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Charge transfer in lunar materials - Interpretation of ultraviolet-visible spectral properties of the moon
... Ti 4+ ion has an electronic configuration of 3d 0 and does not demonstrate absorption bands due to d-d transitions in the ligand field. Ti 4+ ions participate in O-Ti 4+ charge transfer bands positioning in the UV range of spectrum at~300 nm [54] and in intervalence charge transfer Ti 4+ -Ti 3+ transitions responsible for coloration in the visible spectral range [55]. Ti 3+ ions have an electronic configuration of 3d 1 and demonstrate one broad band in the visible range of spectrum due to the 2 T 2g → E g transition of the Ti 3+ ions in octahedral symmetry. ...
... The absorption band of Ti 3+ ion in tetrahedral (T d ) symmetry caused by the 2 T 2g → E g transition is located in the near-infrared region of the spectra [55]. The O-Ti 3+ charge transfer band is expected in UV range of spectrum at~240 nm [54]. ...
... The same raw materials were used for the preparation of both glasses, which means that the iron content in both glasses was similar. The absorption edge in precursor glass and in glass-ceramics originates from O 2− →Ti 4+ and O 2− →Ti 3+ charge transfer bands [54]. Taking into account that the O 2− →Ti 4+ charge transfer band is located at longer wave-lengths than the O 2− →Ti 3+ one, it can be suggested that the content of Ti 4+ ions in the LAS and the LAS ox glasses is very similar, i.e., the content of Ti 3+ in the LAS glass is rather low. ...
Lithium aluminosilicate glasses nucleated by TiO2 are usually melted in oxidizing conditions. The reducing conditions of glass melting, which allow to obtain ions of variable valence in lower oxidation states, can influence the ability of titania to provide proper phase assemblage, structure and properties of lithium aluminosilicate glass-ceramics. The aim of this study is to reveal this influence. The model glass containing TiO2 was melted with and without the addition of As2O3. Using heat treatments between 680 °C and 1300 °C, XRD, SEM and DSC data, Raman and absorption spectroscopy, transparent glass-ceramics based on nanocrystals of β-quartz solid solutions (ss) and/or γ-Al2O3 with spinel structure and opaque glass-ceramics based on nanocrystals of β-spodumene ss were obtained and characterized. Three-phase immiscibility develops during secondary heat treatments. Al2TiO5 crystallizes from aluminotitanate amorphous regions simultaneously with the appearance of β-quartz ss, while traces of anatase and then rutile appear at elevated temperatures. Phase assemblage and the sequence of phase transformations do not depend on the redox conditions of glass melting, while the rate of these transformations is significantly higher in glass melted without the addition of As2O3. Absorption in glass melted without the addition of As2O3 and the corresponding glass-ceramics originate from octahedrally coordinated Ti³⁺ ions and Ti³⁺-Ti⁴⁺ pairs in glass and nanocrystals of γ-Al2O3, Al2TiO5 and β-quartz ss. Transparent glass-ceramics with a thermal expansion coefficient of ~0.3 × 10⁻⁶ K⁻¹ were obtained from both glasses.
... Ti 4+ ion has electronic configuration 3d 0 and does not demonstrate absorption bands due to d-d transitions in the ligand field. Ti 4+ ions participate in O-Ti 4+ charge transfer bands located in the UV spectral range at ~300 nm [68] and in intervalence charge transfer Ti 4+ -Ti 3+ transitions responsible for coloration in the visible spectral range [69]. Ti 3+ ions have electronic configuration 3d 1 and demonstrate one broad absorption band in the visible spectral range due to the 2 T2g → Eg transition of the Ti 3+ ions in octahedral site symmetry. ...
... The absorption band of Ti 3+ ion in tetrahedral (Td) coordination caused by the Eg → 2 T2g transition is located in the near infrared region of the spectra [69]. The O-Ti 3+ charge transfer band is expected in UV spectral range at ~240 nm [68]. ...
... We used the same raw materials for the preparation of both glasses, which means that the iron content in both glasses was similar. The absorption edge in initial glasses and in glass-ceramics is formed by O 2-→Ti 4+ and O 2-→Ti 3+ charge transfer bands [68]. Taking into account that O 2-→Ti 4+ charge transfer band is located at longer wavelengths than the O 2-→Ti 3+ one, we may suggest that the content 19 of Ti 4+ ions in the LAS and the LASox glasses is very similar, i.e., the content of Ti 3+ in the LAS glass is rather low. ...
TiO2 is an effective nucleating agent to obtain glass-ceramics of the lithium aluminosilicate sys-tem. Reducing conditions of glass melting, which allow to get ions of variable valence in lower oxidation state, can influence the ability of titania to provide proper phase assemblage, structure and properties of lithium aluminosilicate glass-ceramics. The model glass nucleated by TiO2 was melted with and without addition of As2O3. Using heat-treatments from 680° to 1300 °C, XRD, SEM and DSC data, Raman and absorption spectroscopy, transparent glass-ceramics based on nanocrystals of β-quartz and/or γ-Al2O3 with spinel structure and opaque glass-ceramics based on nanocrystals of β-spodumene were obtained and characterized. Three-phase immiscibility develops during secondary heat-treatments. Al2TiO5 crystallizes from aluminotitanate amor-phous regions simultaneously with appearance of β-quartz solid solutions, while traces of ana-tase and then rutile appear at elevated temperatures. Phase assemblage and sequence of phase transformations are independent of the redox conditions of glass melting, while the rate of these transformations is significantly higher in glass melted without addition of As2O3. Absorption in the visible and near-IR spectral ranges in glass melted without addition of As2O3 and corre-sponding glass-ceramics originates from octahedrally coordinated Ti3+ ions and Ti3+-Ti4+ pairs in glass and nanocrystals of γ-Al2O3, Al2TiO5 and β-quartz. Transparent glass-ceramics with thermal expansion coefficient of ~0.3 × 10-6 K-1 were obtained.
... The absorption edge and absorption in the visible spectral range, up to 500 nm, is caused by various color centers giving rise to absorption bands of different origins. These include oxygen-metal charge transfer (OMCT) bands caused by O-Ti 3+ (at ~240 nm [34]), O-Ti 4+ (at ~300 nm [34]), O-Fe 2+ (at ~235 nm) and O-Fe 3+ (at ~270 nm), overlapping with bands located at longer wavelengths. These bands from 480 to 625 nm correspond to the 2 T2g→Eg transition of Ti 3+ ion in the ligand field (LF) in octahedral symmetry (Oh) [35], [36]. ...
... The absorption edge and absorption in the visible spectral range, up to 500 nm, is caused by various color centers giving rise to absorption bands of different origins. These include oxygen-metal charge transfer (OMCT) bands caused by O-Ti 3+ (at ~240 nm [34]), O-Ti 4+ (at ~300 nm [34]), O-Fe 2+ (at ~235 nm) and O-Fe 3+ (at ~270 nm), overlapping with bands located at longer wavelengths. These bands from 480 to 625 nm correspond to the 2 T2g→Eg transition of Ti 3+ ion in the ligand field (LF) in octahedral symmetry (Oh) [35], [36]. ...
Transparent glass-ceramics based on Fe2+:Mg-petalite and/or Fe2+:MgAl2O4 nanocrystals were obtained from the initial glass by single and two-stage heat-treatments at temperatures from 800 to 1000 °C. ZrTiO4 and spinel crystallized during the DSC scan up to 1000 °C. Spinel nanocrystals 9-12 nm in size also appeared during single and two-stage heat-treatments at temperatures of 850 - 1000 °C. Mg-petalite crystallites ~30 nm in size evolved in the narrow temperature range from 850 to 900 °C during single-stage holding periods. A maximum fraction of Mg-petalite crystallized at 850 °C. Once formed, Mg-petalite is preserved upon further heating and holding even at 1000 °C for 6 h. Mg-petalite and spinel transformed into sapphirine and highly siliceous residual glass during heating at 1100 °C. Competing crystallization mechanisms are discussed. In materials with a weakly developed liquid-liquid phase-separated structure, crystallization of Mg-petalite from the magnesium aluminosilicate glass predominates, and spinel becomes an additional phase. Spinel crystallizes as the main phase from glasses with the developed liquid-liquid phase-separated structure. Its crystallization is accompanied by the formation of highly siliceous glass, from which Mg-petalite crystallization is impossible. Intense absorption band with maximum at ~1.9 μm due to fourfold coordinated Fe2+ ions in spinel nanocrystals is used as a spectral indicator of spinel formation. Glass-ceramics are relevant for the development of saturable absorbers intended for lasers operating at 1.6-2.4 μm.
... To identify ilmenite-rich spectra, we focused on the 1-and 2-μm reflectance features of ilmenite, which are produced by broad absorption bands between 0.5 μm and 0.6 μm because of the presence of Fe 2+ , Ti 3+ , and Ti 4+ and between 1.3 μm and 1.6 μm because of Fe 2+ Adams & McCord, 1971;Isaacson et al., 2011;Izawa et al., 2021;Loeffler et al., 1974). Notably, an ilmenite containing Ti 3+ shows a maximum reflectance (a 1-μm peak) at λ = 1.0 μm (Izawa et al., 2021;Robertson et al., 2022). ...
... Indeed, ilmenite has diagnostic spectral features in the visible to near-infrared wavelength region. One feature is a 1-μm peak, and the other feature is increased reflectance toward 2 μm; these features are accounted for by broad absorption bands between 0.5 μm and 0.6 μm as a result of the presence of Fe 2+ , Ti 3+ , and Ti 4+ and between 1.3 and 1.6 μm as a result of the presence of Fe 2+ Adams & McCord, 1971;Isaacson et al., 2011;Izawa et al., 2021;Loeffler et al., 1974;Riner et al., 2009;Wagner et al., 1987). Indeed, Surkov et al. (2020) conducted a feasibility study of ilmenite abundance mapping using Moon Mineralogy Mapper (M3) data based on an ilmenite absorption band at ∼ 1.6 μm due to Fe 2+ . ...
We studied the global distribution and geological features of lunar surface sites whose spectra indicate an ilmenite‐rich composition. Hyperspectral data obtained by the Kaguya Spectral Profiler were used for data mining to identify diagnostic features of a 1‐ and 2‐μ {\upmu }m spectral reflectance of ilmenite, revealing the global distribution of sites showing ilmenite‐rich spectra. The results show that regions with ilmenite‐rich spectra are concentrated at the margins of impact basins on the lunar nearside, whereas no such regions are identified in the Feldspathic Highland Terrain or the South Pole‐Aitken basin. Using multiband images and a digital terrain model obtained by the Kaguya Multiband Imager and Terrain Camera, we examined the geological features of each site showing ilmenite‐rich spectra and found that most of the sites are distributed on pyroclastic deposits overlying highland materials. Spectra interpreted as glass‐rich material are prevalent in and around areas having ilmenite‐rich spectra. However, sites showing ilmenite‐rich spectra do not correspond to mare regions with TiO2 ‐rich basalts. These results may indicate that the concentration of ilmenite in pyroclastic deposits is high enough to exhibit diagnostic features of 1‐ and 2‐μ {\upmu }m spectral reflectance of ilmenite, whereas the concentration in mare regions with TiO2 ‐rich basalt is not. Since pyroclastic deposits are expected to be extensive, deep unconsolidated deposits of relatively block‐free debris, resulting in high processing efficiency in the hydrogen reduction processes, our data may be useful for developing an efficient exploration strategy for ilmenite as a lunar resource.
... Calculated molecular orbitals for FeO 6 10− (Fe 2+ ) and FeO 6 9− (Fe 3+ ) clusters showed thresholds of 4.6 eV (~ 37,000 cm −1 ) and 3.1 eV (25,000 cm −1 ), respectively, for the fully allowed lowest-energy transitions identified as the 6t 1u(↓) → 2t 2g(↑) oxygen-to-metal charge transfer (Abragam and Bleaney 2012). Similar calculations showed that the oxygen-tometal charge-transfer transitions can be ordered in ascending energy as: Fe 3+ oct < Fe 3+ tet < Ti 4+ oct < Fe 2+ oct < Ti 3+ oct , where the indexes oct, and tet, stand for octahedral and tetrahedral metal-oxygen clusters, respectively (Loeffler et al. 1974). ...
... This would be entirely consistent with a ligand to Fe 3+ charge-transfer transition. For the BTE sample (Fig. 3b), on the other hand, a second band onset can also be clearly observed above 3.7 eV and could be, in principle, related to FeO 4 5− (Fe 3+ ), TiO 6 8− (Ti 4+ ) and FeO 6 10− (Fe 2+ ) chargetransfer transitions (Loeffler et al. 1974). Since this 3.7 eV band is too far in the UV, it has almost no effect on the color and, thus, is deliberately left out of the discussion herein. ...
The origin of gamma irradiation-induced strong yellow color in brazilianite from Brazil is investigated. The irradiation-induced optical absorption band responsible for the color shows an onset in the blue spectral region at about 2.5 eV and maximum centered in the UV at ~ 4.24 eV. From the ratio between the squared value of the line width (W2) and the peak energy (M), a value of about 0.09 eV is estimated, which is consistent with an absorption band caused by a Schirmer´s-type O− bound small polaron. By electron paramagnetic resonance (EPR), we are able to confirm its microscopic structure. This O− hole center is in fact the Al3+–O−X2+–P5+ hole center already identified by EPR earlier in the literature (where X2+ stands for a nearby divalent cation with negligible abundance of magnetic isotopes). The EPR spectrum of the Al3+–O−X2+–P5+ hole center, along with the O− bound small polaron absorption band responsible for the yellow color, appears simultaneously at high concentrations after gamma irradiation, and vanishes together for thermal annealing above 300 °C, returning after re-irradiation in a reversible way. Their appearance is concomitant with the H0 centers and Ti3+ electron centers, and possible charge-compensating centers.
... Albedo the high-temperature silicates has two strongest ABs in the 350-1100-nm range: the UV band centered at 200 nm, which is induced by oxygen-to-metal electron charge transfer (see, e.g., Loeffler et al., 1974), and the combined AB of pyroxene and olivine centered at 1000 nm, which is caused by electronic transitions in Fe 2+ in the crystal field of minerals (Platonov, 1976;Bakhtin, 1985;Burns, 1993). On the contrary, in the reflectance spectrum of the low-temperature silicates (most often, hydrated), the latter AB is absent, but there is an AB caused by intervalence charge transfer Fe 2+ → Fe 3+ with a center at 700 nm. ...
Physical parameters and characteristics of asteroids as solid atmosphereless celestial bodies are traditionally studied with the same methods as those used for investigating most of the other celestial objects, though they have certain specific features. The main attention is paid to spectrophotometry, as the most effective tool to study remotely the composition, evolution, and origin of asteroids. However, very important information about asteroids was also obtained by other observational methods, such as photometry, polarimetry, radiometry, and radar. Because of this, in addition to spectrophotometry, we discuss here photometry, polarimetry, and radiometry, which, on the one hand, are very close in methodology and, on the other hand, there has been a trend to their integrated use. In connection with the discovery of sublimation–dust activity on a number of asteroids and the periodic formation of a dust exosphere around these asteroids near perihe lion (see, e.g., Busarev et al., 2021), we also consider a methodologically new approach to estimating the chemical and mineralogical composition of particles in the exosphere of these asteroids and, indirectly, of their surface material.
... The band due to the E g → 2 T 2 g transition of Ti 3+ ions in tetrahedral (T d ) site symmetry is located at ~1000 nm [85,86]. The O-Ti 3+ oxygen to metal charge transfer (OMCT) band is in the UV spectral range at ~240 nm [87]. ...
In order to develop glass-ceramics containing ions of variable valence in lower oxidation states, it is important to know how changing the redox conditions of glass melting affects structure and properties of glass-ceramics. The zinc aluminosilicate glass nucleated by TiO2 was melted with and without addition of As2O3 and heat-treated from 720° to 1350°C to obtain gahnite-based glass-ceramics. DSC, XRD analysis, Raman spectroscopy and TEM studies revealed that variation of glass melting redox conditions affects kinetics of liquid phase separation and rutile crystallization, composition and structure of gahnite and rutile, crystallization of the residual glass, and does not affect kinetics of gahnite crystallization, gahnite and rutile fractions and structure of glass-ceramics. In glass-ceramics prepared from glasses melted without As2O3, absorption in the visible and near-IR spectral ranges is due to octahedrally coordinated Ti3+ ions in gahnite nanocrystals. The study is important for development of rare-earth-free phosphors.
... Albedo the high-temperature silicates has two strongest ABs in the 350-1100-nm range: the UV band centered at 200 nm, which is induced by oxygen-to-metal electron charge transfer (see, e.g., Loeffler et al., 1974), and the combined AB of pyroxene and olivine centered at 1000 nm, which is caused by electronic transitions in Fe 2+ in the crystal field of minerals (Platonov, 1976;Bakhtin, 1985;Burns, 1993). On the contrary, in the reflectance spectrum of the low-temperature silicates (most often, hydrated), the latter AB is absent, but there is an AB caused by intervalence charge transfer Fe 2+ → Fe 3+ with a center at 700 nm. ...
Physical parameters and characteristics of asteroids as solid atmosphereless celestial bodies are traditionally studied with the same methods as those used for investigating most of the other celestial objects, though they have certain specific features. The main attention is paid to spectrophotometry, as the most effective tool to study remotely the composition, evolution, and origin of asteroids. However, very important information about asteroids was also obtained by other observational methods, such as photometry, polarim-etry, radiometry, and radar. Because of this, in addition to spectrophotometry, we discuss here photometry, polarimetry, and radiometry, which, on the one hand, are very close in methodology and, on the other hand, there has been a trend to their integrated use. In connection with the discovery of sublimation-dust activity on a number of asteroids and the periodic formation of a dust exosphere around these asteroids near perihelion (see, e.g., Busarev et al., 2021), we also consider a methodologically new approach to estimating the chemical and mineralogical composition of particles in the exosphere of these asteroids and, indirectly, of their surface material.
We report on a comparative study of a transparent Fe:MgAl2O4 (spinel) ceramics and a transparent nanophase Fe:MgAl2O4-based glass-ceramics. The 0.1 mol% Fe:MgAl2O4 ceramics was synthesized by hot pressing (at 1500 °C/50 MPa) of powders obtained by the sol-gel method using LiF as a sintering aid. The Fe:MgAl2O4 ceramic is a single-phase material (cubic structure, sp. gr. Fd3‾m, a = 8.083 Å) with a mean grain size of ∼50 μm. The ceramic exhibits a broadband transparency of 0.2–6.0 μm and a high in-line transmission at ∼1 μm of 74.4%. The iron ions are presented in the ceramics in the single state of Fe²⁺ species in tetrahedral (Td) sites. A broad absorption band spanning from ∼1.2 to 3.7 μm assigned to the ⁵E → ⁵T2 (⁵D) transition of Fe²⁺ ions in Td sites is observed, corresponding to a ground-state absorption cross section of 0.28×10⁻¹⁸ cm² at 1.90 μm. The glass-ceramics were prepared by secondary two-stage heat-treatments of the magnesium aluminosilicate glass nucleated by titanium oxide and doped with 0.1 mol% FeO. Transparent Fe:MgAl2O4-based glass-ceramics obtained at the temperature of the second stage of 800 – 1000 °C were multi-phase materials containing two crystalline nanophases, i.e., spinel (mean size: 3.7 – 7.4 nm) and magnesium aluminotitanate solid solution (mean size: 6.4 - 20.6 nm), as well as residual silica-rich glass. Glass-ceramics obtained at the temperature of the second stage of 1050 °C were transparent and based on Fe-doped sapphirine. For glass-ceramics, absorption has a much more complex character as it is caused by interplay of iron and titanium ions in different valence states, coordination sites and locations. The iron ions enter the spinel nanocrystals but unlike the ceramic, in the form of both VIFe²⁺ and IVFe²⁺ species. The developed ceramics and glass-ceramics are promising for saturable absorbers of mid-infrared (2-3 μm) lasers.
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