EPR of substitutional and charge compensated Fe3+ in anatase (TiO2)*

doi:10.1016/S0022-3697(71)80099-9

Journal of Physics and Chemistry of Solids

Volume 32, Issue 11, 1971, Pages 2529-2538

https://doi.org/10.1016/S0022-3697(71)80099-9 Get rights and content

Abstract

Paramagnetic resonances were observed in natural single crystals of anatase and are interpreted as due to regular substitutional Fe³⁺(I) and to Fe³⁺ combined with an oxygen vacancy at a nearest neighbour site (II). The observed unusually strong temperature dependence of the spin-Hamiltonian parameter b₂⁰ of (I) and the temperature dependence of the orientation of the magnetic axes of the spectrum (II) are both explained by assuming a shift within the unit cell of the equilibrium positions of the oxygen ions with temperature.

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Magnetism and temperature dependence of nano-TiO<inf>2</inf>: Fe EPR spectra
2022, Materials Chemistry and Physics
Nanocrystalline TiO₂ samples of various compositions (up to 5 at.% Fe) with anatase structure and an average particle size of about 20 nm were synthesized using sol-gel technology. The magnetic properties of nano-TiO₂: Fe was studied by electron paramagnetic resonance (EPR) and Faraday balance. The existence of an inhomogeneous magnetic state in TiO₂: Fe samples of different compositions were revealed by EPR spectroscopy in a wide temperature range. The analysis of the EPR spectra in the L -, X - and Q-bands allowed us to calculate the quadratic fine structure (D, E) parameters, “axial” and “rhombic” ones, respectively. The value of D turned out to be quite small, which indicates an insignificant anisotropy, which can be ignored describing the magnetic properties of TiO₂:Fe. It was shown that the temperature behavior of different separate components of the integral EPR spectra can be qualitatively interpreted in the model of coexistence in the TiO₂: Fe system, mainly, dimers with a strong negative exchange interaction and isolated paramagnetic centers. No ferromagnetic state in TiO₂: Fe-based samples after etching of aE-prepared state were detected.
EPR and NEXAFS spectroscopy of BiNb<inf>1-x</inf>Fe<inf>x</inf>O<inf>4-δ</inf> ceramics
2019, Physica B: Condensed Matter
Citation Excerpt :
The values of D estimated by us for the Fe3+ ion positions in the triclinic lattice of bismuth niobate were much higher (0.72 and 0.47 cm−1) and were characterized by a relatively high rhombic distortion (E/D ∼ 0.2 and 0.1, respectively). Similar values of D and E were determined for the octahedral complexes of Fe3+ with vacancies of oxygen in the anatase and rutile TiO2 lattices [23]. The triclinic phase of BiNbO4 has two types of octahedral positions of Nb5+, which differ in the average FeO-bond length and in the degree of distortion of the polyhedron: the lengths of niobium-oxygen bonds (Nb(1)O) in the first octahedron vary from 0.18 nm to 0.231 nm and the average is 0.201 nm; in the second octahedron (Nb(2)–O), they vary from 0.18 nm to 0.223 nm with 0.198 nm in average [12].
The methods of EPR and NEXAFS spectroscopy were employed for the study of BiNb_1-xFe_xO_4-δ solid solutions of triclinic and orthorhombic modifications. The analysis of the NEXAFS Fe2p-spectra of iron oxides and the iron-containing solid solutions revealed that Fe atoms were mainly in an oxidation state of +3. In the EPR spectra of the BiNb_1-xFe_xO_4-δ samples of orthorhombic modification, a broad line with the center at g of 2.16–2.37 was registered along with the low-intensity signal with a g-factor of ∼4.3 observed on its low-field wing. EPR spectra of the triclinic structure samples exhibited a complex system of lines in the range of 80–300 mT. An intense complex line having features at g of 4.74–4.80, 4.04–4.10 and 3.76–3.80 was recognized in the low-field signal. The low-field wing of the line was complicated by a shoulder with g of 5.3 and by a line in the form of an absorption signal with g of 7.7–8.0. As a result of the modeling of EPR spectra, the spin-Hamiltonian parameters of Fe(III) in the structure of BiNb_1-xFe_xO_4-δ of triclinic modification were obtained.
Study of paramagnetic defect centers in as-grown and annealed TiO<inf>2</inf> anatase and rutile nanoparticles by a variableerature X-band and high-frequency (236 GHz) EPR
2016, Journal of Magnetism and Magnetic Materials
Citation Excerpt :
They are due to the Fe3+ ion in Ti4+ substitutional position, with (1 line, 160 mT) and without (4 lines) coupling to an oxygen vacancy in nearest vicinity. The EPR line at 160 mT is similar to that observed for charge-compensated Fe3+ ion in anatase powder [42]. Only one EPR line, due to adsorbed oxygen (O2−), was observed at 77 K at 324.5 mT, as shown in Fig. 11b.
Detailed EPR investigations on as-grown and annealed TiO₂ nanoparticles in the anatase and rutile phases were carried out at X-band (9.6 GHz) at 77, 120–300 K and at 236 GHz at 292 K. The analysis of EPR data for as-grown and annealed anatase and rutile samples revealed the presence of several paramagnetic centers: Ti³⁺, O⁻, adsorbed oxygen (O₂⁻) and oxygen vacancies. On the other hand, in as-grown rutile samples, there were observed EPR lines due to adsorbed oxygen (O₂⁻) and the Fe³⁺ ions in both Ti⁴⁺ substitutional positions, with and without coupling to an oxygen vacancy in the near neighborhood. Anatase nanoparticles were completely converted to rutile phase when annealed at 1000° C, exhibiting EPR spectra similar to those exhibited by the as-grown rutile nanoparticles. The high-frequency (236 GHz) EPR data on anatase and rutile samples, recorded in the region about g=2.0 exhibit resolved EPR lines, due to O⁻ and O₂⁻ ions enabling determination of their g-values with higher precision, as well as observation of hyperfine sextets due to Mn²⁺ and Mn⁴⁺ ions in anatase.
Structural characterization of titania by X-ray diffraction, photoacoustic, Raman spectroscopy and electron paramagnetic resonance spectroscopy
2015, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
A titania mineral (obtained from East coast, Orissa, India) was investigated by X-ray diffraction (XRD), photoacoustic spectroscopy (PAS), Raman and Electron Paramagnetic Resonance (EPR) studies. XRD studies indicated the presence of rutile (91%) and anatase (9%) phases in the mineral. Raman investigation supported this information. Both rutile and anatase phases have tetragonal structure (rutile: space group P4₂/mnm, a = 4.5946(1) Å, c = 2.9597(1) Å, V = 62.48(1) (Å)³, Z = 2; anatase: space group I4₁/amd, 3.7848(2) Å, 9.5098(11) Å, V = 136.22(2) (Å)³, Z = 4). The deconvoluted PAS spectrum showed nine peaks around 335, 370, 415,485, 555, 605, 659, 690,730 and 785 nm and according to the ligand field theory, these peaks were attributed to the presence of V⁴⁺, Cr³⁺, Mn⁴⁺ and Fe³⁺ species. EPR studies revealed the presence of transition metal ions V⁴⁺(d¹), Cr³⁺(d³), Mn⁴⁺(d³) and Fe³⁺(d⁵) at Ti⁴⁺ sites. The EPR spectra are characterized by very large crystal filed splitting (D term) and orthorhombic distortion term (E term) for multiple electron system (s > 1) suggesting that the transition metal ions substitute the Ti⁴⁺ in the lattice which is situated in distorted octahedral coordination of oxygen. The possible reasons for observation of unusually large D and E term in the EPR spectra of transition metal ions (S = 3/2 and 5/2) are discussed.
Modeling zero-field splitting parameters for dopant Mn 2+ and Fe 3+ ions in anatase TiO <inf>2</inf> crystal using superposition model analysis
2012, Chemical Physics Letters
Citation Excerpt :
The bond lengths Ti–O at the octahedron’s equatorial are slightly shorter than those at the octahedron’s apices. The interstitial sites differ from the substitutional ones only in the bond lengths [8], which are longer (0.2792 nm) at the apices along the [0 0 1]-axis (see Figure 1) than those for the substitutional sites (0.1966 nm). The unit cell parameters measured at room temperature [18–22] are given in Table 1.
The local environment around Fe³⁺ and Mn²⁺ ions doped into anatase TiO₂ crystal is investigated and interpretation of EPR data, suggesting remote charge compensation, is provided. Using superposition model, the zero-field splitting (ZFS) parameters are modeled for Fe³⁺ and Mn²⁺ at the substitutional Ti⁴⁺ and interstitial sites (D_2d). Several approaches are used to match the modeled $b_{2}^{0}$ ZFS parameter with experimental values and thus to discriminate between various structural models. Theoretical predictions of the $b_{4}^{q}$ parameters are also provided and tentative matching with experimental data is considered. The feasible values of the structural distortion parameters are predicted. The uniqueness and thus reliability of the predicted ZFS and distortion parameters are discussed.
Investigation of the EPR and local defect structures for (FeO<inf>6</inf>)9- and (MnO<inf>6</inf>)10- clusters in TiO<inf>2</inf> crystal at different temperature
2008, Chemical Physics Letters
Based on the 252 × 252 complete energy matrices for a d⁵ configuration ion in tetragonal ligand–field, the relationship between the defect structure and the EPR parameters $b_{2}^{0}$ , $b_{4}^{0}$ has been investigated. From our calculation, the local structure around impurity ion has been determined. Meanwhile, the temperature influence which varies with the kinds of doping ions on distortion has been discussed and the change curves of the zero-field splitting energies ΔE₁, ΔE₂ as functions of the local structure distortion parameter ΔR for TiO₂:Fe³⁺ and TiO₂:Mn²⁺ systems have been plotted.

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^*: Research support by the National Research Council of Canada.

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Journal of Physics and Chemistry of Solids

Abstract

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Cited by (23)

Magnetism and temperature dependence of nano-TiO<inf>2</inf>: Fe EPR spectra

EPR and NEXAFS spectroscopy of BiNb<inf>1-x</inf>Fe<inf>x</inf>O<inf>4-δ</inf> ceramics

Study of paramagnetic defect centers in as-grown and annealed TiO<inf>2</inf> anatase and rutile nanoparticles by a variableerature X-band and high-frequency (236 GHz) EPR

Structural characterization of titania by X-ray diffraction, photoacoustic, Raman spectroscopy and electron paramagnetic resonance spectroscopy

Modeling zero-field splitting parameters for dopant Mn <sup>2+</sup> and Fe <sup>3+</sup> ions in anatase TiO <inf>2</inf> crystal using superposition model analysis

Investigation of the EPR and local defect structures for (FeO<inf>6</inf>)<sup>9-</sup> and (MnO<inf>6</inf>)<sup>10-</sup> clusters in TiO<inf>2</inf> crystal at different temperature

EPR of substitutional and charge compensated Fe3+ in anatase (TiO2)**

Abstract

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Trans. Faraday Soc.

Can. J. Phys.

Trans Faraday Soc.

EPR of substitutional and charge compensated Fe³⁺ in anatase (TiO₂)^**