The stabilization and immobilization of high‐level radioactive wastes in solid forms have become one of the most pressing industrial problems. Different crystalline mineral phases have been proposed as actinide‐bearing crystalline hosts for waste materials. Self‐radiation damage from alpha‐decay of the incorporated actinides (such as U, Th) and other rare earth elements can lead to metamict state (amorphous state) and can affect the durability and long‐term performance of these actinide‐bearing phases. To investigate the impact of radiation on the nuclear waste forms and to obtain a better comprehension of the damage process and amorphization mechanism are important. The issues of interactions between high‐energy particles and solids and radiation‐induced structural modifications in crystalline‐to‐amorphous state are, in fact, an important and active area of fundamental researches. To study metamict state and metamictization is also important for geochemistry as the U‐Pb isotope system is commonly used for age dating. Although radiation effect and naturally occurring radiation damage (the process is known as metamictization) have been the subject for many research investigations, there remain important and fundamental issues which need to be understood, for example, the structural changes at the atomic level caused by metamictization, the crystal structure of radiation‐induced amorphous phases, solubility and diffusion of radioactive elements in damaged host phases, the effect of pressure and temperature on metamictization process and interaction of water and fluids with nuclear waste forms. Raman spectroscopy is found to be a very powerful tool for study and analysis of the damage effect and metamictization. This chapter describes and reviews recent Raman applications in zircon and titanite, which are proposed for nuclear waste forms. These applications are focused on radiation effect and structural damage process caused by alpha‐decay process as well as recrystallization due to thermal annealing.
- Raman spectroscopy
- radiation damage
Today, there are over 430 commercial nuclear power reactors in 31 countries which provide over 10% of the world electricity . As a result, huge amounts of highly radioactive nuclear wastes are produced each year. One of the critical issues in nuclear energy industry is the safe disposal of nuclear wastes, especially high‐level nuclear wastes (HLNW). The below are some minerals and synthetic materials proposed or developed as actinide‐bearing crystalline hosts for waste materials [2–4]: zircon (ZrSiO4), titanite (CaTiSiO5), baddeleyite (Zr,Hf,…)O2, hafnon (HfSiO4), perovskite [(Ca,Gd,…)(Al,Fe,Ti,…)O3], zirconolite [CaZrTi2O7 and CaZrSi2O7], apatite [Ca10(PO4)6(OH,F,Cl,B)2], pyrochlore [CaZrGd2Ti2O7, Gd2Zr2O7, and La2Zr2O7‐Nd2Zr2O7], monazite [(La,Ce,…)PO4], and garnet [(Ca,Fe,Gd,…)3 (Al,Fe,Si,…)5O12]. Among them, zircon is one of the most studied and modeled minerals. Currently, pyrochlore has attracted attention because of its radiation damage resistance.
Self‐radiation from alpha‐decay of the incorporated actinides can lead to lattice damage resulting in structural change and transformation from crystalline state to an amorphous or aperiodic state, that is, metamict state (the process is known as metamictization) [5–7].
The effects of radiation damage on the structure of metamict minerals can be seen as systematic changes of its physical properties : an increase in cell parameters and broadening of X‐ray diffraction patterns [9–12]; a decrease in Raman and infrared intensities and dramatic band broadening [13, 14]; decreases in refractive index and birefringence [9, 15]; absorption of hydrous species [16–19]; an increase in fracture toughness ; a decrease in density [9, 10]; a variation of TEM diffraction patterns [10, 21, 22]; an increase of leach rate ; changes in bulk modulus and hardness ; a change of 29Si NMR features [11, 25]; changes of diffuse X‐ray scattering from single crystals ; occurrences of Huang type diffuse X‐ray diffraction ; a change in EXAFS ; a variation of Mössbauer spectra [11, 28]; and a variation of positron annihilation lifetime . Therefore, the durability and performance of these actinide‐bearing phases can be altered by self‐radiation damage from alpha‐decay of the incorporated actinides. To gain better comprehension of the effect of radiation on crystal structure at the atomic level, the related damage process and damage mechanism are issues of critical importance.
Vibrational spectroscopy (Raman and infrared (IR) spectroscopy) is a very powerful tool in the analysis and study of structural variations related to medium‐ and short‐range order . Early analysis of alpha‐decay damaged materials by Raman spectroscopy can be traced back to about two decades ago [31, 32]. The main advantages of vibrational spectroscopy (Raman and infrared spectroscopy)  are their fast response time (which can be in the range of ~10-12 s), short correlation length scale (which is in the order of a few unit cells), and good sensitivity to hydrous and hydroxyl species (e.g., H2O and OH). In contrast to diffraction methods which are generally sensitive to periodicity of lattices and the crystallinity of a specimen, vibrational spectroscopy is mainly associated with the strength and length of interatomic bonds, as well as the atomic masses of the sample. Therefore, vibrational spectroscopy can give valuable information on phonon energy, bulk structure, chemical composition, and surface for not only crystalline materials, but also disordered phases. The method has, in fact, been widely applied in studying disordered and amorphous materials such as glasses.
This chapter illustrates recent applications of Raman spectroscopy in the study of radiation‐damaged or metamict zircon and titanite. Being different from simple identification of damaged phases with Raman techniques, the investigations are focused on important issues such as: What happens at the atomic level during radiation damage and recrystallization? What are the possible structural modifications during metamictization? And whether decomposition into oxides is the final result of radiation damage. The experimental results provide a better understanding of the mechanism of radiation damage and the recrystallization processes.
2. Effects of alpha‐decay radiation on Raman spectra of zircon and titanite
Zircon (ZrSiO4) is a common accessory mineral in igneous rocks, in metamorphic rocks, and as detrital grains in sedimentary rocks. The work  has showed that zircon crystallizes to a tetragonal structure with space group or
The effect of alpha‐decay radiation damage on the structure of zircon is evidenced by a decrease in band frequencies, a line broadening of Raman modes and a decrease in Raman intensity (Figure 1). Well‐crystallized zircon samples have sharp and well‐resolved Raman modes. With increasing alpha‐decay radiation dose (the radiation dose of natural minerals is mainly related to sample's rock age and concentration of U and Th, and it can be calculated [9, 10]), the stretching modes of SiO4 tetrahedra near 975 and 1008 cm-1 become weaker and broader, while the lower frequency modes become gradually weaker and could hardly be analyzed for high‐dose cases. The broad Raman feature concurring near 950 cm-1 (the insert part in Figure 1) indicates the formation of amorphous phases in high‐dose samples. The behavior of the band also suggests that SiO4 tetrahedra remain in highly damaged zircon samples. As the 950 cm-1 feature is significantly away from the intense bands near 1008 cm–1 in terms of wavenumber, its appearance implies a new linkage of SiO4 tetrahedra.
The dose dependence of the frequency and full width at half maximum of the 1008 cm
Raman band widths [full width at half maximum (FWHM)] and frequencies (especially those of the ν3 band of SiO4 near 1008 cm-1) in zircon have been used for investigating the relationship between U‐Pb isotopic discordance and metamictization [35–37]. More work is desirable to gain a better understanding of the behavior of this band during radiation damage and recrystallization, and the potential influence of chemical impurities on the band.
Another good example of Raman study of alpha‐decay radiation damage is the application in metamict titanite. Titanite is a calcium titanium nesosilicate mineral, CaTiSiO5. Taylor and Brown  synthesized pure titanite, and their X‐ray data show that it is monoclinic (space group
Natural titanite occurs in igneous and metamorphic rock and incorporates a variety of impurity ions such as U, Th and other rare earth elements (REE). The structure of natural titanite is often metamict, as a result of self‐radiation damage associated with the alpha‐decay of the incorporated REEs. Raman spectra of crystalline or undamaged natural titanite (Figure 3) show spectral features similar to those of synthetic pure
3. Issue of possible decomposition in radiation‐damaged zircon
In the study of radiation effect and metamict state, what happens at the atomic level is an unclear and important question. Researchers have focused on issues such as possible changes in the coordination number of Zr , radiation‐induced disordering rather than amorphization , damage‐related distorted and disoriented isolated silica tetrahedra , the fraction of amorphized phase [14, 46], as well as whether metamictization leads to phase separation or damaged minerals decompose into their oxides, and what is the structural state of the decomposed phases [47, 48]. This issue of radiation‐induced decomposition was, in fact, debated over decades . Based on infrared data on metamict zircon, a two‐stage damage process was proposed . It was suggested that the first stage produces, throughout the lattice, highly stressed and expanded zircon with distorted SiO4 tetrahedra, while the second stage was suggested to result in the decomposition of ZrSiO4 to ZrO2 and SiO2, probably together with some aperiodic ZrSiO4 . However, the decomposition of damaged zircon was often reported in zircon samples only annealed experimentally at high temperatures, in which the decomposition commonly leads to different polymorphs of ZrO2 and glassy silica. Monoclinic ZrO2 was found in heavily damaged samples heated to 1373 K . In a high‐temperature study, Vance and Anderson  observed cubic and tetragonal ZrO2 at 1073 and 1373 K, respectively. It was reported that highly metamict zircon contained randomly orientated ZrO2 when annealed at 1173 K, and further annealing at 1523 K resulted in monoclinic ZrO2, as well as a silica glass phase . Ellsworth et al.  suggested that decomposition of metamict zircon into ZrO2 and glassy SiO2 could be one possible path for recrystallization. In contrast, a high‐temperature neutron work by  suggested that zircon decomposes into crystalline β‐cristobalite (rather than silica glass) and tetragonal ZrO2. An X‐ray powder diffraction study at high temperatures reported the appearance of pseudo‐cubic ZrO2 . Meldrum et al.  observed a decomposition of zircon into tetragonal ZrO2 when irradiating zircon with heavy ions at around 950 K. As discussed by different works [56, 57], some of these previous works based on X‐ray diffraction measurements might have experienced difficulties in the determination of cubic and tetragonal ZrO2.
Vibrational (Raman and infrared) spectroscopy is a good analytical tool for resolving these problems related to decomposition of ZrSiO4 into ZrO2 and SiO2, as pointed out by Ref. , because Raman and infrared spectra of zirconia (ZrO2) have been well studied previously and also because vibrational spectroscopy has short length scales. ZrO2 has three common polymorphs at different temperatures. The room‐temperature phase is monoclinic, while the tetragonal and cubic phases occur at high temperatures . The theoretical calculations [59, 60] have given the optical phonon modes (for zero wave vector) for each polymorph of zirconia ZrO2. The monoclinic ZrO2 has space group /
The data in Figure 1 indicate that it is apparent that there is a lack of signals of ZrO2 in highly damaged zircons. This shows that ZrO2 and SiO2 are not the final products of metamictization in zircon. Results from infrared spectroscopy of radiation‐damaged zircon [14, 47] also support this observation. As mentioned early, although decomposed zircons were commonly reported in lab‐treated samples, tetragonal ZrO2 was recorded in only one natural sample , with an unknown thermal history, among a large number of natural zircon samples with different degrees of damage analyzed in Refs. [14, 47, 48]. In order to explore and examine high‐temperature behavior of damaged zircon and the possible causes for decomposition in zircon, systematic works were carried out by different groups [47, 36, 61, 62]. These works show that thermal annealing of heavily damaged zircon at high‐temperature experiments may lead to the decomposition of metamict zircon into tetragonal ZrO2 and glassy SiO2 at 1200 K, and upon further heating tetragonal ZrO2 transforms to monoclinic ZrO2 near 1400 K. Undamaged and weakly damaged zircons are less likely to show decomposition during high‐temperature annealing. It was reported that the decomposition‐induced silica tended to evaporate on further heating [61, 63]. As naturally damaged samples might experience high‐temperature processes, some reported decomposed metamict zircon could be due to natural thermal annealing prior to experiments. In contrast to high‐temperature annealing, the presence of ZrO2 in some natural zircons might be due to the reaction of fluids with metamict zircons, because radiation damage may alternate the chemical stability of zircon. The observation of ZrO2 in dissolution experiments was reported in highly damaged zircon samples . Raman data of hydrothermally leached metamict zircon  showed the formation of monoclinic ZrO2. In general, the decomposition of zircon into ZrO2 and SiO2 is related to radiation‐damaged zircons. Their crystal lattice is heavily damaged and has more defects. As a result, the durability of these heavily damaged samples is expected to be affected, and they are vulnerable to the impact of external physical and chemical conditions (e.g., water, solutions, high temperature, and even high pressure).
Raman data of metamict titanite (Figure 3) also show there is a lack of formation of oxides in highly damaged samples . The findings are supported by X‐ray measurements and TEM  and infrared data . The observation further shows that phase decomposition into oxides is not the final state of alpha‐decay damage in these materials, and the materials are safe to be used as nuclear waste forms. It has been found that decomposition of radiation‐damaged zircon is commonly related to high‐temperature heating (see a below section).
4. Metamict state
versus glass state of CaTiSiO5 and ZrSiO4
One of the interesting issues related to radiation damage and metamictization is the similarities or differences between the amorphous phases produced by alpha‐decay radiation damage and those produced through thermally quenched glass melts. Previous studies  have led to unanswered questions and concerns: for example, What is the relationship between the structural characteristics of disordered materials and the type of irradiation or physical process used to produce them?
As a type of disordered or amorphous materials, metamict minerals were commonly considered as “glass‐like” materials in early studies on naturally occurring radiation effect and metamictization, and the metamict state was referred as a glassy state, which is somehow similar to that obtained by rapid quenching of high‐temperature melts . With experimental data gathering, evidences have emerged that indicate their important differences between the aperiodic states. The dispersion depends on the physical processes, which produce the amorphous states. Vibrational spectroscopy (Raman and infrared) is very useful to study this issue because of its sensitivity to local structures.
Raman data of metamict titanite (CaTiSiO5) and its glass analogue (with the same chemical compositions) produced by quenching melts show that the two types of materials have different vibrational features (especially in the Ti
Zircon has a high melting temperature (above 2700–2800 K), and crystalline ZrO2 and a liquid of SiO2 may coexist for a composition of ZrSiO4 at a temperature region of ~1960 K and ~2670 K . It is difficult to quench zircon melts without decomposition into ZrO2 and SiO2; however, glass‐like zircon (ZrSiO4) has been recorded with Raman spectroscopy in laser‐treated zircon in a study with laser melting, near the boundaries between the unmolten and molten regions (where a relatively large temperature gradient could exist) . The large temperature gradient is expected to increase the quench rate and facilitate a “freeze” of the local configuration of the ZrSiO4 melts before the decomposition takes place. These experimental results indicate that the metamict state is different from the glassy state obtained by quenching melts. Apparently, the processes and structural states associated with metamictization and irradiation amorphization are more complex than those in common thermal glasses. The formation of the metamict state involves not only amorphization, but also defect accumulation caused by alpha‐particle damage and further radiation or irradiation may lead to damage as well as recrystallization.
So far, although the issue remains under debates, there have been substantial evidences indicating characteristic discrepancies between two types of amorphous states (metamict and glass states) , for example: (i) the two types of materials commonly have spectral and structural discrepancies [40, 43, 69, 70]; (ii) high‐energy heavy ion irradiation may lead to significant modifications in local structures of glasses [71–74]; (iii) upon heating, radiation‐damaged minerals tend to recrystallize epitaxially and recover to their original cryptographic orientations [48, 51, 75], while during high‐temperature treatments, glasses commonly undergo a glass transition; and (iv) for common glasses, their glass transition temperatures are roughly defined, while various responses at different temperatures are seen in metamict minerals. For example, radiation‐induced defects in metamict zircon may be annealed or healed at temperatures as low as 600 K accompanied by changes in the oxidation state of U ions; partial decomposition of ZrSiO4 into SiO2 and SiO2 in heavily damaged zircon may take place at 1050 K; diffusion and conversions of hydrogen‐related species together with dehydroxylation may occur between 1200 and 1600 K (e.g., [16, 18, 52, 76, 77], see for the transition point ).
5. Effect of thermal annealing on metamict zircon and titanite
Thermal annealing of metamict minerals at high temperatures is commonly used in studies of radiation‐damaged minerals . Its aims are to restore the original crystal structure for the purpose of studying recrystallization temperature, activation energy, types of radiation‐induced local defects and phase identification, and to obtain a good comprehension of the recrystallization process and mechanism. Extensive studies were carried out to investigate changes at the atomic level in metamict materials during high‐temperature annealing [10, 15, 36, 40, 44, 47, 48, 50, 52, 54, 55, 65, 75, 78–81]. However, controversies remain regarding the recrystallization path and activation energy.
Thermal annealing results in recrystallization of metamict zircon (Figure 5). The effect of annealing temperature on the structural recovery of damaged zircon can be clearly seen in the frequency and FWHM of the ν3 Si
Spectroscopic data [47, 48, 61] revealed different recrystallization processes between partially and heavily damaged zircons, that is, the recrystallization process depends on the cumulative radiation dose (Figure 5, 6a
and 6b). Being similar to metamict zircon , the thermal response of the damaged titanite (CaTiSiO5) is affected by their initial degrees of damage, that is, at the same treatment conditions, weakly or partially damaged samples are more likely to recover to crystalline titanite as compared with highly metamict samples . Intermediately and heavily damaged titanite samples show a recovery of Ti-O and Si-O bands after annealing at 1300–1400 K, and these recovered crystals are consistent with the
The thermally induced structural recovery and recrystallization of metamict zircon and titanite is also characterized a recovery of the anisotropy of the sample, which is restored during annealing, as evidenced by the recovery of orientational dependence of IR (as well as Raman) spectra along with the original crystallographic orientations as shown in Figure 7, which indicates an epitaxial recrystallization. This behavior indicates that in highly metamict zircon and titanite, crystalline nanodomains with original crystallographic orientations might still exist.
In conclusion, Raman spectroscopy, as shown above, is a very powerful tool for study of radiation damage in actinide‐bearing phases and for estimation of their long‐term durability of their physical properties and chemical stability. This type of Raman applications can provide a better understanding of the mechanism of radiation damage and thermal recrystallization processes. It has a wide usage in condensed mater physics, material science, nuclear material sciences, mineralogist, and geochemistry. It has also been used analysis other radiation‐damaged minerals, such as fergusonite [82, 83], actinide‐bearing monazite , titanoaeschynite (Nd) and nioboaeschynite (Ce) , uranyl titanate mineral davidite‐(La) , aeschynite‐(Y) and polycrase‐(Y) , and steenstrupine , and pyrochlore (Zietlow et al., personal communication).
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