E. Mattievich

Mössbauer studies of the solid solutions of synthetic hureaulite, [FexMn5-x][PO4H]2[PO4]2[H2O]4

Abstract Nine samples of synthetic hureaulites, [Fe x Mn 5 -x ][PO 4 H] 2 [PO 4 ] 2 [H 2 O] 4 , in the interval 1 ≤ x ≤5, were prepared by hydrothermal methods and analysed by Mossbauer spectroscopy in order to determine the occupation of the sites by Fe 2+ as a function of iron concentration, as well as the influence of these substitutions on the hyperfine parameters.

Magnetism in the series (FexMn1−x)5(PO4)2(PO4H)2(H2O)4

Abstract Differential magnetic susceptibility measurements in synthetic polycrystals of the hureaulite series (FexMn1−x)5(PO4)2(PO4H)2(H2O)4 (0 ≤ x ≤ 1) are reported for the first time. Its structure consists essentially of a condensation of pentamers formed by distorted edge-sharing octahedra whose centres are occupied by Mn2+ or Fe2+ ions and disposed in three different crystallographic sites in the proportion (III):(II):(I) = 2:2:1. The results from high temperature susceptibility measurements show an effective magnetic moment ranging from 11.89 to 13.53 μB/f.u., suggesting that the spins in the pentamers are all parallel except one, which was attributed to the central one, that would have its spin reversed. The compound with x = 1 (ferrous compound) behaves as a normal canted antiferromagnet down to the Neel temperature, 12.7 K. Below this temperature the susceptibility tends to zero with the temperature. It was shown that the replacement of Fe2+ by Mn2+ introduces ferromagnetic correlations in the neighbourhood of the critical temperature due to anisotropic exchange interactions (weak ferromagnetism). The compound for x = 0 (manganous compound) is a weak ferromagnet with Neel temperature of 6.0 K. It was also observed that for x ≤ 0.40 a second phase transition begins to appear due, possibly, to a rearrangement of spins.

Magnetic studies on synthetic phosphoferrite Fe 3 2+ (H2O)3(PO4)2

Synthetic crystals of phosphoferrite were studied by Mössbauer spectroscopy for temperatures ranging from 4.2 to 17.5 K. The analysis of the behaviour of the hyperfine fields for the two unequivalent sites near the Néel temperature (17.4 K) allowed the determination of the critical exponentβ for each site asβ1=0.171 andβ2=0.314.

Mössbauer studies in volcanic material of the peruvian volcano Quimsachata

We report here Mössbauer studies of lava samples from the extinct volcano Quimsachata (Per≈u). The Mössbauer spectra of samples from the outer part of the crater show the presence of olivine, pyroxene and one of them exhibits superparamagnetic maghemite. A piece of extruded rock from the inner part shows the spectrum of magnetite that could be consequence of the dehydration of ferric hydroxides (α-FeOOH) forming Fe2O3 and subsequent reduction to magnetite in the volcanic reducing atmosphere.