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.     

X-ray studies of the semiconductors snas,InTe, TlS and TISe UP TO 43 GPa

Abstract

The structures of the semiconductors SnAs, InTe, TlS and TlSe have been investigated using high-pressure (HP) diffraction technique-a gasketed diamond anvil cell (DAC)'. The pressure was measured by ruby fluorescence technique. The first order reversible transition from NaCl to CsCl structure was found in SnAs with volume discontinuity 5% the two-phase area extends from 32 to 43 GPa. The volume change V/v,(P) of the SnAs is shown in Figure 1. In agreement with the gomology rule3, the same pressure effect has been found in Sn_xSb1?x atP=9 GPa2.     

Phase transitions of YbX (X = P,As and Sb) with a NaCl-type structure at high pressures

The powder X-ray diffraction of YbX (X?=?P, As and Sb) with a NaCl-type structure has been studied with synchrotron radiation up to 63?GPa at room temperature. YbSb undergoes the first-order structural phase transition from the NaCl-type (B1) to the CsCl-type (B2) structure at around 13?GPa. The structural change to the B2 structure occurs with the volume collapse of about 1% at 13?GPa. The transition pressure of YbSb is surprisingly lower than that of any other heavier LnSb (Ln?=?Dy, Ho, Er, Tm and Lu). The pressure-induced phase transitions in YbP and YbAs are observed at around 51?GPa and 52?GPa respectively. The transition pressure of both compounds is much higher than that of YbSb. The high-pressure structural behaviour of YbX (X?=?P, As and Sb) is discussed. The volume versus pressure curve for YbX with the NaCl-type structure is fitted by a Birch equation of state. The bulk moduli of these compounds with the NaCl-type structure are 104?GPa for YbP, 85?GPa for YbAs and 52?GPa for YbSb.     

High pressure phase transition and elastic properties of thorium chalcogenides

The structural and elastic properties of thorium chalcogenides at high pressure, have been investigated using a suitable inter-ionic potential. The calculated equation of state, phase transition pressures for B1-B2 transition and bulk moduli for ThX (X=S,Se,Te) compounds agree well with the experimental results. ThTe, which crystallizes in the CsCl structure, does not show any structural transition up to 48 GPa. The present analysis does not show any anomalous features in elastic properties arising from ‘f’ electrons.     

High-pressure study of neptunium and plutonium compounds

Abstract

Neptunium and plutonium monopnictides and monochalcogenides were studied by x-ray diffraction at pressures up to 57 GPa. All of them exhibit structural phase transitions under pressure. The arsenides and tellurides have a CsCl (B2) type high-pressure structure. Sb as an anion favours a tetragonal high-pressure structure. The compressibilities were determined for all of the compounds studied. The results are compared to those obtained for the corresponding thorium and uranium compounds.     

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.     

Performance of different types of detectors in a high-pressure X-ray study of phase transition

Abstract

Phase transitions in praseodymium and lanthanum under pressure have been studied using a synchrotron powder X-ray diffraction technique. A structure refinement of the distorted fcc phase of Pr using diffraction data collected with an imaging plate (IP) detector demonstrate that among some possible structures the rhombohedral structure with space group R3m best reproduces the observed diffraction pattern. The distorted fcc-fcc phase transition in La was observed as a function of the temperature at 23 GPa using a CCD-based detector. A five-minute exposure sufficiently long to measure the intensities of very weak superlattice reflections from the distorted fcc phase, which has been found to transform to the fcc phase at 550 K. The performance of the IP and a CCD-based detector are compared and their future developments discussed.     

Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.     

New phase transition into the b.c.c. structure in antimony at high pressure

X-ray diffraction study of Sb has been made under the pressure of up to 43 GPa. A new phase transition from the Sb(III) into a b.c.c. phase occurs at about 28 GPa. The b.c.c. phase is found to be stable for the wide range of pressure from 28 GPa to at least 43 GPa.     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

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.     

Pressure-induced phase transitions in some AnS (An = Th,U, Np,Pu) Compounds

Pressure-induced structural phase transitions at high-pressure in monosulfides of thorium, uranium, neptunium and plutonium (AnS) have been studied theoretically by an inter-ionic potential theory with modified ionic charge introduced to include the Coulomb screening effect due to localized ‘f’ electrons. These AnS compounds undergo a phase transition from sodium chloride (NaCl) to cesium chloride (CsCl) structure at a very high-pressure. The present theoretical investigation carried out up to 120?GPa reveals that these compounds undergo NaCl–CsCl phase transitions at 100, 81, 75 and 105?GPa for ThS, US, NpS and PuS, respectively. The first-order pressure derivatives calculated from the present theory agree well with observed data.     

Volume compression of thallium to 45 GPa

Abstract

High-pressure structural transition and volume compression for thallium were investigated to 45 GPa in a diamond anvil cell using the angular dispersive X-ray diffraction technique. Except for the known polymorphic transition at 3.7 GPa, no other structural change was observed in this pressure range. The equation of state of the high pressure phase has been obtained: its initial bulk modulus, B₀ = 33.1 GPa, is lower by 10% than that of the hexagonal phase at normal pressure.     

High-pressure phase transitions in UP1-xSx

Abstract

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP_1-x -S_x (X=0.1; 0.25; 0.4) up to 53 GPa UP_1-x S_x is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP_1-x S_x with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP^1-x S_x 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur.     

Dynamical stability and phase transition of ZrC under pressure

A comprehensive first principles study of structural, elastic, electronic, and phonon properties of zirconium carbide (ZrC) is reported within the density functional theory scheme. The aim is to primarily focus on the vibrational properties of this transition metal carbide to understand the mechanism of phase transition. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus, electronic band structure, and phonon dispersion curves (PDC) of ZrC in rock-salt (RS) and high-pressure CsCl structures are determined. The pressure-dependent PDCs are also reported in NaCl phase. The phonon modes become softer and finally attain imaginary frequency with the increase of pressure. The lattice degree of freedom is used to explain the phase transition. Static calculations predict the RS to CsCl phase transition to occur at 308?GPa at 0?K. Dynamical calculations lower this pressure by about 40?GPa. The phonon density of states, electron–phonon interaction coefficient, and Eliashberg's function are also presented. The calculated electron–phonon coupling constant λ and superconducting transition temperature agree reasonably well with the available experimental data.     

Advances in data reduction of high-pressure x-ray powder diffraction data from two-dimensional detectors: a case study of schafarzikite (FeSb(2)O(4))

Methods have been developed to facilitate the data analysis of multiple two-dimensional powder diffraction images. These include, among others, automatic detection and calibration of Debye-Scherrer ellipses using pattern recognition techniques, and signal filtering employing established statistical procedures like fractile statistics.All algorithms are implemented in the freely available program package Powder3D developed for the evaluation and graphical presentation of large powder diffraction data sets.As a case study, we report the pressure dependence of the crystal structure of iron antimony oxide FeSb(2)O(4) (p≤21?GPa, T = 298?K) using high-resolution angle dispersive x-ray powder diffraction. FeSb(2)O(4) shows two phase transitions in the measured pressure range. The crystal structures of all modifications consist of frameworks of Fe(2+)O(6) octahedra and irregular Sb(3+)O(4) polyhedra. At ambient conditions, FeSb(2)O(4) crystallizes in space group P4(2)/mbc (phase I). Between p = 3.2?GPa and 4.1?GPa it exhibits a displacive second order phase transition to a structure of space group P 2(1)/c (phase II, a = 5.7792(4)??, b = 8.3134(9)??, c = 8.4545(11)??, β = 91.879(10)°, at p = 4.2?GPa). A second phase transition occurs between p = 6.4?GPa and 7.4?GPa to a structure of space group P4(2)/m (phase III, a = 7.8498(4)??, c = 5.7452(5)??, at p = 10.5?GPa). A nonlinear compression behaviour over the entire pressure range is observed, which can be described by three Vinet equations in the ranges from p = 0.52?GPa to p = 3.12?GPa, p = 4.2?GPa to p = 6.3?GPa and from p = 7.5?GPa to p = 19.8?GPa. The extrapolated bulk moduli of the high-pressure phases were determined to K(0) = 49(2)?GPa for phase I, K(0) = 27(3)?GPa for phase II and K(0) = 45(2)?GPa for phase III. The crystal structures of all phases are refined against x-ray powder data measured at several pressures between p = 0.52?GPa, and 10.5?GPa.     

Pressure induced phase transition in n-type InP at low temperature studied by photoluminescence

Abstract

Low temperature (5 K) photoluminescence of n-type InP has been studied in the 0–12 GPa range. The luminescence band energy follows the pressure dependence of the band gap, provided effective mass variations are taken into account. No evidence of a direct to indirect gap crossover has been observed up to the phase transition pressure (10.3 GPa). The phase transition is uncompleted at 12 GPa     

A study on the formation and stability of quasicrystal in Al4Mn and Al6Cr

Abstract

The condition of the formation of quasicrystal in Al₄Mn and Al₆Cr under high static pressure has been investigated for the first time. I-phase and T-phase have been observed in electron diffraction experiment. The structures of Al₄Mn quenched at about 100 K/s are different under various pressure from 0.95GPa to 4.45GPa. The phase transition from I- and T-phase to crystal phase has also been investigated.     

Structural phase transitions and the equation of state of SnTe at high pressures up to 2 mbar

Synchrotron X-ray diffraction studies of the structure of SnTe have been performed at room temperature and high pressures under the conditions of quasihydrostatic compression up to 193.5 GPa created in diamond anvil cells. Two structural phase transitions have been detected at P ≈ 3 and 23 GPa. The first phase transition is accompanied by a stepwise decrease in the volume of the unit cell by 4% because of the orthorhombic distortion of the initial SnTe-B1 cubic structure of the NaCl type. It has been found that two intermediate rhombic phases of SnTe with the space groups Cmcm and Pnma coexist in the pressure range of 3–23 GPa. The second phase transition at 23 GPa occurs from the intermediate rhombic modification to the SnTe-B2 cubic phase with the CsCl structure type. This phase transition is accompanied by an abrupt decrease in the volume of the unit cell by 8%. The pressure dependence of the volumes per formula unit at room temperature has been determined.