Abstract:We report an analysis of the absorption edge anisotropy in CuGeO3 based on a comparison
of polarized optical data with electronic structure calculations in the framework of the
extended Hückel tight binding model. Taking the z axis as parallel to the CuO4 chains, we
ascribe the absorption edge in the magnetic chain direction to O 2p
x
, 2p
y
→ singly filled Cu
3d transitions and the edge in the transverse direction to O 2p
z
→ singly filled Cu 3d
excitations. The dual slope in the transverse direction is a d… Show more
“…At low temperature, the gap sharpens and moves to higher energy. The shape of the optical gap is similar to that of other synthetic metal materials. − 1 300 K absorption spectrum of C 8 -VONT. The inset shows a close-up view of one of the high-energy excitations, which changes modestly with sheet distance.…”
We measured the optical properties of mixed-valent vanadium oxide nanotubes to investigate the charge degrees of freedom in these novel materials. The electronic structure strongly resembles that of other bulk, low-dimensional, and molecular vanadates. The 1.2-eV band is assigned as a superposition of V 4+ d f d and V 4+ f V 5+ charge-transfer excitations, and the features above 3 eV are attributed to O 2p f V 3d charge-transfer excitations. The 5-eV excitation shows a modest sheet distance dependence, red-shifting with increasing sheet distance. We find that the optical gap is ∼0.56 eV at room temperature and ∼0.65 eV at 4.2 K. It does not depend systematically on tube size. At the same time, selected V-O-V stretching modes sharpen and red-shift with increasing sheet distance. We attribute these trends to the microscopic manifestations of strain, which changes with curvature. A lowfrequency mode is observed at 113 cm -1 , which is assigned as the radial breathing mode of the VO x nanotubes.
“…At low temperature, the gap sharpens and moves to higher energy. The shape of the optical gap is similar to that of other synthetic metal materials. − 1 300 K absorption spectrum of C 8 -VONT. The inset shows a close-up view of one of the high-energy excitations, which changes modestly with sheet distance.…”
We measured the optical properties of mixed-valent vanadium oxide nanotubes to investigate the charge degrees of freedom in these novel materials. The electronic structure strongly resembles that of other bulk, low-dimensional, and molecular vanadates. The 1.2-eV band is assigned as a superposition of V 4+ d f d and V 4+ f V 5+ charge-transfer excitations, and the features above 3 eV are attributed to O 2p f V 3d charge-transfer excitations. The 5-eV excitation shows a modest sheet distance dependence, red-shifting with increasing sheet distance. We find that the optical gap is ∼0.56 eV at room temperature and ∼0.65 eV at 4.2 K. It does not depend systematically on tube size. At the same time, selected V-O-V stretching modes sharpen and red-shift with increasing sheet distance. We attribute these trends to the microscopic manifestations of strain, which changes with curvature. A lowfrequency mode is observed at 113 cm -1 , which is assigned as the radial breathing mode of the VO x nanotubes.
“…Electronic structure calculations were carried out using the extended Hückel tight-binding method. This method has been shown to be successful in the past for understanding the behavior of other transition metal oxides. − …”
We report the electronic and vibrational properties of Na 2 V 3 O 7 nanotubes and compare the response with other layered and nonlayered vanadates. The electronic structure of Na 2 V 3 O 7 displays a strong similarity to that of nontubular vanadates. We assign the 1.2-eV band as a V d f d excitation and the 3.3-and 3.9-eV bands as O 2p f V 3d charge-transfer structures. Although band structure calculations predict additional fine structure in the density of states because of the tubular morphology, such features are not observed in the absorption spectrum. The vibrational spectrum displays triplet mode splitting as a result of reduced site symmetry, consistent with the three sightly different vanadium atoms that form the basic structural unit. A low-frequency rattling mode is observed in Na 2 V 3 O 7 at 88 cm -1 . This unique characteristic of the Na + -ion intercalated tubes might be connected with the ionic conductivity.
“…It is slightly lower for the dehydrated compounds, giving 3.52 and 3.47 eV, respectively (Table 1). For comparison, Rudko [24] observed an absorption edge near 3.5 eV for the charge transfer CuO (tenorite) [25] and CuGeO 3 , respectively [26].…”
Section: Resultsmentioning
confidence: 99%
“…It is slightly lower for the dehydrated compounds, giving 3.52 and 3.47 eV, respectively (Table 1). For comparison, Rudko [24] observed an absorption edge near 3.5 eV for the charge transfer insulator CuGeO 3 . The absorption structures at high energy just before the energy gap may be attributed to simultaneous ligand field transitions involving both metal centres of the dimer at twice the monomer transition energy (SPE), because their oscillator strengths are too weak for charge transfer (CT) transitions.…”
Low-dimensional quantum spin systems with the Cu 2+ central ion are still in the focus of experimental and theoretical research. Here is reported on growth of mm-sized single-crystals of the low-dimensional S = 1/2 spin compound Cu 6 (Ge,Si) 6 O 18 ·6H 2 O by a diffusion technique in aqueous solution. A route to form Si-rich crystals down to possible dioptase, the pure silicate, is discussed.Motivated by previously reported incorrect assignments of UV-VIS spectra, the assignment of dd excitations from such spectra of the hexahydrate and the fully dehydrated compound is proposed in comparison to dioptase and selected Cu(II) oxo-compounds using bond strength considerations. Non-doped cuprates as layer compounds show higher excitation energies than the title compound. However, when the antiferromagnetic interaction energy as J z ·ln (2)
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