High-pressure behavior of intermediate scapolite : compressibility, structure deformation and phase transition

Scapolites are common volatile-bearing minerals in metamorphic rocks. In this study, the high-pressure behavior of an intermediate member of the scapolite solid solution series (Me47), chemical formula (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01], has been investigated up to 17.79 GPa, by means of in situ single-crystal synchrotron X-ray diffraction. The isothermal elastic behavior of the studied scapolite has been described by a III-order Birch\u2013Murnaghan equation of state, which provided the following refined parameters: V0 = 1110.6(7) \uc53, KV0 = 70(2) GPa (\u3b2V0 = 0.0143(4) GPa 121) and KV\u2032 = 4.8(7). The refined bulk modulus is intermediate between those previously reported for Me17 and Me68 scapolite samples, confirming that the bulk compressibility among the solid solution increases with the Na content. A discussion on the P-induced structure deformation mechanisms of tetragonal scapolite at the atomic scale is provided, along with the implications of the reported results for the modeling of scapolite stability. In addition, a single-crystal to single-crystal phase transition, which is displacive in character, has been observed toward a triclinic polymorph at 9.87 GPa. The high-pressure triclinic polymorph was found to be stable up to the highest pressure investigated

Ettringite at high pressure: structure evolution and elastic behaviour

In order to predict the elastic properties of the complex multi-component Portland cement, database of the thermodynamic parameters of the main constituents is needed. Ettringite (ideally Ca6Al2(SO4)3(OH)12\ub727H2O, with a=b \uf07e 11.21 and c \uf07e 21.43 \uc5, Sp. Gr. P31c) is a common crystalline phases in Portland cements. It contains more than 45 wt% of H2O. In the early hydration stages, the crystallization of ettringite governs the set rate of the highly reactive Ca3Al2O6 phase (also known as \u201cC3A\u201d), whereas in aged cements its formation is associated to degradation processes1. The crystal structure of ettringite is rather complex and it consists of [Ca3[Al(OH)6]\ub712H2O]-columns (in which Al(OH)6-octahedra are alternated with triplets of Ca(OH)4(OH2)4-polyhedra) and sulphate groups connected by a complex H-bonding net2. Previous studies on the behavior of ettringite at high pressure reported only the isotropic compressional behavior of ettringite 3,4. Because of that, the linear bulk moduli (Ka and Kc) and a full description of the deformation mechanisms at the atomic scale are still missing. We compressed a single crystal of ettringite up to 4.2 GPa by means of in-situ synchrotron X-ray diffraction, using a diamond-anvil cell and the mix methanol:ethanol (4:1) as P-transmitting fluid. Ettringite shows a marked anisotropic compressional pattern (Ka 21(1) GPa, Kc 47(1) GPa), which dramatically changes at P>3 GPa (Fig. 1). At P>3 GPa, the bulk modulus KV of ettringite drops from 26.6(5) to 10.4(8) GPa. Such a softening is governed by the structural changes which affect mainly the elastic behavior on the ab plane (Ka drops from 21(1) to 7.3(8) GPa whereas Kc decreases only moderately). The structure refinements reveal that the elastic softening reflects the collapse of the H-bonding net, due an average decrease of the Odonor\ub7\ub7\ub7Oacceptor distances (up to 0.20 \uc5 in some cases), which mainly affect the interaction between the sulphate groups and the Ca(OH)4(OH2)4-polyhedra lying in the ab plane

High-pressure versus isoelectronic doping effect on the honeycomb iridate Na2_2IrO3_3

We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate Na$_2$IrO$_3$ by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of Na$_2$IrO$_3$ is discussed in terms of a Mott insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev-interactions. With increasing Li content $x$, (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ moves deeper into the Mott insulating regime and there are indications that up to a doping level of 24\% the compound comes closer to the Kitaev-limit. The optical conductivity spectrum of single crystalline $\alpha$-Li$_2$IrO$_3$ does not follow the trends observed for the series up to $x=0.24$. There are strong indications that $\alpha$-Li$_2$IrO$_3$ is less close to the Kitaev-limit compared to Na$_2$IrO$_3$ and closer to the quasimolecular orbital picture. Except for the pressure-induced hardening of the phonon modes, the optical properties of Na$_2$IrO$_3$ seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.Comment: 12 pages, 6 figures, accepted for publication in Phys. Rev.

Crystal structure of LaTiO_3.41 under pressure

The crystal structure of the layered, perovskite-related LaTiO_3.41 (La_5Ti_5O_{17+\delta}) has been studied by synchrotron powder x-ray diffraction under hydrostatic pressure up to 27 GPa (T = 295 K). The ambient-pressure phase was found to remain stable up to 18 GPa. A sluggish, but reversible phase transition occurs in the range 18--24 GPa. The structural changes of the low-pressure phase are characterized by a pronounced anisotropy in the axis compressibilities, which are at a ratio of approximately 1:2:3 for the a, b, and c axes. Possible effects of pressure on the electronic properties of LaTiO_3.41 are discussed.Comment: 5 pages, 6 figure

Raman excitation spectroscopy of carbon nanotubes: effects of pressure medium and pressure

Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added - water, hexane, sulphuric acid - and when the nanotubes are unbundled in water with surfactant and sonication. The shift is about 2 - 3 cm-1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously.Comment: 6 pages, 3 Figures, Proceedings of EHPRG 2011 (Paris

Nesting Induced Peierls-type Instability for Compressed Li-CI16

Alkalies are considered to be simple metals at ambient conditions. However, recently reported theoretical and experimental results have shown an unexpected and intriguing correlation between complex structures and an enhanced superconducting transition temperature in lithium under pressure. In this article we analyze the pressure induced Fermi surface deformation in bcc lithium, and its relation to the observed cI16 structure. According to our calculations, the Fermi surface becomes increasingly anisotropic with pressure and develops an extended nesting along the bcc [121] direction. This nesting induces a phonon instability of both transverse modes at N, so that a Peierls-type mechanism is proposed to explain the stability of Li-cI16.Comment: Proceedings of Fukuoka 2006 Conference on Novel Pressure-induced Phenomena in Condensed Matter Systems. To be published in J. Phys. Soc. Jpn. 2 pages and 3 figure

Quantum and Classical Orientational Ordering in Solid Hydrogen

We present a unified view of orientational ordering in phases I, II, and III of solid hydrogen. Phases II and III are orientationally ordered, while the ordering objects in phase II are angular momenta of rotating molecules, and in phase III the molecules themselves. This concept provides quantitative explanation of the vibron softening, libron and roton spectra, and increase of the IR vibron oscillator strength in phase III. The temperature dependence of the effective charge parallels the frequency shifts of the IR and Raman vibrons. All three quantities are linear in the order parameter.Comment: Replaced with the final text, accepted for publication in PRL. 1 Fig. added. Misc. text revision

Methanol adsorption at High Pressure in MFI- Zeolites

In some olefins-production processes, MFI-zeolites are currently used as catalysts, representing an appealing alternative to the high-energy demanding Steam Cracking process, from which the 95% of the total worldwide olefins production relies1,2. Furthermore, MFI zeolites have been used in the methanol-to-olefins (MTO) synthesis process, to obtain olefins directly from methanol, thus bypassing oil as raw precursor. At ambient conditions, only the surfaces of the zeolite crystallites are believed to be active in the methanol-to-olefins process. However, pressure can enhance methanol adsorption in the structural cavities of zeolites3, thus enhancing the active surface directly in contact with methanol. This may bear a significant impact in the industrial applications of this zeolite as a catalyst: a \u201ccold\u201d intrusion of methanol into the zeolite cavities might pave the way to ultimately increase the efficiency of the MTO conversion process. In this study, we synthesized and investigated, by in situ synchrotron powder-XRD experiments with a diamond-anvil cell, the high-pressure behaviour of six MFI-zeolites with different chemical composition (Na-Al-MFI, Na-Fe-MFI, Na-B-MFI, H-Al-MFI, H-B-MFI, and Silicalite-1-MFI). Consistently with previous studies4, all the synthesized zeolites are monoclinic (space group P21/n11) at ambient pressure. A monoclinic-to-orthorhombic (P21/n11-to-Pnma) phase transition (MOPT) was reported to occur at P > 1 GPa in MFIzeolites. On the basis of the in-situ X-ray diffraction data, we ascertain that: i) all the MFI zeolites compressed in silicone oil (acting as non-penetrating fluid) have overall the same bulk compressibility, ii) methanol penetrate through the structural voids (leading to an apparent lower compressibility, Fig. 1) and, among the different zeolites, the magnitude of the adsorption phenomenon is different, i.e. it is governed by the different chemical composition, iii) the MOPT is influenced by the chemical composition of the zeolites and by the absorption of methanol. The experimental findings of this study represent the first step to select the optimal chemical composition of a potential MFI-catalyst for the MTO conversion process operating at high-pressure conditions

Dolomite-IV : Candidate structure for a carbonate in the Earth's lower mantle

We report the crystal structure of dolomite-IV, a high-pressure polymorph of Fe-dolomite stabilized at 115 GPa and 2500 K. It is orthorhombic, space group Pnma, a =10.091(3), b = 8.090(7), c = 4.533(3) \uc5, V = 370.1(4) \uc53 at 115.2 GPa and ambient temperature. The structure is based on the presence of threefold C3O9 carbonate rings, with carbon in tetrahedral coordination. The starting Fe-dolomite single crystal during compression up to 115 GPa transforms into dolomite-II (at 17 GPa) and dolomite-IIIb (at 36 GPa). The dolomite-IIIb, observed in this study, is rhombohedral, space group R3, a =11.956(3), c =13.626(5) \uc5, V =1686.9(5) \uc53 at 39.4 GPa. It is different from a previously determined dolomite-III structure, but topologically similar. The density increase from dolomite-IIIb to dolomite IV is ca. 3%. The structure of dolomite-IV has not been predicted, but it presents similarities with the structural models proposed for the high-pressure polymorphs of magnesite, MgCO3. A ring-carbonate structure match with spectroscopic analysis of high-pressure forms of magnesite-siderite reported in the literature, and, therefore, is a likely candidate structure for a carbonate at the bottom of the Earth's mantle, at least for magnesitic and dolomitic compositions