1,721,226 research outputs found
Single crystal diffraction at earth’s mantle conditions
No full textRecent improvements at synchrotron beamlines and on Diamond Anvil Cells techniques allow single crystal diffraction experiments at non ambient conditions on very small crystals, one order of magnitude smaller compared to
samples routinely investigated on conventional laboratory sources. Three examples concerning high pressure single
crystal diffractions at Earth’s Mantle conditions will be illustrated and discussed: structural determination of CaFe2O4 and CaCO3 across electron Fe
3+ and Fe2+ spin transition
(around 50 GPa) and elasticity and phase transitions of CaFeO2.5
defective brownmillerite/perovskite at lower mantle conditions
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
T- and P-stability and thermo-elastic behavior of the ABW-compounds TlAlSiO4 and CsAlSiO4
No full textT- and P-stability and thermo-elastic behavior
of the ABW-compounds TlAlSiO4 and CsAlSiO4
Paolo Lotti,a G. Diego Gattaa,b, Domenico Caputoc, Marco Merlinia,
Paolo Apreac, Andrea Lausid, Carmine Colellac
aDipartimento di Scienze della Terra, Università degli Studi di Milano, Milano, Italy
bCNR - Istituto di Cristallografia, Sede di Bari, Bari, Italy
cDipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli “Federico II”, Napoli, Italy
dSincrotrone Trieste S.C.p.A. di Interesse Nazionale, Basovizza, Trieste, Italy
[email protected]
A large number of microporous compounds sharing the ABW framework topology have so far been reported in the literature. These compounds show a significant chemical variability, leading to interesting magnetic, optical or structural properties (see e.g. [1] and references therein). The ABW framework can be described as made by sheets of six-membered rings of tetrahedra, in which three tetrahedra have apical oxygen atoms pointing upward (U) and three downward (D), according to a “UUUDDD” scheme. The sheets are interconnected through the apical oxygen atoms, giving rise to elliptical 8-membered ring channels, where the extraframework population is hosted. The latter is generally represented by monovalent cations, with (as Li-ABW) or without (as Rb-, Cs- or Tl-ABW) H2O molecules. Only a few studies have so far been devoted to the phase-stability fields and thermo-elastic behavior of ABW compounds, in response to T and P.
In this study, we focused our attention to two synthetic ABW compounds: TlAlSiO4 and CsAlSiO4, which gain interest for the pollutant and/or toxic nature of the hosted extraframework cations (Tl+ or Cs+). TlAlSiO4 has been investigated up to 950 °C (at room-P) and up to 8 GPa (at room-T) by means of in-situ synchrotron powder diffraction with a diamond anvil cell and with a high-temperature furnace [2]. No phase transition has been observed within the T- and P-range investigated. A II-order Birch-Murnaghan equation of state (II-BM EoS) fit of the P-V data led to a refined bulk modulus KV0 = 48.8(2) GPa. A polynomial fit of the T-V data led to a refined volume thermal expansion coefficient αV,25°C = 4.44(3)*10-5 K-1. CsAlSiO4 has been investigated up to 1000 °C (at room-P) and up to 10 GPa (at room-T) by means of in-situ synchrotron powder diffraction [3]. As for the Tl-analogue, no phase transitions have been observed within the T- and P-range investigated. A II-BM EoS fit of the P-V data gave a refined KV0 = 41.3(3) GPa. A polynomial fit of the T-V data led to a refined αV,20°C = 3.63(1)*10-5 K-1. Both the studied ABW-compounds show a remarkably anisotropic thermo-elastic pattern, resembling that of “layered materials” (e.g. phyllosilicates), where the stacking direction of the 6mR-sheets is significantly more compressible and expandable than the sheets plane. Such a behavior appears to be governed by the nature of the ABW topology of the framework. The high stability and flexibility of TlAlSiO4 and CsAlSiO4 at high-T (at room-P) and high-P (at room-T) suggest these compounds as functional materials for the fixation and storage of the Tl+ and Cs+.
[1] V. Kahlenberg, R.X. Fischer, W.H. Baur, Z. Kristallogr. 2001, 216, 489-494.
[2] G.D. Gatta, P. Lotti, M. Merlini, D. Caputo, P. Aprea, A. Lausi, C. Colella, Micropor. Mesopor. Mater. 2014, submitted.
[3] G.D. Gatta, M. Merlini, P. Lotti, A. Lausi, M. Rieder, Micropor. Mesopor. Mater. 2013, 163, 147-152
More 1980s than the 1980s
Full text linkThis article analyzes the close relation between synthwave music and cinema, focusing on the role played by the former in paradigmatic films and TV series from the 1980s and the 2010s. To the two eras correspond two sets of semantic “associations,” which designate the functions and connotations embodied by synthwave in the cinematic context. The first analysis focuses on the context in which the association between the two protagonists of this affair has taken place for the first time, trying to understand why sci-fi cinema is often privileged when it comes to the association with (proto-)synthwave music—especially soundtracks by Tangerine Dream, John Carpenter, Giorgio Moroder and Vangelis, which are often referred to as main sources of inspiration for contemporary synthwave. The result is a first set of two “first-order associations” between synthwave and sci-fi cinema on the one side, and with the 1980s geek culture on the other. The focus then shifts towards the present times, exploring the ways in which synthwave soundtracks remediate 1980s popular culture by employing two “second-order associations” between synthwave and mediated, hyperreal versions of the 1980s and its products. In particular, the protagonists of this second analysis are the functions of synthwave soundtracks as an “age-synecdoche” and as an “aesthetic mediator,” especially in the series Stranger Things, in the movie It Follows, and in the late works by director Nicolas Winding Refn
Thermal expansion and phase transitions in akermanite and gehlenite
No full textThermal expansion has been measured by laboratory and synchrotron X-ray powder diffraction for end-member (a) over circle kermanite (ak, Ca2MgSi2O7) and gehlenite (ge, Ca2Al2SiO7) in the range 20-1,500 K. In ak in the range 340-390 K, there is a negative linear thermal expansion in [001] direction. This is related to the phase transition from an incommensurate modulated structure (IC) to a normal one (N). The volumetric mean thermal expansion coefficients for ak and ge, obtained with a linear fit of the experimental data in the temperature range 298-1,400 K, are respectively 32.1x10(-6) and 28.3x10(-6) K-1. The variation of the c/a ratio with temperature, due to different thermal expansion along the crystallographic axes, can be related to the different behaviour of the tetrahedral layers in the N and IC phases. Analysis of the variation of the superstructure peaks intensity across the phase transition confirms the tricritical behaviour of the IC/N transition in ak
Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K2Mg(CO3)2-Na2Mg(CO3)2 join
No full textAlthough alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na2Mg(CO3)2-K2Mg(CO3)2 join only the Na-end-member eitelite (R3 space group), was reported in nature. The K-end-member (R3m) readily hydrates even at low temperatures, therefore, only baylissite, K2Mg(CO3)2·4H2O, has been observed. Because of the role of (K,Na)Mgdouble carbonates in mantle metasomatism, we performed high P-T experiments on K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)2. Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V0 = 396.2(4), 381.2(5), and 347.1(3) Å3 and K0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)2, respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high-P phase is 1% denser than the low-P polymorph. The NaMg-double carbonate has a phase transition at ∼14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25-600 °C and ambient pressure and are É0 = 14.31(5) × 10-5 K-1 and 16.73(11) × 10-5 K-1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a- and c-axis: É0(a) = 2.84(6) × 10-5 and 4.78(5) × 10-5 K-1 and É0(c) = 10.47(11) × 10-5 and 8.72(5) × 10-5 K-1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively
Low temperature SR-XRPD study of akermanite-gehlenite solid solution
No full textLow temperature thermal expansion coefficients of members of the solid solution gehlenite (ge)-åkermanite (åk) were measured by synchrotron radiation X-Ray powder diffraction. The linear thermal expansion coefficient is maximum for a composition with about 50 % content of åk. In åk-rich compositions an incommensurate modulated structure is present. The ge-åk solid solution shows a non-ideal behaviour, with negative excess volume near the ge end-member
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
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