High temperature Mössbauer spectroscopy of titanomagnetite and maghemite in basalts

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J _s -T) curves with Curie temperatures of about 840 K, has revealed considerable amounts of maghemite (-Fe₂O₃) in many samples. In view of the expected instability of maghemite at temperatures above 620 K, this reversibility came as a surprise. For further studies of the magnetization-temperature relationship of these minerals, we have constructed an elliptical radiation-heated furnace in which Mössbauer spectra can be acquired at temperatures between 300 and 900 K. Measurements at different temperatures have been obtained for two types of basalts, one in which the magnetic minerals are nearly pure magnetite and the other where the room temperature spectrum indicates a mixture of maghemite and magnetite. The two series show different features of the collapse of the internal magnetic hyperfine field, and the composition of minerals in the samples changes during the treatment, showing maghemite.     

Oxidation state of iron in mantle-derived magmas of the Icelandic rift zone

Olivine tholeiites are mantle-derived magmas that are formed by partial melting of their deep sources and which have equilibrated with mineral assemblages at slightly different subcrustal pressure-temperature conditions prior to eruption. The minimum depth of the pre-eruptive reservoirs of these magmas is in the order of 10–15 km and their liquidus temperatures fall within the range of 1180–1240 C. Three types of primitive olivine tholeiites are exposed along the rift zones in Iceland. In the present study, the ferric/ferrous ratios of natural glasses (pillow crusts) of the three types of olivine tholeiites were obtained by Mössbauer spectrometry. This technique is particularly well suited for the analysis of high-Mg glasses since it resolves microcrystallites of olivine which contribute to ferrous iron in chemical analysis. All results fall within 10–15% Fe(III). At the liquidus temperature of these glasses, this ferric/ferrous ratio corresponds to fugacity close to the fayalite-magnetite-quartz-oxygen (FMQ) buffer with an uncertainty of less than one log unit in fO₂. This result confirms that there is no significant difference in the oxidation state of the three magma types.     

Structural properties of LaxSr2−xFeO4±δ at high temperature and under reducing conditions

A series of materials, La_xSr_2−xFeO_4±δ (x=0.6-1.0), have been investigated structurally using neutron and X-ray diffraction techniques as well as Mössbauer spectroscopy as a function of temperature and composition. These materials adopt the I4/mmm K₂NiF₄-type structure over a wide range of temperatures (25-1200°C) and oxygen stoichiometries. In particular, it was observed that at a critical composition of x>0.8 there was a significant shift in the lattice parameters. This was attributed to changes in the Fe³⁺ content and the resultant effect of this on the d_z² orbitals giving a lengthening of the Fe-O bonds. On heating, completely linear behavior in both the a and c cell constants was observed, masking underlying bond length changes.     

The synthesis of 6,9-bis[(aminoalkyl)amino] substituted benzo[g]quinoxaline-, benzo[g]quinazoline- and benzo-[g]phthalazine-5,10-diones via regiospecific displacements

The synthesis of 6,9-difluoro substituted benzo[g]quinoxaline-5,10-diones ( 3A ), benzo[g]quinazoline-5,10-diones ( 3B ) and benzo[g]phthalazine-5,10-diones ( 3C ) have been accomplished. Treatment of 3A, 3B or 3C with diamines or N-(t-butoxycarbonyl)ethylenediamine led to the corresponding 6,9-bis[(aminoalkyl)amino]-substituted analogues related to 2A, 2B and 2C , respectively. The mono-substituted derivatives 4h and 4i could be isolated from displacements commencing from 3A . A competitive ring-opening of the pyrimidine ring of 2C occurred during the reaction with N,N-dimethylethylenediamine. Removal of the BOC-protecting group from 2Ac led to the hydrochloride salt 2Ab . A novel synthetic pathway to 6,9-dihydroxybenzo[g]-phthalazine-5,10-dione ( 21a ) was developed. Conversion of 21a to the ditosylate 21b was readily accomplished. Treatment of 21b with N,N-dimethylethylenediamine or N-(t-butoxycarbonyl)ethylenediamine led to 2Ca and 2Cc , respectively. Removal of the BOC-protecting group from 2Cc with trifluoroacetic acid followed by ion-exchange led to the hydrochloride salt 2Cb . Treatment of ditosylate 21b with N-(t-butoxycar-bonyl)ethylenediamine also led to the mono-substituted analogue 25a along with a small amount of the O-S cleavage product 25b . Treatment of 25a with N,N-dimethylethylenediamine led to the unsymmetrically substituted derivative 25c which was converted into the trifluoroacetate salt 25d.     

Iron-57 Mössbauer spectroscopic study of fluorinated strontium orthoferrite

A nuclear spin maser of a new type, that employs a feedback scheme based on optical nuclear spin detection, has been fabricated. The spin maser is operated at a low static field of 30 mG by using the optical detection method. The frequency stability and precision of the spin maser have been improved by a highly stabilized current source for the static magnetic field. An experimental setup to search for an electric dipole moment (EDM) in ¹²⁹Xe atom is being developed.     

Disordered chromite in the Martian meteorite Allan Hills 84001

The magnetic structure of NiFe₂O₄ nanoparticles has been investigated by means of Mössbauer spectra at T?=?4.2 K in applied fields up to 12 T. Four samples were studied, with mean particle diameters ranging from 4.3 to 8.9 nm. All spectra could be decomposed into three sextets, two corresponding to the ferrimagnetic sublattices of Fe ions in the spinel structure (core) and the third one to randomly frozen spins near the particle surface (shell). The shell thickness, calculated from the fraction of disordered spins, was found to be about one-third of the particle radius at H _app?=?0 and to decrease with the applied field toward a common limit of ~0.4 nm. The mean canting angle relative to the field was also found to decrease for increasing fields, at a rate inversely correlated to the particle size.     

Tin-doped spinel-related oxides of composition M3O4 (M = Mn, Fe, Co)

Tin-doped compounds of spinel-related M₃O₄ (M = Fe, Mn, Co) have been studied by ¹¹⁹Sn and ⁵⁷Fe Mössbauer spectroscopy in the temperature range of 20–600 K. The ¹¹⁹Sn Mössbauer spectra recorded down to 20 K from the non-iron-containing compounds of Co₃O₄ and Mn₃O₄ contained only doublets showing no transfer of magnetic properties from cobalt or manganese to the dopant tin ions. In contrast, the tin-doped-(FeCo)₃O₄ and (FeMn)₃O₄ gave ¹¹⁹Sn and ⁵⁷Fe Mössbauer spectra, which showed magnetic hyperfine interactions. The Curie temperature has been estimated for the former sample.     

Magnetic interaction in ferrous antimonite, FeSb2O4, and some derivatives

Ferrous antimonite, FeSb(2)O(4), which is isostructural with Pb(3)O(4), and some lead-?and cobalt-doped variants of composition FeSb(1.5)Pb(0.5)O(4) and Co(0.5)Fe(0.5)Sb(1.5)Pb(0.5)O(4) have been examined by (57)Fe and (121)Sb?M?ssbauer spectroscopy. Antimony is present as Sb(3+). The presence of Pb(2+) on the antimony site induces partial oxidation of Fe(2+) to Fe(3+). There is no Verwey-type transition in which electrons are shared between iron in different oxidation states. The quasi-one-dimensional magnetic structure gives rise to situations in which weakly coupled Fe(2+) ions can coexist in a non-magnetic state alongside Fe(3+) ions in a magnetically ordered state.