The relationships between microstructure and crystal structure in zincite solid solutions

Single phase (Zn,Fe)_1−x O zincite solid solution samples have been prepared by high temperature equilibration in air and in reducing atmospheres, followed by quenching to room temperature. The Fe²⁺/Fe³⁺ concentrations in the samples have been determined using wet chemical and XPS techniques. Iron is found to be present in zincite predominantly in the form of Fe³⁺ ions. The transition from an equiaxed grain morphology to plate-like zincite crystals is shown to be associated with increasing Fe³⁺ concentration, increasing elongation in <001> of the hexagonal crystals and increasing anisotropic strain along the c-axis. The plate-like crystals are shown to contain planar defects and zincite polytypes at high iron concentrations.     

Effect of Crystal Quality on HCP-BCC Phase Transition in Solid 4He

The kinetics of HCP-BCC structure phase transition is studied by precise pressure measurement technique in ⁴He crystals of different quality. An anomalous pressure behavior in bad quality crystals under constant volume conditions is detected just after HCP-BCC structure phase transition. A sharp pressure drop of 0.2 bar is observed at constant temperature. The effect observed can be explained if we suppose that microscopic liquid droplets appear on the HCP-BCC interphase region in bad quality crystals. After the interphase region disappearance, these droplets are crystallized with pressure reduction. It is shown that this effect is absent in high quality thermal-treated crystals.     

Structural,transport and spectroscopic properties of Ti substituted magnetite: Fe3−xTixO4

The effect of Ti⁴⁺ ion on the formation of magnetite, which were prepared by solid-state route reaction method, were studied by resistivity, Raman and ⁵⁷Fe Mössbauer spectrometry. Resistivity measured in the range of 10 < T < 300 K for Ti⁴⁺ magnetite Fe_3−xTi_xO₄ exhibit first order phase transformations at the Verwey transition T_v for Fe₃O₄, Fe_2.98Ti_0.02O₄ and Fe_2.97Ti_0.03O₄ at 123 K, 121 K and 118 K, respectively. No first order phase transition was observed for Fe_2.9Ti_0.1O₄ and small polaron model retraces the semiconducting resistivity behavior with activation energy of about 72 meV. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Ti doping. The Raman active mode for Fe_2.9Ti_0.1O₄ at ≅634.4 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅670 cm⁻¹, inferring that Mn²⁺ ions are located mostly on the octahedral sites. ⁵⁷Fe Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Ti⁴⁺ ions are located mostly on the octahedral sites of the Fe_3−xTi_xO₄ spinel structure.     

Non-stoichiometry of lithium-copper ferrites - I - spectrophotometric characterization (γ, X-ray,UV-vis IR)

In the heat-treatment under nitrogen of spinel type lithium-copper ferrite Li_0,25Cu_0,5Fe_2,25O₄, the oxygen loss involves the precipitation of CuFeO₂ and the formation of a non-stoichiometric ferrimagnetic phase. Fe²⁺ ions are revealed in this spinel phase by different spectrometric analyses (γ, X, UV-vis IR).     

Landau–Zener spin transitions in Fe2+–Fe2+ quantum dots controlling dislocation mobility in NaCl:Fe crystals

Magnetic field induces transition in the distorted Fe²⁺–Fe²⁺ pairs (quantum dots) from the initial bonding singlet state to the high spin antibonding state providing decay of the pairs for two separated Fe²⁺ ions. Dislocations moving under internal stresses easily overcome separated Fe²⁺ ions in comparison with Fe²⁺–Fe²⁺ pairs lying close to the glide plane. Non-monotonous field dependence of dislocation displacements under internal stresses governed by short (100 μs) impulse of high magnetic fields up to 31 T was revealed in NaCl:Fe crystals. This non-typical dependence is the fingerprint of the Landau–Zener non-adiabatic spin transition between singlet and high spin states in quantum dots distorted by mechanical stresses of moving dislocations.     

Preparation and characterization of stoichiometric CaFeO3

Stoichiometric CaFeO₃ was prepared and was characterized by crystallographic, magnetic and electrical measurements. A slight tetragonal distortion from the ideal cubic perovskite structure was found. The tetramolecular unit cell has a = 5.325 (3) A and c = 7.579 (5) A. The susceptibility showed a maximum at about 115 K and the temperature dependence of electrical resistivity changed from metallic to semiconductive in the vicinity of the magnetic transition temperature. These indicate a phase transition from the metallic-paramagnetic (the high-temperature phase) to the semiconductive-antiferromagnetic phase. The Mössbauer spectra indicated that a charge disproportionation 2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺ associates with the transition.     

Kinetics of the anatase–rutile transformation in TiO2 in the presence of Fe2O3

The anatase–rutile phase transition in TiO₂ in the presence of Fe₂O₃ was investigated in air and argon atmospheres by means of X-ray diffraction and scanning electron microscopy. Isothermal curves of rutile transformed from anatase as a function of time were obtained between 825 and 950 °C. The data were well fitted by various rate laws. In the presence of Fe³⁺, the anatase–rutile transition temperature is lower and the transformation rate in air is higher than the corresponding one in pure TiO₂. The transformation in the presence of Fe³⁺ in an argon atmosphere is more rapid than in air. The enhancement effect of Fe³⁺ on the anatase–rutile transformation in both atmospheres is understood on the basis of the formation of oxygen vacancies.     

The nature of the change in the conditions for surmounting pinning centers by dislocation in lead at the superconducting transition

The effect of the presence of Sb on the amplitude dependence of ultrasonic attenuation α in the concentration range 1.7×10 ^?3 to 4.25×10 ^?1 at % has been studied systematically on Pb single crystals at 1.4 and 4.2 K (superconducting and normal states) and 77 K. The dependence curve shifts toward greater amplitudesU with increasing concentrationC and to smallerU whenT goes from 1.4 to 4.2 K. The shift toward greaterU at the transition to the normal state is observed for all concentrations. The amplitude-independent attenuation α _i varies with concentration nonmonotonically: IncreasingC causes α _i first to diminish (the minimum atC=0.034 at %) and then to grow. AtC≥0.034at %, α _i does not change at the n-s transition. The studies suggest that the damping factor for dislocations in impure crystals practically does not change at the superconducting transition. Here the damping is essentially due to the dynamic dislocation-impurity interaction. The change in the conditions for a dislocation surmounting the pinning centers at the superconducting transition is determined by the quasistatic mechanism of lowering barriers.     

ESR characterization of Fe(III) ions in polycrystalline cation-deficient FeAl2O4 spinel

Polycrystalline samples of cation-deficient FeAl₂O₄ spinel with Fe³⁺ ions and vacancies on both octahedral and tetrahedral sites have been investigated by electron spin resonance (ESR). For samples annealing at 1100° C and slightly oxidized (⩽0.16 wt % Fe³⁺), the ESR spectrum originates from Fe³⁺ ions located in trigonally distorted octahedral sites ofα-Al₂O₃ formed during the annealing and also from Fe³⁺ ions introduced by oxidation at low temperature in octahedral sites of spinel structure. The samples with concentration between 0.16 and 1.6 wt % Fe³⁺ show the majority of Fe³⁺ ions to be on tetrahedral sites and the rhombic symmetry around the Fe³⁺ ions is attributed to the presence of cation vacancies.     

Effects of the equilibrium concentration of impurities on the reproducibility of thermoluminescence in MgO

A study of thermoluminescent (TL) sensitivity and reproducibility is undertaken in crystals of MgO exposed to ultraviolet radiation in the as-received condition, and heat treated in air or in a mixtue of CO/CO₂ at either 1000 or 1400° C. The changes in the concentrations of transition metal ions such as Fe³⁺, Cr³⁺, Mn²⁺ and V²⁺, caused by the heat treatments, are followed using electron spin resonance. It is shown that the equilibrium concentrations of Fe³⁺ and V²⁺ ions, which can be altered by heat treatments, play an important role in the reproducibility of the TL in this system.     

Optical and transport properties of Fe-doped Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se (<Emphasis Type="Italic">x</Emphasis> = 0.35)

The optical and transport properties of Fe²⁺-doped Cd _x Hg_1?x Se crystals with a midgap Fe²⁺ level have been studied. The results demonstrate that Fe²⁺ ions influence both the optical and transport properties of Cd _x Hg_1?x Se〈Fe²⁺〉. The observed optical absorption bands are due to a donor Fe²⁺ level in the band gap, with a depth E _Fe = 0.21 eV, and to band-band transitions. Thermal anneals in Hg and Se vapors have different effects on the carrier concentration and mobility in the crystals. The effect of annealing on the transport properties of the Fe²⁺-doped crystals differs from that for undoped crystals and is governed by the state of point defects.     

The effect of Cu content on the structure of Ni1−xCuxFe2O4 spinels

A series of Ni_1−xCu_xFe₂O₄ (0 ≤ x ≤ 0.5) spinels were synthesized employing sol-gel combustion method at 400 °C. The decomposition process was monitored by thermal analysis, and the synthesized nanocrystallites were characterized by X-ray diffraction, transmission electron microscopy, infra-red and X-ray photoelectron spectroscopy. The decomposition process and ferritization occur simultaneously over the temperature range from 280 °C to 350 °C. TEM indicates the increase of lattice parameter and particle size with the increase of copper content in accordance with the XRD analysis. Cu²⁺ can enter the cubic spinel phase and occupy preferentially the B-sites within x = 0.3, and redundant copper forms CuO phase separately. A broadening of the O 1s region increases with the increment of copper content compared to pure NiFe₂O₄, showing different surface oxygen species from the spinel and CuO. Cu²⁺ substitution favors the occupancy of A-sites by Fe³⁺.     

Mössbauer studies on Zn-substituted iron molybdate

Mössbauer spectra of Fe_2−yZn_yMoO₄ spinel ferrites were recorded in their paramagnetic state. Paramagnetic Mössbauer spectra of all the samples show broad absorption peaks due to the presence of Fe²⁺ and Fe³⁺ ions on both sites (A and B) of the spinel lattice. All the spectra have been fitted with four doublets, using a least squares fitting program. The isomer shift and quadrupole splitting values show that Fe_A²⁺, Fe_A³⁺, Fe_B³⁺ and Fe_B^2.5+ ions are present. Fe_B^2.5+ represents the presence of the Fe²⁺ and Fe³⁺ ions on the B-sites, which take part in charge hopping. The results of electrical resistivity and magnetic measurements support charge hopping between Fe²⁺ and Fe³⁺ ions on B-sites.     

Contribution de la spectrometrie mössbauer et de la spectrometrie d'absorption X A l'etude de la non-stoechiometrie de CuFe2O4

The effects of oxygen loss on the crystallographic and magnetic properties of CuFe₂O₄, ferrite have been studied by high-temperature x-ray diffraction, X-ray absorption spectrometry and Mössbauer spectrometry.In the heat-treatment under nitrogen, formation of a ferrimagnetic phase is observed with an increased content of Fe²⁺ and precipitation of CuFeO₂. Quenching after sintering leads to a mixture containing CuO and a spinel ferrimagnetic phase in which the oxygen deficiency involves the formation of Cu⁺.     

Investigation of phase separation in the Ge x Cu1–x Fe2O4 system

The spinel series Ge _x Cu_1–x Fe₂O₄ (x=0.0 to 0.9) has been studied in detail by means of Mössbauer spectroscopy, X-ray diffraction and magnetization measurements at room temperature (298 K). Analysis of X-ray diffraction intensity data and Mössbauer intensity data suggest that this system remains in single phase up tox=0.4 then it phase separates into two different phases forx=0.5 to 0.9. Lattice constants of this system deviate from Vegard's law. Mössbauer spectra for x=0.0 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octrahedral ions (B-sites), while forx=0.5 to 0.9 it gives Mössbauer patterns corresponding to two separate phases. The systematic composition dependence of quadrupole interactions and nuclear hyperfine fields of⁵⁷Fe³⁺ ions also support the concept of phase separation forx=0.5 to 0.9. The observed variation of⁵⁷Fe³⁺ hyperfine field on A- and B-sites withx forx=0.0 to 0.4 can be explained qualitatively on the basis of supertransferred hyperfine interactions.     

Some new sulfo-spinels containing iron-group transition metals

The study of the cation site preferences related to Rh³⁺ or Sn⁴⁺ ions in the spinel lattice leads to a series of new spinels. Various sulfo-spinels containing some of the transition elements such as Mn, Fe, Co and Ni, have been synthesized and the cation distributions and u-parameters were determined. Some magnetic and electrical properties were measured for Fe₂RhS₄ and Cd_1?xFe_1+xSnS₄ (0 ≤ x ≤ 0.9).     

Structural and magnetic characterization of LiMn1.825Cr0.175O4 spinel obtained by ultrasonic spray pyrolysis

Quaternary spinel oxide LiMn_1.825Cr_0.175O₄ powder was synthesized by using an ultrasonic spray pyrolysis method, without additional annealing. The crystal structure of the as-prepared powder was revealed by X-ray powder diffraction and identified as a single spinel phase with Fd3m space group. The powders had a spherical morphology with extremely smooth surface appearance and densely congested interior structure. Transmission electron microscopy confirmed that the particle consisted by the cohesion of the primary particles. Magnetic measurements performed in DC field in both zero-field-cooled and field-cooled regimes, as well as AC susceptibility experiments, show that system undergoes spin-glass transition at the freezing temperature T_f = 20 K. The value of the effective magnetic moment μ_eff = 4.34 μ_B obtained from the Curie-Weiss fit in the high temperature region confirms the substitution of Mn³⁺ ions with Cr³⁺ ions.     

Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

A mössbauer study of hyperfine interactions in the boroferrite Fe3−x Ga xBO6(x=0, 0·12)

Fe₃BO₆ shows no thermal hysteresis around the phase transition temperatureT _SR=418 K; Fe_2·88Ga_0·12BO₆ undergoes spin transition atT _SR=403 K resulting in a reduction of theT _SR. Both these features are different from the reported observations on α-Fe₂O₃. The two Fe³⁺ sites show difference in their magnetic interactions. Ga³⁺ replaces Fe³⁺ in a random manner.     

Effects of gamma-irradiation on Al2O3 crystals grown under reducing atmosphere

Gamma-ray irradiation-induced color centers in Al₂O₃ crystals grown by temperature gradient techniques (TGT) under a strongly reducing atmosphere were studied. The transition F⁺ → F takes place during the irradiation process. Glow discharge mass spectroscopy (GDMS) and annealing treatments show that Fe³⁺ impurity ions are present in the crystals. A composite (F⁺-Fe³⁺) defect was presented to explain the origin of the 255 nm band absorption in the TGT-Al₂O₃ crystals.