Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

Crystal structure of the high pressure phases of bismuth bi iii and bi iii′ by high energy synchrotron x-ray diffraction

Abstract

This paper reports the results of a synchrotron X-ray diffraction study on the crystal structures of Bi 111 and Bi 111′ which have been known to form under high pressure but have, for a long time, been unsolved. Powdered samples were compressed in a cubic-type multi-anvil press, MAXID, and diffraction data were collected using an Imaging Plate with monochromatized radiation of an energy of 49.7 keV. It was possible to identify at 3.8 GPa forty-eight reflections for Bi I11 in the sin θ / δ range from 1.6 nm^?1 to 5.6 nm^?1, which were indexed in terms of a tetragonal unit cell with a=0.8659 nm and c═ O·4238 nm (2=10). Analysis based on the observed intensities of the reflections led to a structure in which atoms form a distorted body-centered cubic lattice. It is of the same type as the structure of the high pressure phase of antimony Sb 11. When pressure was increased across the suggested transition pressure 4.3 GPa between Bi III and Bi III′ to 6.6 GPa, no change in the diffraction pattern was observed, indicating that there is no distinction between the two phases as long as the crystal structure is concerned. Discussion is given on the sequence of high pressure phase transitions in the Group Vb elements.     

Bulk moduli and high-pressure phases of the uranium rocksalt structure compounds: II. The monopnictides

Abstract

The high-pressure crystal structures of the compounds UX, where X = N, P, As and Sb, have been studied using X-ray diffraction in the pressure range up to about 60 GPa Rhornbohedral distortions are observed for UN and Up above 29 GPa and lO GPa, respectively. In Up a further transformation to an orthorhombic phase occurs at 28 GPa. UAs and USb transform to the CsCl structure at 20 GPa and 9 GPa, respectively. The latter transformations show a considerable hysteresis when the pressure is released. The scaling behaviour of the bulk modulus has been studied. It is confirmed that a log-log plot of bulk modulus versus specific volume for the cubic phases gives a straight line with a slope near ? 5/3.     

Synchrotron X-ray diffraction of SiO2 to multimegabar pressures

Abstract

α-Quartz was compressed at room temperature in a diamond-anvil cell without a medium to maximum pressures of 31 to 213 GPa and was studied by energy-dispersive synchrotron X-ray diffraction. Broad peaks observed in a previous high-pressure diffraction study of silica glass are evident in the present study of quartz compression, providing in situ confirmation of pressure-induced amorphization above 21 GPa. The 21-GPa crystalline-crystalline (quartz 1–11) transformation previously observed on quasihydrostatic compression of quartz is found to also occur under the current nonhydrostatic conditions, at the identical pressure. With nonhydrostatic compression, however, new sharp diffraction lines are observed at this pressure. The measurements show the coexistence of at least one amorphous and two crystalline phases above 21 GPa and below 43 GPa. The two crystalline phases are identified as quartz II and a new, high-pressure silica phase. The high-pressure phases, both crystalline and amorphous, can be quenched to ambient conditions from a maximum pressure of 43 GPa. With compression above 43 GPa, the diffraction pattern from quartz II is lost and the second crystalline phase persists to above 200 GPa.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

High-pressure phases of thorium and uranium compounds with the rocksalt structure

Abstract

The high-pressure crystal structures of the actinide compounds ThX and UX (X= C, N, P, S, As, Se, Sb, Te) have been studied by X-ray diffraction using synchrotron radiation, in the pressure range up to about 60 GPa Distorted fcc structures were observed for UC (27 GPa), UN (29 GPa), UP (10/28 GPa), US (10 GPa) and ThS (20 GPa). No phase transition has been observed for ThC and ThN. Compounds with As, Se, Sb all transform to the CsCl structure. ThP transform to the CsCl structure at 30 GPa. ThTe has the CsCl structure at ambient pressure and no further phase transition has been observed. UTe transforms to the CsCl structure at 9 GPa.     

Phase transitions of sulfur at high pressure: Influence of temperature and pressure environment

Abstract

Phase transitions of orthorhombic sulfur were investigated above 10 GPa by Raman spectroscopy using red light excitation. Transitions into several phases that have been reported in previous studies using green light excitation, are confirmed. The phase behaviour is observed to depend strongly on the preparation method. In the presence of a pressure transmitting medium (methanol/ethanol, 4:1), a sequence of phases α-S₈ → [intermediate phase (“ip”) + S₆] → [S₆ + high pressure-low temperature phase (“hplt”)] is described and characterized. Without the use of a pressure transmitting medium, the phase sequence α-S₈ → [“ip” + “hplt”] + “hplt” is observed. In addition, contributions of amorphous sulfur are detected around 10 GPa, i.e. at pressures below the transformation of α-S₈ into the above-mentioned phases. Characteristic Raman spectra of the different phases are extracted and documented over a wide pressure range.     

Development of a high pressure stirring cell up to 2 GPa: a new window for chemical reactions and material synthesis

ABSTRACT

A new method for stirring under high pressure conditions has been developed and tested. The key component is a Teflon cell assembly equipped with magnetic stirring function, which is capable to operate across a wide pressure range, up to at least 2?GPa, in a large volume press. The setup enables adjustable stirrer rotation rate and detection of stirring in a sample, e.g. to observe liquid–solid phase transitions at high pressure. The viscosity limit of stirring is ca. 500 times that of water at room temperature (i.e. ～500?mPas). Moreover, we show that zinc oxide nanoparticles hydrothermally synthesized at 0.5?GPa and 100°C under stirring conditions show an order of magnitude smaller size (100?nm) compared to those synthesized under non-stirring conditions (1?μm). The wide pressure range for stirring of viscous media opens interesting possibilities to produce novel materials via hydrothermal synthesis and chemical reactions.     

Structural transition behavior of ZnS nanotetrapods under high pressure

ZnS nanotetrapods synthesized via a solvothermal route have a octahedral core with a zincblende (ZB) structure and four hexprism-shaped arms consisting of alternately stacking ZB and wurtzite (WZ) phases, where the WZ phase has a higher volume percentage. In situ angular-dispersive X-ray diffraction (ADXRD) measurements were carried out to study the structural behavior of ZnS nanotetrapods under high pressure up to 41.3?GPa. The initial WZ structure exhibits a very high mechanical stability to ～11.3?GPa. Both the WZ and ZB structures transform to the rocksalt (RS) structure at ～15.4?GPa. The bulk moduli of the WZ (148.2?±?8.9?GPa) and RS (165.6?±?9.9?GPa) phases are both larger than the previously reported values. These phenomena are discussed based on the alternating epitaxial growth of the WZ and ZB phases in the arms of nanotetrapods. Our study suggests that the internal structure of nanomaterials could also greatly affect their stability and transition behavior.     

Measurements of compressibility of solids and powder compacts by a strain gauge technique at hydrostatic pressure up to 9GPa

Abstract

An experimental technique is described which enables one to measure the pressure-volume (P-V) relationship of solids and powder compacts and the linear compressibility of anisotropic single crystals by means of the resistive strain gauges at hydrostatic pressure up to 9 GPa. The potential of this technique is demostrated for solids possessing pressure induced phase transitions (PbTe, SmSe) and anisotropic crystals (Sb). For the first time P-V relationship is measured for highly compressible powder compact at increase and decrease of pressure.     

Pressure induced metallization of the antiferromagnetic-insulator CoI

Abstract

High pressure electrical measurements were conducted in the antiferromagnetic insulator CoI, using a miniature Diamond Anvil Cell (DAC). The existence of a Mott Transition predicted from high pressure ¹²⁹I Mgssbauer Spectroscopy (MS)¹ has been verified. At about 8 GPa the system becomes metal1ic as evidenced by the temperature behavior of the conductivity. The conductivity at room temperature, however, still increases with increasing pressure, leveling off at 11 GPa. The metallic behavior in the 8 -11 GPa is explained by coexistence of metallic and insulating clusters via a percolating process. Above 11 GPa the material is completely metallic. This mechanism is consistent with the MS findings.     

Compressibility of Fe1.087Te: a high pressure X-ray diffraction study

Fe_1.087Te exhibits three phases in the pressure range from ambient to 16.6?GPa and becomes amorphous at higher pressures. All three phases have tetragonal symmetry. The low pressure T-phase is stable in the pressure range 0≤P<4.1?GPa and is found to be relatively soft having zero pressure bulk modulus B ₀=36(1)?GPa. The intermediate cT-phase is less compressible with B ₀=88(5)?GPa and stable in the pressure range 4.1≤P<10?GPa while a more compressible phase was observed between 10 and 16.6?GPa.     

Solidification of selenium melt under pressure up to 6.5 GPa

Abstract

High purity selenium samples were melted under high pressure (≤6.4 GPa) and quenched at various rates ranging from 2 K s^?1 to 500 K s^?1 and the recovered material was examined by X-ray diffraction and electron microscopy. In the entire range of pressure and cooling rate, the melt was found to solidify into a polycrystalline aggregate of the trigonal phase of selenium. The samples obtained by slow cooling of the melt at 6.4 GPa contain, in addition to crystalline phase, regions which appear to be amorphous.     

TiC-MgO composite: an X-ray transparent and machinable heating element in a multi-anvil high pressure apparatus

ABSTRACT

TiC-MgO composite was developed as a heating element for X-ray study in the multi-anvil high pressure apparatus. We synthesized TiC-MgO blocks (50–70 wt.% of TiC) by compression in a cold isostatic press followed by baking in a gas flow furnace. Heaters of tubular shape were manufactured from the synthesized blocks either by lathe or numerically controlled milling machine. The so-produced heating elements have been proved to generate temperatures up to 2250?K at 10?GPa, condition where classical graphite heaters are not suitable anymore due to graphite-diamond transition. These new heaters have been successfully used for in situ X-ray radiography and diffraction measurements on liquid Fe alloys, exploiting excellent X-ray transparency.     

Decreasing electrical resistivity of gold along the melting boundary up to 5 GPa

ABSTRACT

The electrical resistivity of gold was experimentally measured at high pressures from 2 to 5?GPa and temperatures ～300?K above melting. The resistivity decreased as a function of pressure and increased as a function of temperature as expected. The temperature dependence of resistivity in the solid and liquid phases are comparable to 1?atm results. The observed melting temperatures at each pressure agree well with previous experimental and theoretical studies. The essential result of this study is that resistivity decreases along the pressure-dependent melting boundary, conflicting with a prediction of invariant behavior as reported in the literature. This result is discussed in terms of the interaction between s and d-bands as both pressure and temperature increase along the melting boundary. The thermal conductivity of gold was calculated from the measured electrical resistivity using the Wiedemann-Franz law. The temperature-induced effect on the thermal conductivity at high temperatures is as expected in both the solid and liquid phase while the pressure-effect shows some variability.     

High pressure and low-temperature polymorphism in cyclopentanol

The polymorphism of cyclopentanol (C₅H₁₀O) has been investigated as a function of temperature at ambient pressure and as a function of hydrostatic pressures to 3.7?GPa at room temperature. Differential scanning calorimetry (DSC) and Raman spectra reveal that two plastic phases and two fully ordered crystalline phases are formed during cooling. High pressure Raman and infrared spectra show that cyclopentanol undergoes two-phase transformations. At around 0.6?GPa, the liquid cyclopentanol transforms to a solid plastic structure. On further compression to 1.9?GPa, one fully ordered crystalline phase is observed. Based on pieces of evidence such as peak splitting and emergence of new peaks, it can be concluded that the ordered crystalline structure has a lower symmetry. In addition, the decrease in the wavenumber of the O–H stretching modes at low temperature and high pressure suggests the ordered crystalline phases are characterized by the formation of hydrogen-bonded molecular chains.     

Comparative Study on Pressure-Induced Structural Changes between C 6 O 2 I 4 and C 6 I 6

We have investigated the pressure effects on the structural properties of C 6 O 2 I 4 up to 39 GPa by powder x-ray diffraction measurements, which were compared with those of C 6 I 6 . The diffraction patterns of C 6 O 2 I 4 indicated a phase transition starting at 26.8 GPa. The mixed state of the low- and high-pressure phases continued up to 39 GPa well above an insulator-to-metal transition pressure of 33 GPa. The C 6 O 2 I 4 molecule remains planar structure in the low-pressure phase below 26.8 GPa in contrast to the non-planar molecular structure of C 6 I 6 at ambient and high pressures.     

High volumetric hydrogen density phases of magnesium borohydride at high-pressure: A first-principles study

The previously proposed theoretical and experimental structures,bond characterization,and compressibility of Mg(BH 4) 2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations.It is found that the ambient pressure phases of meta-stable I4 1 /amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure.Enthalpy calculation indicates that the ground state of F 222 structure proposed by Zhou et al.[2009 Phys.Rev.B 79 212102] will transfer to I4 1 /amd at 0.7 GPa,and then to a P-3m1 structure at 6.3 GPa.The experimental P 6 1 22 structure(α-phase) transfers to I4 1 /amd at 1.2 GPa.Furthermore,both I4 1 /amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure.Their theoretical volumetric hydrogen densities reach 146.351 g H 2 /L and 134.028 g H 2 /L at ambient pressure,respectively.The calculated phonon dispersion curve shows that the I4 1 /amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa.So the I4 1 /amd phase may be synthesized under high pressure and retained to ambient pressure.Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range.In addition,they each have an anisotropic compressibility.The c axis of these structures is easy to compress.Especially,the c axis and volume of P-3m1 phase are extraordinarily compressible,showing that compression along the c axis can increase the volumetric hydrogen content for both I4 1 /amd and P-3m1 structures.     

Crystal structure prediction of ReN at high pressure: a new incompressible phase

Based on the swarm-intelligence-based CALYPSO method the NbO, R3m and NiAs phases for ReN are predicted. The R3m phase of ReN at high pressure is firstly found. The structural, mechanical and electronic properties of ReN with the three phases are studied systematically. Moreover, it is also firstly found that pressure stimulated ReN to undergo twice phase transitions, from NbO to R3m phase at 43.3?GPa and from R3m to NiAs phase at 53.6?GPa. The three phases of ReN are verified to be mechanically stable and a promising low-compressible material at ambient conditions. According to the electron density of states and electron localization functions we have found that their structural stability and high hardness is on account of the strong covalent bonding of Re-N and N-N.     

Phase diagram of RDX (hexahydro-1,3,5-trinitro-s-triazine) at high pressures and temperatures

Abstract

The phase diagram of RDX-h₆ (hexahydro-1,3,5-trinito-s-triazine) and RDX-d₆ has been studied by Raman spectroscopy to more than 13 GPa at 295 K and 7.5 GPa between 150 and 450 K. Two stable high pressure phases have been found. γ-RDX or RDX-III forms from α-RDX above 3.8 GPa below 380 K. β-RDX forms when α- or γ-RDX are heated, can be retained metastably at low temperatues, and may be related to a very unstable form occasionally recovered at ambient pressure. Deuterium isotopic substitution and shear increase the temperature where β-RDX begins to form on heating.