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Raman spectra of molecules or clusters incorporated into cubic zeolites are usually studied using microcrystalline powder samples, light being completely unpolarized because of multiple reflection on microcrystal surfaces. Therefore, all information about symmetry of the Raman-active vibrations of species and orientation of species in zeolite cavit...
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Context 1
... being important. Moreover, the possibility of the determination of ratios between the Raman tensor components is demonstrated and distinct polarization and angular dependencies are obtained for the S 8 librations representing interaction of the rings with zeolite. RS of A–Se single crystals at different excitation wave- lengths of 632.8 and 514.5 nm are shown in Fig. 7. As has been found earlier, 15 the Raman bands of A–Se can be attributed to two main species of selenium stabilized in zeolite A, namely Se 12 and Se 8 rings. Se 8 is a well-known molecule that is stable in crystalline monoclinic Se and amorphous Se. The Se 12 ring is not as stable as Se 8 and it is not observed in any condensed form of Se. This is why we call this specie ‘‘cluster.’’ The proposed structure of Se 12 is similar to that of cyclododecasulfur S 12 ( D 3 d point group ͒ . 18–20 A number of reasonable arguments for the formation of Se 12 rings in zeolite A were shown in our previous studies. Another important argument for possibility of the formation of Se 12 rings is the fact of recent synthesis of crystals containing such rings sur- rounded by the ͓ Mo 3 S 13 ͔ 2 Ϫ clusters. 21 Concerning Se 12 rings in A–Se, the previously published Raman data supporting the formation of these rings were obtained using zeolite powder samples. Further experimental evidence for the formation of Se 12 in zeolite A, which is based on the polarized Raman data of A–Se single crystals, is necessary. Our strategy in this section is based on the assignment of certain Raman bands of A–Se to the vibrations of Se 12 rings, which was made in Ref. 15. We aim to show that the polarization and angular dependencies of the intensities of the Raman bands correspond well to this assignment, Se 12 rings being oriented by their threefold axes along the threefold axes of zeolite. As has been shown in the previous work, the 514.5 nm light of the Ar laser interacts strongly with Se 12 rings and at certain power densities causes their destruction. However, in this work, we have found that at a very low power density of 514.5 nm line ͑ laser power was ϳ 0.1 mW and the defocused laser light spot was ϳ 5 m ͒ resonant enhancement of the Se 12 Raman bands occurs without any destruction of the Se 12 rings. This circumstance is very helpful for the analysis of the behavior of the Se 12 bands. Indeed, the bands of Se 8 at 75, 111, 122, and 268 cm Ϫ 1 , which are clearly seen in the spectrum excited with 0 ϭ 632.8 nm ͓ Fig. 7 ͑ a ͔͒ , practically are unnoticeable in the spectrum excited with 0 ϭ 514.5 nm ͓ Fig. 7 ͑ b ͔͒ . There are four strong bands in RS of A–Se shown in Fig. 7 ͑ b ͒ , which are centered at 27, 55, 88, and 258 cm Ϫ 1 . These bands have been attributed earlier 15 to the Se 12 ring librations ( E g ), A 1 g bond-bending mode, E g bond-bending mode, and A 1 g bond-stretching mode, respectively. ͑ Types of vibrations are the same as those of S 12 . 19 ͒ Using the polarized RS of A–Se, we can check the correctness of the assignment of the 55 and 258 cm Ϫ 1 bands to A 1 g modes of Se 12 . A 1 g modes with only diagonal components of the Raman tensor should be much more active for the XX configuration than for the XY configuration. Indeed, the bands at 55 and 258 cm Ϫ 1 are much stronger for the XX configuration than for the XY configuration for both ϭ 0° and ϭ 45°. This observation is a very strong argument for the correctness of the assignment of these bands to the A 1 g modes of Se 12 . For analysis of the angular dependencies of the Se 12 Raman band intensities, it is reasonable to use the symmetric bond-bending mode band at 55 cm Ϫ 1 ͓ Fig. 7 ͑ b ͔͒ , which is similar to the 222 cm Ϫ 1 band of S 8 , displays mainly in-plane Raman activity ͓ Fig. 8 ͑ a ͔͒ , and does not overlap with other Raman bands. It is clearly seen from RS of A–Se for different polarization configurations ͓ Fig. 7 ͑ b ͔͒ that the behavior of the 55 cm Ϫ 1 band of Se 12 differs from the behavior of the 222 cm Ϫ 1 band of S 8 ͓ Fig. 4 ͑ b ͔͒ . Indeed, maximal intensity of the 222 cm Ϫ 1 band of S 8 corresponds to the XX configuration at ϭ 0° and minimal intensity ͑ almost disappearing ͒ to the XY configuration at ϭ 0°. However, maximal intensity of the 55 cm Ϫ 1 band of Se 12 corresponds to the XX configuration at ϭ 45° and minimal intensity to the XY configuration at ϭ 45°. It means that the orientation of the Se 12 rings in zeolite cavities differs from the orientation of the S 8 rings. The experimental angular dependencies of the 55 cm Ϫ 1 band intensity and the calculated dependencies for the orientation of Se 12 rings by the threefold axis of the ring parallel to the threefold axis of zeolite according to Eqs. ͑ A7 ͒ and ͑ A8 ͒ are shown in Fig. 9. Agreement is good and this is evidence for the orientation of the Se 12 ring by its threefold axis along the threefold axis of zeolite A. ͓ Four possible orientations of Se 12 rings in zeolite are schematically shown in Fig. 10 ͑ a ͒ . ͔ This conclusion is in accordance with the conclusion made from speculations regarding the symmetry and size of Se 12 and those of the zeolite A large cavity. 7,14,15 The corresponding drawing is shown in Fig. 10 ͑ b ͒ . It is interesting to note that the symmetric bond-stretching mode band at 258 cm displays behavior similar to that of the 55 cm Ϫ 1 band suggesting quite strong anisotropy of the corresponding Raman tensor. There is one more band of Se 12 ͑ at 143 cm Ϫ 1 ͒ in RS of A–Se ͑ Fig. 7 ͒ , which is assigned to the A 1 g mode. 15 This is a very weak band and we do not discuss in detail the behavior of this band. However, it is easy to note that this band can be recognized in the XX spectra at ϭ 45° and practically is unnoticeable in other spectra. The highest activity of the 143 cm Ϫ 1 band for this configuration and this angle is a good argument for the correctness of the assignment of the band to the A 1 g mode. Two strong bands at 27 and 88 cm Ϫ 1 ͑ Fig. 7 ͒ , which have been assigned to the E g modes of Se 12 , 15 display com- parable activity for both XX and XY configurations. Indeed, the 27 cm Ϫ 1 band is very strong for both XX ( ϭ 45°) and XY ( ϭ 0°) configurations and the 88 cm Ϫ 1 band is strong for both XX ( ϭ 0°) and XY ( ϭ 45°) ͑ Fig. 7. ͒ . This is in accordance with the presence of both diagonal and off- diagonal components in the Raman tensor for E g -type vibrations ͑ see the Appendix ͒ . The angular dependencies of the bond-bending mode band at 88 cm Ϫ 1 can be treated in the same way as for the 154 cm Ϫ 1 band of S 8 . Our calculations show that the ratio between the components of the Raman tensor of the 88 cm Ϫ 1 band c / d ϳ 0.1 ͓ see Eqs. ͑ A9 ͒ and ͑ A10 ͔͒ satisfies the experimental angular dependencies in agreement with the orientation of the Se 12 rings by the threefold axis along the threefold axis of zeolite. The E g libration band centered at ϳ 27 cm Ϫ 1 , which actually consists of two components at ϳ 23 and ϳ 30 cm Ϫ 1 , also shows angular dependencies agreeing with the orientation of the Se 12 rings by the threefold axis along the threefold axis of zeolite. The ratio between the tensor components c / d 5 fits well theoretical Eqs. A9 and A10 and experimental angular dependencies. However, we should note that the correct analysis of the angular dependencies is not as simple as that for the internal vibrations of the molecules because of the strong influence of the zeolite host. Moreover, the E g torsional mode of Se 12 can contribute to the 27 cm Ϫ 1 band. For A–Se, the analysis of the external vibration of Se 12 can be embarrassed because of the possible contribution of the Se 8 rings even to the spectrum excited with the 514.5 nm light ͓ Fig. 7 ͑ b ͔͒ , in which contribution of the internal vibrations bands of Se 8 is insignificant. Concerning the Se 8 molecule with structure and types of vibrations similar to S 8 , we do not show detailed analysis of the angular dependencies of the Raman bands of this specie. We just note that behavior of the band of the A 1 symmetric bond-bending mode at 111 cm Ϫ 1 is similar to that of the corresponding band of S 8 at 222 cm Ϫ 1 , suggesting the orientation of the fourfold axis of Se along the fourfold axis of the zeolite. The behavior of other distinct bands of Se 8 the E 2 bond-bending mode at 75 cm Ϫ 1 and A 1 bond-stretching mode at 268 cm Ϫ 1 ͒ is also similar to the behavior of the corresponding bands of S 8 at 154 and 480 cm Ϫ 1 , respectively. We can also note a detail that demonstrates the insignificant difference between the behavior of Se 8 and S 8 , namely the band at 75 cm Ϫ 1 displays splitting to two components at 73 cm Ϫ 1 ( B 2 for the C 4 v point group ͒ and 76 cm Ϫ 1 ( B 1 ) inverse to the similar splitting of the 154 cm Ϫ 1 band of S 8 to 153 cm Ϫ 1 ( B 1 ) and 156 cm Ϫ 1 ( B 2 ). To summarize this section, using low power excitation of the 514.5 nm line, we have obtained RS of A–Se display- ing only Se 12 ring bands. Polarization and angular dependencies of intensities of these bands confirm their assignment made earlier for the zeolite powder samples 15 and therefore provide us with further evidence for the stabilization of the Se 12 rings in the zeolite A cavities. The Se 12 rings are found to be oriented by their threefold axes along the threefold axes of zeolite. Behavior of the Se 8 ring bands observed at the 632.8 nm excitation is consistent with the orientation of these rings by their fourfold axes along the fourfold axes of zeolite. We have proposed to use Raman microprobe polarization measurements for studying the structure and orientation of species incorporated into cubic zeolites. Using this method, we have determined polarization and angular dependencies of the Raman bands of the S 8 and Se 12 ring species incorporated into zeolite A. The Raman bands of the S 8 rings, in accordance with x-ray diffraction data, 12 display behavior corresponding to the orientation of ...
Context 2
... interaction of the rings with zeolite. RS of A–Se single crystals at different excitation wave- lengths of 632.8 and 514.5 nm are shown in Fig. 7. As has been found earlier, 15 the Raman bands of A–Se can be attributed to two main species of selenium stabilized in zeolite A, namely Se 12 and Se 8 rings. Se 8 is a well-known molecule that is stable in crystalline monoclinic Se and amorphous Se. The Se 12 ring is not as stable as Se 8 and it is not observed in any condensed form of Se. This is why we call this specie ‘‘cluster.’’ The proposed structure of Se 12 is similar to that of cyclododecasulfur S 12 ( D 3 d point group ͒ . 18–20 A number of reasonable arguments for the formation of Se 12 rings in zeolite A were shown in our previous studies. Another important argument for possibility of the formation of Se 12 rings is the fact of recent synthesis of crystals containing such rings sur- rounded by the ͓ Mo 3 S 13 ͔ 2 Ϫ clusters. 21 Concerning Se 12 rings in A–Se, the previously published Raman data supporting the formation of these rings were obtained using zeolite powder samples. Further experimental evidence for the formation of Se 12 in zeolite A, which is based on the polarized Raman data of A–Se single crystals, is necessary. Our strategy in this section is based on the assignment of certain Raman bands of A–Se to the vibrations of Se 12 rings, which was made in Ref. 15. We aim to show that the polarization and angular dependencies of the intensities of the Raman bands correspond well to this assignment, Se 12 rings being oriented by their threefold axes along the threefold axes of zeolite. As has been shown in the previous work, the 514.5 nm light of the Ar laser interacts strongly with Se 12 rings and at certain power densities causes their destruction. However, in this work, we have found that at a very low power density of 514.5 nm line ͑ laser power was ϳ 0.1 mW and the defocused laser light spot was ϳ 5 m ͒ resonant enhancement of the Se 12 Raman bands occurs without any destruction of the Se 12 rings. This circumstance is very helpful for the analysis of the behavior of the Se 12 bands. Indeed, the bands of Se 8 at 75, 111, 122, and 268 cm Ϫ 1 , which are clearly seen in the spectrum excited with 0 ϭ 632.8 nm ͓ Fig. 7 ͑ a ͔͒ , practically are unnoticeable in the spectrum excited with 0 ϭ 514.5 nm ͓ Fig. 7 ͑ b ͔͒ . There are four strong bands in RS of A–Se shown in Fig. 7 ͑ b ͒ , which are centered at 27, 55, 88, and 258 cm Ϫ 1 . These bands have been attributed earlier 15 to the Se 12 ring librations ( E g ), A 1 g bond-bending mode, E g bond-bending mode, and A 1 g bond-stretching mode, respectively. ͑ Types of vibrations are the same as those of S 12 . 19 ͒ Using the polarized RS of A–Se, we can check the correctness of the assignment of the 55 and 258 cm Ϫ 1 bands to A 1 g modes of Se 12 . A 1 g modes with only diagonal components of the Raman tensor should be much more active for the XX configuration than for the XY configuration. Indeed, the bands at 55 and 258 cm Ϫ 1 are much stronger for the XX configuration than for the XY configuration for both ϭ 0° and ϭ 45°. This observation is a very strong argument for the correctness of the assignment of these bands to the A 1 g modes of Se 12 . For analysis of the angular dependencies of the Se 12 Raman band intensities, it is reasonable to use the symmetric bond-bending mode band at 55 cm Ϫ 1 ͓ Fig. 7 ͑ b ͔͒ , which is similar to the 222 cm Ϫ 1 band of S 8 , displays mainly in-plane Raman activity ͓ Fig. 8 ͑ a ͔͒ , and does not overlap with other Raman bands. It is clearly seen from RS of A–Se for different polarization configurations ͓ Fig. 7 ͑ b ͔͒ that the behavior of the 55 cm Ϫ 1 band of Se 12 differs from the behavior of the 222 cm Ϫ 1 band of S 8 ͓ Fig. 4 ͑ b ͔͒ . Indeed, maximal intensity of the 222 cm Ϫ 1 band of S 8 corresponds to the XX configuration at ϭ 0° and minimal intensity ͑ almost disappearing ͒ to the XY configuration at ϭ 0°. However, maximal intensity of the 55 cm Ϫ 1 band of Se 12 corresponds to the XX configuration at ϭ 45° and minimal intensity to the XY configuration at ϭ 45°. It means that the orientation of the Se 12 rings in zeolite cavities differs from the orientation of the S 8 rings. The experimental angular dependencies of the 55 cm Ϫ 1 band intensity and the calculated dependencies for the orientation of Se 12 rings by the threefold axis of the ring parallel to the threefold axis of zeolite according to Eqs. ͑ A7 ͒ and ͑ A8 ͒ are shown in Fig. 9. Agreement is good and this is evidence for the orientation of the Se 12 ring by its threefold axis along the threefold axis of zeolite A. ͓ Four possible orientations of Se 12 rings in zeolite are schematically shown in Fig. 10 ͑ a ͒ . ͔ This conclusion is in accordance with the conclusion made from speculations regarding the symmetry and size of Se 12 and those of the zeolite A large cavity. 7,14,15 The corresponding drawing is shown in Fig. 10 ͑ b ͒ . It is interesting to note that the symmetric bond-stretching mode band at 258 cm displays behavior similar to that of the 55 cm Ϫ 1 band suggesting quite strong anisotropy of the corresponding Raman tensor. There is one more band of Se 12 ͑ at 143 cm Ϫ 1 ͒ in RS of A–Se ͑ Fig. 7 ͒ , which is assigned to the A 1 g mode. 15 This is a very weak band and we do not discuss in detail the behavior of this band. However, it is easy to note that this band can be recognized in the XX spectra at ϭ 45° and practically is unnoticeable in other spectra. The highest activity of the 143 cm Ϫ 1 band for this configuration and this angle is a good argument for the correctness of the assignment of the band to the A 1 g mode. Two strong bands at 27 and 88 cm Ϫ 1 ͑ Fig. 7 ͒ , which have been assigned to the E g modes of Se 12 , 15 display com- parable activity for both XX and XY configurations. Indeed, the 27 cm Ϫ 1 band is very strong for both XX ( ϭ 45°) and XY ( ϭ 0°) configurations and the 88 cm Ϫ 1 band is strong for both XX ( ϭ 0°) and XY ( ϭ 45°) ͑ Fig. 7. ͒ . This is in accordance with the presence of both diagonal and off- diagonal components in the Raman tensor for E g -type vibrations ͑ see the Appendix ͒ . The angular dependencies of the bond-bending mode band at 88 cm Ϫ 1 can be treated in the same way as for the 154 cm Ϫ 1 band of S 8 . Our calculations show that the ratio between the components of the Raman tensor of the 88 cm Ϫ 1 band c / d ϳ 0.1 ͓ see Eqs. ͑ A9 ͒ and ͑ A10 ͔͒ satisfies the experimental angular dependencies in agreement with the orientation of the Se 12 rings by the threefold axis along the threefold axis of zeolite. The E g libration band centered at ϳ 27 cm Ϫ 1 , which actually consists of two components at ϳ 23 and ϳ 30 cm Ϫ 1 , also shows angular dependencies agreeing with the orientation of the Se 12 rings by the threefold axis along the threefold axis of zeolite. The ratio between the tensor components c / d 5 fits well theoretical Eqs. A9 and A10 and experimental angular dependencies. However, we should note that the correct analysis of the angular dependencies is not as simple as that for the internal vibrations of the molecules because of the strong influence of the zeolite host. Moreover, the E g torsional mode of Se 12 can contribute to the 27 cm Ϫ 1 band. For A–Se, the analysis of the external vibration of Se 12 can be embarrassed because of the possible contribution of the Se 8 rings even to the spectrum excited with the 514.5 nm light ͓ Fig. 7 ͑ b ͔͒ , in which contribution of the internal vibrations bands of Se 8 is insignificant. Concerning the Se 8 molecule with structure and types of vibrations similar to S 8 , we do not show detailed analysis of the angular dependencies of the Raman bands of this specie. We just note that behavior of the band of the A 1 symmetric bond-bending mode at 111 cm Ϫ 1 is similar to that of the corresponding band of S 8 at 222 cm Ϫ 1 , suggesting the orientation of the fourfold axis of Se along the fourfold axis of the zeolite. The behavior of other distinct bands of Se 8 the E 2 bond-bending mode at 75 cm Ϫ 1 and A 1 bond-stretching mode at 268 cm Ϫ 1 ͒ is also similar to the behavior of the corresponding bands of S 8 at 154 and 480 cm Ϫ 1 , respectively. We can also note a detail that demonstrates the insignificant difference between the behavior of Se 8 and S 8 , namely the band at 75 cm Ϫ 1 displays splitting to two components at 73 cm Ϫ 1 ( B 2 for the C 4 v point group ͒ and 76 cm Ϫ 1 ( B 1 ) inverse to the similar splitting of the 154 cm Ϫ 1 band of S 8 to 153 cm Ϫ 1 ( B 1 ) and 156 cm Ϫ 1 ( B 2 ). To summarize this section, using low power excitation of the 514.5 nm line, we have obtained RS of A–Se display- ing only Se 12 ring bands. Polarization and angular dependencies of intensities of these bands confirm their assignment made earlier for the zeolite powder samples 15 and therefore provide us with further evidence for the stabilization of the Se 12 rings in the zeolite A cavities. The Se 12 rings are found to be oriented by their threefold axes along the threefold axes of zeolite. Behavior of the Se 8 ring bands observed at the 632.8 nm excitation is consistent with the orientation of these rings by their fourfold axes along the fourfold axes of zeolite. We have proposed to use Raman microprobe polarization measurements for studying the structure and orientation of species incorporated into cubic zeolites. Using this method, we have determined polarization and angular dependencies of the Raman bands of the S 8 and Se 12 ring species incorporated into zeolite A. The Raman bands of the S 8 rings, in accordance with x-ray diffraction data, 12 display behavior corresponding to the orientation of the rings by their fourfold axes along the fourfold axes of zeolite, point group symmetry of the rings structure being reduced from D 4 d to C 4 v . Symmetries of the vibrations determined from the polarization and ...
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Citations
... Zeolites provide a unique opportunity to form and accommodate uniform guest species in their cavities/channels where the species are oriented due to the crystalline nature of zeolites. [7][8][9][10][11][12][13] For example, only zeolites allowed obtaining polarized Raman spectra (RS) and optical absorption spectra (OAS) of isolated Te chains and rings. [7][8][9][10][11] Moreover, fabrication of such regular high-density Te species arrays can be considered as an important direction in the development of new functional materials, so-called cluster crystals. ...
... We performed measurements in four different polarization configurations: aa, cc, ab and cd (see the inset in Fig. 3(a)) similar to the polarization-orientation Raman study of LTA with sulphur LTA-S 12 and LTA with selenium LTA-Se. 12,44,45 Experimentally, we rotated the LTA-Te crystals in the ab plane with incident and scattered light polarizations (1) parallel and (2) perpendicular to each other. The procedure was described in detail earlier. ...
... The procedure was described in detail earlier. 12 Theoretical RS spectra of LTA-Te for the aa, cc, ab and cd polarization configurations were obtained via summation of the Raman responses of three Te 8 rings in their three possible orientations in LTA crystals [ Fig. 3(b)]. ...
The Te8 ring molecule (cluster) is poorly investigated due to the lack of experimental data. Here, we report an experimental and theoretical study of a regular array of oriented Te8 rings formed in the ∼1.14 nm diameter cavities of zeolite LTA, which are arranged in a cubic lattice with a spacing of ∼1.2 nm. Single crystals of LTA with encapsulated tellurium (LTA-Te) were studied using Raman spectroscopy (RS) and optical absorption spectroscopy (OAS). The experimental LTA-Te spectra were found to be in agreement with those calculated using density functional theory (PBE0 hybrid functional and def2-TZVP basis sets) for the crown-shaped Te8 ring molecule with D4d symmetry. Using polarization-orientation RS, we show that the Te8 rings are oriented by their major axes along the 4-fold axes of cubic LTA. We also show that the site symmetry of Te8 in LTA-Te is lower than D4d. Te8 bond-bending modes are well described in the harmonic approximation, while bond-stretching modes are mixed due to the reduced ring symmetry and, probably, anharmonicity. Importantly, OAS data of LTA-Te display dependence on the Te8 concentration, implying the interaction of the rings from neighbouring LTA cavities with the generation of the valence and conduction electron bands of such a cluster crystal.
... Another important advantage of this method is a high density and strict periodicity of the cavities and channels. Zeolites are suitable crystalline containers for both stabilization of known species and formation of new ones like Te 6 [24], Te 8 [6,31] and Se 12 [23,34,42]. In this paper, we study orientation, polarized Raman spectra (RS), optical absorption spectra (OAS) and photo-induced effects of the zeolite-confined Se 6 ring clusters which are still insufficiently studied. ...
... As was shown in a number of works, zeolite-confined selenium usually represents a variety of different molecules in comparable concentrations in the same sample [24,25,29,31,32,34,37]. Fortunately, CHA is an exception in this sense. ...
... Indeed, the plane of the Se 6 ring is confined in the 0.7 nm space in the aaxis direction and not confined at all in the c-axis direction in the MOR channel while it is strictly confined in the 0.67 nm space in both directions perpendicular to the c-axis in the CHA cavity. We have to note that the presence of a weak band at ~76 cm − 1 in the ca-spectrum (Fig. 4b, red curve) is a clear sign of an insignificant presence of Se 8 rings [25,31,34]. The coexistence of the Se 6 , and Se 8 rings along with Se chains in the MOR channels was discussed in Ref. [29]. ...
Polarized Raman and optical absorption spectra of oriented Se6 ring clusters incorporated into free spaces of CHA, MOR and AFI zeolite single crystals are studied. The ring orientation was determined via polarization dependence of the bond-bending mode Raman activities which are high in the configuration with both incident and scattered lights polarized parallel to the ring plane. Se6 rings seem to be compressed in CHA cavities, their original D3d symmetry remaining preserved with the ring 3-fold axis orientation along the 3-fold axis of CHA. No symmetry distortion and less compression were found for the Se6 rings in the AFI channels, with their 3-fold axes being oriented along the 3-fold axis of AFI. A noticeable structural distortion is found for Se6 rings in MOR channels which are oriented by their 3-fold axes along the b-axis of MOR. Two broad peaks at 3.1–3.2 eV and 3.5–3.65 eV with rather clear polarization dependence were found in the optical absorption spectra of Se6 rings. Photo-induced Se6 ring fragmentation is demonstrated at illumination with the photon energies of ∼1.96 eV and ∼2.41 eV at the temperature of ∼77 K, formation of Se2⁻ (Se3) and Se8 molecules being observed in CHA and AFI, respectively.
... However, for none of them any features in the vicinity of 213 cm À1 were reported, the closest in frequency being vibrational modes at 233-237 cm À1 (for trigonal Se), 250 cm À1 (for chains) and 260 cm À1 (for Se 8 rings) [31][32][33][34]. Neither was any Raman band near 213 cm À1 observed for Se 8 or Se 12 rings incorporated in cubic zeolite A [35], nor for linear Se chains in cancrinite [29], nor for gelatin-capped Se nanocrystals [36]. On the contrary, Raman spectra of Se 3 clusters incorporated into alkali metal halide crystals, reveal a band of similar width in the range of 218-241 cm À1 [30]. ...
As follows from Raman studies, the formation of cadmium chalcogenide nanocrystals in borosilicate glass under thermal treatment at 625–700 °C can be accompanied by precipitation of elemental Se or Te in the form of molecular Se2 or Te2 clusters or larger aggregations. The clusters are in most cases formed when the thermal treatment temperature and/or duration values are beyond the range most suitable for the formation of the II–VI nanocrystals. Larger aggregations are formed at intense thermal treatment, precipitation of tellurium being achieved much easier than that of selenium.
... Raman studies of selenium clusters consisting of a small number of atoms (from 2 to 12) were reported for various host media. In particular, for Se 8 and Se 12 rings incorporated in cubic zeolite A, Raman bands at 48, 78, 154, 222, and 480 cm −1 (for Se 8 rings) and at 27, 55, 88, and 258 cm −1 (for Se 12 rings) were observed [38]. For linear Se chains in cancrinite (one-dimensional-channel-type zeolite) nanochannels, an intense Raman band near 250 cm −1 , typical for Se chains, was registered [39]. ...
While studying the effect of thermal treatment at 625–700°C on the formation of borosilicate glass-embedded CdSe or CdSe1−x
S
x
nanocrystals, pronounced bands at 323 and 646cm−1 were observed in the Raman spectra. They are assigned to Se2 clusters on the base of their frequency positions, widths, intensities, and resonance behavior. The precipitation of Se2 molecular clusters in borosilicate glass is shown to occur when the heat treatment temperature and/or duration are beyond
the range, most suitable for the formation of CdSe or CdSe-rich CdSe1−x
S
x
nanocrystals.
... Among those, pioneering work by Bogomorov et al. has demonstrated that a chalcogen behaves as an interesting guest [7][8][9], partly because the chalcogen forms molecules with ring and chain structures, which may not produce many dangling bonds when nano-structured. Substantial successive studies have been published [10][11][12][13][14][15][16][17][18][19]. ...
... Second, Al/Si ratios in the zeolites have been 0.2-1 ( Fig. 1), being more-or-less polar, which is likely to exert substantial ionic effects to covalent chalcogen molecules [11,17,18], as theoretically predicted [20]. Third, many of previous studies, with a few exceptions [14][15][16]19], have employed powder-like zeolites with typical sizes of ~1 μm [7,9,10,12,13]. Accordingly, chalcogens deposited onto the surface of zeolites may modify or govern measured properties. ...
Sulfur, selenium, and tellurium were loaded into sub-mm size ZSM-5 single crystals and the optical properties have been comparatively studied. S and Te show similar features, while Se is unique. S and Te have optical absorption edges at wavelengths of ～400 nm with transmission dips at ～450 nm, while Se has the edge at ～550 nm. The three materials provide photoluminescence at visible wavelengths, with intensities of S and Te being stronger than that of Se by two orders. These optical properties imply that S and Te in the zeolite form small atomic units such as S3 and Te2, while Se condenses into single-chain structures.
... semiconductors organized as quantum dots or chains in zeolites channels. Semiconductor materials were encapsulated in synthetic zeolites such as AlPO 4 -5, zeolites A and Y [1][2][3] with their uniform and large channels (>7 A), but also the naturally occurring mordenite with its compressed channels (7 · 6.5 A) was successfully used for molecular organization of atoms and molecules. ...
Single crystals of self-synthesized mordenite-Na were used for incorporation of elemental selenium. The mordenite sample was first dehydrated at 280 °C and selenium was subsequently incorporated as gas phase at 450 °C for 72 h. Bright orange-colored Se-loaded mordenite was quantitatively analyzed by an electron microprobe yielding Na6Al6Si42O96·[Se7.9]. X-ray data collection of mordenite-Na and Se-loaded mordenite-Na single-crystals were performed at 120 K with synchrotron radiation (λ=0.79946 Å) using the single-crystal diffraction line at SNBL (ESRF, Grenoble), where diffracted intensities were registered with a MAR image plate. The structures of mordenite-Na and Se-mordenite-Na were both refined in the monoclinic space group Cc converging at R1=5.25% (mordenite-Na), and R1=6.65% (Se-mordenite-Na). A strongly broadened Raman band at approximately 254 cm−1 confirmed the existence of Se chains in the 12-membered channels along the c-axis. Several, low-populated, disordered Se chains with a length up to 10 Å and seven Se atoms were located in the large mordenite channels. During structure refinement nearest and next nearest neighbor Se–Se distances were fixed at 2.34 and 3.62 Å, respectively. Other distances and angles remained unconstrained. Because of electrostatic interaction with the framework and influence of extraframework occupants such as Na+ and H2O molecules, the chains show different geometrical Se arrangement with highly variable dihedral angles. Any other Se species such as Se6 or Se8 rings were neither confirmed by structure refinement nor by Raman spectroscopy. There was no indication of a trigonal Se chain geometry within the 12-membered ring channel.
... This is not the case for some other zeolites, for example zeolite A after dehydration and infilling with Se displays a high optical quality permitting it to perform detailed polarized Raman measurements. 12 Optical quality of Can-Se samples can be significantly improved if the crystals are prepared by the direct hydrothermal synthesis ͓we call them Can-Se͑s͔͒. Another important point is that the Can-Se samples with different parameters prepared by different methods are needed for better understanding of processes taking place in nanochannels. ...
Cancrinite crystals possessing parallel nanochannels are attractive for incorporation of guest materials and preparation of one-dimensional structures. In this work, we study variety of cancrinite crystals synthesized with Se inside their channels. Single crystal x-ray diffraction, polarized Raman, optical absorption, and luminescence spectra are investigated. It is shown that Se is stabilized in the form of Se22− and Se2− dimers located in the center of the channel and oriented along the channel. Different absolute and relative concentrations of Se22− and Se2− are obtained for different samples. The Se22− dimers at high concentration show tendency to organize linear chains. At low temperatures, quite strong interdimer bonding for both Se22− and Se2− is observed. Another important low-temperature effect is appearance of additional Raman bands, which are attributed to the vibrations of linear Se22− chains distorted by the incommensurate potential of cancrinite. Strong near-infrared polarized luminescence is observed for all samples. Photoionization of dimers is shown to be important step in the mechanism of the luminescence. © 2002 American Institute of Physics.
... Second, a set of intense positive-going bands emerged in the difference spectrum at 153, 220, 440, and 472 cm -1 . Based on extensive Raman spectroscopic studies of polymeric sulfur compounds [19][20][21][22], we assigned the observed low frequency modes primarily to S 8 rings. The intensive 472 cm -1 peak was demonstrated to be characteristic of S 8 rings and belong to S-S symmetric stretching vibration [21], while the lower intensity 222 and 154 cm -1 bands were assigned to S-S-S symmetric and asymmetric bending modes, respectively [20]. ...
... Based on extensive Raman spectroscopic studies of polymeric sulfur compounds [19][20][21][22], we assigned the observed low frequency modes primarily to S 8 rings. The intensive 472 cm -1 peak was demonstrated to be characteristic of S 8 rings and belong to S-S symmetric stretching vibration [21], while the lower intensity 222 and 154 cm -1 bands were assigned to S-S-S symmetric and asymmetric bending modes, respectively [20]. It should be noted that some contribution to the observed FT-Raman spectrum from polymeric linear sulfur species and polysulfides was also possible. ...
1 FT-Raman spectroscopy was applied for adsorption studies of anions at va-rious anion-exchange membranes (ACS, AMX, AMI-7001, and MA-40) from an aqueous modified fixer solution containing Br – , S 2 O 3 2– , SO 4 2– , and SO 3 2– ions. Difference spectra constructed from membranes incubated in the modi-fied fixer and 1M KBr reference solutions revealed presence of bands corres-ponding to internal vibrations of S 2 O 3 2– and SO 4 2– anions. A shift of S-O symmetric stretching vibration to higher frequencies compared with the ion in solution, and an opposite shift of stretching S-S mode evidenced coordination of S 2 O 3 2– ion with ACS, AMX, and AMI-7001 membranes through the S atom. Formation of sulfur clusters, predominantly in the form of S 8 rings, inside the MA-40 membrane was observed. The present results demonstrate a potential of FT-Raman spectroscopy for probing the competition adsorption and coordination of anions at anion-exchange membranes.
Recently, LTA-Se(1–8) samples with 1–8 Se atoms per cavity (simplified unit cell, large cavity + sodalite cage) obtained via adsorption at the temperature of ∼450 °C were reported. It was shown that single Se8 or single Se12 ring are formed in the large LTA cavities, Se8/Se12 ring concentration ratio decreasing with an increase in the Se loading density. Contrary, in the present work, using Se vapour adsorption at ∼550 °C, we succeeded in encapsulation of ∼17 Se atoms per cavity (LTA-Se(17)) with a significant increase in the Se8/Se12 concentration ratio manifesting double Se8-ring cluster formation in the most of the LTA large cavities, which is a step towards cluster crystal fabrication. According to our polarization/orientation Raman spectroscopic study of LTA-Se(17) single crystals, the orientations of the Se8 and Se12 appeared to be similar to those in previously investigated LTA-Se(1–8). Importantly, luminescent Se2⁻ anions, oriented along the LTA 4-fold axes and located in the sodalite cages, are detected via Raman polarization/orientation dependencies of LTA-Se(17). Bright Se2⁻ light emission with a maximum at ∼1.56 eV and vibronic structure is observed in the 1.3–1.8 eV spectral range. We show that the anions experience a compression in LTA which is slightly relaxing with a decrease in temperature producing an anomalous Raman band downshift. The compression of Se2⁻ in LTA is weaker/stronger than that in sodalite/cancrinite, luminescence band photon energy depending on its strength. High concentration of regularly arranged Se2⁻ in LTA suggests considering LTA-Se(17) as an important novel light-emitting material.
