High-pressure Polymorph Research Articles

High pressure structure studies of three SrGeO3 polymorphs – Amorphization under pressure

We report on the synthesis and high pressure behavior of three polymorphs of SrGeO3. At ambient pressure, SrGeO3 crystallizes in the monoclinic structure pseudo-wollastonite. Two high pressure polymorphs, triclinic walstromite, and cubic perovskite were synthesized using a large volume multi-anvil press. The crystal structures of the three polymorphs were investigated with powder X-ray diffraction as a function of pressure using diamond anvil cells. It was found that the pseudo-wollastonite polymorph becomes amorphous at 10 GPa and equation of state fitting of the volume data yielded a bulk modulus of K0 = 47(4) GPa, reported for the first time. Compression of the walstromite structure showed the structure to be very compressible with two distinct phase transitions at around 10–12 GPa and 35–38 GPa. The data suggest that the structure then becomes amorphous although it retains a small degree of long-range order to the highest pressure studied. The perovskite polymorph was very incompressible and equation of state fitting of the volume data yielded a high bulk modulus of K0 = 194(3) GPa. All the experimental data was compared to density functional theory calculations, which were observed to fit well with the experiments.

Stöfflerite, (Ca,Na)(Si,Al)4O8 in the hollandite structure: A new high-pressure polymorph of anorthite from martian meteorite NWA 856

AbstractWe present the crystal structure, composition, and occurrence of stöfflerite, the naturally occurring Ca-aluminosilicate with hollandite-type structure. Stöfflerite is a high-pressure polymorph of anorthite and an approved mineral. The type material was found in the shergottitic martian meteorite Northwest Africa 856. Type stöfflerite (Sto60Lin40) assumes space group I4/m with unit-cell dimensions a = 9.255(1) Å, c = 2.742(3) Å, V = 235.1(2) Å3, and Z = 2.

Empirical constrains on volume changes in pressure-induced solid state transformations

ABSTRACT By means of comparing molar volumes at reference ambient conditions (298 K, 1.105 Pa) of 160 pressure-driven polymorphic transitions in chalcogenides, germanates, silicates, simple molecular compounds, elemental non-metals and a few metals it is shown that for reconstructive transitions between 0.1 and 150 GPa, mean volume contraction ranges around 11% for about 85% of all examined materials. The mean volume change is 7–9% for the first and second transition in trimorphic, and 4–5% for the third transition in tetramorphic systems. Less than 15% of the examined materials deviate systematically from these general correlations and follow a correlation ΔV ≈ 3·exp(ΔV’/15) (with ΔV the volume reduction upon the lower and ΔV’ upon the higher pressure transitions). These materials include metastable high pressure polymorphs from shock compression, materials with frustrated ordering, materials with macroscopic disorder like water-ices, and high-pressure electrides. Thus, the correlation of volumes that governs these phase transitions is independent on structure types but ruled by the configurational entropy of their ambient or intermediate structures.

In-situ SAXS investigation of high-pressure triglyceride polymorphism in milk cream and anhydrous milk fat

The effect of high-pressure on triglyceride polymorphism in both milk cream and anhydrous milk fat was compared using small-angle X-ray scattering up to maximum observed pressures of 1430 and 1480 MPa, respectively. The presence of water was found to have a significant effect on polymorphism at high pressures, with pressure effects having a strong influence for anhydrous milk fat (AMF). The susceptibility of specific polymorphs to compression was influenced by their triglyceride tilt relative to the lattice plane (tilting lead to greater compression) as well as the presence of water.

Magnetic Structures of Mn11Ta4O21 and Interpretation as an Hexagonal A-site Manganite.

Mn11Ta4O21 is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer (cchchch)2 stacking sequence that is related to BaVO3 and BaCrO3 high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at TN1 = 88 K and TN2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below TN1 and TN2, respectively. A complementary description with 14H-(hhccccc)2 sequence of only Mn octahedra provides a direct comparison with BaMnO3-δ hexagonal perovskites and naturally explains the AFM order. Below TN2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

Structural characteristics and elasticities of coesite and coesite-II at high pressure

Silica, SiO2, has attracted significant interest as one of the most important compounds in the fields of condensed-matter physics, materials science, and geoscience etc. Here, we theoretically investigate crystal structural characteristics and elastic properties of coesite and its high-pressure polymorph coesite-II at high pressures, which is critical to improve the understanding of densification mechanism of silica at high pressure. The driving mechanism for the pressure-induced transition pathways of coesite comes from both structural features and elastic effect. The phase transition of coesite to cosite-II is triggered by the shortening of Si1–O1 bond distance, and accompanied by elastic instability from a combination of softening elastic constants C44 and C46. The structural modulation of coesite-II along the b-axis direction, as proposed experimentally, results from, on the one hand, the bending of Si2–O15–Si3 angles and shortening of Si5–O4 and Si7–O4 distance with increasing pressure and, on the other hand, elastic softening associated with phonon instability. These results provide key insights into the densification mechanism of silica under high pressure.

A novel high-pressure polymorph of TaSi2

We report a novel high-pressure cubic cF24 phase of TaSi2 which is stable above 185 GPa through first-principles swarm intelligence structure search method. Results on enthalpy difference show that the structural transition of TaSi2 from the ambient hexagonal C40 phase to cF24 phase is first order with a volume drop of 4.1%. Under compression, the formation of cubic cF24 phase originates from the dramatic polymerization of one dimensional helical Si chain in the low-pressure C40 phase creating peculiar 3D Si tetrahedrons framework embedded in diamond-like Ta sublattice. The emergence of 3D Si tetrahedrons framework in cF24 phase are suggested to be the main causes for its weaker metallic nature compared to the hexagonal C40 phase.

High-pressure crystal polymorph of the protic ionic liquid: Ethylammonium nitrate

A high pressure (HP) crystal polymorph was obtained for the protic ionic liquid ethylammonium nitrate (EAN). Crystal structures of the HP phases were determined by X-ray diffraction. The formation of either a single crystal or polycrystal depended on the temperature and pressure pathway. In a previous work, a low temperature (LT) crystal polymorph was identified, and multiple crystallization pathways were observed by simultaneous X-ray diffraction and differential scanning calorimetry measurements. The non-reversible HP-crystal polymorph was different from the LT-one. Despite the simple molecular structure of EAN, HP-crystal polymorph is observed in addition to LT-crystal polymorph and multiple LT-crystallization pathways.

Mechanical behavior and phase change of alkali-silica reaction products under hydrostatic compression.

Alkali-silica reaction (ASR) causes severe degradation of concrete. The mechanical property of the ASR product is fundamental to the multiscale modeling of concrete behavior over the long term. Despite years of study, there is a lack of consensus regarding the structure and elastic modulus of the ASR product. Here, ASR products from both degraded field infrastructures and laboratory synthesis were investigated using high-pressure X-ray diffraction. The results unveiled the multiphase and metastable nature of ASR products from the field. The dominant phase undergoes permanent phase change via collapsing of the interlayer region and in-planar glide of the main layer, under pressure >2 GPa. The bulk moduli of the low- and high-pressure polymorphs are 27±3 and 46±3 GPa, respectively. The laboratory-synthesized sample and the minor phase in the field samples undergo no changes of phase during compression. Their bulk moduli are 35±2 and 76±4 GPa, respectively. The results provide the first atomistic-scale measurement of the mechanical property of crystalline ASR products.

Quartz metastability at high pressure: what new can we learn from polarized Raman spectroscopy?

The pressure dependence of the polarized Raman scattering of quartz was studied under hydrostatic conditions up to 9 GPa. We extended the available pressure calibrations, usually limited to the two most intense peaks, to a larger number of modes, providing polynomial functions that describe the relationship between pressure P and wavenumber shift $$\Delta \omega$$ for the 128-, 206-, 265-, 464-, 696-, 809-, 1080- and 1161- $$\hbox {cm}^{-1}$$ modes. For the first time, the pressure behavior of the LO-TO splitting is characterized up to 9 GPa under hydrostatic conditions. The pressure-induced wavenumber changes and derived phonon compressibilities show that longitudinal and transverse modes can be used interchangeably for the calculation of strains through the Gruneisen tensor to derive the strains in crystals, such as mineral inclusions, under non-hydrostatic conditions. A careful examination of the linewidths as a function of pressure shows that they are sensitive to the metastability of the quartz structure with respect to the high-pressure silica polymorphs. It is proposed that strong multiphonon interactions contribute to the stability of the structure of quartz at ambient conditions.

Original Oxo-Centered Frameworks in Bi3(VO4)O3 and Bi3.5(AsO4)(OH)0.5O3.5 by Supercritical Steam.

Two new bismuth compounds, oxovanadate Bi3(VO4)O3 and oxoarsenate Bi3.5(AsO4)(OH)0.5O3.5, were prepared using supercritical hydrothermal pressure. Dealing with the anionic sublattice, both crystal structures are built on anti-oxo-OBi4/OBi3 or -OBi4/OBi5 units connected together in infinite corrugated 2D layers surrounded by isolated XO4 (X = V or As) tetrahedra. These edifices complete a series initiated by the recent Bi3(PO4)O3 prepared under similar conditions. With the latter being assigned to the "simplest" bioxophosphate in terms of structural complexity, this aspect was investigated among the other compounds in their chemical ternaries. These phases are suggested to be high-pressure polymorphs, not possible to tackle when working at ambient pressure and temperature conditions.

Evidence of shock-compressed stishovite above 300\u2009GPa

SiO2 is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1–10 ME). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P42/mnm) and its thermodynamic stability is of great interest for understanding the seismic and dynamic structure of planetary interiors. Previous studies on stishovite via static and dynamic (shock) compression techniques are contradictory and the observed differences in the lattice-level response is still not clearly understood. Here, laser-induced shock compression experiments at the LCLS- and SACLA XFEL light-sources elucidate the high-pressure behavior of stishovite on the lattice-level under in situ conditions on the Hugoniot to pressures above 300 GPa. We find stishovite is still (meta-)stable at these conditions, and does not undergo any phase transitions. This contradicts static experiments showing structural transformations to the CaCl2, α-PbO2 and pyrite-type structures. However, rate-limited kinetic hindrance may explain our observations. These results are important to our understanding into the validity of EOS data from nanosecond experiments for geophysical applications.

Polymorphism of feldspars above 10 GPa

Feldspars are rock-forming minerals that make up most of the Earth’s crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Previous structural studies of feldspars are limited to ~10 GPa, and have shown that the dominant mechanism of pressure-induced deformation is the tilting of AlO4 and SiO4 tetrahedra in a tetrahedral framework. Herein, based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi2Al2O8), albite (NaAlSi3O8), and microcline (KAlSi3O8). The phase transitions are induced by severe tetrahedral distortions, resulting in an increase in the Al and/or Si coordination number. High-pressure phases derived from feldspars could persist at depths corresponding to the Earth upper mantle and could possibly influence the dynamics and fate of cold subducting slabs.

Optical signatures of phase transitions and structural modulation in elemental tellurium under pressure

A room-temperature infrared spectroscopy study of elemental tellurium at pressures up to 8.44 GPa in the energy range 0.015-2 eV is reported. Optical signatures of the high-pressure polymorphs are investigated and compared to the results of density-functional band-structure calculations. A Drude peak is first seen in the optical conductivity around 3.5 GPa indicating a semiconductor-to-metal transition within trigonal Te-I. A sharp increase in the Drude spectral weight and dc-conductivity around 4.3 GPa signals the transformation toward the triclinic Te-II polymorph. An absorption peak around 0.15 eV appears above 5 GPa concomitant with the gradual transformation of Te-II into the structurally similar but incommensurately modulated Te-III. Microscopically, this peak can only be reproduced within a sufficiently large commensurate approximant, suggesting the low-energy optical response as a fingerprint of the structural modulation.

Phase stabilities ofMgCO3andMgCO3-II studied by Raman spectroscopy, x-ray diffraction, and density functional theory calculations

Carbonates are the major hosts of carbon on Earth's surface and their fate during subduction needs to be known to understand the deep carbon cycle. Magnesite (${\mathrm{MgCO}}_{3}$) is thought to be an important phase participating in deep Earth processes, but its phase stability is still a matter of debate for the conditions prevalent in the lowest part of the mantle and at the core mantle boundary. Here, we have studied the phase relations and stabilities of ${\mathrm{MgCO}}_{3}$ at these $P,T$ conditions, using Raman spectroscopy at high pressures ($\ensuremath{\sim}148\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$) and after heating to high temperatures ($\ensuremath{\sim}3600\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) in laser-heated diamond anvil cell experiments. The experimental Raman experiments were supplemented by x-ray powder diffraction data, obtained at a pressure of 110 GPa. Density-functional-theory-based model calculations were used to compute Raman spectra for several ${\mathrm{MgCO}}_{3}$ high-pressure polymorphs, thus allowing an unambiguous assignment of Raman modes. By combining the experimental observations with the density-functional-theory results, we constrain the phase stability field of ${\mathrm{MgCO}}_{3}$ with respect to the high-pressure polymorph, ${\mathrm{MgCO}}_{3}$-II. We further confirm that Fe-free ${\mathrm{MgCO}}_{3}$-II is a tetracarbonate with monoclinic symmetry (space group $C2/m$), which is stable over the entire $P,T$ range of the Earth's lowermost mantle geotherm.

Design of Advanced Li2s/Graphene Composite Cathode for Li-S Batteries

A mixture of graphene and Li2S is pelletized by a volume reduction of 220% to synthesize a high energy density cathode for lithium−sulfur (Li−S) batteries. The compacted graphene/Li2S composite cathode, in addition to providing robust electrical pathways, drives Li2S metastable particles to metastable states (high-pressure polymorphs with a highly deformed mechanical state), triggering a spontaneous conversion of Li2S to S8 in the first charging process. This direct conversion resolves the long-standing problem of excess polysulfide formation during Li2S activation. Moreover, graphene sheets, tightly encapsulating the Li2S particles, effectively confine the active material in subsequent cycles to ensure unprecedented cycling stability at a high loading density for the Li−S battery. Here we show a major breakthrough in Li−S battery technology, providing the high energy density promised by the proposed technology while ensuring the required battery lifetime. Furthermore, successful operation of a pouch-type cell employing the pelletized graphene/Li2S composite cathode with a high loading level clearly demonstrates the convenience of scaling up the proposed pelletization of Li2S or S cathodes to a large-format Li−S battery.

Large Ocean Worlds with High-Pressure Ices

Pressures in the hydrospheres of large ocean worlds extend to ranges exceeding those in Earth deepest oceans. In this regime, dense water ices and other high-pressure phases become thermodynamically stable and can influence planetary processes at a global scale. The presence of high-pressure ices sets large icy worlds apart from other smaller water-rich worlds and complicates their study. Here we provide an overview of the unique physical states, thermodynamics, dynamic regimes, and evolution scenarios specific to large ocean worlds where high-pressure ice polymorphs form. We start by (i) describing the current state of knowledge for the interior states of large icy worlds in our solar system (i.e. Ganymede, Titan and Callisto). Then we (ii) discuss the thermodynamic and physical specifics of the relevant high–pressure materials, including ices, aqueous fluids and hydrates. While doing this we (iii) describe the current state of the art in modeling and understanding the dynamic regimes of high-pressure ice mantles. Based on these considerations we (iv) explore the different evolution scenarios for large icy worlds in our solar system. We (v) conclude by discussing the implications of what we know on chemical transport from the silicate core, extrapolation to exoplanetary candidate ocean worlds, limitations to habitability, differentiation diversity, and perspectives for future space exploration missions and experimental measurements.

Riesite, a New High Pressure Polymorph of TiO2 from the Ries Impact Structure

This paper describes riesite, a new high-pressure polymorph of TiO2 from the Ries impact structure, Germany. Riesite occurs in shock-induced melt veins within xenoliths of bedrock in suevite. It is structurally closely related to srilankite from which it differs by having two distinct cation sites rather than one and through its monoclinic symmetry. It is indicative that riesite forms only upon release from the shock state upon back transformation from akaogiite.

A new high-pressure benzocaine polymorph — towards understanding the molecular aggregation in crystals of an important active pharmaceutical ingredient (API)

Benzocaine (BZC), an efficient and highly permeable anaesthetic and an active pharmaceutical ingredient of many commercially available drugs, was studied under high pressure up to 0.78 GPa. As a result, new BZC polymorph (IV) was discovered. The crystallization of polymorph (IV) can be initiated by heating crystals of polymorph (I) at a pressure of at least 0.45 GPa or by their compression to 0.60 GPa. However, no phase transition from polymorph (I) to (IV) was observed. Although polymorph (IV) exhibits the same main aggregation motif as in previously reported BZC polymorphs (I)-(III), i.e. a hydrogen-bonded ribbon, its molecular packing and hydrogen-bonding pattern differ considerably. The N-H...N hydrogen bonds joining parallel BZC ribbons in crystals at ambient pressure are eliminated in polymorph (IV), and BZC ribbons become positioned at an angle of about 80°. Unfortunately, crystals of polymorph (IV) were not preserved on pressure release, and depending on the decompression protocol they transformed into polymorph (II) or (I).

Computational Investigation of the Electrolyte Properties of Li12P3N9 and Its High Pressure Polymorph Li4PN3 through First-Principles Simulations

Two recently synthesized crystalline materials which belong to the LiPON materials family, Li12P3N9 and it’s high–pressure polymorph Li4PN3, have been computationally examined as solid electrolytes for the usage in Li–ion batteries through first–principles simulations. The simulations of the idealized electrolyte properties of both materials suggests that these materials are promising solid electrolytes. The simulated crystal structures of both materials found to be in good agreement with experimental data, the phase transition between Li12P3N9 and Li4PN3 at high pressure has been validated through the simulations and found to be consistent with computational and experimental literature. The Li–ion migration analysis predicts that these materials are a very promising conductors for Li–ions with a calculated value for the activation energy comparable and even smaller to those of good solid electrolytes in the same materials family. The Li–ion migration analysis also suggests that the Li–ion migration is dominated with the vacancy migration mechanism which takes place along the three diffusion axes in Li12P3N4 and along the c–axis only in Li4PN3. The simulations of idealized interfaces with metallic Li anode demonstrate that these materials are likely to form a metastable interfaces with Li metal.