**1. Introduction**

Today, there are over 430 commercial nuclear power reactors in 31 countries which provide over 10% of the world electricity [1]. 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 (CaTiSiO<sup>5</sup> ), baddeleyite (Zr,Hf,…)O2 , hafnon (HfSiO4 ), perovskite [(Ca,Gd,…)(Al,Fe,Ti,…)O3 ], zirconolite [CaZrTi2 O7 and CaZrSi2 O7 ], apa‐ tite [Ca10(PO4 )6 (OH,F,Cl,B)2 ], pyrochlore [CaZrGd2 Ti2 O7 , Gd2 Zr2 O7 , and La2 Zr2 O7 ‐Nd2 Zr2 O7 ], monazite [(La,Ce,…)PO4 ], and garnet [(Ca,Fe,Gd,…)3 (Al,Fe,Si,…)<sup>5</sup> O12]. Among them, zircon is one of the most studied and modeled minerals. Currently, pyrochlore has attracted atten‐ tion 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 system‐ atic changes of its physical properties [8]: 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 [20]; a decrease in density [9, 10]; a variation of TEM diffraction patterns [10, 21, 22]; an increase of leach rate [23]; changes in bulk modulus and hardness [24]; a change of 29Si NMR features [11, 25]; changes of diffuse X‐ray scattering from single crystals [6]; occurrences of Huang type diffuse X‐ray diffraction [26]; a change in EXAFS [27]; a variation of Mössbauer spectra [11, 28]; and a variation of positron annihilation lifetime [29]. 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 [30]. 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) [30] 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., H2 O and OH). In contrast to diffraction methods which are generally sensitive to periodicity of lattices and the crystallinity of a specimen, vibrational spec‐ troscopy 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 dam‐ aged 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 decomposi‐ tion 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.

**1. Introduction**

104 Raman Spectroscopy and Applications

(HfSiO4

tite [Ca10(PO4

for waste materials [2–4]: zircon (ZrSiO4

hydrous and hydroxyl species (e.g., H2

disordered and amorphous materials such as glasses.

(OH,F,Cl,B)2

tion because of its radiation damage resistance.

)6

monazite [(La,Ce,…)PO4

), perovskite [(Ca,Gd,…)(Al,Fe,Ti,…)O3

Today, there are over 430 commercial nuclear power reactors in 31 countries which provide over 10% of the world electricity [1]. 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

), titanite (CaTiSiO<sup>5</sup>

is one of the most studied and modeled minerals. Currently, pyrochlore has attracted atten‐

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 system‐ atic changes of its physical properties [8]: 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 [20]; a decrease in density [9, 10]; a variation of TEM diffraction patterns [10, 21, 22]; an increase of leach rate [23]; changes in bulk modulus and hardness [24]; a change of 29Si NMR features [11, 25]; changes of diffuse X‐ray scattering from single crystals [6]; occurrences of Huang type diffuse X‐ray diffraction [26]; a change in EXAFS [27]; a variation of Mössbauer spectra [11, 28]; and a variation of positron annihilation lifetime [29]. 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 [30]. 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) [30] 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

generally sensitive to periodicity of lattices and the crystallinity of a specimen, vibrational spec‐ troscopy 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

Ti2 O7 , Gd2 Zr2 O7

], pyrochlore [CaZrGd2

], and garnet [(Ca,Fe,Gd,…)3

], zirconolite [CaZrTi2

(Al,Fe,Si,…)<sup>5</sup>

O and OH). In contrast to diffraction methods which are

), baddeleyite (Zr,Hf,…)O2

, and La2

and CaZrSi2

Zr2 O7 ‐Nd2 Zr2 O7 ],

O12]. Among them, zircon

O7

, hafnon

O7 ], apa‐

## **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 [33] has showed that zircon crys‐ tallizes to a tetragonal structure with space group <sup>19</sup> *D*4h or *I*41 /*amd* (with *Z* = 4), contain‐ ing a chain of alternating, edge‐sharing SiO4 tetrahedra and ZrO8 triangular dodecahedra extending parallel to the *c*‐axis. Actinides such as U, Th and Pu can substitute Zr and locate in the Zr site. Because of their uranium and thorium content, some zircons undergo metamictization. Group theory predicts twelve Raman‐active normal modes in zircon at *k* = 0: 2A1g + 4B1g + B2g + 5E<sup>g</sup> [34]. These Raman modes can be simply classified as inter‐ nal modes and external modes. There are five (2B1g + 3E<sup>g</sup> ) external modes and seven (2A1g + 2B1g + B2g + 2E<sup>g</sup> ) internal modes. For Raman measurements of natural samples which are damaged by radiation of incorporated actinides, it is a good practice to use laser excita‐ tion with different wavelengths to ensure that spectral features recorded are due to phonon modes rather than features due to luminescence and impurities‐related color centers (e.g., the work [8] used 514, 488, 457 and 632 nm lasers in its Raman measurement). Nine of the twelve predicted Raman modes can be seen in the most crystalline natural zircon (**Figure 1**). They are internal modes: 1008 cm-<sup>1</sup> (B1g, ν3 stretching of SiO4 ), 975 cm-<sup>1</sup> (A1g, ν1 stretching), 439 cm-<sup>1</sup> (A1g, ν2 bending), and 269 cm-<sup>1</sup> (B2g, ν2 bending) and external modes: 393, 355, 225, 214 and 202 cm–1. The other predicted Raman bands appear too weak to be observed in a common experimental arrangement.

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-<sup>1</sup> 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-<sup>1</sup> (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-<sup>1</sup> 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.

**Figure 1.** Micro‐Raman spectra (with 488 nm excitation) of metamict zircon between 50 and 1100 cm-<sup>1</sup> (Ref. [8], modified). The dosage is in the units of 1018 alpha‐events g-<sup>1</sup> .

The dose dependence of the frequency and full width at half maximum of the 1008 cm-**<sup>1</sup>** stretching band of SiO4 are shown in **Figure 2**. The data indicate that the Si**‐**O bond strength exhibits a weakness, while the specific volume of the crystal increases, although radiation damage does not destroy SiO4 and short‐range ordering associated with the tetrahedral framework remains. The observation suggests that the increase in bond distances is prob‐ ably depolarized by a rotation of the SiO4 tetrahedra within the zircon structure. The data clearly show that the unit cell swelling in damaged zircon is associated with the SiO4 tetra‐ hedra which formed new linkage and play an important role in the zircon structure rather than isolated molecular complexes.

Raman band widths [full width at half maximum (FWHM)] and frequencies (especially those of the ν3 band of SiO4 near 1008 cm-<sup>1</sup> ) in zircon have been used for investigating the relation‐ ship 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.

**Figure 2.** Phonon frequency and full width at half maximum (FWHM) of the ν3 stretching band (B1g) of SiO4 in radiation‐ damaged zircon as a function of radiation dose (Ref. [8], modified). The lines are visual guides.

The dose dependence of the frequency and full width at half maximum of the 1008 cm-**<sup>1</sup>**

**Figure 1.** Micro‐Raman spectra (with 488 nm excitation) of metamict zircon between 50 and 1100 cm-<sup>1</sup>

.

exhibits a weakness, while the specific volume of the crystal increases, although radiation

framework remains. The observation suggests that the increase in bond distances is prob‐

hedra which formed new linkage and play an important role in the zircon structure rather

Raman band widths [full width at half maximum (FWHM)] and frequencies (especially those

ship 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.

clearly show that the unit cell swelling in damaged zircon is associated with the SiO4

are shown in **Figure 2**. The data indicate that the Si**‐**O bond strength

and short‐range ordering associated with the tetrahedral

) in zircon have been used for investigating the relation‐

tetrahedra within the zircon structure. The data

tetra‐

(Ref. [8], modified).

stretching band of SiO4

106 Raman Spectroscopy and Applications

of the ν3

damage does not destroy SiO4

The dosage is in the units of 1018 alpha‐events g-<sup>1</sup>

than isolated molecular complexes.

band of SiO4

ably depolarized by a rotation of the SiO4

near 1008 cm-<sup>1</sup>

Another good example of Raman study of alpha‐decay radiation damage is the application in metamict titanite. Titanite is a calcium titanium nesosilicate mineral, CaTiSiO<sup>5</sup> . Taylor and Brown [38] synthesized pure titanite, and their X‐ray data show that it is monoclinic (space group *P*21 /*a*, *Z* = 4) with unit‐cell parameters *a* = 7.057, *b* = 8.707, *c* = 6.555 Å, β = 113.81°. The crystal structure of *P*21 /*a* titanite phase contains chains of corner‐sharing TiO<sup>6</sup> octahedra par‐ allel along [1 0 0], which are cross‐linked by edge‐sharing CaO<sup>7</sup> ‐polyhedra extending parallel to [1 0 1]. On heating, the *P*21 /*a* phase undergoes a phase transition to an *A*2/*a* phase near 500 K [38]. The two phases have different optical active representations. For the *P*21 /*a* phase *optic* Γ  = 24*A*<sup>g</sup>  + 24*B*<sup>g</sup>  + 23*A*u + 22*B*u (*A*<sup>g</sup> and *B*<sup>g</sup> are Raman‐active, and *A*u and *B*u IR‐active), whereas for the *A*2/*a* phase *optic* Γ  = 9*A*<sup>g</sup>  + 12*B*<sup>g</sup>  + 11*A*u + 13*B*u [39]. Therefore, the *P*21 /*a* phase is expected to have 48 Raman‐active modes, whereas the *A*2/*a* phase contains 21 Raman modes. Although pure synthetic titanite is in *P*21 /*a* symmetry, some well‐crystalline natural titanite samples were surprisingly reported to be in the *A*2/*a* structure [11]. This significant difference in the total number of the Raman modes for the *P*21 /*a* and *A*2/*a* phases is important and help‐ ful for identifying the presence of the two phases [40].

Natural titanite occurs in igneous and metamorphic rock and incorporates a variety of impu‐ rity 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 *P*21 /*a* titanite as reported previously [39]. The Raman work [41] suggested that anisotropy is preserved upon metamictization and that the structural state of highly metamict titanite should not be considered as quasi‐amor‐ phous. It was reported that the local structure of the amorphized regions contains a high degree of short‐range order [42]. The effect of alpha‐decay radiation on Raman spectrum of titanite is characterized by a dramatic decrease in intensity and line broadening (**Figure 3**).

**Figure 3.** Raman spectra of fine powders of titanites (CaTiSiO<sup>5</sup> ) with different degrees of radiation damage (Ref. [40], modified). The top spectra are from well‐crystallized samples, which show that the space group is the *P*21 /*a* symmetry [40]. Metamictization causes a loss of spectral details and a line broadening in the spectra of titanites. The Ti‐O stretching band near 605 cm−1 shifts to a higher frequency, while a extra band near 574 cm−1 (which is due to the *A*2/*a* phase) appears in partially damage samples. A relatively intense band is recorded near 675 cm−1 with a FWHM of about 80 cm−1 in heavily damaged titanite (in the bottom of the plot). The large FWFM indicates that this feature is related to radiation‐ induced disordered or amorphous phase. Crystalline titanites have the bands near 858 and 912 cm−1, which are due to stretching vibrations of SiO4 tetrahedra. These bands shift to lower frequencies in intermediately damaged samples and appear as a broad feature near 845 cm−1 in heavily damage samples (bottom).

The intense Ti‐O band near 605 cm−1 shows the largest intensity decrease, indicating radia‐ tion effect on the TiO<sup>6</sup> octahedra. This change is consistent with the behavior of the infrared‐ active Ti‐O stretching mode near 670 cm−1, which is mostly affected by radiation damage and shifts to 710 cm−1 in heavily damaged titanite [43]. A Raman work on metamict titanite [44] proposed that radiation‐induced periodic faults in the crystalline matrix of metamict titanite are related to the disturbance of SiO4 ‐TiO<sup>6</sup> ‐SiO4 ‐TiO<sup>6</sup> rings comprising TiO<sup>6</sup> octahedra from different chains, whereas the radiation‐induced amorphization is associated with the partial change of Ti coordination from octahedral to pyramidal and/or tetrahedral, which in turn vio‐ lates the Ti‐O‐Ti intrachain linkages. In addition to these changes, radiation damage in titanite leads to the appearance of extra Raman signals (e.g., 574 cm−1) in intermediately damaged samples (**Figure 3**). This behavior is not seen in metamict zircon. These additional phonon modes in these partially metamict titanite samples are, in fact, characteristic Raman bands of the *A*2/*a* phase [40]. The results indicate that as a result of the radiation, the *P*21 /*a* titanite first transforms to the *A*2/*a* structure, and then, with further radiation, the *A*2/*a* phase becomes an amorphous phase. Beirau et al. [45] reported in situ high‐temperature Raman data of radia‐ tion‐damaged titanite, and they found a structural anomaly near 500 K in partially metamict titanite, which was attributed to the *P*21 /*a‐A*2/*a* transition. These above findings explained why some of natural crystalline titanites were found to appear the *A*2/*a* structure [11], rather than the *P*21 */a* phase. This could be due to the fact that natural titanite crystals commonly experi‐ enced radiation damage, which resulted in an alteration of the *P*21 */a* crystal structure [40].

in the total number of the Raman modes for the *P*21

108 Raman Spectroscopy and Applications

**Figure 3.** Raman spectra of fine powders of titanites (CaTiSiO<sup>5</sup>

appear as a broad feature near 845 cm−1 in heavily damage samples (bottom).

stretching vibrations of SiO4

modified). The top spectra are from well‐crystallized samples, which show that the space group is the *P*21

[40]. Metamictization causes a loss of spectral details and a line broadening in the spectra of titanites. The Ti‐O stretching band near 605 cm−1 shifts to a higher frequency, while a extra band near 574 cm−1 (which is due to the *A*2/*a* phase) appears in partially damage samples. A relatively intense band is recorded near 675 cm−1 with a FWHM of about 80 cm−1 in heavily damaged titanite (in the bottom of the plot). The large FWFM indicates that this feature is related to radiation‐ induced disordered or amorphous phase. Crystalline titanites have the bands near 858 and 912 cm−1, which are due to

tetrahedra. These bands shift to lower frequencies in intermediately damaged samples and

ful for identifying the presence of the two phases [40].

show spectral features similar to those of synthetic pure *P*21

Natural titanite occurs in igneous and metamorphic rock and incorporates a variety of impu‐ rity 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**)

[39]. The Raman work [41] suggested that anisotropy is preserved upon metamictization and that the structural state of highly metamict titanite should not be considered as quasi‐amor‐ phous. It was reported that the local structure of the amorphized regions contains a high degree of short‐range order [42]. The effect of alpha‐decay radiation on Raman spectrum of titanite is characterized by a dramatic decrease in intensity and line broadening (**Figure 3**).

/*a* and *A*2/*a* phases is important and help‐

) with different degrees of radiation damage (Ref. [40],

/*a* symmetry

/*a* titanite as reported previously
