Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: pressure- and temperature-dependent optical investigations

The infrared response of the organic conductor (TMTSF)$_2$PF$_6$ and the Mott insulator (TMTTF)$_2$PF$_6$ are investigated as a function of temperature and pressure and for the polarization parallel and perpendicular to the molecular stacks. By applying external pressure on (TMTTF)$_2$PF$_6$, the Mott gap rapidly diminishes until the deconfinement transition occurs when the gap energy is approximately twice the interchain transfer integral. In its deconfined state (TMTTF)$_2$PF$_6$ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. For (TMTSF)$_2$PF$_6$ this dimensional crossover is observed either with increase in external pressure or with decrease in temperature. We quantitatively determine the dimensional crossover line in the pressure-temperature diagram based on the degree of coherence in the optical response perpendicular to the molecular stacks.Comment: 12 pages, 15 figure

Plasmons in Sodium under Pressure: Increasing Departure from Nearly-Free-Electron Behavior

We have measured plasmon energies in Na under high pressure up to 43 GPa using inelastic x-ray scattering (IXS). The momentum-resolved results show clear deviations, growing with increasing pressure, from the predictions for a nearly-free electron metal. Plasmon energy calculations based on first-principles electronic band structures and a quasi-classical plasmon model allow us to identify a pressure-induced increase in the electron-ion interaction and associated changes in the electronic band structure as the origin of these deviations, rather than effects of exchange and correlation. Additional IXS results obtained for K and Rb are addressed briefly.Comment: 5 pages, 4 figure

Structural investigations on ϵ\epsilon-FeGe at high pressure and low temperature

The structural parameters of $\epsilon$-FeGe have been determined at ambient conditions using single crystal refinement. Powder diffraction have been carried out to determine structural properties and compressibility for pressures up to 30 GPa and temperatures as low as 82 K. The discontinuous change in the pressure dependence of the shortest Fe-Ge interatomic distance might be interpreted as a symmetry-conserving transition and seems to be related to a magnetic phase boundary line.Comment: 4 pages, 5 figure

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

Compressibility of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4−xAlxOxN7−x] (x = 2; M = Sr, Ba)

The compressibilities of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4−xAlxOxN7−x] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42 GPa were generated using the diamond–anvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order Birch–Murnaghan equation-of-state to the p–V data results in V0 = 302.91 (6) Å3, B0 = 176 (2) GPa and B′ = 4.4 (2) for SrYb[Si4N7]; V0 = 310.4 (1) Å3, B0 = 161 (2) GPa and B′ = 4.6 (2) for SrYb[Si4−xAlxOxN7−x]; and V0 = 317.3 (5) Å3, B0 = 168 (2) GPa and B′ = 4.7 (2) for BaYb[Si4−xAlxOxN7−x]. While the linear compressibilities of the a and c axes of BaYb[Si4−xAlxOxN7−x] are very similar up to 30 GPa, distinct differences were observed for SrYb[Si4N7] and SrYb[Si4−xAlxOxN7−x], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role

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

Two pressure-induced structural phase transitions in TiOCl

We studied the crystal structure of TiOCl up to pressures of $p$=25~GPa at room temperature by x-ray powder diffraction measurements. Two pressure-induced structural phase transitions are observed: At $p_{c1}$$\approx$15~GPa emerges an 2$a$$\times$2$b$$\times$$c$ superstructure with $b$-axis unique monoclinic symmetry (space group P2$_1$/$m$). At $p_{c2}$$\approx$22~GPa all lattice parameters of the monoclinic phase show a pronounced anomaly. A fraction of the sample persists in the ambient orthorhombic phase (space group $Pmmn$) over the whole pressure range.Comment: 5 pages, 5 figures; accepted for publication in Phys. Rev.

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.

Pressure-induced phase transitions and high-pressure tetragonal phase of Fe1.08Te

We report the effects of hydrostatic pressure on the temperature-induced phase transitions in Fe1.08Te in the pressure range 0-3 GPa using synchrotron powder x-ray diffraction (XRD). The results reveal a plethora of phase transitions. At ambient pressure, Fe1.08Te undergoes simultaneous first-order structural symmetry-breaking and magnetic phase transitions, namely from the paramagnetic tetragonal (P4/nmm) to the antiferromagnetic monoclinic (P2_1/m) phase. We show that, at a pressure of 1.33 GPa, the low temperature structure adopts an orthorhombic symmetry. More importantly, for pressures of 2.29 GPa and higher, a symmetry-conserving tetragonal-tetragonal phase transition has been identified from a change in the c/a ratio of the lattice parameters. The succession of different pressure and temperature-induced structural and magnetic phases indicates the presence of strong magneto-elastic coupling effects in this material.Comment: 11 page