Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts

Pathway for a martensitic quartz-coesite transition

An atomistic pathway for a strain-induced subsolidus martensitic transition between quartz and coesite was found by computing the set of the smallest atomic displacements required to transform a quartz structure into a coesite structure. A minimal transformation cell with 24 [Formula: see text] formula units is sufficient to describe the diffusionless martensitic transition from quartz to coesite. We identified two families of invariant shear planes during the martensitic transition, near the {10[Formula: see text]1} and {12[Formula: see text]2} set of planes, in agreement with the orientation of planar defect structures observed in quartz samples which experienced hypervelocity impacts. We calculated the reaction barrier using density functional theory and found that the barrier of 150 meV/atom is pressure invariant from ambient pressure up to 5 GPa, while the mean principal stress limiting the stability of strained quartz is [Formula: see text] 2 GPa. The model calculations quantitatively confirm that coesite can be formed in strained quartz at pressures significantly below the hydrostatic equilibrium transition pressure.

Synthesis of rare-earth metal compounds through enhanced reactivity of alkali halides at high pressures

Chemical stability of the alkali halides NaCl and KCl has allowed for their use as inert media in high-pressure high-temperature experiments. Here we demonstrate the unexpected reactivity of the halides with metals (Y, Dy, and Re) and iron oxide (FeO) in a laser-heated diamond anvil cell, thus providing a synthetic route for halogen-containing binary and ternary compounds. So far unknown chlorides, Y₂Cl and DyCl, and chloride carbides, Y₂ClC and Dy₂ClC, were synthesized at ~40 GPa and 2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction. Also, FeCl₂ with the HP-PdF₂-type structure, previously reported at 108 GPa, was synthesized at ~160 GPa and 2100 K. The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.

A new high-pressure structure of SiO2directly converted fromα-quartz under nonhydrostatic compression

High-pressure behavior of SiO₂is one of the prototypical subjects in several research areas including condensed matter physics, inorganic chemistry, mineralogy, materials science, and crystallography. Therefore, numerous studies have been performed on the structure evolution of SiO₂under pressure. Here, we show a new structure directly converted fromα-quartz under uniaxial compression. Ourab initiocalculations elucidate a simple transition pathway fromα-quartz to the Fe₂P-type phase, and an intermediate state with the Li₂ZrF₆-type structure appears in this structure conversion. Some interesting properties are found on this intermediate state. (1) The Li₂ZrF₆-type phase is metastable probably due to a volumetric unbalance between the Li and Zr sites but becomes more energetically stable thanα-quartz over ∼12 GPa. (2) It is vibrationally stable at 0 GPa, suggesting that this phase can be recovered down to ambient condition once synthesized. (3) The crystal structures of Li₂ZrF₆-type SiO₂and phase D, one of dense magnesium hydrous silicates, are found identical, suggesting the stabilization of their solid solution under high-P,Tcondition.

Electronic, Structural, and Mechanical Properties of SiO_{2} Glass at High Pressure Inferred from its Refractive Index

We report the first direct measurements of the refractive index of silica glass up to 145 GPa that allowed quantifying its density, bulk modulus, Lorenz-Lorentz polarizability, and band gap. These properties show two major anomalies at ∼10 and ∼40  GPa. The anomaly at ∼10  GPa signals the onset of the increase in Si coordination, and the anomaly at ∼40  GPa corresponds to a nearly complete vanishing of fourfold Si. More generally, we show that the compressibility and density of noncrystalline solids can be accurately measured in simple optical experiments up to at least 110 GPa.

Edge-sharing BO4 tetrahedra and penta-coordinated silicon in the high-pressure modification of NaBSi3O8

High-pressure modification of NaBSi3O8 results in the first example of a borosilicate compound containing edge-sharing BO4 tetrahedra and SiO5 polyhedra.

Percolation transitions in compressed SiO2 glasses

Amorphous-amorphous transformations under pressure are generally explained by changes in the local structure from low- to higher-fold coordinated polyhedra^1-4. However, as the notion of scale invariance at the critical thresholds has not been addressed, it is still unclear whether these transformations behave similarly to true phase transitions in related crystals and liquids. Here we report ab initio-based calculations of compressed silica (SiO₂) glasses, showing that the structural changes from low- to high-density amorphous structures occur through a sequence of percolation transitions. When the pressure is increased to 82 GPa, a series of long-range ('infinite') percolating clusters composed of corner- or edge-shared tetrahedra, pentahedra and eventually octahedra emerge at critical pressures and replace the previous 'phase' of lower-fold coordinated polyhedra and lower connectivity. This mechanism provides a natural explanation for the well-known mechanical anomaly around 3 GPa, as well as the structural irreversibility beyond 10 GPa, among other features. Some of the amorphous structures that have been discovered mimic those of coesite IV and V crystals reported recently^5,6, highlighting the major role of SiO₅ pentahedron-based polyamorphs in the densification process of vitreous silica. Our results demonstrate that percolation theory provides a robust framework to understand the nature and pathway of amorphous-amorphous transformations and open a new avenue to predict unravelled amorphous solid states and related liquid phases^7,8.

High-Pressure Insertion of Dense H2 into a Model Zeolite

Our combined high-pressure synchrotron X-ray diffraction and Monte Carlo modeling studies show super-filling of the zeolite, and computational results suggest an occupancy by a maximum of nearly two inserted H₂ molecules per framework unit, which is about twice that observed in gas hydrates. Super-filling prevents amorphization of the host material up to at least 60 GPa, which is a record pressure for zeolites and also for any group IV element being in full 4-fold coordination, except for carbon. We find that the inserted H₂ forms an exotic topologically constrained glassy-like form, otherwise unattainable in pure hydrogen. Raman spectroscopy on confined H₂ shows that the microporosity of the zeolite is retained over the entire investigated pressure range (up to 80 GPa) and that intermolecular interactions share common aspects with bulk hydrogen, while they are also affected by the zeolite framework.

Mixed Coordination Silica at Megabar Pressure

Silica (SiO_{2}), as a raw material of silicon, glass, ceramics, abrasive, and refractory substances, etc., is of significant importance in industrial applications and fundamental research such as electronics and planetary science. Here, using a crystal structure searching method and first-principles calculations, we predicted that a ground state crystalline phase of silica with R3[over ¯] symmetry is stable at around 645-890 GPa, which contains six-, eight-, and nine-coordinated silicon atoms and results in an average coordination number of eight. This mixed-coordination silica fills in the density, electronic band gap, and coordination number gaps between the previously known sixfold pyrite-type and ninefold Fe_{2}P-type phases, and may appear in the core or mantle of super-Earth exoplanets, or even the solar giant planets such as the Neptune. In addition, we also found that some silicon superoxides, Cmcm SiO_{3} and Ccce SiO_{6}, are stable in this pressure range and may appear in an oxygen-rich environment. Our finding enriches the high-pressure phase diagram of silicon oxides and improves understanding of the interior structure of giant planets in our solar system.

Stabilization of pentazolate anions in the high-pressure compounds Na2N5 and NaN5 and in the sodium pentazolate framework NaN5·N2

Synthesis and characterization of nitrogen-rich materials is important for the design of novel high energy density materials due to extremely energetic low-order nitrogen-nitrogen bonds. The balance between the energy output and stability may be achieved if polynitrogen units are stabilized by resonance as in cyclo-N5- pentazolate salts. Here we demonstrate the synthesis of three oxygen-free pentazolate salts Na2N5, NaN5 and NaN5·N2 from sodium azide NaN3 and molecular nitrogen N2 at ∼50 GPa. NaN5·N2 is a metal-pentazolate framework (MPF) obtained via a self-templated synthesis method with nitrogen molecules being incorporated into the nanochannels of the MPF. Such self-assembled MPFs may be common in a variety of ionic pentazolate compounds. The formation of Na2N5 demonstrates that the cyclo-N5 group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.

High pressure phase transitions of paracelsian BaAl2Si2O8

Three new polymorphs of aluminosilicate paracelsian, BaAl₂Si₂O₈, have been discovered using synchrotron-based in situ high-pressure single crystal X-ray diffraction. The first isosymmetric phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The phase transition is associated with the formation of pentacoordinated Al³⁺ and Si⁴⁺ ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO₄ and SiO₄ tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P2₁/c) to orthorhombic (Pna2₁). The structure of paracelsian-III consists of SiO₆ octahedra, AlO₆ octahedra and distorted AlO₄ tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si⁴⁺ and Al³⁺ coordination, which show rather complex behaviour with the general tendency towards increasing coordination numbers. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna2₁ to Pn. The transition has a displacive character. In the course of the phase transformation pathway up to 32 GPa, the structure of polymorphs becomes denser: paracelsian-II is based upon elements of cubic and hexagonal close-packing arrangements of large O²⁻ and Ba²⁺ ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB.

Penta- and hexa-coordinated beryllium and phosphorus in high-pressure modifications of CaBe2P2O8

Beryllium oxides have been extensively studied due to their unique chemical properties and important technological applications. Typically, in inorganic compounds beryllium is tetrahedrally coordinated by oxygen atoms. Herein based on results of in situ single crystal X-ray diffraction studies and ab initio calculations we report on the high-pressure behavior of CaBe₂P₂O₈, to the best of our knowledge the first compound showing a step-wise transition of Be coordination from tetrahedral (4) to octahedral (6) through trigonal bipyramidal (5). It is remarkable that the same transformation route is observed for phosphorus. Our theoretical analysis suggests that the sequence of structural transitions of CaBe₂P₂O₈ is associated with the electronic transformation from predominantly molecular orbitals at low pressure to the state with overlapping electronic clouds of anions orbitals.

Beryllium in inorganic compounds is usually coordinated to four oxygen atoms, but higher coordination numbers have been predicted. Here the authors observe a pressure induced stepwise transition in CaBe₂P₂O₈ where Be coordination changes to trigonal-bipyramidal and octahedral, implying that d orbitals are not mandatory for high coordination.