M. A. B. de Araujo

Crystallization of ferroelectric LiNbO3 in niobophosphate glasses

Niobophosphate glasses [(xNb2O5(0.5 − x)P2O5)0.5Li2O]: yFe2O3 were prepared by the melt-quenching method and analysed by means of infrared spectroscopy and x-ray powder diffraction. LiNbO3 crystals were detected for high niobium contents samples. The use of both techniques gives new information about the structure of niobium-phosphate glasses and glass-ceramics.

Spectroscopic Studies of Iron Niobophosphate Glasses

We investigated iron niobiophosphate glasses [0.1Nb 2 O 5 . 0.4P 2 O 5 . 0.5Li 2 O]yFe 2 O 3 (y = 0, 0.1, 0.2, 0.3, 0.4, 0.5) prepared in ambient atmosphere using the melt quenching technique. The samples were studied using X-ray powder diffraction Fe Mossbauer and infrared spectroscopy, in order to investigate the effect of the increasing concentration of α-Fe 2 O 3 in the glass matrix. According to Mossbauer results high spin Fe 3+ and high spin Fe 2+ at distorted octahedral sites are present in all samples. The distortion of the Fe 3+ and Fe 2+ ions sites increases from low (y = 0.1) to high (y = 0.3) iron content samples. The measurements of the infrared absorption, X-ray powder diffraction and Mossbauer spectroscopy also indicate that LiFePO 4 and α-Fe 2 O 3 crystalline phases are present for high iron content samples (y = (1.4, 0.5). The limit of solubility of α-Fe 2 O 3 in these glasses is around 23 mol%.

Structure of iron niobophosphate glasses investigated by DTA, infrared and Mössbauer spectroscopy

Activation energies for crystallization of 50 glasses, with x = 0, 5 and 10 mol%, were determined by differential thermal analysis (DTA). The structure was investigated by infrared and M?ssbauer spectroscopy. Samples with x = 5 mol% of show larger activation energy . Absorptions at P-O-P, P=O, and units are presents in these glasses. P=O absorptions disappear at compositions with x = 5 and 10 mol% of , which shows absorption around , attributed to units. According to M?ssbauer results high spin and at distorted octahedral sites are present in samples with x = 5 and 10 mol% of , thus working as a network modifier.

Thermal transformations of natural jacobsite (Mn, Fe)3O4

Mossbauer spectroscopy and x-ray powder diffraction have been used to study the natural manganese ferrite known as jacobsite (Mn, Fe)3O4, after calcination at temperatures varying from 400°C up to 1160°C. Calcination at temperatures between 400°C and 900°C results in the formation of haematite and bixbyite (Mn, Fe)2O3. The room temperature Mossbauer spectra of samples calcined above 900°C indicate that the haematite and the bixbyite transform, as the calcination temperature is increased, gradually into synthetic manganese ferrite. The results suggest that Mossbauer spectroscopy of ferrites can supply information about the thermal history of such samples.

Cation Distribution in Natural Ferrites

Mössbauer spectra of jacobsite have been recorded at temperatures of 4.2, 125, 180, 240 and 300 K and with an external magnetic field of 60 kOe applied parallel to the incident radiation at 4.2 K. The results show the presence of two different magnetic hyperfine interactions associated with the iron nuclei. Spectra of samples in external magnetic fields indicate the same sign for the magnetic hyperfine interactions, implying that both iron ions occupy only the octahedral sublattice. These results are compared with data from synthetic stoichiometry manganese ferrite, and a model of cation distribution, assuming the existence of vacancies, is proposed. According to this model, the mineral jacobsite is rich in manganese compared with stoichiometric MnFe2O4 ferrite.

Study of substituted pentacyanoferrate /II/ complexes by Mössbauer spectroscopy

The57Fe Mössbauer spectra of substituted pentacyanoferrate /II/ complexes [/CN/5 FeII L]3− have been obtained for L=adenine, guanine, purine, caffeine, ethylene sulphide, dimethyl sulphoxide, and ammonia. The Mössbauer parameters are utilized to classify the various ligands according to their σ and π bonding abilities. A linear correlation between the Mössbauer isomer shift and the tetragonal distortion caused by the ligand L is proposed. Comparison between this linear correlation and the approximative method of Wentworth and Piper1 is made.

Analysis of impact-induced Fe2+ disorder in the pyroxene of the Ibitira meteorite

Mössbauer spectroscopy and X-ray diffraction analysis indicate that the only iron compound present in the Ibitira meteorite is the pyroxene (Mg, Fe, Ca)2Si2O6 known as pigeonite [1]. The Mössbauer spectrum shows Fe2+ in the two different crystallographic sites, M1 and M2, and the population ratio Fe2+ (M1)/Fe2+ (M2) indicates a reasonable degree of cation order. Comparison between our results and results obtained from pyroxene shocked under controlled conditions in the laboratory [2] suggests that, the meteorite was subjected to a impact of low intensity.

Low temperature Mössbauer spectra of the Ibitira meteorite (achondrite)

The Mossbauer spectra of a powdered sample of the Ibitira meteorite (achondrite) have been obtained at different temperatures, ranging from 4.2 K up to 300 K. X-ray diffraction analysis indicates that the meteorite consists mostly of anorthite (CaAl2Si2O8) and pigeonite [(Mg, Fe, Ca)2Si2O6)] The Mossbauer spectrum at room temperature consists of an asymmetric doublet with parameters characteristic of iron in the oxidation state Fe(II). The asymmetry is due to the overlapping of two quadrupole doublets arising from Fe(II) in the two different crystallographic sites (M1 and M2) of pigeonite. The temperature dependence of the Mossbauer parameters is different for the two sites, so they are clearly distinguishable at low temperatures (<55 K). No magnetic effects were observed between 300 K and 14 K. The spectrum obtained at 4.2 K exhibits magnetic relaxation. The mechanism of relaxation has not yet been deeply investigated. It should probably be spin-spin relaxation as it was recently observed in terrestrial pyroxenes [1].

Structural and electrical properties of iron molybdenum phosphate glasses

SummaryIron molybdenum phosphate glasses [xMoO3 · (0.6 -x)P2O5 · 0.4Li2O] :yFe2O3 with 0 ≤x ≤ 0.6 andy = 0.03 (mol%) prepared in ambient atmosphere using the melt quenching technique were studied by using DC electrical conductivity,57Fe Mössbauer and infrared spectroscopies. The DC conductivity depends on the MoO3 concentrationx. It was observed that, with increasingx, the ratio Fe2+ /(Fe3+ + Fe2+) and the DC conductivity increase. Infrared spectroscopy and X-ray powder diffraction indicate that a Li2 MoO4 crystalline phase is present for high MoO3 content samples (x = 0.5, 0.6).

Mössbauer analysis of crystallization processes in iron-soda-lime-silicate glasses

Abstract57Fe Mössbauer spectroscopy was used to observe the crystallization process in glasses with composition denoted by [0.50SiO2, 0.45Na2O, 0.05CaO]100-x(Fe2O3)x with x equal to 4, 10 and 16 mol%. It is shown that Mössbauer spectroscopy is a very suitable technique for detecting and checking the very early stages of crystallite formation in amorphous materials. The crystallization process was observed and qualitatively analysed by means of classical volume nucleation theory.