Davide Levy

Synthetic MgAl2O4 (spinel) at high-pressure conditions (0.0001-30 GPa): A synchrotron X-ray powder diffraction study

Abstract The equation of state and the structural behavior of synthetic MgAl2O4 have been investigated using synchrotron X-ray powder diffraction data collected to 30 GPa at room temperature. The Birch-Murnaghan, Vinet, and Poirier-Tarantola models have been fitted to the observed P-V data. The Birch-Murnaghan equation of state, with V0 fixed at its experimental value, yields K0 = 190.8(±1.2) GPa, K0 = 6.77(±0.15) and K″0 = -0.075 GPa-1 (implied value). The compression of spinel occurs with a negligible change of the fractional coordinate of oxygen. Therefore the structural shrinking is a function of cell edge shortening alone. The results presented here are compared with those from the literature.

Crystal structure refinement of Maya Blue pigment prepared with deuterated indigo, using neutron powder diffraction

Maya Blue, a synthetic pigment produced by the ancient Mayas in pre-Columbian America, is the combination of a fibrous clay (palygorskite) and an organic blue dye (indigo). The main features of the structure of Maya Blue are already known, although the specific interactions occurring between clay and dye have yet to be completely explained. The details of the structure were studied using the Rietveld method on neutron powder diffraction patterns collected at POLARIS (Spallation Neutron Source ISIS) on a Maya Blue sample freshly synthesized using deuterated indigo. The position of the dye molecule in the structure of Maya Blue and the nature of its interactions with the clay framework were described. In the pigment, indigo lies within the palygorskite channels partially substituting the zeolitic water previously expelled during the synthesis. The dye and the water mutually compete to occupy the clay microchannels, although some portions may be empty, containing neither indigo nor water. Furthermore, the encapsulation of indigo increases the disorder in the disposition of the residual zeolitic water. The occupancy of indigo results to be higher in the orthorhombic than in the monoclinic polymorph, presumably for the faster loss of zeolitic water in the former polymorph during heating. The quantity of indigo in the pigment is rather low ( b axis. The stability of Maya Blue is guaranteed by strong H-bonds formed between the clay structural water and the indigo C=O group. All the present results are coherent with data obtained in previous studies.

P-V equation of State, thermal expansion, and P-T stability of synthetic zincochromite (ZnCr2O4 spinel)

Abstract The elastic properties and thermal behavior of synthetic zincochromite (ZnCr2O4) have been studied by combining room-temperature high-pressure (0.0001.21 GPa) synchrotron radiation powder diffraction data with high-temperature (298.1240 K) powder diffraction data. Elastic properties were obtained by fitting two Equations of State (EoS) to the P-V data. A third-order Birch-Murnaghan model, which provides results consistent with those from the Vinet EoS, yields: K0 = 183.1(±3.5) GPa, K’ = 7.9(±0.6), K’’ = .0.1278 GPa-1 (implied value), at V0 = 577.8221 Å3 (fixed). Zincochromite does not exhibit order-disorder reactions at high temperature in the thermal range explored, in agreement with previous studies. The volume thermal expansion was modeled with αV = α0 + α1T + α2/T-2, where only the first coefficient was found to be significant [α0 = 23.0(4) 10-6K-1]. Above 23 GPa diffraction patterns hint at the onset of a phase transition; the high pressure phase is observed at approximately 30 GPa and exhibits orthorhombic symmetry. The elastic and thermal properties of zincochromite were then used to model by thermodynamic calculations the P-T stability field of ZnCr2O4 with respect to its oxide constituents (Cr2O3 and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics.

Structure of magnetite (Fe3O4) above the Curie temperature: a cation ordering study

A pristine magnetite (Fe3O4) specimen was studied by means of Neutron Powder Diffraction in the 273–1,073xa0K temperature range, in order to characterize its structural and magnetic behavior at high temperatures. An accurate analysis of the collected data allowed the understanding of the behavior of the main structural and magnetic features of magnetite as a function of temperature. The magnetic moments of both tetrahedral and octahedral sites were extracted by means of magnetic diffraction up to the Curie temperature (between 773 and 873xa0K). A change in the thermal expansion coefficient around the Curie temperature together with an increase in the oxygen coordinate value above 700xa0K can be observed, both features being the result of a change in the thermal expansion of the tetrahedral site. This anomaly is not related to the magnetic transition but can be explained with an intervened cation reordering, as magnetite gradually transforms from a disordered configuration into a partially ordered one. Based on a simple model which takes into account the cation-oxygen bond length, the degree of order as a function of temperature and consequently the enthalpy and entropy of the reordering process were determined. The refined values are ΔH0xa0=xa0−23.2(1.7) kJxa0mol−1 and ΔS0xa0=xa0−16(2) Jxa0K−1xa0mol−1. These results are in perfect agreement with values reported in literature (Mack et al. in Solid State Ion 135(1–4):625–630, 2000; Wu and Mason in J Am Ceramic Soc 64(9):520–522, 1981).

The equation of state of PbTiO3 up to 37 GPa: a synchrotron x-ray powder diffraction study

High-pressure synchrotron x-ray powder diffraction patterns were collected using ID09 of ESRF (Grenoble, France) for a powder sample of PbTiO3, placed in a diamond anvil cell. The patterns were collected at room temperature using nitrogen (up to 37 GPa) and methanol–ethanol solution (up to 7 GPa) as pressure-transmitting media. The bulk moduli were calculated for the first time using the Vinet equation of state and they were compared to those of isostructural compounds. The trend of the spontaneous polarization as a function of pressure confirms that the ferroelectric–paraelectric phase transition at 11.2 GPa possesses a second-order character.

Crystal structure refinement of a sepiolite/indigo Maya Blue pigment using molecular modelling and synchrotron diffraction

Maya Blue is an artificial pigment used in Pre-Columbian America, renowned for its chemical stability. The pigment can be considered a precursor of modern inclusion compounds as a hosting microporous clay (palygorskite or sepiolite) shelters the guest indigo dye (≤2 wt%) within its micro-channels. While most papers on Maya Blue are focused on the interaction between indigo and palygorskite, this study describes the pigment structural features when sepiolite is the host structure. Synchrotron X-ray powder diffraction patterns were collected on both pristine sepiolite and sepiolite + indigo (2 wt%) pigment. The pigment structure was investigated with the Rietveld method, basing on both molecular mechanics and the refined structure of sepiolite. The evidence obtained shows that: (i) indigo molecules, encapsulated within the micro-tunnels, stay close to a TOT strip in order to receive H-bonds from the structural OH 2 ; (ii) there is no evidence for direct metal-oxygen bonds between the sepiolite Mg and the indigo C=O groups, as the applied heating (≤190 °C) does not remove structural OH 2 ; (iii) the indigo molecule is affected by 4-fold disorder, as it occupies only one of four partially superposed equivalent sites; (iv) indigo and the zeolitic H 2 O compete to occupy the channels; refined occupancies showed that the dye fills 27 vol% of the channels whereas 73 vol% is occupied by H 2 O. Calculated indigo weight % (1.9) is in close agreement with experimental data; (v) indigo encapsulation modifies zeolitic H 2 O sites, increasing the number and strength of mutual hydrogen bonds; (vi) difference-Fourier maps computed removing indigo contribution confirmed the position of the molecule inside the channels.

Equation of state and compressibility of phlogopite by in-situ high-pressure X-ray powder diffraction

The elastic properties of a natural phlogopite have been studied by in-situ high-pressure X-ray powder diffraction experiments on the ID9 beamline at the ESRF, at room temperature. Several EoS models (Birch-Murnaghan, Vinet, Poirier-Tarantola) have been fitted to the experimental P-V data and the results obtained are presented and discussed. The third-order Birch-Murnaghan EoS, assuming V 0 fixed at its experimental value, yields K 0 = 49.7(±0.5) GPa, K’ 0 =8.59(±0.19), K’ 0 =-0.5953 GPa −1 (implied value). The axial compressibilities at room conditions, determined by the third-order Birch-Murnaghan EoS, result in β EoS a0 =3.48(5), β EoS b0 =3.2(1), β EoS c0 =13.2(1) 10 −3 GPa −1 . Equilibrium thermodynamic calculations have been carried out to show how the new elastic parameters here reported affect the stability field of phlogopite.

Elastic properties of andradite and grossular, by synchrotron X-ray diffraction at high pressure conditions

The bulk elastic properties of natural andradite and grossular have been investigated by synchrotron radiation powder diffraction at high pressure conditions, up to 37 GPa, on the ID9 beamline (ESRF). The P-V data have been interpolated by the Birch-Murnaghan EOS and the Vinet EOS, and the bulk modulus and its first derivative versus P determined. The results obtained by the different EOS models are compared with one another, and with earlier issues. For andradite and grossular the Birch-Murnaghan EOS yields K 0 = 158.5(±1.0) GPa and K 0 ’ = 5.77(±0.10) [ K 0 “=−0.056 GPa −1 implied value], and K 0 =169.3(±1.2) and K 0 ’ = 5.92(±0.14) [ K 0 ”=-0.056 GPa −1 implied value], respectively. The elastic properties here reported are compared with previous issues. The cell edge compressibilities, which have been determined by fitting the experimental lattice parameters with polynomials in P , are β a = 0.00208(2) GPa −1 , for andradite, and β a = 0.00194(3) GPa −1 , for grossular.

Kinetics of the Decomposition of Crocidolite Asbestos: A Preliminary Real-Time X-Ray Powder Diffraction Study

This work is a preliminary kinetic study of the crocidolite decomposition followed in situ at high temperature using real time conventional powder diffraction and DTA in the temperature range 720-795 °C. The data analysis using the Avrami models indicates that the rate limiting step of the reaction is monodimensional ion diffusion (n=0.5) with an activation energy of 129 (10)kcal/mole.

P-V and T-V equations of State of natural biotite : An in-situ high-pressure and high-temperature powder diffraction study, combined with Mossbauer spectroscopy

Abstract The P-V and T-V equations of state of a natural biotite sample (Mg/Fe ratio ≈ 1) have been studied using in-situ high-pressure (0.0001.11 GPa) synchrotron radiation powder diffraction at the European Synchrotron Radiation Facilities (ESRF) in Grenoble, France, and in-situ high-temperature (298.610 K) laboratory X-ray powder diffraction. A third-order Birch-Murnaghan model [V0 = 498.7(1) Å3, measured value] provides the following elastic parameters: K0 = 49(1) GPa, K’ = 8.1(5). The volume thermal expansion is satisfactorily described by a constant value resulting in 37(2) 10.6 K-1. Mössbauer spectroscopy proves that REDOX reactions have occurred upon heating, presumably 2(OH- + Fe2+) → 2O2- + 2Fe3+ + H2 ↑ and/or 4Fe2+ + 2OH- + O2 → 4Fe3+ + 3O2- + H2O. On the basis of the elastic and thermal parameters measured we have modeled the deformation contribution (Gdeform) to the Gibbs energy. The third-order Birch-Murnaghan model with V0 fixed at its experimental value and the model with refined V0 do not significantly differ from one another in terms of Gdeform. A comparison based on Gdeform between biotite and phlogopite shows a better compliance to P of the former, though balanced in mineral reactions by a difference of molar volume, i.e., V0(biotite) > V0(phlogopite).