196,515 research outputs found
Impact of lattice vibrations on equation of state of the hardest boron phase
No full textAn 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
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
Phase transition at high pressure in Cu2CO3(OH)(2) related to the reduction of the Jahn-Teller effect
No full textHydroxycarbonates with the general formula Me-2(CO3)(OH)(2) are widely used materials in industrial processes and are widespread in nature. The Cu term, malachite, Cu2CO3(OH)(2), is monoclinic, P2(1)/a. Substitution of Cu2+ with other bivalent cations such as Mg, Zn, Fe, Cu or Ni is possible and leads to a different structure type, rosasite, P2(1)/a or P2(1)/b11 in the same cell setting as malachite. Rosasite structure is topologically similar to malachite, but the symmetry elements are oriented differently with respect to structural units. The stability of the malachite-like structure (MS) compared with the rosasite-like structure (RS) has been suggested to be related to the Jahn-Teller effect in CuO6 coordination polyhedra. For this reason the hypothesis of the phase transition of malachite, Cu2CO3(OH)(2), to a rosasite structure at high pressure, as a result of the reduced Jahn-Teller effect, has been tested and confirmed by powder and single-crystal diffraction structural studies: above 6 GPa the malachite structure is no longer stable and transforms to a RS structure. RS Cu2CO3(OH)(2) is 3% more dense than malachite and the bulk modulus is remarkably higher, 80 (2) GPa compared with 48 (4) GPa. The longer apical Cu-O bonds in the distorted Me1 octahedral site are progressively shortened with increasing pressure, revealing a decrease in the Jahn-Teller effect at high pressure. The transition has a first-order character, is reversible with a significant hysteresis, and there is no evidence of any intermediate phase between the two structures. We then have further evidence that in the Me-2(CO3)(OH)(2) compounds, the two main structural types, MS and RS, are closely related. The former structure is stabilized only when Cu is the prevalent cation in the octahedral sites, and it can transform directly to the RS as a function of thermodynamic changes
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
Phase stability of hydrated borates at high pressure
No full textHydrated borates are a class of minerals made by clusters or chains of Bφx groups (φ represents an oxygen, an H2O molecule or an OH-) organized either in tetrahedra or in planar trigonal groups. Hydrated borates are believed to be a cheaper alternative to B4C for radiation-shielding concretes (Okuno et al., 2005), due to the large cross section (~3840 barns) for thermal neutrons of the isotope 10B, which represents about 20% of
the boron in nature. A comprehensive characterization of the crystal-chemistry, elastic properties, stability and structural behavior of natural borates at varying T and P conditions is advisable for modelling and understanding their role when utilized as aggregates in radiation-shielding concretes (Torrenti et al., 2010), in which the components are subject to pressure (by static compression) and temperature (by irradiation). Interestingly, all hydrated borates studied so far at high-pressure display one (or more) phase transition, and the pressure at which the phase transitions occur seems to be correlated to the H2O content of the minerals (e.g., Comboni et al., 2020, 2021). During the phase transitions, the most dramatic structural change is the increase
of the coordination number of part of the IIIB to IVB, by the interaction between the IIIB and one H2O molecule or OH- group, underlying the importance of the hydrogen bond network in the stability of the crystalline structure.
Comboni D., Pagliaro F., Gatta G.D., Lotti P., Milani S., Merlini M., Battiston T., Glazyrin K. & Liermann H.P. (2020) - High-pressure behavior and phase stability of Na2B4O6(OH)2·3H2O (kernite). J. Am. Ceram. Soc., 103, 5291-5301.
Comboni D., Poreba T., Pagliaro F., Battiston T., Lotti P., Gatta G.D., Garbarino G. & Hanfland M. (2021) - Crystal structure of the high-P polymorph of Ca2B6O6(OH)10·2(H2O) (meyerhofferite). Acta Crystallogr., B77, 940-945.
Okuno K. (2005) - Neutron shielding material based on colemanite and epoxy resin. Radiat. Prot. Dosim., 115, 258-261.
Torrenti J. & Nahas G. (2010) - Durability and Safety of Concrete Structures in the Nuclear Context. Int. Conf. Concr. under Sev. Cond., Merida, Mexico, 3-18
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
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
ID15b, crystallography and Earth sciences
No full textIn the last decades, experiments at non-ambient conditions have greatly benefited from the improvement
of home-lab instruments and large-scale facilities allowing to investigate matter at extreme PT conditions
(megapascal and temperatures ranging from few to thousands kelvin). Experiments performed at non-ambient
conditions devoted to unveil the structure, properties and the deformation mechanisms of minerals and
synthetic compounds, improve our knowledge regarding the evolution of planets and allow to tailor future new
cutting-edge materials. In this context, Earth sciences have (and still can) greatly benefited from a number of
dedicated beamlines, such as ID15b (ESRF), devoted to the determination of structural properties of minerals
at non-ambient PT conditions using angle-dispersive-diffraction and diamond anvil cells. ID15b is capable to
provide high-quality data, thanks to a bright and focalized X-ray beam (λ ∼ 0.410 Å) that can be made as small
as 2x3 μm2
and an EIGER2 X 9M CdTe (340x370 mm) flat panel detector. The extremely high brightness of
the EBS-ESRF source, the first fourth-generation high-energy synchrotron in the world, allows to perform a
completed single crystal data collection in few minutes. In addition to conventional membrane-driven diamond
anvil cells, the beamline is equipped with a He-cooled cryostat and external resistive heating devices which
allow to perform high-pressure experiments at low temperatures (down to 10K) and high temperatures (up to
600K). An ex-situ Nd-YAG laser system can anneal samples inside the diamond anvil cell at high-temperature,
further increasing the range of the investigable T-induced effects allowing to investigate minerals suggested
at shallow-crust to mantle-like conditions. At ID15b, Earth-sciences researchers can find a cooperating and
competent staff willing to make fruitful suggestion and help during experiments. Allocations of beam time is
open to every scientist via submission of standard or long-term Research Proposal. This poster is meant to be
a showcase of the ID15b beamline, featuring what it can provide for all the Earth-science researchers
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