J. Kaczmarek

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 .

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.

Nonstoichiometric solid solutions during formation of Cd/Ni ferrites

Abstract Cadmium-nickel ferrites prepared by different procedures from coprecipitated amorphous x Cd(OH) 2 ·(1− x )Ni(OH) 2 ·2Fe(OH) 3 mixtures have been investigated by X-ray, magnetic and thermogravimetry techniques. In the preparations obtained by hydrothermal methods at 100 and 150°C there is a marked tendency to form solid solutions between the nickel and cadmium ferrites for lower values of x , but for higher x the lattice parameters suddenly decrease, indicating the formation of solid solutions between CdFe 2 O 4 and maghemite, γ-Fe 2 O 3 . It has been found that during a gradual heating of dried, initially amorphous mixtures of hydroxides, several distinct nonstoichiometric intermediate phases of Cd/Ni ferrites arose before the final product, corresponding to the nominal composition was formed. Prolonged thermal treatment at T ⩾1000°C, because of the Cd volatility, leads again to the defect structure solid solutions between the cadmium-nickel ferrites and maghemite.

The defect structure and vacancy distribution in the low-temperature hausmanite

Abstract The distribution of Mn 3+ cations and of cationic vacancies different from those in the anhydrous hausmanite, α-Mn 3 O 4 , has been evidenced in the tetragonal spinel phase obtained by the controlled oxidation of Mn(OH) 2 . The single-phase product contains hydroxyl groups in the anionic sublattice and could be defined as hydrohausmanite, α-Mn 3 (O, OH) 4 . Based on measurements of the integrated intensities of X-ray reflections it has been found that the cationic vacancies in hydrohausmanite are caused mainly by the presence of structural OH - groups. The distribution of vacancies on the tetrahedral and octahedral spinel cationic sublattices may be expressed in the form: Mn 2+ 8−3x+y Mn 3+ 5x 2 −y □ x 2 [ Mn 3+ 16− x 2 □ x 2 ] ( OH ) y O 32−y , where x = vacancies / unit - cell 0643 0642 V 3 and y = hydroxyl groups / unit - cell .

Hydrothermal conversion of amorphous NiFe2–xAlx(OH)8 into crystalline phases

Mixtures of amorphous hydroxides prepared from FeIIIAlIII and NiII nitrate solutions in proportion to obtain a series of compounds with 0.1 (if necessary 0.05) increments for x in NiFe2–xAlx(OH)8, have been subjected to hydrothermal ageing at 150 °C in mother-liquor for one or six months. X-Ray, IR, TG and magnetic studies revealed no influence of ageing time on the properties of the crystalline phases obtained. It was proved that for certain values of x, it is possible to obtain hydrothermally a pure phase of nickel ferrite-aluminate solid solutions, displaying magnetic characteristics similar to those of solid solutions produced by standard ceramic methods. At x= 0.5 the spinel phase deteriorates, the haematite structure appears and the nickel ions engage (till x= 2) in the formation of layered double hydroxides (LDHs), accompanied from x= 0.8 by the boehmite phase. At x= 1.8 the spinel phase appears again, having a lattice parameter corresponding to 0.72 Al3+ in the ferrite structure.

Characterization of ferrimagnetic phases from hydrothermal storage of Cd/Ni/Fe/Al-hydroxides

Abstract Ferrimagnetic phases formed from amorphous CdxNi1−xFe2−yAly(OH)8 hydroxides with y = 0, 0.2 and 0.4 in 0.2 steps for x, coprecipitated at pH 10 with NaOH from corresponding nitrates and stored one month at 150 °C in mother solution, were studied by XRD, IR, TG and magnetic measurements. The hydrothermally obtained spinel phases displayed the same magnetic properties as the series sintered at 1000 °C only for y = 0.0 and 0.2. It is assumed that the highest incorporation of Cd2+ ions into nickel ferrite occurs for x = 0.6–0.8. The comparison of the unit cell parameters with a standard straight a = f(x) line for the CdFe2O4-NiFe2O4 solid solutions sintered at 1000 °C, indicates that the isomorphous incorporation of cadmium reaches about 0.4 mol. For the products obtained from precursors with nominal x = 1.0, the formation of a solid solution between cadmium ferrite and maghemite has been assumed to justify the ferrimagnetism of samples. The first member of the solid solution series, NiFe2O4 (x = 0.0, y = 0.0) was used to prove that the hydrothermally obtained ferrites contain OH− groups substituting O2− ions in the spinel lattice.