Nature of the high-pressure transition in Fe2O3 hematite

We present a new method to separate the crystallographic and electronic phase transitions in hematite using x-ray emission spectroscopy and x-ray diffraction. Our observations, based on the behavior of a metastable high-pressure phase in the stability domain of the low-pressure phase, show that the electronic transition is preempted by the crystallographic transition. The former occurs only afterwards in the high-pressure phase, possibly as a result of a Mott transition. The idea that the electronic transition drives the transition in hematite is therefore invalidated. Such methods should help elucidate the mechanics and the driving forces behind a number of first-order high-pressure phase transitions.     

Surface and bulk electrical properties of the hematite phase Fe2O3

Electrical properties of Fe₂O₃ were studied by using several electrical methods such as electrical conductivity, thermopower (Seebeck effect) and work function. The studies were performed at elevated temperatures (1053–1153 K) and under controlled oxygen activity (10²10⁵ Pa). Samples of different thickness varying between 10³ nm and 1 mm were taken for the measurements of both electrical conductivity and thermopower. It has been found that the exponent of the po₂ dependence resulting from the work function measurements (1/n ) is about 1/2. Both thermopower and electrical conductivity data are well consistent with work function data for the thin film (1000 nm) of Fe₂O₃. The charge transport in Fe₂O₃ has been interpreted in terms of small polaron mechanism. Analysis of measured electrical parameters, regarding the thickness of studied specimens, indicates that the near-surface layer of Fe₂O₃ exhibits much higher deviation from stoichiometry than the bulk phase and resulting strong interaction between charge carriers. This effect has been interpreted in terms of segregation of intrinsic lattice defects to the surface, and presumably also to grain boundaries, of Fe₂O₃.     

The charge states of iron in insulators implanted with57Co and57Fe

Single crystals of α-Al₂O₃ and LiNbO₃ were implanted with⁵⁷Co (dose: up to 2×10¹⁵ atoms/cm²) and with⁵⁷Fe (dose: 2×10¹⁵ atoms/cm²) ions. The Mössbauer spectra revealed the disordered atomic environment. Fe²⁺ and Fe³⁺ charge states were observed. The spectra were compared to the spectra of crystals doped with⁵⁷Co. It was remarkable that in the doped α-Al₂O₃ Fe³⁺ states with slow spin-spin relaxation have appeared. The CEMS study of the samples implanted with⁵⁷Fe resulted in Fe²⁺ ionic states indicating that a fraction of Co atoms can also be in Co²⁺ state.     

Optical absorption spectrum of hematite, αFe2O3 near IR to UV

Optical absorption spectra have been measured on thin (011) single crystal platelets and on highly oriented (110) thin films of αFe₂O₃. We have observed and assigned some of the absorption bands predicted by ligand field theory and SCF-Xα calculations. The temperature dependence of the 11760 cm^?1 single crystal band has been fitted to the function $\text{? = ?}{}_0\text{(1 + exp (?}\text{θ}\text{T}\text{))}$ with $\text{?}{}_0\text{= 0.85 × 10}{}^{\mathrm{?4}}$ and θ = 200 K (139 cm^?1). We have measured the photocurrent as a function of wavelength and have found several peaks that coincide with optical absorption bands.     

Characterization of Fe states in dilute 57 Mn implanted SnO 2 film

States of dilute Fe in SnO₂ have been monitored using ⁵⁷Fe emission Mössbauer spectroscopy following implantation of ⁵⁷Mn (T _1/2 = 85.4 s) in the temperature range from 143 K to 711 K. A sharp annealing stage is observed at ~330 K where the Fe^3?+?/Fe^2?+? ratio shows a marked increase. It is suggested that this annealing stage is due to the dissociation of Mn-V_O pairs during the lifetime of ⁵⁷Mn; the activation energy for this dissociation is estimated to be 0.9(1) eV. Fe^3?+? is found in a paramagnetic state showing spin-lattice relaxation rates consistent with an expected T ² dependence derived for a Raman process. In addition, a sharp lined doublet in the Mössbauer spectra is interpreted as due to recoil produced interstitial Fe.     

Ferryl (Fe=O) termination of the hematite alpha-Fe2O3(0001) surface

Using scanning tunneling microscopy and infrared reflection absorption spectroscopy we have observed that the alpha-Fe2O3(0001) surface exhibits ferryl (Fe=O) groups, which may coexist with domains of the Fe-terminated surface. We therefore fully support ab initio calculations recently reported in the literature [W. Bergmeyer, H. Schweiger, and E. Wimmer, Phys. Rev. B 69, 195409 (2004)]. The close similarity to the results on the (0001) surfaces of Cr2O3 and V2O3 strongly suggests that the M=O termination under certain oxygen pressure conditions is the most stable for the close-packed surfaces of transition metal oxides with the corundum structure.     

Oxidation states of Fe in LaNi1−x Fe x O3

We discuss a new method of producing spin-polarized radioactive nuclear probes which capitalizes characteristic features of projectile fragmentation reaction at intermediate energies. Experimental results are given for the behavior of polarization as a function of the fragment momentum, target mass number, and beam energy. As an application of this method, we also report on results ofg-factor measurements for several neutron-rich nuclei. A full account of theg-factor measurement will be presented elsewhere.     

Magnetic ordering of Fe atoms in YBa2(Cu1−x Fe x )3O3 in superand semiconducting states

Magnetic behavior of Fe atoms in YBa₂(Cu_1−x Fe _x )₃O _y system in superconducting (SC) and non-SC states is studied. In SC region one magnetic ordering point T_m1 is observed for all Fe atoms in Cu1 sites: T_m1=10 K and 16 K for x=0.05 and x=0.10, respectively. In the non-SC state for x=0.05 Cu1 and Cu2 sublattices are magnetically independent and two magnetic ordering points are found: T_m1=20 K and T_m2=405K. For x=0.10 a strong magnetic interaction between Cu1 and Cu2 sublattices appears and only one ordering point T_m2=435 K is observed.     

Al‐for‐Fe substitution in hematite: the effect of low Al concentrations in the Raman spectrum of Fe2O3

Five different mixed Al‐hematites with an aluminium molar content varying from 0 to 10% were investigated by micro‐Raman spectroscopy to study the effect of Al‐for‐Fe substitution on the hematite lattice. A red shift of hematite vibrational wavenumbers and a line broadening were observed; also a shoulder located near 430 cm⁻¹ and a broad band at 670 cm⁻¹ developed. The variation of the spectral features is discussed in terms of a local disorder correlated to the insertion of Al³⁺ ions into the Fe(O)₆ octahedra constituting hematite structure. A multivariate analysis was also carried out on the spectral data to distinguish between the doped samples analysed. Copyright © 2007 John Wiley & Sons, Ltd.     

Local trapping sites in Y2O3 and Sc2O3 ceramics

Heat-stimulated luminescence (HSL) of x-ray-irradiated Y₂O₃, Y₂O₃:Eu, and Sc₂O₃ ceramics samples is investigated. An analysis of the form of the elementary contours making the HSL curves shows that recombination processes at the HSL peaks with maxima at 110, 122, 176, 185, 200, and 247 K in Y₂O₃. ceramics and with those at 121, 136, 200, 215, and 221 K in Sc₂O₃ are described in terms of linear kinetics. The spectral composition of the HSL of the ceramic obtained is studied. Some methods are employed to determine the activation energies and frequency factors corresponding to the HSL peaks. It is established that the recombination processes occurring upon release of the trapping sites in the Y₂O₃ and Sc₂O₃ ceramic samples are conditioned by diffusion-controlled tunneling recombination due to heat-stumulated migration of V_k-centers. L'vov I. Franko State University, 50, Dragomanov St., L'vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 348–353, May–June, 1997.     

Conductivity of Fe2O3-doped YSZ

Effect of 2 mol.% Fe₂O₃ addition on conductivity of yttria stabilized zirconia ceramics (YSZ, 6–12 mol.% Y₂O₃) was established. For Fe doped material, it was shown that conductivity demonstrates maximum at 10 mol.% Y₂O₃, and its value increases by 1.4 times. Conductivity maximum corresponds with the minimal volume of crystal lattice. Discussion of results was performed in terms of the model that assumes the control effect of hydrostatic pressure on oxygen ions diffusion.     

Mössbauer study of iron in high oxidation states in the K–Fe–O system

Oxidation of metallic iron by potassium superoxide leads to the formation of ferrate(V). Under room temperature this compound is unstable and instantly decomposes by disproportionation mechanism. Grinding the substance into powder accelerates the decomposition process.     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001     

Local states of iron ions in perovskite Bi0.815Y0.085La0.10FeO3

Bi_0.815Y_0.085La_0.10FeO₃ perovskite is studied by ⁵⁷Fe Mössbauer spectroscopy at 87, 295, and 670 K. The measured temperature of the magnetic phase transition (the Néel temperature) is T _N = 666 ± 5 K. It is found that substituting Y³⁺ ions for 0.085 at % Bi³⁺ in the Bi_0.9La_0.1FeO₃ perovskite destroys the spatially spin-modulated structure while the rhombohedral crystal structure is retained. Two structurally nonequivalent states of iron ions are found above the Neel temperature. Below the Neel temperature, there are four magnetically nonequivalent states of trivalent iron ions.     

Local states of iron ions in multiferroics Bi1−x La x FeO3

Mössbauer studies of perovskites Bi_1?x La _x FeO₃ (x = 0, 0.10, 0.20, 0.61, 0.90, 1.00) were conducted at 295 and 87 K. The spatial spin-modulated structure (SSMS) observed in perovskites BiFeO₃ and Bi_0.9La_0.1FeO₃ leads to a specific distribution of hyperfine fields P(B) with two peaks. Substitution of La for Bi (x = 0.2) destructs the SSMS. The concentration dependences of the hyperfine field (B), isomer shift (?) and quadrupole shift (δ) were measured. The iron ions are in the trivalent state. The local magnetic moments μ(Fe) of the Fe³⁺ ions are determined.