5.1 Spectra and their variability
The measured base spectra of uranophane-α, uranophane-β, sklodowskite and haiweeite as shown in Fig. 8 are expected to be already good estimates for true pure spectra. Both uranophane polymorphs can be distinguished from the closely related sklodowskite as well as from haiweeite on the basis of luminescence spectra. While we are aware that those spectra have been derived from natural material, measured peak positions are well in line with literature values (Table 2). All four mineral species showed some variability in their spectra. Especially, the overall red-shift is a common feature that can be interpreted by an additional defect-induced broadening, possibly related to radiation damage or weathering. Interestingly, the most prominent variability occurs in the case of uranophane-α, especially exhibiting a double peak structure. Our finding of a double peak structure is supported by previous data as well on natural (Haberlandt et al., 1950; Frankland et al. 2022) as on synthetic material (Kuta et al. 2013).
At this point we emphasize, that the prominent double peak structure was observed in the case of uranophane-α only, but for all uranophane-α available to this study, independent of locations. In several cases, the peak of the second spectral species at around 501 nm dominated the spectrum, reducing the ZPL of uranophane-α (at around 495 nm) to barely more than a “shoulder”. While the luminescence spectrum in such cases might mimic uranophane-β, Raman and XRD-data clearly indicated uranophane-α as main species. For practical use of luminescence spectroscopy e.g. in field-monitoring applications, this observation has to be considered to avoid misinterpreting spectra of uranophane-α as uranophane-β. It might be helpful to explicitly address the absence of this “shoulder” for assignment of uranophane-β (provided accordance in the main spectrum).
5.2 Spectral Reconstruction
To further address the double peak structure observed on uranophane-α, we apply the model outlined in section 2.3. Selected spectra of uranophane-α and uranophane-β are decomposed into respective phononic bands with additional hot-band contributions (Fig. 14). Applying the model, we are aware that in inorganic crystals localized defects and substitutions might introduce changes to the model. An additional contribution to account for general spectral broadening and redshift, thus reflecting general loss of crystallinity or further alteration, might be necessary (Frankland et al. 2022). However, this analysis is not meant as modelling in strict sense, but rather intended to illustrate spectral relations.
As we focus on systematic influences, we chose the base spectra of uranophane-β (spectrum #117-6) and uranophane-α (#086 − 4) together with spectrum #065, a representative uranophane-α spectrum with distinct double-peak structure, but low overall redshift.
First, the spectrum of uranophane-β was parameterized in sequences of phononic and hot-peaks using Lorentzian peak shapes. Positions of the phononic peaks were taken from measured data, their average energetic separation ωphonon of about 97 meV corresponds rather well to the energy of the UO22+ symmetric stretching vibration as determined by Raman (compared to 99 meV from Colmenero et al., 2019). The sequence of intensities with ratios 0.89:0.93:0.48:0.15:0.03 (compare to Višňák and Sobek 2016) with empiric peak-width of 25 meV delivers already the main characteristics of the spectrum (blue components in Fig. 14a). To estimate hot-peak contributions (compare Fig. 5), the obtained phononic sequence is scaled by 0.06 with empiric width of 30 meV and shifted by the estimated hot phonon energy ω’phonon of 66 meV (magenta components in Fig. 14a). By this scaling we assure, that the hot-band contribution remains limited to several percent. The hot phonon energy estimate is taken from data with the width taken as fitting parameter. The full composed spectrum (red) matches the measured data (black in Fig. 14a) very well.
In a similar manner the base-spectrum of uranophane-α is parameterized (Fig. 13b). Main phononic bands (blue) are characterized by a separation of 98 meV in accordance with Raman data and intensity ratios (0.74:0.76:0.33:0.1:0.02) are similar to those of uranophane-β. Hot band contributions are shifted by 66 meV.
As the contribution of the hot bands to intensity is limited by the Boltzmann distribution to a few percent (Višňák and Sobek 2016; Haubitz et al. 2018), an additional component is necessary. This becomes evident regarding the relative intensities of bands necessary to reconstruct the uranophane-α spectrum. The leading hot-band transition of highest probability (\({1}^{\text{'}}\to 0\), at around 2.56 eV; Fig. 14b) is comparably small as in the case of uranophane-β. In contrast, the following bands necessary for reconstruction (e. g. \({1}^{\text{'}}\to 1\), at around 2.48 eV; Fig. 14b) carry a rather large contribution (about half the amplitude of the related phononic band). Assigning those large contributions to multi-phonon hot-bands would contradict thermal limitations – especially as the leading hot-band is of small intensity. Therefore, we conclude that an additional component – a second spectroscopic species – is necessary that shows a phononic structure with a well-defined relation to the phononic bands of uranophane-α.
As empirical observation we state that the spectral fingerprint (peak positions, spacing and relative peak intensities) of this second spectroscopic species (necessary to reconstruct uranophane-α spectra) matches well with the base spectrum of uranophane-β. Thus, we propose to directly use the scaled uranophane-β phononic sequence as additional component representing the second spectroscopic species.
In detail, we reconstruct the uranophane-α spectrum using the main phononic sequence of uranophane-α plus corresponding hot bands (position and spacing from measured data, intensity scaled by 0.06 with respect to phononic peaks) and adding the scaled phononic spectrum of uranophane-β. Here, we use exactly the reconstructed base spectrum of uranophane-β described above (i.e. in the case of #117-6), uniformly scaled by a factor 0.45 (dashed brown bands in Fig. 14b). Again, the composed spectrum (red) matches rather well the measured data (black in Fig. 14b).
In an analogous manner, the spectrum of uranophane-α #065 with prominent double peak spectrum is reconstructed (Fig. 14c). Notably, the ZPL is red-shifted by about 5 meV, separations of the phononic bands (blue in Fig. 14c) are around 95 meV, intensity ratios are 0.38:0.88:0.4:0.12:0.03. Red shift and reduced phononic energy might reflect pronounced presence of defects. Again, following the concept outlined, a small intrinsic hot-band contribution is added (magenta) plus the uniformly scaled (factor 0.72) and shifted (6.5 meV) uranophane-β phononic sequence representing the second spectroscopic species (brown in Fig. 14c). Uniform scaling and the necessity of a uniform shift of the spectrum of the second spectroscopic species shows that the second spectroscopic species is incorporated in the crystal lattice. The reconstructed spectrum (red) approaches the measured data (black in Fig. 14c) rather well, even though some discrepancies become visible. Remarkably, the main phononic contributions are not dominating the double peak structure. Instead, the spectrum seems more related to uranophane-β.
5.3 Incorporated clusters of uranophane-β as proposed origin
The pronounced spectral variability of uranophane-α with respect to the other two uranyl silicates haiweeite and sklodowskite points to a pronounced variability on structural level, as the uranyl luminescence is sensitive to its local environment. We briefly summarize the observations concerning the second spectroscopic species.
Reconstructing spectra of uranophane-α, a second spectroscopic species is necessary to limit hot-band contributions within thermal constraints. This second spectroscopic species is well described by the uniformly scaled phononic sequence of uranophane-β and it red-shifts with the shift of the uranophane-α phononic peaks. Additionally, this signature occurs and varies locally on individual crystals. Thus, the species is incorporated in the uranophane-α crystal. The contribution of a second spectroscopic species was observed to variable degree in all uranophane-α spectra taken from samples of 16 different locations and the spectral characteristics of the second spectroscopic species is conserved in the related data (see Fig. 7). Thus, we exclude chemical substitutions as direct source of the spectral contribution, as the double peak structure appears with one systematic signature, only varying in intensity. Next, the spectrum of the second spectroscopic species is well represented by (the phononic sequence of) the base spectrum of uranophane-β. The pronounced and variable double peak structure occurred in the case of uranophane-α only. We emphasize that none of the examined uranophane-α samples showed a spectrum, where a contribution of uranophane-β could be excluded. No such double peak structure was visible in the spectra of sklodowskite or haiweeite – for both species no polymorphic forms exist either.
In this light, the hypothesis of strongly varying hot-band contributions as explanation seems unsatisfactory. Such variations should similarly occur in the other mineral species, especially of equivalent uranophane topology, i.e. in sklodowskite. However, this is not observed.
Likely, a possible contribution of a second type of UO22+ luminescent center should apply as well in the case of uranophane-β and sklodowskite, thus not explaining the exclusive behavior of uranophane-α.
Therefore, we propose, that indeed small clusters of a uranophane-β like phase are present in uranophane-α to varying degree. As both uranophane share chemistry and topology, such defects – essentially exchanging locally the building sequence ..udud.. by ..uudd.. – might be stabilized by the polymorphism. Those clusters contribute to the luminescence with their respective spectra. The photon energy corresponds to the photon energy of uranophane-α hot-band luminescence, allowing for energy transfer. As these clusters represent defect structures incorporated in the uranophane-α lattice, the spectral signature shifts with the overall spectral red-shift in a disturbed lattice.
Regarding the reverse case of uranophane-α clusters in uranophane-β, no contribution to the spectrum is expected. The bandgap energy of uranophane-α is larger than that of uranophane-β (2.68 eV, Colmenero et al. 2019), related to a higher ZPL energy of uranophane-α as compared to uranophane-β. Additionally, spectral features of uranophane-α do not match hot-bands of uranophane-β hampering reverse energy transfer.
For sklodowskite, no such mechanism is observable, as no polymorphic form exists. Haiweeite on the other hand has a different topology, so the reasoning is not simply transferable.
Our hypothesis of uranophane-β like clusters incorporated in uranophane-α is supported by observations by Wall and colleagues (2010), who report traces of uranophane-β in synthesis of uranophane-α.
5.4 Polymorphism and spectral variability: possible relations
Looking at UO22− in the well-defined environment of sklodowskite, all possible sites are predetermined (neglecting obvious defects like vacancies or impurities). The situation in uranophane is slightly different, as two polymorphs exist. Without obvious defects like vacancies or impurities, a localized transition between the two characteristic structures uranophane-α and uranophane-β might be possible. The uranophane topology is conserved in both cases. We consider the characteristic structural elements of the two uranophane structures: an alternating succession of up- and down-oriented SiO4-tetrahedra (Fig. 2). While the characteristic motif of uranophane-α is ..ud.. the equivalent motif of uranophane-β is ..uudd... Thus, at the growth front or at a domain interface, seeds of uranophane-β structure might readily form simply by misorientation or stacking defects of one chain unit, so e.g. ..ududu becomes by addition of one unit ..ududuu, where the occurrence of uu might already favor continuation in uranophane-β type phase.
Colmenero and colleagues (2019) showed uranophane-α to be the thermodynamically slightly preferred form of uranophane. So, a seed of uranophane-β structure in uranophane-α is not favored and growth will be limited by thermodynamic conditions. Small variations rapidly stop further propagation of this domain. On the other hand, the energy difference appears to be rather low, -12.0 kJmol− 1 at zero temperature and pressure according to Colmenero and colleagues (2019). Thus, further variations in environmental conditions might again support reappearance of a uranophane-β domain, its growth eventually again being stopped and so forth. Such a mechanism might produce small, randomly distributed but localized domains of uranophane-β like phases in the general uranophane-α structure. Notably, the mechanism might occur similarly under natural as well as under laboratory conditions of crystal growth. The scenario described here is supported by the observation of typically different morphology of natural uranophane-α (crystals with high aspect ratio) with respect to that of natural uranophane-β (more bulky crystals), indicating differences in growth mechanisms and pointing to a rather fast growth of uranophane-α crystals (Schindler et al. 2004; Schindler et al. 2004b). Additionally, approaches to obtain uranophane-β by synthesis resulted rather in uranophane-α phase (Cesbron et al., 1993).
As potential second pathway, intrinsically induced radiation damage and healing of related defects might play their role (Utsunomiya et al. 2003; Sureda et al. 2011). By radioactive decay, structure is damaged while the chemical elements basically remain at the site, thus particle numbers can be regarded as approximately conserved. Under the condition, where no particles can be imported, healing of such defects might differ from growth in an open system. Even though this is rather speculative, some support comes from comparison to other systems (Nasdala et al., 2013; Lenz and Nasdala 2015; Lenz et al., 2020).
The situation is substantially different in the cases of sklodowskite and haiweeite. No polymorphic forms are available during growth or restructuring. Thus, a defect site created by a disturbance remains restricted to a point defect, as growth is not supported by a second polymorphic structure, or it vanishes. In consequence, structural variations that affect luminescence emission remain very limited. Thus, spectra are conserved to a higher degree in the case of sklodowskite and haiweeite as compared to uranophane-α.