High-pressure Synchrotron X-ray Diffraction Research Articles

Structural and Transport Properties of the Weyl Semimetal NbAs at High Pressure **Supported by the National Natural Science Foundation of China under Grant Nos 11422429 and 11421404, the National Basic Research Program of China under Grant Nos 2012CB821402 and 2015CB921401, the China Postdoctoral Science Foundation under Grant No 2014M560288, the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai

We perform a series of high-pressure synchrotron x-ray diffraction (XRD) and resistance measurements on the Weyl semimetal NbAs. The crystal structure remains stable up to 26 GPa according to the powder XRD data. The resistance of NbAs single crystal increases monotonically with pressure at low temperature. Up to 20 GPa, no superconducting transition is observed down to 0.3 K. These results show that the Weyl semimetal phase is robust in NbAs, and applying pressure may not be a good way to obtain a topological superconductor from Weyl semimetal NbAs.

High pressure synthesis of amorphous TiO2 nanotubes

Amorphous TiO2 nanotubes with diameters of 8-10 nm and length of several nanometers were synthesized by high pressure treatment of anatase TiO2 nanotubes. The structural phase transitions of anatase TiO2 nanotubes were investigated by using in-situ high-pressure synchrotron X-ray diffraction (XRD) method. The starting anatase structure is stable up to ∼20GPa, and transforms into a high-density amorphous (HDA) form at higher pressure. Pressure-modified high- to low-density transition was observed in the amorphous form upon decompression. The pressure-induced amorphization and polyamorphism are in good agreement with the previous results in ultrafine TiO2 nanoparticles and nanoribbons. The relationship between the LDA form and α-PbO2 phase was revealed by high-resolution transmission electron microscopy (HRTEM) study. In addition, the bulk modulus (B0 = 158 GPa) of the anatase TiO2 nanotubes is smaller than those of the corresponding bulks and nanoparticles (180-240 GPa). We suggest that the unique open-ended nanotube morphology and nanosize play important roles in the high pressure phase transition of TiO2 nanotubes.

Negative linear compressibility in a crystal of α-BiB3O6.

Negative linear compressibility (NLC), a rare and important mechanical effect with many application potentials, in a crystal of α-BiB3O6 (BIBO) is comprehensively investigated using first-principles calculations and high-pressure synchrotron X-ray diffraction experiments. The results indicate that the BIBO crystal exhibits the second largest NLC among all known inorganic materials over a broad pressure range. This unusual NLC behaviour is due to the rotation and displacement of the rigid [BO3] and [BO4] building units that result in hinge motion in an umbrella-like topology. More importantly, the parallel-polar lone-pair electrons on the Bi3+ cations act as “umbrella stands” to withstand the B-O hinges, thus significantly enhancing the NLC effect. BIBO presents a unique example of a “collapsible umbrella” mechanism for achieving NLC, which could be applied to other framework materials with lone-pair electrons.

High-pressure behavior of cuprospinel CuFe2O4: Influence of the Jahn-Teller effect on the spinel structure

The Jahn-Teller effect at Cu2+ in cuprospinel CuFe2O4 was investigated using high-pressure single-crystal synchrotron X-ray diffraction techniques at beamline BL10A at the Photon Factory, KEK, Japan. Six data sets were collected in the pressure range from ambient to 5.9 GPa at room temperature. Structural refinements based on the data were performed at 0.0, 1.8, 2.7, and 4.6 GPa. The unit-cell volume of cuprospinel decreases continuously from 590.8(6) to 579.5(8) A3 up to 3.8 GPa. Least-squares fitting to a third-order Birch-Murnaghan equation of state yields the zero-pressure volume V = 590.7(1) A3 and bulk modulus K = 188.1(4.4) GPa with K ′ fixed at 4.0. The structural formula determined by electron microprobe analysis and site occupancy refinement is represented as T (Fe3+0.90Cu2+0.10) M (Fe3+1.10Fe2+0.40Cu2+0.50)O4. Most of the Cu2+ are preferentially distributed onto the octahedrally coordinated ( M ) site of the spinel structure. With pressure, the arrangement of the oxygen atoms around the M cation approaches a regular octahedron. This leads to an increase in the electrostatic repulsion between the coordinating oxygen ions and the 3 d z2 orbital of M Cu2+. At 4.6 GPa, a cubic-tetragonal phase transition is indicated by a splitting of the a axis of the cubic structure into a smaller a axis and a longer c axis, with unit-cell parameters a = 5.882(1) A and c = 8.337(1) A. The tetragonal structure with space group I 41/ amd was refined to R 1 = 0.0332 and wR 2 = 0.0703 using 38 observed reflections. At the M site, the two M -O bonds parallel to the c -axis direction of the unit cell are stretched with respect to the four M -O bonds parallel to the a-b plane, which leads to an elongated octahedron along the c -axis. The cubic-to-tetragonal transition induced by the Jahn-Teller effect at Cu2+ is attributable to this distortion of the CuO6 octahedron and involves Cu 3 d z2 orbital, ab initio quantum chemical calculations support the observation. At the tetrahedrally coordinated ( T ) site, on the other hand, the tetrahedral O- T -O bond angle increases from 109.47° to 111.7(7)°, which generates a compressed tetrahedral geometry along the c -axis. As a result of the competing distortions between the elongated octahedron and the compressed tetrahedron, the a unit-cell parameter is shortened with respect to the c unit-cell parameter, giving a c / a ′ ratio ( ![Formula][1] ) slightly greater than unity as referred to cubic lattice ( c / a ′ = 1.002). The c / a ′ value increases to 1.007 with pressure, suggesting further distortions of the elongated octahedron and compressed tetrahedron. [1]: /embed/mml-math-1.gif

Superconductivity emerging from a suppressed large magnetoresistant state in tungsten ditelluride.

The recent discovery of large magnetoresistance in tungsten ditelluride provides a unique playground to find new phenomena and significant perspective for potential applications. The large magnetoresistance effect originates from a perfect balance of hole and electron carriers, which is sensitive to external pressure. Here we report the suppression of the large magnetoresistance and emergence of superconductivity in pressurized tungsten ditelluride via high-pressure synchrotron X-ray diffraction, electrical resistance, magnetoresistance and alternating current magnetic susceptibility measurements. Upon increasing pressure, the positive large magnetoresistance effect is gradually suppressed and turned off at a critical pressure of 10.5 GPa, where superconductivity accordingly emerges. No structural phase transition is observed under the pressure investigated. In situ high-pressure Hall coefficient measurements at low temperatures demonstrate that elevating pressure decreases the population of hole carriers but increases that of the electron ones. Significantly, at the critical pressure, a sign change of the Hall coefficient is observed.

New High-Pressure Polymorph of the Nonlinear Optical Crystal BaTeMo2O9

High-pressure synchrotron X-ray diffraction on the nonlinear optical crystal α-BaTeMo2O9 (α -BTM) has revealed a new polymorph of γ-BTM having a monoclinic space group of P21. This first order phase transition from the orthorhombic Pca21 to the new monoclinic P21 structure occurs at 3.8 GPa pressure. The mechanism of the phase transition under high pressure is attributed to the anisotropic pressure response along different crystallographic directions.

Breakdown of three-dimensional Dirac semimetal state in pressurizedCd3As2

We report an observation of a pressure-induced breakdown of the three-dimensional Dirac semimetal (3D-DSM) state in Cd3As2, evidenced by a series of in situ high-pressure synchrotron x-ray diffraction and single-crystal transport measurements. We find that Cd3As2 undergoes a structural phase transition from a metallic tetragonal phase in space group I41/acd to a semiconducting monoclinic phase in space group P21/c at critical pressure 2.57 GPa; above this pressure, an activation energy gap appears, accompanied by distinct switches in Hall resistivity slope and electron mobility. These changes of crystal symmetry and corresponding transport properties manifest the breakdown of the 3DDSM state in pressurized Cd3As2.Received 4 January 2015Revised 5 April 2015DOI:https://doi.org/10.1103/PhysRevB.91.165133©2015 American Physical Society

Compressibility and equation of state of beryl (Be3Al2Si6O18) by using a diamond anvil cell and in situ synchrotron X-ray diffraction

High-pressure single-crystal synchrotron X-ray diffraction was carried out on a single crystal of natural beryl compressed in a diamond anvil cell. The pressure-volume (P-V) data from room pressure to 9.51 GPa were fitted by a third-order Birch-Murnaghan equation of state (BM-EoS) and resulted in unit-cell volume V-0 = 675.5 +/- 0.1 angstrom(3), isothermal bulk modulus K-0 = 180 +/- 2 GPa, and its pressure derivative K-0' = 4.2 +/- 0.5. We also calculated V-0 = 675.5 +/- 0.1 angstrom(3) and K-0 = 181 +/- 1GPa with fixed K-0' at 4.0 and then obtained the axial moduli for a (K-a0)-axis and c (K-c0)-axis of 209 +/- 1 and 141 +/- 2 GPa by "linearized" BM-EoS approach. The axial compressibilities of a-axis and c-axis are beta(a) = 1.59 x 10(-3) GPa(-1) and beta(c) = 2.36 x 10(-3) GPa(-1) with an anisotropic ratio of beta(a):beta(c) = 0.67:1.00. On the other hand, the pressure-volume-temperature (P-V-T) EoS of the natural beryl has also been measured at temperatures up to 750 K and at pressures up to 16.81 GPa, using diamond anvil cell in conjunction with in situ synchrotron angle-dispersive powder X-ray diffraction. The P-V data at room temperature and at a pressure range of 0.0001-15.84 GPa were then analyzed by third-order BMEoS and yielded V-0 = 675.3 +/- 0.1 angstrom(3), K-0 = 180 +/- 2 GPa, K-0' = 4.2 +/- 0.3. With K-0' fixed to 4.0, we also obtained V-0 = 675.2 +/- 0.1 angstrom(3) and K-0 = 182 +/- 1 GPa. Consequently, we fitted the P-V-T data with high-temperature BM-EoS approach using the resultant K-0' (4.2) from room-temperature BM-EoS and then obtained the thermoelastic parameters of V-0 = 675.3 +/- 0.2 angstrom(3), K-0 = 180 +/- 1 GPa, temperature derivative of the bulk modulus (partial derivative K/partial derivative T)(P) = -0.01 7 +/- 0.004 GPa K-1, and thermal expansion coefficient at ambient conditions alpha(0) = (2.82 +/- 0.74) x 10(-6) K-1. Present results were also compared with previous studies for beryl. From the comparison of these fittings, we propose to constrain K-0 = 180 GPa and K-0' = 4.2 for beryl. And we also observed that beryl exhibits anisotropic thermal expansion at relatively low temperatures, which is very consistent with previous studies. Furthermore, no phase transition was observed in the entire pressure and temperature range (up to 16.84 GPa and 750 K) of this study for the natural beryl.

Intermolecular stabilization of 3,3'-diamino-4,4'-azoxyfurazan (DAAF) compressed to 20 GPa.

The room temperature stability of 3,3'-diamino-4,4'-azoxyfurazan (DAAF) has been investigated using synchrotron far-infrared, mid-infrared, Raman spectroscopy, and synchrotron X-ray diffraction (XRD) up to 20 GPa. The as-loaded DAAF samples exhibited subtle pressure-induced ordering phenomena (associated with positional disorder of the azoxy "O" atom) resulting in doubling of the a-axis, to form a superlattice similar to the low-temperature polymorph. Neither high pressure synchrotron XRD, nor high pressure infrared or Raman spectroscopies indicated the presence of structural phase transitions up to 20 GPa. Compression was accommodated in the unit cell by a reduction of the c-axis between the planar DAAF layers, distortion of the β-angle of the monoclinic lattice, and an increase in intermolecular hydrogen bonding. Changes in the ring and -NH2 deformation modes and increased intermolecular hydrogen bonding interactions with compression suggest molecular reorganizations and electronic transitions at ∼ 5 GPa and ∼ 10 GPa that are accompanied by a shifting of the absorption band edge into the visible. A fourth-order Birch-Murnaghan fit to the room temperature isotherm afforded an estimate of the zero-pressure isothermal bulk modulus, K0 = 12.4 ± 0.6 GPa and its pressure derivative K0' = 7.7 ± 0.3.

Pressure-Induced Diversity of π-Stacking Motifs and Amorphous Polymerization in Pyrrole

The behavior of pyrrole under high pressure has been investigated by in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman scattering up to 34 GPa. A solid-to-solid transition at about 6.2 GPa with a large collapse of volume (∼40%) from Pnma to P21/c has been found after a liquid-to-solid transition at 0.6 GPa, which is caused by the molecular rotational repacking of π-stacking. This new phase P21/c plays a central role in the following pressure-induced polymerization due to the formation of a closed dimer, a very important precursor. The threshold of C···C distance with steric hindrance in dimer is about 1.62 Å at ∼10.2 GPa. After this steric hindrance is overcome, a crystal–amorphous transformation starts at ∼14.3 GPa. When completely released from 34 GPa, the recovered solid product with single bond is identified by in situ Raman measurement.

Pressure-induced phase coexistence in BaFe1.8Co0.2As2

We report high-pressure powder synchrotron X-ray diffraction measurements of overdoped BaFe1.8Co0.2As2 under quasi-hydrostatic pressure up to 40.1 GPa. Our results indicate that a tetragonal (T) to collapsed tetragonal (CT) phase transition occurs at 16.8 GPa and the two phase coexist until 30 GPa, which has not been previously observed in iron arsenide compounds. Both the lattice parameters a and c show discontinuous change for the T and CT phases. The decrease of the c lattice parameter is as large as 12.2% owing to the uniaxial pressure effect. The axial ratio c/a of the T phase exhibits similar features to the other 122-type compounds below 16.8 GPa, whereas there is a very small increase with increasing pressure in the two phase coexistence region. Because of the relationship between the axial ratio and superconductivity, the abnormal expansion may be related to the sudden increase of the strength of antiferromagnetic spin fluctuations.

Structural phase transition of BaZrO3 under high pressure

We studied the phase transition behavior of cubic BaZrO3 perovskite by in situ high pressure synchrotron X-ray diffraction experiments up to 46.4 GPa at room temperature. The phase transition from cubic phase to tetragonal phase was observed in BaZrO3 for the first time, which takes place at 17.2 GPa. A bulk modulus 189 (26) GPa for cubic BaZrO3 is derived from the pressure–volume data. Upon decompression, the high pressure phase transforms into the initial cubic phase. It is suggested that the unstable phonon mode caused by the rotation of oxygen octahedra plays a crucial role in the high pressure phase transition behavior of BaZrO3.

Pure Hexagonal Phase of EuF3 Modulated by High Pressure

We have performed in situ high-pressure synchrotron X-ray diffraction (XRD) and photoluminescence (PL) measurements to explore the behaviors of europium fluoride (EuF3). At ambient conditions, EuF3 consists of two representative phases of the rare earth trifluorides family. The orthorhombic phase (α-EuF3) evolves into its hexagonal phase (β-EuF3) in the pressure range 2.7–7.8 GPa, which involves the variations of europium positions in one layer and changes in the coordination number of europium from nine to eleven. The existence of pure β-EuF3 at high pressure allows solving detailed structural information of β-EuF3 for the first time. In comparison with the excellent hydrostatic condition in methanol–ethanol–water, a somewhat poor pressure condition in silicon oil contributes to the sluggishness of the phase transition, restrains the compression of lattice constants, and controls the phase content ratio of the quenched sample. Significantly, the PL measurements show that the pressure-induced purity of EuF3 is a benefit to the enhancement of its luminescence intensity. This work provides precise structural and PL information on β-EuF3, getting a deeper insight into the nature of these two typical structures with respect to rare-earth trifluorides.

The high-pressure behavior of bloedite: A synchrotron single-crystal X-ray diffraction study

High-pressure single-crystal synchrotron X-ray diffraction was carried out on a single crystal of bloedite [Na 2 Mg(SO 4 ) 2 4H 2 O] compressed in a diamond-anvil cell. The volume-pressure data, collected up to 11.2 GPa, were fitted by a second- and a third-order Birch-Murnaghan equation of state (EOS), yielding V 0 = 495.6(7) A 3 with K 0 = 39.9(6) GPa, and V 0 = 496.9(7) A 3 , with K 0 = 36(1) GPa and K ′ = 5.1 (4) GPa −1 , respectively. The axial moduli were calculated using a Birch-Murnaghan EOS truncated at the second order, fixing K ′ equal to 4, for a and b axes and a third-order Birch-Murnaghan EOS for c axis. The results were a 0 = 11.08(1) and K 0 = 56(3) GPa, b 0 = 8.20(2) and K 0 = 43(3) GPa, and c 0 = 5.528(5), K 0 = 40(2) GPa, K ′ = 1.7(3) GPa −1 . The values of the compressibility for a , b , and c axes are β a = 0.0060(3) GPa −1 , β b = 0.0078(5) GPa −1 , β c = 0.0083(4) GPa −1 with an anisotropic ratio of β a :β b :β c = 0.72:0.94:1. The evolution of crystal lattice and geometrical parameters indicates no phase transition up to 11 GPa. Sulfate polyhedra are incompressible, whereas the Mg polyhedral bulk modulus is 95 GPa. The sodium polyhedron is the softest part of the whole structure with a bulk modulus of 41 GPa. Pressure decreases significantly the distortion of Na coordination. Up to 10 GPa, the donor-acceptor oxygen distances decrease significantly and the difference between the two water molecules decreases with an increase in the strengths of hydrogen bonds. At the same time, the bond lengths from Na and Mg to O atoms of the water molecules decrease faster than other bonds to these cations suggesting that there is a coupling between the Na-Ow and Mg-Ow bond strengths and the “hydrogen transfer” to acceptor O atoms.

Photoluminescence studies of Y2O3:Eu3+ under high pressure

The Eu-doped yttria (Y2O3:Eu3+) has been investigated by the in situ high-pressure angle dispersive synchrotron X-ray diffraction (XRD) and the photoluminescence (PL) spectroscopy. The red shift and intensity ratio variation of emissions with increasing pressure were observed and elucidated. It was found that the red shift of emissions is related to the expansion of the f orbit of the Eu3+ and the intensity ratio variation of emissions is ascribed to the change of the crystal field under high pressure. The pressure-induced changes in spectrum are related to the phase transition, which was confirmed by XRD pattern. The two high pressure phases were identified as the monoclinic (C2/m) phase and hexagonal (P-3m1) phase by the Rietveld refinement.

A novel pressure-induced phase transition in CaZrO3

High pressure synchrotron X-ray diffraction measurements on CaZrO3 were carried out in a diamond anvil cell up to 50.1 GPa at room temperature. It was found that the orthorhombic phase can be stable up to 30 GPa. A new pressure-induced phase transition was observed in CaZrO3 beyond 30 GPa. The high pressure structure of CaZrO3 was determined to be a monoclinic phase which is distinct from the high pressure structures that were previously reported for other perovskite oxides. Upon release of pressure, the high pressure phase transforms into the initial orthorhombic phase. A fit of the compression data to the third-order Birch–Murnaghan equation of state yields a bulk modulus K0 of 193(14) GPa. We propose that the unique distorted structure probably plays a crucial role in the high pressure behavior of CaZrO3. Especially, the distinct phase transformation may be related to the rotation or tilting of the ZrO6 octahedra.

Pressure-induced amorphous-to-amorphous reversible transformation in Pr75Al25

A pressure-induced amorphous-to-amorphous reversible transformation was revealed in Pr75Al25 metallic glass (MG) using in situ high-pressure synchrotron x-ray diffraction technique. The transition began at about 21 GPa with a ∼ 5% volume collapse and ended at about 35 GPa. This transition is reversible with hysteresis. Based on the high-pressure behaviors of Ce-based metallic glasses and Pr metal here, we suggest that the pressure-induced polyamorphic transition in Pr75Al25 MG stems from 4f-electron delocalization of Pr metal which leads to abrupt change in bond shortening. These results obtained here provide new insights into the underlying mechanism of the amorphous-to-amorphous phase transition in metallic glasses and will trigger more theoretical and experimental investigations for such transition.

The pressure-induced phase transition studies of In2S3 and In2S3:Ce nanoparticles

A novel method, gas–liquid phase chemical deposition is developed to prepare In2S3 and In2S3:Ce nanoparticles. The structural, morphology and composition feature of these two nanoparticles is studied by XRD, HRTEM, and XPS. In situ high-pressure synchrotron X-ray diffraction studies were carried out by using a diamond-anvil cell. The doping does not influence the tetragonal-to-cubic phase transition path while results in a lower phase transition pressure of In2S3:Ce nanoparticles (4.3GPa) than that of In2S3 nanoparticles (7.1GPa). The bulk moduli of tetragonal phases are B0=87.1±4.3GPa and B0=55.6±4.1GPa, respectively. The distinct high-pressure behaviors can be explained in term of the doped ions, causing lattice distortion and reducing structural stability of the In2S3 nanoparticles and further accelerating the phase transition.

First principle study of elastic and thermodynamic properties of FeB4 under high pressure

The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB4 at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB4 are also determined in the pressure range of 0–100 GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB4 is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB4 possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB4 is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB4 are successfully obtained in the present work.

In situ high-pressure synchrotron X-ray diffraction study of the structural stability in NdVO4 and LaVO4

Room-temperature angle-dispersive X-ray diffraction measurements on zircon-type NdVO4 and monazite-type LaVO4 were performed in a diamond-anvil cell up to 12GPa. In NdVO4, we found evidence for a non-reversible pressure-induced structural phase transition from zircon to a monazite-type structure at 6.5GPa. Monazite-type LaVO4 also exhibits a phase transition but at 8.6GPa. In this case the transition is reversible and isomorphic. In both compounds the pressure induced transitions involve a large volume collapse. Finally, the equations of state and axial compressibilities for the low-pressure phases are also determined.