1,721,128 research outputs found
Single-crystal diffraction at megabar conditions by synchrotron radiation
No full textCrystal structure determination at extreme pressures is currently possible at synchrotron beamlines optimized for such a purpose. We report the description of the experimental setup available at European Synchrotron Radiation Facility ID09 beamline (Grenoble, France) and, with two examples, we illustrate the state-of-the-art experiments currently performed at third-generation synchrotrons. The first example concerns the determination of the equation of state and the structural behavior of low-spin Fe-bearing siderite in the megabar pressure range. Siderite, in fact, undergoes a first-order isosymmetric transition at 45 GPa, and, above this pressure, it features Fe2+ in electronic low-spin configuration. The local configuration of Fe coordination polyhedra, determined by structural refinements, significantly deviates from a regular octahedron. Nevertheless, no further structural transition is detected up to the maximum pressure reached in our experiments, 135 GPa. The analysis of the Fe-O bond length extrapolated to ambient pressure, which indicates that the difference in ionic radii between the high- and the low-spin state of Fe 2+ is 0.172 Å, in excellent agreement with the tabulated data by Shannon and Prewitt [Effective ionic radii in oxides and fluorides. Acta Crystallogr. 1969;B25:925-946]. The second example concerns the determination and refinement of the oP8 structure adopted by sodium in the pressure interval 118-125 GPa, using an experimental dataset collected at 118 GPa. The orthorhombic [a=4.7687(15) Å, b=3.0150(6) Å, c=5.2423(7) Å, V=75.4(3) Å3] oP8 structure is topologically related to the MnP structure, with two non-equivalent atoms in the unit cell. Despite the weak scattering factor of Na atoms, the quality of the data also allows meaningful displacement parameters refinements (R1=4.6%, 14 parameters, 190 diffractions, and 105 unique) demonstrating that the current accuracy of diffraction data at extreme pressures can be comparable with ambient condition measurements
Single crystal diffraction studies of phase transition of minerals across Fe high-low spin transition at high pressure
No full textThe spin state of Fe in structure of minerals relevant for the lower mantle mineralogy, is known to undergo a high to low spin state change. This phenomena is often coupled to a remarkable volume contraction and from a structural point of view, often is associated to isosymmetrical phase transition. Recent improvements at X-Ray beamlines for diffraction at extreme conditions at synchrotron facilities allow the possibility to perform single crystal diffraction and determine crystal structure of minerals at extreme conditions, including also structural studies across first or second order phase transition. The accurate knowledge of crystal structure and of phase behaviour at high pressure is a very important step in order to: 1-understand the physical properties; 2- have an accurate experimental constraint on numerical simulation. We report here three examples of structure determination by single crystal X-Ray diffraction at extreme conditions concerning phase transition related to Fe spin state change, measured at ID09A beamline (ESRF, France). CaFe2O4 undergoes a spin transition at 50 GPa. XRD before and after indicate the symmetry and crystal structure is the same. The transition is marked by 10 % volume contraction. The use of He as pressure transmitting media strongly reduced strain induced by pressure and let the crystal survive this transition, allowing for the first time direct determination of Fe-O bond length changes related to variation of spin state. The main structural difference between high and low spin structure is simply a collapse of FeO6 polyhedra. FeCO3 has been also investigated, and the results are also compared with already present in literature. FeCO3 undergoes a transition around 45 GPa, with a remarked hysteresis. In the pressure range 20-45 however an anomalous behaviour is noticed, probably related to a different spin interaction due to reduced Fe-Fe distances. Fe1-xS pyrrhotite has been investigated in two different structure (a monoclinic and a incommensurately modulated hexagonal structure). Both samples present a continuous increase of compressibility in the pressure range 0-8 GPa. Above the volume data can be fitted with a conventional EoS, and, proved also by spectroscopic measurement, Fe is present in low spin state. Pyrrhotite have been also investigated at high pressure and high temperature and the effect of temperature is to shift the pressure of transition towards higher values. Crystal structure refinement below and above spin transition indicate that there is a local significative rearrangement of the structure evidenced also by a strong increase of modulation intensity in incommensurate pyrrhotite
The MnCO3-II high-pressure polymorph of rhodocrosite
No full textWe investigated the behavior of MnCO3 in the pressure range 0-50 GPa and ambient temperature by synchrotron X-ray single-crystal diffraction technique. MnCO3 maintains the calcite-type structure (R3c symmetry) up to 44 GPa. Above this pressure we observed a phase transition. The highpressure phase, MnCO3-II, is triclinic, with cell parameters a = 2.928(2), b = 4.816(4), c = 5.545(4) Å, α = 101.71(6)°, β = 94.99(6)°, γ = 89.90(6)°, and V = 76.28(10) Å3 at 46.8 GPa. The structure is solved with the charge flipping algorithm. MnCO3-II is isostructural with CaCO3-VI. The density increase on phase transition is 4.4%. The occurrence of CaCO3-VI structure in MnCO3 composition indicates that CaCO3-VI structure is also adopted by carbonates with cations smaller than calcium
CaCO 3-III and CaCO 3-VI, high-pressure polymorphs of calcite: Possible host structures for carbon in the Earth's mantle
No full textCalcite, CaCO 3, undergoes several high pressure phase transitions. We report here the crystal structure determination of the CaCO 3-III and CaCO 3-VI high-pressure polymorphs obtained by single-crystal synchrotron X-ray diffraction. This new technical development at synchrotron beamlines currently affords the possibility of collecting single-crystal data suitable for structure determination in-situ at non-ambient conditions, even after multiphase transitions. CaCO 3-III, observed in the pressure range 2.5-15GPa, is triclinic, and it presents two closely related structural modifications, one, CaCO 3-III, with 50 atoms in the unit cell [a=6.281(1)Å, b=7.507(2)Å, c=12.516(3)Å, α=93.76(2)°, Β=98.95(2)°, γ=106.49(2)°, V=555.26(20)Å 3 at 2.8GPa], the second, CaCO 3-IIIb, with 20 atoms [a=6.144(3)Å, b=6.3715(14)Å, c=6.3759(15)Å, α= 93.84(2)°, Β=107.34(3)°, γ=107.16(3)°, V=224.33(13)Å 3 at 3.1GPa]. Different pressure-time experimental paths can stabilise one or the other polymorph. Both structures are characterised by the presence of non-coplanar CO 3 groups. The densities of CaCO 3-III (2.99g/cm 3 at 2.8GPa) and CaCO 3-IIIb (2.96g/cm 3 at 3.1GPa) are lower than aragonite, in agreement with the currently accepted view of aragonite as the thermodynamically stable Ca-carbonate phase at these pressures. The presence of different cation sites, with variable volume and coordination number (7-9), suggests however that these structures have the potential to accommodate cations with different sizes without introducing major structural strain. Indeed, this structure can be adopted by natural Ca-rich carbonates, which often exhibit compositions deviating from pure calcite. Mg-calcites are found both in nature (Frezzotti et al., 2011) and in experimental syntheses at conditions corresponding to deep subduction environments (Poli et al., 2009). At these conditions, the low pressure rhombohedral calcite structure is most unlikely to be stable, and, at the same time, Mg and Fe solubility in aragonite is hindered energetically in the 9-fold coordination site. Above 15GPa, and up to the maximum pressure investigated (40GPa), we observe the high-pressure polymorph CaCO 3-VI, triclinic [a=3.3187(12)Å, b=4.8828(14)Å, c=5.5904(14)Å, α=103.30(2)°, Β=94.73(2)°, γ=89.21(2)°, V=87.86(20)Å 3 at 30.4GPa] with 10 atoms in the unit cell. It is characterised by coplanar CO 3 groups but the structure is no longer layered, as in the lower pressure polymorphs. The density of the CaCO 3-VI structure (3.78g/cm 3 at 30.4GPa) is higher than aragonite. For this reason it could be supposed that a region may exist where this polymorph replaces aragonite in the Earth's intermediate mantle. The lower coordination number for the Ca site [7+2] instead of [9] in aragonite suggests that this structure could be easily adopted by an extended solid-solution range from calcite towards the dolomite [CaMg(CO 3) 2]-ankerite [CaFe(CO 3) 2] compositional join. The transitions from calcite to CaCO 3-III, CaCO 3-IIIb and CaCO 3-VI are perfectly reversible and after pressure release we always observe the calcite structure, with the sample recovered as a single-crystal. Indeed, it is highly unlikely that these structures can be observed in samples recovered from high-pressure environments
Synthetic MgAl2O4 (spinel) at high pressure conditions (0.0001-30 GPa): a X-ray powder diffraction by synchrotron radiation
No full textThe 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, K′0 = 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
Impact of lattice vibrations on equation of state of the hardest boron phase
Full text linkAn accurate equation of state (EOS) is determined for the high-pressure orthorhombic phase of boron, B(28), experimentally as well as from ab initio calculations. The unique feature of our experiment is that it is carried out on the single crystal of B(28). In theory, we take into consideration the lattice vibrations, often neglected in first-principles simulations. We show that the phonon contribution has a profound effect on the EOS of B(28), giving rise to anomalously low values of the pressure derivative of the bulk modulus and greatly improving the agreement between theory and experiment.Original Publication:Eyvas Isaev, Sergey Simak, Arkady Mikhaylushkin, Yu. Kh. Vekilov, E. Yu. Zarechnaya, L. Dubrovinsky, N. Dubrovinskaia, M. Merlini, M. Hanfland and Igor Abrikosov, Impact of lattice vibrations on equation of state of the hardest boron phase, 2011, Physical Review B. Condensed Matter and Materials Physics, (83), 13, 132106.http://dx.doi.org/10.1103/PhysRevB.83.132106Copyright: American Physical Societyhttp://www.aps.org
Phase transition of synthetic zinc ferrite spinel (ZnFe2O4) at high pressure, from synchrotron X-ray powder diffraction.
No full textSynthetic Zn-ferrite (ideally ZnFe2O4; mineral name: franklinite) was studied up to 37 GPa, by X-ray powder diffraction at ESRF (Grenoble, France), on the ID9 beamline; high pressure was achieved by means of a DAC. The P-V equation of state of franklinite was investigated using the Birch-Murnaghan function, and the elastic properties thus inferred [K0 = 166.4(± 3.0) GPa K′0 = 9.3(± 0.6) K′′0 = -0.22 GPa-1] are compared with earlier determinations for MgA1-spinel and magnetite. The structural behaviour of Zn-ferrite as a function of pressure was studied by Rietveld refinements, and interpreted in the light of a phase transition from spinel to either CaTi2O4- or MnFe2O4-like structure; this transformation occurs above 24 GPa
Orthovanadate wakefieldite-(Ce) in symplectites replacing vanadium-bearing omphacite in the ultra-oxidized manganese deposit of Praborna (Aosta Valley, Western Italian Alps)
Full text linkBecause of their unique structure and properties, rare-earth (REE) orthovanadates are extensively employed since decades in advanced ceramics, in particular in the laser industry in replacement of Nd:YAG. Ca-bearing rare-earth orthovanadate with empirical formula (Ce0.279 Ca0.271 Y0.267 Gd0.057 Nd0.055 Dy0.032 Sm0.027 La0.020 Th0.027 Sr0.002) (V5+0.908 Cr3+0.067 Fe3+0.017 As5+0.005) O4 • n H2O has been found in metacherts from Praborna (Italian Alps), as micrometer-sized euhedral crystals in clinopyroxene + plagioclase symplectites replacing eclogite-facies vanadium-bearing omphacite (Aegirine55–48Jadeite42–33Diopside10–8 with V2O3 ≤ 1.39 wt%). We applied synchrotron radiation single crystal micro-diffraction technique, recently optimized at ID09A beamline (ESRF, France), to determine the crystal structure of this mineral. It is tetragonal and isostructural with zircon, with a = 7.2233(12) Å, c = 6.3949(18) Å, V = 333.66(13) Å3, Z = 4, spatial group I41/amd and it has been therefore identified as Ca- and Y-bearing wakefieldite-(Ce) (ideally CeV5+O4). Cell parameters are in agreement with those of synthetic Ce0.7Ca0.3VO4. Raman spectra of the studied wakefieldite-(Ce) are comparable with natural and synthetic wakefieldite-(Ce) spectra and revealed the presence of OH groups and/or water of hydration, which is also suggested by the low totals in microprobe analyses. Mass balance indicates that wakefieldite-(Ce) is a by-product of the omphacite breakdown; omphacite and Mn-rich epidote, a minor reactant, provided vanadium and REE respectively. Petrological observation and thermodynamic modeling suggest that the mineral, coexisting with hematite, Mn-rich epidote and braunite, formed during retrogression to greenschist-facies conditions at ultra-oxidized conditions (∆FMQ ≥ +16 log units), which are often observed in Mn-oxide ores. Wakefieldite is an effective scavenger of REE in oxidized geological environments at P–T conditions that range from sedimentary to medium-grade metamorphic settings, even where the REE bulk concentration is negligible. Its rarity reflects both the overall low abundance of vanadium and the scarcely recorded ultra-oxidized conditions in metamorphic rock systems, where REE phosphates (i.e., monazite, xenotime) are commonly found instead
Synchrotron X-ray powder diffraction study of natural P2/n omphacites at high pressure conditions
No full textSynchrotron X-ray powder diffraction experiments at high pressure conditions (0.0001-13 GPa) were performed at ESRF (Grenoble-F), on the beamline ID9, to investigate the bulk elastic properties of natural P2/n- omphacites, with quasi-ideal composition. The monoclinic cell parameters a, b, c and β were determined as a function of pressure, and their compressibility coefficients are 0.00277(7), 0.00313(8), 0.00292(5) and 0.00116(4) GPa-1, respectively. The third-order BirchMurnaghan equation of state was used to interpolate the experimental P - V data, obtaining Ko = 116.6 (±2.5) GPa and K1o = 6.03 (±0.60). Ko was also determined by means of the axial and angular compressibilities [122.5(±1.7) GPa], and of the finite Lagrangian strain theory [121.5(±1.0) GPa]. The discrepancies on Ko are discussed in the light of a comparison between techniques to determine the bulk modulus of crystalline materials from static compression diffraction data
High-pressure behavior of davyne [CAN-topology] : an in situ single-crystal synchrotron diffraction study
No full textThe high-pressure elastic behavior and the pressure-induced structural evolution of a natural P63/m davyne was investigated by in situ single-crystal synchrotron diffraction with a diamond anvil cell. A P-induced displacive phase transition from a P63/m to a P63 structure occurred between room-P and 0.38(2) GPa. The post-transition P6 3-davyne showed a large isothermal (293 K) stability field as function of pressure, being stable at least up to 7.18(2) GPa. The elastic behavior was described by a III-order Birch-Murnaghan equation of state fit, leading to the following refined elastic parameters: V0 = 761.6(5) Å3, KV0=46.5(11)GPa and KV′=3.7(3); a 0 = 12.814(2) Å, Ka0=50.3(9)GPa and Ka′=4.0(3); c0 = 5.3561(9) Å, Kc0=40.3(7)GPa and Kc′=3.2(2). The refined isothermal volume bulk modulus (46.5(3) GPa) is comparable to those so far reported for other cancrinite-group compounds. The elastic anisotropy at room-P conditions can be described as K a0:Kc0=1.25:1, and was found to increase with pressure. The bulk volume compression is mainly accommodated by the tilting of the quasi-rigid framework tetrahedra. A description of the P-induced deformation mechanisms at the atomic scale and a comparison with the pressure-induced behavior of previously studied cancrinite-group minerals are carried out
- …
