E. Wolska

Vibrational spectra of lithium ferrites: infrared spectroscopic studies of Mn-substituted LiFe5O8 ☆

Abstract Investigations on the series of manganese-substituted lithium ferrites reveal the destructive effect of manganese ions on the ordering of Li+ cations in the spinel-type crystal lattice. A spectacular order–disorder transition with increasing substitution of Mn3+ ions has been observed with infrared spectroscopy, in the region of lattice vibrations. The preference of manganese ions for the octahedral coordination, appears to be associated with the migration of Li+ from octahedral to tetrahedral positions. Results for the LiFe5−yMnyO8 solid solution have been confronted with an order–disorder transition in the pure lithium ferrite.

X-ray powder diffraction study of cation distribution and the Fd3m→P4132 symmetry reduction in Li0.5Fe2.5O4/LiMn2O4 spinel solid solutions

Abstract Structural changes in solid solutions formed between the cubic normal spinel, LiMn 2 O 4 , and ordered inverse spinel, Li 0.5 Fe 2.5 O 4 , have been investigated by X-ray powder diffraction and infrared spectroscopic techniques. By refining the X-ray diffraction patterns with Rietveld profile analysis, the lattice parameters, interatomic distances and cation distribution were determined. The Fe-for-Mn substitution increases the spinel unit-cell constant from 8.21A to 8.34A for the Fe/(Fe+Mn) mole ratio of 0 and 1, respectively. A distinct departure from Vegard’s law appears for Fe/(Fe+Mn)≥0.6. The additional ‘superstructure’ reflections on the X-ray patterns confirm an ordering of lithium ions in octahedral sites, bringing about the lowering of Fd 3 m symmetry to the P 4 1 32/ P 4 3 32 space group. Li + ions are distributed over both octahedral and tetrahedral cationic positions of spinel lattice, although the end-members of the solid solution series, LiMn 2 O 4 and Li 0.5 Fe 2.5 O 4 , contain Li + coordinated tetrahedrally and octahedrally, respectively. The ordering of Li + ions in the 4 b Wyckoff’s positions of a cubic primitive unit cell ( P 4 1 32/ P 4 3 32), observed for the increasing Fe 3+ content, has been confirmed for that system with infrared spectroscopy. The number of infrared active vibrations increases significantly with the lowering of crystal symmetry caused by the 1:3 ordering of Li + in octahedral sites.

Synchrotron X-ray powder diffraction studies on the phase transitions in LiMn2O4

Abstract Structural phase transitions of the spinel lithium-manganese oxide, LiMn 2 O 4 ( Fd3m at room temperature) in the temperature range of 300–20 K, were investigated using synchrotron and laboratory X-ray powder diffraction. Splitting of the spinel X-ray lines 311, 400, 440, 531, 533 and 622, recorded during the test, evidenced the reversible formation of cubic, orthorhombic and tetragonal structures. The first transformation starts at ∼290 K as a cubic ( Fd3m )→orthorhombic ( Fddd ) symmetry reduction, and both phases coexist down to 260 K. Below 100 K a second structural transition, orthorhombic→tetragonal ( F4 1 /ddm ) was observed, with the coexistence of these two phases down to ∼40 K. The supercell corresponding to 3 a ×3 a × a ( a , spinel unit-cell constant) has been considered for both orthorhombic and tetragonal polymorphs of LiMn 2 O 4 , based on the appearance of (2.10.0) superstructure X-ray reflection.

Defect structures in cadmium-nickel ferrites

Abstract The cation-deficient solid solutions of cadmium ferrite (CdFe 2 O 4 ) and nickel ferrite (NiFe 2 O 4 ) obtained by prolonged thermal treatment from stoichiometric samples have been investigated by X-ray powder diffraction, magnetic measurements and chemical analysis. The stoichiometric Cd x Ni 1−x Fe 2 O 4 samples were prepared from coprecipitated x Cd(OH) 2 ·(1− x )Ni(OH) 2 ·2Fe(OH) 3 -hydroxides, sintered at 1000°C during one hour. It has been found that because of the CdO volatility, stoichiometric preparations with x = 0.9 and x = 1.0 should be synthesized with the excess of 8% and 10% Cd(OH) 2 respectively. The linear increase of spinel lattice constant with increasing Cd-content and the observed X-ray reflection intensities confirm the stoichiometry and the tetrahedral coordination of Cd 2+ -ions. Thermal treatment above 1000°C leads to the defect spinel structures with the Cd 2+ -deficiency, composed from solid solutions with the general formula Cd 2+ x−α Fe 3+ 1−x+α [Ni 2+ 1−x Fe 3+ 1+x−α/3 □ α/3 ]O 4 , corresponding to a solid solution of stoichiometric (Cd, Ni)-ferrite with the maghemite, γFe 2 O 3 .

Relationship of crystal structure to interionic interactions in the lithium-manganese spinel oxides

Lithium manganese oxides in the form of cubic spinel phases (space group Fd3m) are formed in a LixMn3-xO4 system for rather limited values of x. Structural investigations by X-ray powder diffraction, applied to the Li-Mn-O compounds, indicate the formation of a second crystalline phase, Li2MnO3 (space group C2/m), with the increasing lithium content. Total Li+ content per unit cell and the cation distribution over a spinel lattice in LixMn3-xO4 have been studied by measurements of integrated intensities of X-ray reflections, and by structure refinement using Rietveld profile analysis. In an attempt to understand the factors affecting cation distribution in the spinel lattice, we applied the computer modelling techniques and investigated the Li+, Mn3+ and Mn4+ ion distribution by calculating the lattice energy, combined with energy minimisation procedures, using the General Utility Lattice Program (GULP), a program designed for simulation of ionic and semi-ionic solids, based on interatomic potential models.

X-ray powder diffraction and Mössbauer studies on the formation of Cd0.5Ni0.5Fe2O4/Zn0.5Ni0.5Fe2O4 spinel solid solutions☆

Abstract Structural investigations by X-ray powder diffraction, magnetic measurements and by Mossbauer spectroscopy, applied to a series of solid solutions formed between two mixed spinels, zinc–nickel ferrite and cadmium–nickel ferrite, indicates the difference in the cation arrangement in the solid solution obtained hydrothermally, compared to that of ferrites sintered at high temperatures. The ferrimagnetic crystalline Cd–Zn–Ni ferrite series, of a composition of Cd 0.5- α Zn α Ni 0.5 Fe 2 O 4 , have been prepared by hydrothermal treatment of the coprecipitated amorphous Cd–Zn–Ni–Fe hydroxide mixtures. The hydrothermally obtained samples displaying a defected spinel structure, with clearly noticeable non-stoichiometry, may be considered as precursors for the preparation of stoichiometric products by further thermal treatment. The first approach to the Mossbauer spectra of the system (Cd 0.5- α Zn α Fe 0.5 ) tetr [Ni 0.5 Fe 1.5 ] oct O 4 analysis has been undertaken.

Determination of solid solution limits based on the thermal behaviour of aluninium substituted iron hydroxides and oxides

The formation of substitutional solid solutions of the isostructural oxyhydroxides α-FeOOH-α-AlOOH (goethite-diaspore and γ-FeOOH-γ-AlOOH (lepidocrocite-boehmite) was investigated by X-ray powder diffraction technique and by thermal analysis.The unit cell parameters of both orthorhombic structures of α-Fe1−x-AlxOOH (Pbnm) and γ-Fe1−xAlxOOH (Cmcm) decrease clearly with the increase of aluminium content up tox=0.10.Thermal analysis reveals the sensitivity of DTA and DTG effects to the composition of solid solutions. The temperatures of topotactic formation of rhombohedral α-(Fe, Al)2O3 and of the defect-spinel γ-(Fe, Al)2O3 during the dehydroxylation of α-(Fe, Al)OOH and γ-(Fe, Al)OOH respectively increase as the aluminium concentration rises up to 10 mol%. The dehydration curves of samples with greater aluminium content exhibits the same maximumΔm/ΔT as for the 10 mol% Al-preparation, indicating indirectly the end-members of the solid solution ranges.ZusammenfassungMittels Röntgendiffraktion und Thermoanalyse wurde die Bildung von Substitutions-Mischkristallen der isostrukturellen Oxyhydroxide α-FeOOH-α-AlOOH (Goethit-Diaspor) und γ-FeOOH-γ-AlOOH (Lepidokrokit-Böhmit) untersucht. Die Elementarzellenparameter beider rhombischer Strukturen von α-Fe1−xAlxOOH (Pbnm) und γ-Fe1−xAlxOOH (Cmcm) werden mit zunehmenden Aluminiumgehalt bis zux=0.10 eindeutig kleiner.Thermoanalyse zeigt die Empfindlichkeit von DTA- und DTG-Effekten gegenüber der Zusammensetzung der Mischkristalle. Steigt die Aluminiumkonzentration bis 10 mol% an, so erhöhen sich auch die Temperaturen für die topotaktische Bildung von rhomboedrischem α-(Fe,Al)2O3 und des Defekt-Spinells γ-(Fe,Al)2O3 bei der Dehydroxylierung von α-(Fe,Al)OOH und γ-(Fe,Al)OOH. Die Dehydratationskurven von Proben mit einem größeren Aluminiumgehalt zeigen dasselbe Δm/ΔT-Maximum wie für die Proben mit 10 mol%, was indirekt auf die Grenze des Mischkristallbereiches deutet.

Ferrimagnetic spinels in hydrothermal and thermal treatment of MnxFe2−2x(OH)6−4x

Products of hydrothermal treatment of the initial amorphous system MnxFe2−2x(OH)6−4x for 0≤x1 in 0.1x intervals, and products of their further thermal treatment, were examined by chemical analysis, X-ray, IR, and DTA techniques supported by magnetic measurements. After hydrothermal growth for lowx, hematite and goethite phases occurred. Although the goethite phase was still identifiable atx=0.6, formation of a solid solution with the isostructural groutite was not found. The ferrimagnetic spinel phase, which resists heating up to 400‡C, was present at 0.5≤x≤0.9. At higher temperatures, it transformed into the rhombohedral hematite type phase or into the cubic bixbyite phase. AtT≥900‡C, a ferrimagnetic spinel structure reappeared up tox=0.8. For x=0.9, the low- and high-temperature forms of the hausmannite phase occurred, forx= 1 passing from one form into another through Mn5O8 and partritgeite.For a primary mixture Mn0.5Fe(OH)4, corresponding to the manganese ferrite structure, the lattice parameter of which passes from 8.43 å through 8.33 å to 8.50 å, the probable crystallochemical formula was suggested.

X-ray diffraction study on the distribution of lithium ions in LiMn2O4/LiFe5O8 spinel solid solutions

Abstract Solid solutions formed between the cubic spinel LiMn 2 O 4 and the ordered spinel Li 0.5 Fe 2.5 O 4 have been investigated by X-ray powder diffraction, infrared spectroscopy and magnetic techniques. The series shows a spinel structure with the unit-cell constant increasing from a = 8.209 A to a = 8.335 A, with the increasing Fe/(Fe + Mn) ratio. For 0 ≤ Fe/(Fe + Mn) ≤ 0.6 the cubic spinel structures (space group Fd3m) were formed, while in samples with higher Fe content the ordering of lithium ions leads to the lowering of symmetry (space group P4 1 32). The measurements of integrated intensities of X-ray reflections show the distribution of Li + ions in both octahedral and tetrahedral cationic positions of spinel lattice, although the end-members of the series, LiMn 2 O 4 and LiFe 5 O 8 , contain Li + coordinated tetrahedrally and octahedrally, respectively.

Effect of the anionic sublattice hydroxylation on the goethite → maghemite transformation in the AlxFe1−xOOH system

Abstract The pseudomorphic transformation of the aluminium-substituted α-oxyhydroxide of iron (Al-goethite) into the tetragonal spinel γ-ferric oxide (Al-maghemite) was studied by X-ray powder diffraction, infrared spectroscopy, thermogravimetry and magnetic measurements. It was found that the order of cationic vacancies and the tetragonal superstructure in γ-(Fe 1− x Al x ) 2 O 3 were preserved in the whole range of substitutional solid solution, i.e. for 0 ⩽ x ⩽ 0.1. The conclusive factor affecting the cation and vacancy ordering in the defect spinel Al-maghemite appears to be the degree of hydroxylation of anionic sublattice of the Algoethite precursors.