused by the vibrations of Zr–LaO₆ due to Zr–O and La–O stretching vibration mixed with O–La–O bending vibration.^[34] The mode at 308 cm⁻¹ could be assigned to the infrared mode F_1u, which is caused by La–O′ and O-O′ vibration.^[34] The selection rules result in some vibrational modes being optically inactive based on symmetry.^[35] Random displacement disorder can result in the relaxation of the selection rules, and previously inactive Raman modes are expected to appear in Raman spectra.^[35,36] Hence, it is assumed that 200-MeV Kr ion irradiation induces the La and O′ sites to be displaced in pyrochlore.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (a) Plots of intensity versus Raman shift of La2Zr2O7 pyrochlore samples irradiated by 200-MeV Kr ions with (dE/dx)e of 16.0 keV/nm at various fluences. Dependence of (b) FWHM and (c) relative intensity of band on Raman shift of La2Zr2O7 pyrochlore samples irradiated by different ion fluences.

According to Fig. 2, the intensity of the band at 651 cm⁻¹ and 740 cm⁻¹ increase with ion fluence increasing. The band at wavenumber 651 cm⁻¹ is assigned to be infrared active F_1u mode related to a mixture of La–O and Zr–O bond stretching with bending vibrations.^[6,29–34] It is noted that wavenumber at 740 cm⁻¹ is assigned to be of amorphous mode. It is suggested that disordered structure occurs in the pyrochlore irradiated with higher ion fluence.^[35] In addition, the frequency of modes at 651 cm⁻¹ and 740 cm⁻¹ shift towards lower wavenumbers with ion fluence increasing, according to Fig. 2. Raman shift is related to the structure of the target material. It is thought that the octahedron deformation occurs in the pyrochlore due to Kr ion irradiation. Infrared active mode F_1u shifting towards lower frequency indicates that the energy needed for photon to vibrate is lower, suggesting that the observed structure becomes unstable due to the irradiation of heavy ions.

The influences of irradiation parameters on the vibrational modes of La₂Zr₂O₇ pyrochlore are further verified. The La₂Zr₂O₇ pyrochlore sample is subjected to the irradiation of 1750-MeV Bi ions ((dE/dx)_e = 39.6 keV/nm). Figure 3 shows the normalized Raman spectrum of La₂Zr₂O₇ pyrochlore irradiated by Bi ions with increasing fluences. After being irradiated by Bi ions with increasing fluences, all crystalline vibrational modes broaden in width and decrease in intensity. It is indicated that swift heavy ion irradiation introduces an amorphous structure into La₂Zr₂O₇ pyrochlore. This behavior has also been observed in other irradiated pyrochlores. No shift occurs towards the strongest peak at 300 cm⁻¹ as marked by the dash line in Fig. 3. The F_1u mode at 192 cm⁻¹ is not present in Bi ion irradiated pyrochlore. A broad peak-like structure with a maximum at ∼ 740 cm⁻¹ is still existent, which has been observed in 200-MeV Kr ion experiments and is related to the formation of amorphous phase. Another F_1u mode at ∼ 651 cm⁻¹ exists over the entire fluence range, and the peak intenstity increases and the peak width broadens with fluence increasing. At higher ion fluence 3 × 10¹² ions/cm², the peaks at 651 cm⁻¹ and 740 cm⁻¹ are combined into a broad peak. It is suggested that amorphous fraction in the irradiated pyrochlore increases with ion fluence increasing. Quantitative analysis of the peaks is not made because the background noise grows up with fluence increasing. Nevertheless, the evolution of the intensity profiles comfirme that the intensities of the crystalline vibrational modes decrease systematically with fluence increasing.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. Plots of intensity versus Raman shift of La2Zr2O7 pyrochlore recorded before and after the irradiation of Bi ions with 39.6 keV/nm at various fluences.

To observe the detailed microstructure of the irradiated pyrochlore, TEM is used to explore the pyrochlore irradiated by Bi ion with (dE/dx)_e of 39.6 keV/nm. As expected, continuous tracks with amorphous structure in the core are found as shown in Fig. 4. Figure 4(b) shows the magnified image of the track at about 5 μm far from the irradiated surface. The structure in the track core is amorphous and surrounded by defect fluorite shell. The TEM morphology of the amorphous tracks also verifies the results shown in Fig. 3. The core–shell morphology shows the general structure of the latent tracks caused by heavy ion irradiation in pyrochlore. Previous literature has reported the latent tracks consisted with fluorite and amorphous structures.^[19,37,38] An amorphous core surrounded by an anion-deficient fluorite shell has a well-known core–shell structure of the latent tracks in heavy ion irradiated pyrochlore. The core–shell morphology of the latent tracks is associated with the localized “melting” and rapid recrystallization process following heavy ion penetration.^[38] According to the thermal spike model, the rapid quenching of a “molten” zone give rise to the amorphous core, while the epitaxial recrystallization at the molten/solid interface produces a surrounding shell. Multiple factors determine the final morphology of the latent tracks in irradiated pyrochlore. On the one hand, the energy density and the cooling rate strongly influence the track morphology. Rapid cooling leads to an amorphous track core, while the slower cooling will cause the disordered structure to form. High energy density induced by Bi ions with higher (dE/dx)_e leads to a rapid cooling rate along the ion path. Thus, amorphous track core is observed in the Bi ion irradiated La₂Zr₂O₇ pyrochlore. On the other hand, the final structure of ion tracks depends on the competition between melting process and recrystallization process, which is dependent on the chemical composition of pyrochlores, in particular, the cation ionic radius r_A/r_B.^[39] The amorphous core becomes larger, and the defect shell becomes less pronounced with r_A/r_B increasing.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. (a) Cross-section morphology of La2Zr2O7 pyrochlore irradiated by Bi ions with (dE/dx)e of 39.6 keV/nm at fluence of 1 × 1012 ions/cm2. (b) Morphology of tracks at about 5 μm far from the irradiated surface in high resolution.

In this wrok, La₂Zr₂O₇ bulk samples are irradiated by various kinds of heavy ions (e.g., Ar, Kr, Bi) in a large energy regime in HIRFL to systematically study the vibrations and phase transitions in heavy ion irradiated pyrochlore. For La₂Zr₂O₇ pyrochlore irradiated with (dE/dx)_e of 16.0 keV/nm, infrared modes F_1u at wavenumbers 192, 308, and 651 cm⁻¹ appear in Raman spectra due to the irradiation induced atom displacement and octahedron deformation. The F_2g band at 265 cm⁻¹ rises up due to oxygen diffusion or adsorption caused by ion irradiation. For La₂Zr₂O₇ pyrochlore irradiated with higher (dE/dx)_e, e.g., 39.6 keV/nm, pyrohlore–amorphous transition occurs according to Raman study. The TEM measurements of Bi ion irradiated pyrochlore verify the generation of latent track with amorphous structure in the core.