The shape and extended fine structure of the manganese K x-ray absorption discontinuity in some divalent compounds

We report in the present paper the shape and extended fine structure of the manganese K absorption discontinuity in four octahedral divalent compounds, viz. MnCl₂·4H₂O, MnSO₄·H₂O, MnC₂O₄·2H₂O and α-MnS. Molecular orbital theory is used to interpret the various features of the main absorption discontinuity. The extended X-ray absorption fine structure (EXAFS) is interpreted with the help of the graphical method given by Lytle et al.     

Local structure of Mn in (Ga,Mn)As probed by X-ray absorption spectroscopy

Local structure of Mn atoms in Ga_1−xMn_xAs epilayers was studied using the X-ray absorption fine structure (XAFS) at Mn K-edge. X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques were used. XAFS spectra for different Mn sites has been calculated and compared with the experimental data. Multi-parameter fitting of the EXAFS data indicates that 15-25% of Mn atoms are in interstitial sites in the as grown films. The Mn-As bond length has been found longer than Ga-As bond length in GaAs for both substitutional (Mn_Ga) and interstitial (Mn_I) sites.     

Homogeneity regions in manganites of rare-earth elements

Based on X-ray powder diffraction analysis of homogeneous phases and heterogeneous compositions with the general formula Ln_2?x Mn _x O_3±δ (Ln = Sc, Pr, Nd, Sm, Eu; 0.90 ≤ x ≤ 1.20; Δx = 0.02), which were obtained by a ceramic synthesis from oxides in air in the temperature range 900–1400°C, the solubility boundaries of oxides Ln₂O₃ and manganese in LnMnO_3±δ are determined. The results are represented in the form of fragments of phase diagrams of the systems Sc-Mn-O, Pr-Mn-O, Nd-Mn-O, Sm-Mn-O, and Eu-Mn-O in air. It is assumed that the solubility of Ln₂O₃ oxides in LnMnO_3±δ is determined by defects of the crystalline structure, and that of manganese oxides is determined by the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺ with subsequent partial substitution of Ln³⁺ ions by divalent manganese in cuboctahedral sites of a perovskite-like crystal lattice.     

Mn-doped (Ba,Y)Fe12O19 hexaferrites: Crystal structure and oxidation states of Mn and Fe

We have fabricated Ba_0.95Y_0.05Fe_12-xMn_xO₁₉ samples with large Mn-doping amounts of x = 4 and 6, using the mechanical milling and heat treatment. X-ray diffraction analysis indicated the samples crystallized in the M-type hexaferrite structure. The Mn doping caused the modification, shift and broadening of some characteristic phonon-vibration modes, which were recorded by Raman spectroscopy. This is due to an incorporation of Mn ions into the M-type structure that disorders the periodic lattice and changes symmetry. Basing on X-ray absorption spectroscopy, we have found Fe in all samples stable with an oxidation state 3+ (Fe³⁺). Though Mn²⁺ and Mn³⁺ ions coexist, the concentration of Mn²⁺ in x = 4 is larger than that in x = 6. The analysis of Fourier-transform spectra have demonstrated the replacement of Mn^2+,3+ ions for Fe³⁺ in the M-type structure. The sites of Mn^2+,3+ ions in this structure have been discussed.     

Local surrounding of Mn in LaMn1−xCoxO3 compounds by means of EXAFS on Mn–K

A systematic study of LaMn_1?xCo_xO₃ perovskite series by means of X-ray absorption spectroscopy in the extended X-ray absorption fine structure (EXAFS) range of the K-absorption edge of Mn is reported. The Mn–K edge absorption measurements in the EXAFS region were performed to study the local surrounding of Mn ions. Polycrystalline powder samples of LaMn_1?xCo_xO₃ (x=0, 0.02; 0.2; 0.4; 0.5; 0.6; 0.8) prepared by solid-state reaction were used. The EXAFS spectra were analyzed with the FEFF8 computer program. The Mn–O distances of Mn to the nearest oxygen surroundings were evaluated for the samples in the series and compared with the Co–O distances obtained by EXAFS in V. Procházka et al., JMMM 310 (2007) 197 and with results of X-ray powder diffraction in C. Autret, J. Phys. Condens. Matter 17 (2005) 1601.     

Luminescence of yttrium aluminum borate single crystals doped with manganese

The optical absorption and luminescence spectra of single crystals of yttrium aluminum borate YAl₃(BO₃)₄ doped with manganese ions are measured. It is established that the optical absorption spectra of yttrium aluminum borate single crystals doped with manganese ions are determined primarily by the contribution from the Mn⁴⁺ ions. The luminescence spectra of the YAl₃(BO₃)₄ single crystals doped with manganese exhibit narrow lines attributed to the Mn⁴⁺ ions and an extended multiband structure which is associated primarily with the contribution from the Mn²⁺ ions.     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Ab initio calculation and synchrotron X-ray spectroscopy investigations of tin oxides near the Sn <Emphasis Type="Italic">L</Emphasis><Subscript>3</Subscript> absorption edges

The results of theoretical and experimental investigations of the electron-energy structure of the conduction band of tin oxides have been presented. The Sn L ₃ X-ray absorption near-edge fine structure (XANES) has been calculated for the first time for single crystals of metallic tin and tin monoxide, as well as for the orthorhombic and tetragonal phases of tin dioxide, using the linearized augmented plane wave (LAPW) method. The fine structure of the XANES spectra has been compared with the specific features of the energy distribution of the local partial densities of states of the tin compounds under investigation. A joint analysis of the results of the simulation and the experimental X-ray synchrotron Sn L ₃ XANES spectra of commercial bulk samples of metallic tin and tin oxides SnO and SnO₂ has been carried out.     

Relation between charge ordering and local lattice disorder in manganites studied by EXAFS

We report extended X-ray absorption fine structure measurements at the Mn K absorption edge and in a wide temperature range on La_0.25Ca_0.75MnO₃ samples with a charge ordering transition temperature, T_CO of the Mn³⁺ and Mn⁴⁺ ions around 225 K. The mean Mn-O distances do not show major changes at the transition; however, our results show the presence of an anomalous local lattice disorder and distortion below T_CO, possibly determined by the breaking of the symmetries of the high temperature state, that is the splitting of the Mn-O distances.     

Chemical and structural properties of ternary post-transition metal oxide thin films: InZnO,InGaO and GaZnO

The chemical states of ternary post-transition metal oxide thin films of InGaO, GaZnO and InZnO were investigated using X-ray photoelectron spectroscopy. Detailed binding energy (BE) analyses revealed certain evolution in chemistry in the ternary oxides compared to the reference binary oxides of In₂O₃, ZnO, or Ga₂O₃. In particular, O 1s BEs were changed with the compositions, which suggests that the charge transfer (CT) between In³⁺/Ga³⁺/Zn²⁺ and O²⁻ ions is significant. Results of extended X-ray absorption fine structure analyses further showed that the first shell coordination (cation–O bond) is roughly maintained even though the ternary oxide films were structurally disordered. This implies that the CT process via O²⁻ ions can influence the charge reconstructions in the ternary oxide systems.     

Theoretical considerations regarding the defect structure of M1−xO cubic oxides for small departures from stoichiometry

This study examines the discrepancy between two models of defect structure proposed for M_1?xO cubic oxides with small departures from stoichiometry. One, involving vacancy aggregates such as 4:1 or 6:2 clusters is deduced by Callow et al. from the calculated defect formation energies. The other, involving free unassociated vacancies V″_M and V′_M is used by many authors to interpret their experimental results.In the particular case of M_1?xO, for which accurate thermodynamic data are available for 1273 K< T < 1423 K, it is shown that, between these two models, only the latter is consistent with the experimental ΔG(O₂) values and that the concentrations of clusters are negligible over the whole homogeneity range. Furthermore, the unassociated defect formation energies calculated by Catlow et al. [27] do not satisfactorily account for the experimental values of ΔH(O₂).     

Luminescence and vibration spectra of zinc manganese diphosphates

The synthesis conditions for new luminescent materials, zinc manganese diphosphates Zn_{2? x}Mn_xP₂O₇ · 5H₂O) (0 ≤ x ≤ 2.0), are considered. The photoluminescence and its excitation spectra, IR absorption, and Raman spectra of these materials are studied. The red luminescence band with a peak at about 700 nm is shown to be due to the radiative transitions in Mn²⁺ ions, which are set in an octahedral oxygen environment. The range of concentration quenching of Mn²⁺ ion radiation is determined. Correlation of the luminescent and vibration properties of these compounds is discussed.     

球形α-Fe2O3超微粒的红外光学特性和表面模吸收

用改进的水解法制备了大小均匀、形状规则的球状α-Fe₂O₃超微粒;分别测量了两种不同粒度超微粒的红外透射谱;观测到了球状α-Fe₂O₃超微粒的全部表面模;讨论了微粒的聚集对吸收的影响;并用柱状微晶来近似链状聚集解释了光谱中的两个展宽吸收带;介绍了将多原子立方结构超微粒的表面模理论推广应用到单轴晶体结构的α-Fe₂O₃超微粒的情况。对表面模吸收频率进行了计算,理论数据同实验结果 关键词：     

EPR of Mn4+ in LiF:U,Mn

A new manganese center in LiF single crystals doped with U₃O₈ and MnO₂ has been observed at room and liquid nitrogen temperatures. The trigonal EPR spectrum consists of one very anisotropic fine structure transition with six well-resolved hyperfine components. An analysis of the spectra recorded at X and Q bands shows that the zero field splitting is larger than the electronic Zeeman interaction.By comparing a theoretical calculation of the g values and the hyperfine structure with the experimental X and Q band data it was concluded that the ion is Mn⁴⁺. The presence of Mn⁴⁺ in these crystals confirms an assumption of Runciman regarding the possibility of introducing high valency ions in alkali fluorides containing uranium with an excess of oxygen.     

Investigation of Rhodamine 6G in oleic acid solution fluorescence under high pressure

Interesting behavior has been observed in the absorption spectra of the solution of Rhodamine 6G [C₂₈H₃₁N₂O₃Cl] in oleic acid [C₁₈H₃₄O₂] [K. Wieja et al., Pressure-induced changes in electronic absorption spectrum in oleic acid, High Press. Res. 30 (2010), pp. 130–134]. Application of high pressures to the solution has caused the reshaping of the absorption spectra curve, and the maximum of absorption has been shifted towards the longer wavelength. One of the most possible explanations of the changes in the absorption spectra is the formation of charge-transfer complexes in the solution. Pressure-induced changes of the VIS absorption spectrum in the mixture of Rhodamine 6G and oleic acid have been observed in comparison with the absorption spectrum of the mixture of Rhodamine 6G in ethanol. Moreover, changes have been observed in the fluorescence spectrum of the examined mixture, which indicated a fluorescent electronic band of a CT complex.     

Spectral studies on Mn ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption, and luminescence spectral studies of Mn²⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xMnO₂ (x=1.0, 2.0, 3.0, 4.0, and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g value at g_eff≈2.02 with six line hyperfine structure. A weak resonance signal with effective g value at g_eff≈4.3 is also observed for higher concentrations of Mn²⁺ ions. The EPR spectra of x =3.0 mol% of Mn²⁺ in sodium-lead borophosphate glass sample have been studied at various temperatures. It is observed that the resonance signal intensity decreases with increase in temperature. The optical absorption spectrum exhibits bands characteristic of Mn²⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibits single broad band in the green region.     

Multiplet splitting patterns exhibited by the first row transition metal oxides in X-ray photoelectron spectroscopy

High resolution photoelectron spectra from transition metal ions in TiO₂, V₂O₅, VO₂, V₂O₃, MnO, Mn₂O₃, MnO₂, Cr₂O₃, FeO, CoO, NiO, CuO, Cu₂O, FeSrO₃, and Cu doped CaTiO₃ were re-examined using a constrained curve fitting approach. Effective fits of the multiplet splitting present could be attained for the oxides with unfilled 3d bands if multiple final states were assumed. The type of transitions implied, also suggest the classification of these oxides during core level photoelectron emission as either; Mott–Hubbard (V₂O₅, VO₂, V₂O₃, Cr₂O₃, and FeO), intermediate (MnO, Mn₂O₃, and MnO₂) or charge transfer type compounds (FeSrO₃, CoO, NiO, CuO and Cu doped CaTiO₃). These transitions along with relationships defining the splitting energy with respect to the total spin and binding energy are discussed.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

Effect of allied and alien ions on the EPR spectrum of Mn-containing lithium-manganese spinel oxides

Electron paramagnetic resonance spectroscopy was used for studying the effect of allied and alien ions on the EPR spectrum of Mn⁴⁺-containing lithium-manganese spinel oxides. Manganese spinel oxides with paramagnetic Mn⁴⁺ and diamagnetic substituents in the 16d spinel sites were studied: Li[Mg_0.5Mn_1.5]O₄, Li[Mg_0.5−xCo_2xMn_1.5−x]O₄, 0<x≤0.5, and Li[Li_1/3Mn_5/3]O₄. Ni²⁺-ions with integer-spin-ground state (S=1) were selected as alien ions: Li[Mg_0.5−xNi_xMn_1.5]O₄ (0≤x≤0.5), Li[Li_(1−2x)/3Ni_xMn_(5−x)/3]O₄ (0≤x≤0.5), and Li[Ni_0.5Mn_1.5−yTi_y]O₄ (0≤y≤1.0). It was shown that in Ni-substituted oxides the low temperature EPR response comes from magnetically correlated Ni-Mn spins, while at high registration temperature Mn⁴⁺ ions give rise to the EPR profile. Analysis of the EPR line width allows differentiating between the contributions of the density of paramagnetic species and the strength of the exchange interactions in magnetically concentrated systems. The density of allied and alien paramagnetic species has no effect on the EPR line width in cases when the strengths of antiferro- and ferromagnetic interactions on an atomic site are close. On the contrary, when antiferro- or ferromagnetic interactions on an atomic site are dominant, the EPR line width increases with the density of paramagnetic species.     

Uranium oxides investigated by X-ray absorption and emission spectroscopies

X-ray absorption and resonant X-ray emission measurements at the O 1s edge of the uranium oxides UO₂, U₃O₈ and UO₃ are presented. The spectral shapes of the O Kα X-ray emission spectra of UO₃ exhibit significant excitation energy dependence, from an asymmetric to a symmetric form, which differs from those of UO₂ and U₃O₈. This energy dependence is attributed to a significant difference in the oxygen-uranium hybridization between two different sites in the crystal structure of UO₃. The spectral shapes of UO₂ and U₃O₈ are also found to be different but without significant energy dependence. The experimental spectra of the valence and conduction bands of the uranium oxides are compared to the results of electronic structure calculations available in the literature.