Navegando por Autor "Alboom, Antoine Van"
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Item Defciency of water molecules in the crystallographic structure of vauxite.(2018) Alboom, Antoine Van; Costa, Geraldo Magela da; Grave, Eddy DeA vauxite mineral sample from Huanuni, Bolivia, was studied by XRD, TGA and Mössbauer spectroscopy. The XRD revealed the sample as having the typical triclinic structure of vauxite. The chemical formula was determined as (Fe0.88Mn0.01)Al1.99(PO4)2(OH)1.75(H2O)5.31, implying some Fe2+, OH− and H2O deficiencies. The TGA curve showed ca. 27% loss of weight over a temperature range from 80 to 400 °C, supposedly due to the loss of water and hydroxyl groups. For the first time, Mössbauer spectra for vauxite were collected over a wide temperature range between 9 and 310 K. No magnetic ordering was detected. The spectra could be successfully and consistently analyzed by a superposition of four doublet subspectra. On the basis of the relation between the center shift and the mean Fe-ligand distance on the one hand and the center shift values for the various doublets on the other hand, one doublet was assigned to Fe(2). For the other doublets, it is proposed that, as a result of the H2O deficiency in the structure of the present vauxite sample, vacancies are present in the second coordination spheres of some Fe(1) and that these vacancies affect the quadrupole splitting of the corresponding Fe(1) cations, thus causing three Fe(1) doublet components in the Mössbauer spectra. The temperature variations of center shift and quadrupole splitting of the various doublet contributions are presented and discussed.Item Low-temperature Mossbauer study of heterosite, (Fe, Mn)PO4.(2013) Grave, Eddy De; Costa, Geraldo Magela da; Alboom, Antoine Van; Vandenberghe, Robert EmileThe heterosite phase occurring in a pegmatitic rock sample was characterized by X-ray diffraction, by energy-dispersive X-ray spectroscopy and by Mossbauer spectroscopy. The orthorhombic unit-cell parameters, expressed in A˚ , were found as a = 9.733 (1), b = 5.837 (1) and c = 4.776 (1). The composition was determined to be (Fe0.54Mn0.43Mg0.04)PO4. Mossbauer spectra recorded at temperatures T of 65K and higher consist of two broadened quadrupole doublets. Their isomer shifts ı are both diagnostic for the ferric state. The dominant doublet (∼60% of total area) exhibits an average quadrupole splitting _EQ,av of 1.62mm/s at room temperature, while the weaker broader doublet has _EQ,av = 0.68 mm/s. For temperatures T≤60K the spectra are composed of a broad sextet and a central quadrupole doublet. The doublet persists down to the lowest applied temperature of 17 K. It is concluded that this doublet is due to an Fe-bearing phase other than heterosite and which gives rise to the inner doublet appearing in the spectra recorded at T≥65 K. The broad sextets, attributable to the heterosite phase, were fitted with model-independent hyperfine-field distributions. However, it was consistently experienced that using the common Lorentzian-shaped elementary sextets composing the distribution, could not adequately reproduce the observed line shapes. Instead, the calculations had to be based on the diagonalization of the complete hyperfine-interaction Hamiltonian. This is due to the unusually strong quadrupole interaction. The as-such calculated hyperfine parameters of the heterosite phase at 17K may be summarized as follows: maximum-probability hyperfine field Bhf,m = 473 kOe, isomer shift ıFe = 0.54 mm/s, average quadrupole coupling constant ½e2qQ = 1.50 mm/s, asymmetry parameter of the EFG _ = 0.80, and polar angles of the hyperfine field with respect to the EFGs principal axes frame˝=40◦ and _ =90◦. The temperature variation of the hyperfine field was interpreted in terms of the Bean–Rodbell (BR) model. The BR parameter, _BR, was found to be 0.90, indicating a first-order magnetic transition at TN = 59.7 K. The temperature variation of the isomer shift is explained by the second-order Doppler shift ıSOD. Using the Debye model for the lattice vibrational spectrum for calculating ıSOD, the characteristic Mossbauer temperature _M was found to be 400 K, which is unusually low for a ferric compound.Item Mössbauer spectroscopic study of natural eosphorite, [(Mn,Fe)AlPO4(OH)2H2O].(2015) Alboom, Antoine Van; Resende, Valdirene Gonzaga de; Costa, Geraldo Magela da; Grave, Eddy DeA 57Fe Mössbauer spectroscopic study of natural eosphorite, (Mn,Fe2+)AlPO4(OH)2H2O, is reported. The Mössbauer spectra were collected at temperatures between 295 and 4.2 K. At temperatures exceeding 30 K, the Mössbauer spectra consist of a somewhat broadened quadrupole doublet with a splitting of 1.73 mm/s at 295 K. From the spectrum recorded at 250 K in an external magnetic field of 60 kOe, it is derived that the sign of the principal component of the electric field gradient (EFG) is negative and that the EFG’s asymmetry parameter is large (~0.5). From these findings, it is concluded that the octahedral coordination of the ferrous cations exhibits in first order a trigonal compression with a further lowering from axial symmetry due to an additional distortion. The spectrum collected at 4.2 K shows the existence of magnetic ordering. It was analyzed in terms of the full hyperfine interaction Hamiltonian, yielding a magnetic hyperfine field value of 146 kOe and EFG characteristics that are fully in line with those obtained from the external-field spectrum. In the temperature range from 10 to 30 K, the spectra indicate the occurrence of relaxation effects. They could be satisfactorily described in terms of the Blume-Tjon (BT) model for electronic relaxation, assuming a superposition of a slow- and a fast-relaxation component. The observed temperature dependence of the isomer shift is adequately described by the Debye model for the lattice vibrations. The characteristic Mössbauer temperature was found as (360 ± 20) K and the zero-Kelvin intrinsic isomer shift as (1.480 ± 0.005) mm/s. The variation of the quadrupole splitting with temperature is explained by the thermal population of the Fe2+ electronic states within the T2g orbital triplet, which is split by the trigonal crystal field in a singlet ground state and an upper doublet state, the latter being further split into two orbital singlets by an additional distortion. The energies of these latter excited states with respect to the ground state are calculated to be 280 ± 20 and 970 ± 50 cm–1, respectively, based on a point-charge calculation of the 5D level scheme. This calculation confirms the sign of the Vzz component of the electric field gradient being negative as it is determined from the external magnetic field measurement and from the magnetic spectrum acquired at 4.2 K. Iron phosphates are widely spread minerals in the Earth’s crust and are expected to occur in soils and rocks on Mars as well. Mössbauer spectroscopy as complementary source of information to the results of other analytical techniques, could therefore be useful to identify and characterize the environmental Fe-bearing phosphates.