The BC8 silicon allotrope has a lattice thermal conductivity 1-2 orders of magnitude lower than that of diamond-cubic silicon. In the current work, the phonon density of states, phonon dispersion, and lattice thermal conductivity are investigated by inelastic neutron scattering measurements and first-principles calculations. Flat phonon bands are found to play a critical role in the reduction of lattice thermal conductivity in BC8-Si. Such bands in the low-energy range enhance the phonon scattering between acoustic and low-energy optical phonons, while bands in the intermediate-energy range act as a scattering bridge between the high- and low-energy optical phonons. They significantly enlarge the phonon-phonon scattering phase space and reduces the lattice thermal conductivity in this novel silicon allotrope. This work provides insights into the significant reduction of the lattice thermal conductivity in BC8 -Si, thus expanding the understanding of novel silicon allotropes and their development for electronic devices. (C) 2021 Elsevier Ltd. All rights reserved.

This study revisits subduction processes at the Hellenic Subduction Zone (HSZ) including tearing, segmentation, and backstepping, by refining the geometry of the Nubian slab down to 150-180 km depth using well-located hypocentres from global and local seismicity catalogues. At the western termination of the HSZ, the Kefalonia Transform Fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it, respectively. A discontinuity is suggested to exist between the two slabs at shallow depths. The Kefalonia Transform Fault is interpreted as an active Subduction-Transform-Edge-Propagator-fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it. A model reconstructing the evolution of the subduction system in the area of Peloponnese since 34 Ma, involving the backstepping of the subduction to the back-side of Adria, provides seismological evidence that supports the single-slab model for the HSZ and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere. In the area of Rhodes, earthquake hypocentres indicate the presence of a NW dipping subducting slab that rules out the presence of a NE-SW striking Subduction-Transform-Edge-Propagator-fault in the Pliny-Strabo trenches region. Earthquake hypocentres also allow refining the slab tear beneath southwestern Anatolia down to 150-180 km depth. Furthermore, the distribution of microseismicity shows a first-order slab segmentation in the region between Crete and Karpathos, with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east. Thermal models indicate the presence of a colder slab beneath the southeastern Aegean that leads to deepening of the intermediate-depth seismicity. Slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity along the interplate seismogenic zone.

Super-Earths ranging up to 10 Earth masses (M-E) with Earth-like density are common among the observed exoplanets thus far, but their measured masses and radii do not uniquely elucidate their internal structure. Exploring the phase transitions in the Mg-silicates that define the mantle-structure of super-Earths is critical to characterizing their interiors, yet the relevant terapascal conditions are experimentally challenging for direct structural analysis. Here we investigated the crystal chemistry of Fe3O4 as a low-pressure analog to Mg2SiO4 between 45-115 GPa and up to 3000 K using powder and single crystal X-ray diffraction in the laser-heated diamond anvil cell. Between 60-115 GPa and above 2000 K, Fe3O4 adopts an 8-fold coordinated Th3P4-type structure (I-43d, Z = 4) with disordered Fe2+ and Fe3+ into one metal site. This Fe-oxide phase is isostructural with that predicted for Mg2SiO4 above 500 GPa in super-Earth mantles and suggests that Mg2SiO4 can incorporate both ferric and ferrous iron at these conditions. The pressure-volume behavior observed in this 8-fold coordinated Fe3O4 indicates a maximum 4% density increase across the 6- to 8-fold coordination transition in the analog Mg-silicate. Reassessment of the FeO-Fe3O4 fugacity buffer considering the Fe3O4 phase relationships identified in this study reveals that increasing pressure and temperature to 120 GPa and 3000 K in Earth and planetary mantles drives iron toward oxidation.

We examine the thermodynamic and dynamic stability (i.e., phonon dispersion) of mixed silicon-carbon clathrate frameworks using first-principles calculations as a function of pressure and composition. Silicon atoms were substituted on special framework Wyckoff positions in the Type-I and Type-II empty carbon clathrate structures over a broad compositional range, and the enthalpies of the mixed clathrates were compared to pure silicon and carbon clathrates, as well as the thermodynamic ground states. While all mixed clathrates examined were found to be metastable with respect to elemental formation components and/or silicon carbide, certain empty binary host lattices are found to be lower-energy phases than the pure-component clathrate endmembers at high pressure, in particular Type-I C22Si24 and Type-II C32Si104. This enhanced energetic stability is rationalized by a decrease of energy upon doping specific crystallographic positions. When occupied by small guest ions like Li+ and Na+, these mixed C-Si clathrate structures exhibit minima in their formation enthalpies under high-pressure conditions, providing insights into potential synthetic pathways.

Nanothreads are one-dimensional nanomaterials composed of a primarily sp(3) hydrocarbon backbone, typically formed through the compression of small molecules to high pressures. Although nanothreads have been synthesized from a range of precursors, controlling reaction pathways to produce atomically precise materials remains a difficult challenge. Here, we show how heteroatoms within precursors can serve as "thread-directing" groups by selecting for specific cycloaddition reaction pathways. By using a less-reactive diazine group within a six-membered aromatic ring, we successfully predict and synthesize the first carbon nanothread material derived from pyridazine (1,2-diazine, C4H4N2). Compared with previous nanothreads, the synthesized polypyridazine, shows a predominantly uniform chemical structure with exceptional long-range order, allowing for structural characterization using vibrational spectroscopy and X-ray diffraction. The results demonstrate how thread-directing groups can be used for reaction pathway control and the formation of chemically precise nanothreads with a high degree of structural order.

Double seismic zones are two-layered distributions of intermediate-depth earthquakes that provide insight into the thermomechanical state of subducting slabs. We present new precise hypocenters of intermediate-depth earthquakes in the Tonga subduction zone obtained using data from local island-based, ocean-bottom, and global seismographs. The results show a downdip compressional upper plane and a downdip tensional lower planewith a separation of about 30 km. The double seismic zone in Tonga extends to a depth of about 300 km, deeper than in any other subduction system. This is due to the lower slab temperatures resulting from faster subduction, as indicated by a global trend toward deeper double seismic zones in colder slabs. In addition, a line of high seismicity in the upper plane is observed at a depth of 160 to 280 km, which shallows southward as the convergence rate decreases. Thermal modeling shows that the earthquakes in this "seismic belt" occur at various pressures but at a nearly constant temperature, highlighting the important role of temperature in triggering intermediate-depth earthquakes. This seismic belt may correspond to regions where the subducting mantle first reaches a temperature of similar to 500 degrees C, implying that metamorphic dehydration of mantle minerals in the slab provides water to enhance faulting.

Manipulation by external pressure of the optical response of 2D Metal Halide Perovskites (MHPs) is a fascinating route to tune their properties and promote the emergence of novel features. We investigate here DA(2)PbI(4) and DA(2)GeI(4) (DA = decylammonium) perovskites in the pressure range up to similar to 12 GPa by X-ray powder diffraction, absorption, and photoluminescence spectroscopy. Although the two systems share a similar structural evolution with pressure, the optical properties are rather different and influenced by Pb or Ge. DA(2)PbI(4) shows a progressive red shift from 2.28 eV (P = 0 GPa) to 1.64 eV at 11.5 GPa, with a narrow PL emission, whereas DA(2)GeI(4), changes from a non-PL system at ambient pressure to a clear broadband emitter centered around 730 nmwith an intensity maximum at about 3.7GPa. These results unveil the role of the central atom on the nature of emission under pressure in 2D MHPs containing a long alkyl chain.

The strongest evidence to support the classical plume hypothesis comes from seismic imaging of the mantle beneath hot spots. However, imaging results are often ambiguous and it is questionable whether narrow plume tails can be detected by present-day seismological techniques. Here we carry out synthetic tomography experiments based on spectral element method simulations of seismic waves with period T > 10s propagating through geodynamically derived plume structures. We vary the source-receiver geometry in order to explore the conditions under which lower mantle plume tails may be detected seismically. We determine that wide-aperture (4,000-6,000km) networks with dense station coverage (<100-200km station spacing) are necessary to image narrow (<500km wide) thermal plume tails. We find that if uncertainties on traveltime measurements exceed delay times imparted by plume tails (typically <1s), the plume tails are concealed in seismic images. Vertically propagating SKS waves enhance plume tail recovery but lack vertical resolution in regions that are not independently constrained by direct S paths. We demonstrate how vertical smearing of an upper mantle low-velocity anomaly can appear as a plume originating in the deep mantle. Our results are useful for interpreting previous plume imaging experiments and guide the design of future experiments.

Metamorphic dehydration reactions in a subducting slab release fluids that trigger arc volcanism and are thought to be responsible for intermediate-depth seismicity. The fluid flow from the source is controlled by buoyancy and compaction pressure which is modified by viscous and elastic effects. In this paper, we investigate how fluid migrates in viscoelastic slab by using 2-D and 3-D numerical models based on a theory of two-phase flow. When bulk viscosity is sufficiently low, viscosity plays a dominant role and fluid goes up almost vertically soon after its release producing porosity waves. When a higher bulk viscosity is assumed, a large amount of fluid is trapped in a high porosity region produced by the fluid source and migrates along the source except for a case where the ratio of permeability (K) to fluid viscosity () is relatively low. We also find that porosity increases in the deeper part of the fluid source in cases with intermediate and low values of K/. In 3-D, fluid focusing occurs where the slab bends away from the trench causing a local increase in porosity and compaction pressure. These findings may help us explain several types of observations in subduction zones including slow earthquakes at the plate interface, low seismic wave velocities in the oceanic crust, double seismic zones in the slab, and shallow subduction angle at the bend of the slab.

The scattering of PKP waves in the lower mantle produces isolated signals before the PKIKP phase. We explore whether these so-called PKIKP precursors can be related to wave scattering off mid ocean ridge basalt (MORB) fragments that have been advected in the deep mantle throughout geologic time. We construct seismic models of small-scale (>20 km) heterogeneity in the lower mantle informed by mantle mixing simulations from Brandenburg et al. (2008) and generate PKIKP precursors using 3D, axisymmetric waveform simulations up to 0.75 Hz. We consider two end-member geodynamic models with fundamentally different distributions of MORB in the lower mantle. Our results suggest that the accumulation of MORB at the base of the mantle is a viable hypothesis for the origin of PKP scattering. We find that the strength of the PKIKP precursor amplitudes is consistent with P wave speed heterogeneity of 0.1-0.2%, as reported previously. The radial distribution of MORE has a profound effect on the strength of PKIKP precursors. Simulation of PLUMP precursors for models with an increasing MORB concentration in the lower-most 500 km of the mantle appears to reproduce most accurately the strength of PKIKP precursors in Global Seismic Network waveforms. These models assume that MORB has an excess density of at least 7%. Additional simulations of more complex geodynamic models will better constrain the geodynamic conditions to explain the significant variability of PKP scattering strength. (C) 2017 Elsevier B.V. All rights reserved.