High-pressure X-ray Diffraction Study Research Articles

Iso-structural phase transition in pyrochlore iridates (Sm _{1-x}$Bix)2Ir2O7 (x = 0, 0.02, and 0.10): high-pressure Raman and x-ray diffraction studies

The pyrochlore iridates,A2Ir2O7, show a wide variety of structural, electronic, and magnetic properties controlled by the interplay of different exchange interactions, which can be tuned by external pressure. In this work, we report pressure-induced iso-structural phase transitions at ambient temperature using synchrotron-based x-ray diffraction (up to ∼20 GPa) and Raman-scattering measurements (up to ∼25 GPa) of the pyrochlore series (Sm_{1-x}Bix)2Ir2O7(x= 0, 0.02, and 0.10). Our Raman and x-ray data suggest an iso-structural transition in Sm2Ir2O7atPc∼ 11.2 GPa, associated with the rearrangement of IrO6octahedra in the pyrochlore lattice. The transition pressure decreases to ∼10.2 and 9 GPa forx= 0.02 and 0.10, respectively. For all the samples, the linewidth of three phonons associated with Ir-O-Ir (A1gandEg) and Ir-O (T2g4) vibrations show anomalous decrease up toPc, due to decrease in electron-phonon interaction.

Role of GdO addition in the structural stability of cubic Gd2O3 at high pressures: Determination of the equation of states of GdO and Gd2O3

We report a high-pressure X-ray diffraction study on bixbyite-type Gd2O3 showing that the addition of 4.3 mol% of GdO increases the pressure stability of Gd2O3. In our experiments the onset of the phase transition to the previously reported high-pressure trigonal polymorph was detected upon compression at 15.1(1) GPa, with the low- and high-pressure phases coexisting up to 20 GPa. The observed transition is reversible upon decompression. Our finding extends the high-pressure stability of the low-pressure phase, which in previous studies was reported to undergo phase transitions from 2.6 to 13.4 GPa. This is important for many of the technological applications of Gd2O3 including gas turbine engines, light water reactors, and nuclear waste immobilization. Additionally, we find that GdO undergoes an irreversible amorphization beyond 15.1(1) GPa. Finally, accurate equations of state were determined from experiments and density-functional theory calculations. These calculations also provided information on elastic constants and moduli. A systematic discussion of the bulk modulus and its pressure derivative is also presented.

Pressure-induced phase transition, its mechanism and compressibility studies on nanocrystalline and bulk U3O8

High Pressure X-Ray Diffraction (HPXRD) studies were conducted on powder samples of U3O8 with crystallite sizes of 54 nm, 82 nm, as well as bulk U3O8. The investigation revealed that U3O8 undergoes a phase transition to a fluorite phase through an intermediate tetragonal structure, with the (200) plane of the orthorhombic structure playing a pivotal role. This transition occurs through the movement of uranium atoms while maintaining the overall integrity of the lattice. The phase transition is sluggish, and an extensive coexistence regime of the ambient and High-Pressure (HP) fluorite phase is observed in nano form as compared to bulk- U3O8. Moreover, the ambient phase of nano- U3O8 exhibits an elevated bulk modulus of 92 GPa, in contrast to the bulk sample's modulus of 78 GPa. Similarly, for the high-pressure phase, the bulk modulus is 419 GPa for nano- U3O8 and 367 GPa for bulk- U3O8. These findings provide valuable insights into the phase behavior and compressibility of U3O8, thereby carrying potential implications for nuclear technologists.

High-pressure structural phase transition on Bi14MoO24

In situ high-pressure powder synchrotron X-ray diffraction studies on tetragonal Bi14MoO24 have been carried out up to 11.5 GPa at room temperature. A tetragonal (I4/m) to monoclinic (C2/m) structural phase transition has been identified around 5 GPa and possible evidence for a second phase transition is observed at the highest pressure of this study. Both phase transitions are reversible upon decompression. The evolution of the lattice parameters, axial ratio, and volume under pressure have been determined for both the low pressure tetragonal and high-pressure monoclinic phases. A volume collapse of 2.7% observed at the tetragonal to monoclinic phase transition indicates it to be of first-order character. The phase transition involves a shear deformation perpendicular to the b-axis and a displacement of the Mo atoms along the b-axis. The details of the mechanism of the tetragonal-monoclinic transition are discussed. The bulk modulus of the low-pressure tetragonal and high-pressure monoclinic phases have been estimated to be 67(7) GPa and 64(3) GPa, respectively. The linear compressibilities of both phases show an anisotropic response to pressure. The obtained results are compared with the high-pressure behavior of its isomorph counterpart Bi14CrO24 and with high-temperature behavior on Bi14MoO24.

Equation of state of ammonia dihydrate up to 112 GPa by static and dynamic compression experiments in diamond anvil cells

Understanding the high-pressure behavior of ammonia hydrates is relevant for modeling the interior of solar and extrasolar icy bodies. We present here the results of high-pressure x-ray diffraction studies on ammonia-dihydrate (ADH) at room temperature (298 K) up to 112 GPa, employing both static and dynamic compression experiments performed in diamond anvil cells. The derived pressure-volume (P-V) compression curves are in excellent agreement regardless of the compression technique. In contrast to early theoretical predictions, our results indicate the stability of the disordered molecular alloy (DMA) phase, a body centered cubic (bcc) structure, and the absence of self-ionization in the investigated pressure range. By combining the P-V data from seven different compression runs, we derive the first equation of state for ADH-DMA based on the third-order Birch-Murnaghan formalism with best-fit parameters: ${V}_{0}=23.96\ifmmode\pm\else\textpm\fi{}0.03$ (${\AA{}}^{3}$/molecule), ${B}_{0}=9.95\ifmmode\pm\else\textpm\fi{}0.14$ GPa, and ${B}_{0}^{\ensuremath{'}}=6.59\ifmmode\pm\else\textpm\fi{}0.03$. The instantaneous bulk modulus directly derived from the quasicontinuous compression curves displays a smooth increase upon compression that further supports the absence of structural transitions.

Magnetic ordering in terbium at high pressures and low temperatures

We have investigated magnetic ordering in the heavy lanthanide terbium (Tb) for pressures up to 70 GPa and temperatures down to 20 K by synchrotron x-ray diffraction at the Advanced Photon Source, Argonne National Laboratory. The x-ray diffraction studies were complemented by neutron diffraction studies at the Spallation Neutron Source, Oak Ridge National Laboratory. We focused on the higher-pressure phases of Tb, such as the double hexagonal close-packed (dhcp, above 16 GPa), distorted face-centered cubic (hR24, above 30 GPa), and orthorhombic (oF16, above 51 GPa) phases. We observed the phenomenon of spontaneous magnetostriction in the dhcp and hR24 phases, where magnetic ordering at low temperatures gives rise to subtle splitting in x-ray diffraction peaks. The neutron diffraction study of the dhcp phase at 20 GPa revealed magnetic peaks that can be assigned a propagation vector k=(12,0,12), and a magnetic ordering temperature that is consistent with the magnetostriction effect observed with x-ray diffraction. The high-pressure and low-temperature x-ray diffraction study at 70 GPa in the collapsed oF16 phase did not show any evidence of magnetostriction, indicating a significant decrease in the magnetic moment of the 4f-shell due to band broadening at ultra-high compression.

Switching On/Off of a Solvent Coordination in a Au(I)-Pb(II) Complex: High Pressure and Temperature as External Stimuli.

The benzonitrile solvate {[{Au(C6F5)2}2{Pb(terpy)}]·NCPh}n (1) (terpy = 2,2':6',2″-terpyridine) displays reversible reorientation and coordination of the benzonitrile molecule to lead upon external stimuli. High-pressure X-ray diffraction studies between 0 and 2.1 GPa reveal a 100% of conversion without loss of symmetry, which is totally reversible upon decompression. By variable-temperature X-ray diffraction studies between 100 and 285 K, a partial coordination is achieved.

High-Pressure X-ray Diffraction and DFT Studies on Spinel FeV2O4

We have studied the behaviour of the cubic spinel structure of FeV2O4 under high-pressure by means of powder X-ray diffraction measurements and density-functional theory calculations. The sample was characterized at ambient conditions by energy-dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray diffraction experiments. One of the main findings of this work is that spinel FeV2O4 exhibits pressure-induced chemical decomposition into V2O3 and FeO around 12 GPa. Upon pressure release, the pressure-induced chemical decomposition appears to be partially reversible. Additionally, in combination with density-functional theory calculations, we have calculated the pressure dependence of the unit-cell volumes of both the spinel and orthorhombic FeV2O4 crystal structures, whose bulk moduli are B0 = 123(9) and 154(2) GPa, respectively, finding the spinel FeV2O4 to exhibit the lowest bulk modulus amongst the spinel oxides. From experimental results, the same information is herein obtained for the cubic structure only. The Raman modes and elastic constants of spinel FeV2O4 have also obtained the ambient conditions.

Strain-induced structural and electronic phase transitions in ZrSe2: high pressure X-ray diffraction and Raman studies

Strain-induced structural and electronic phase transitions in ZrSe2: high pressure X-ray diffraction and Raman studies

Correlating chemical bonds with glass formation in coordination polymers: a combined X-ray electron density and high-pressure single-crystal X-ray diffraction study

Correlating chemical bonds with glass formation in coordination polymers: a combined X-ray electron density and high-pressure single-crystal X-ray diffraction study

Pressure-Driven Phase Transition in Two-Dimensional Perovskite MHy2PbBr4

The application of high pressure allows tuning physicochemical properties of materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies of a two-dimensional (2D) perovskite, MHy2PbBr4, comprising a very small spacer cation (methylhydrazinium, MHy+). This crystal exhibits highly desired ferroelectric and extraordinary multiple linear and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction shows that MHy2PbBr4 undergoes an unusual Pmn21 → P21 phase transition near 4 GPa, associated with the extrusion of some MHy+ cations from the interlayer space into voids located within the inorganic sheets, not reported for any 2D hybrid perovskite. The transport of counter cations leads to a significant increase of Pb–NH2 interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa–1 within the layers. The Raman data confirm the association of the phase transition with strong distortion of the crystal structure and reorganization of the hydrogen bond network, while the absorption spectra of the compressed ambient-pressure Pmn21 phase show the band gap narrowing, followed by its widening in the high-pressure P21 phase. A similar change in the pressure dependence from a red shift to a blue shift is also observed for the free-exciton (FE) photoluminescence (PL). Furthermore, the pressure-induced phase transition leads to a giant enhancement of PL intensity, especially pronounced for the broad-band emission attributed to the self-trapped excitons (STEx). We attribute the effects, observed in absorption and PL spectra, to the shortening of Pb–Br bonds in the ambient pressure phase and increased distortion of the inorganic layers and tilts of PbBr6 octahedra in the high-pressure phase. Overall, our results for a 2D hybrid compound comprising very small spacer cations extend the understanding of the pressure effect on the properties of 2D hybrid perovskites in general and demonstrate a very different behavior under compression compared to the analogues with large organic cations. They revealed that the structure–strain mechanism can be used for engineering new high-pressure phases with unusual structural, mechanical, and optoelectronic properties.

A new transition metal diphosphide α-MoP2 synthesized by a high-temperature and high-pressure technique

Monoclinic α-MoP2, with the OsGe2-type structure (space group C2/m, Z = 4) and lattice parameters a = 8.7248(11) Å, b = 3.2322(4) Å, c = 7.4724(9)Å, and β = 119.263°, was synthesized under a pressure of 4~GPa at a temperature between 1100 °C and 1200 °C. The structure of α-MoP2 and its relationship to other transition metal diphosphides are discussed. Surprisingly, the ambient pressure phase orthorhombic β-MoP2 (space group Cmc21) is denser in structure than α-MoP2. Room-temperature high-pressure x-ray diffraction studies exclude the possibility of phase transition from β-MoP2 to α-MoP2, suggesting that α-MoP2 is a stable phase at ambient conditions; this is also supported by the total energy and phonon calculations.

Raman and first-principles study of the pressure-induced Mott-insulator to metal transition in bulk FePS3

Members of a recently discovered class of two-dimensional materials based on transition metal phosphorous trichalcogenides exhibit an antiferromagnetic ground state and they have potential applications in spintronics. In particular, FePS3 is a Mott insulator with a band gap of ∼ 1.5 eV. In this study, we used Raman spectroscopy and first-principles density functional theoretical analysis to examine the stability of the structure and electronic properties of FePS3 under pressure. Raman spectroscopy detected two phase transitions at 4.6 GPa and 12 GPa, which were characterized by changes in the pressure coefficients of the mode frequencies and the number of symmetry allowed modes. FePS3 transformed from the ambient monoclinic C2/m phase with a band gap of 1.54 eV to another monoclinic C2/m (band gap of 0.1 eV) phase at 4.6 GPa, which was followed by another transition to the metallic trigonal P-31m phase at 12 GPa. Our findings complement those obtained recently in high pressure X-ray diffraction studies. The calculated elastic properties indicated increases in the bulk, shear, and Young's moduli, as well as a significant reduction in the universal elastic anisotropy index as the crystal changed from the ambient monoclinic C2/m phase to the high pressure trigonal P-31m phase.

High-pressure single-crystal synchrotron X-ray diffraction study of lillianite

Abstract In this paper, high-pressure data from a synchrotron X-ray diffraction study on a lillianite (Pb3Bi2S6) single crystal up to ~21 GPa are presented. A phase transition from lillianite (space group Bbmm, LP lillianite) to the high-pressure form β-Pb3Bi2S6 (space group Pbnm, HP lillianite) was confirmed and bracketed between 4.90 and 4.92 GPa. The transition is reversible but of first-order with a hysteresis of ~2.8 GPa. It showed weak effects of pseudo-merohedral twinning that disappeared upon decompression, testifying to a full recovery of the single crystal of lillianite. This makes lillianite an interesting shape-memory material. With a bulk modulus K4.9 = 78(3) GPa and K′ = 5.1(4), β-Pb3Bi2S6 is markedly less compressible than lillianite [K0 = 44(2) GPa, K′ = 7(1)]. Compressional anisotropy increases markedly in β-Pb3Bi2S6 with compressibility along the b axis [M0b = 130(6) GPa and Mb′ = 19(3) in lillianite, M4.9b = 145(4) GPa and Mb′ = 16.0(7) in β-Pb3Bi2S6] significantly larger than that along the other two axes [M0a = 118(5) GPa, Ma′ = 21(3), M0c = 139(12) GPa, and Mc′ = 31(10) in lillianite, M4.9a = 242(12) GPa, Ma′ = 8(1), M4.9c = 242(5) GPa, and Mc′ = 29(1) in β-Pb3Bi2S6]. The behavior of lillianite at high pressure is an interesting case study in relation to non-quenchable ultrahigh-pressure phases likely occurring in the inner Earth, like post-perovskite MgSiO3, the oxide homologue N = 1 of the lillianite series. The β-Pb3Bi2S6 structure, on the other hand, is the N = 3 homologue of the meneghinite series to which the higher-pressure modification of the post-perovskite structure also belongs (homologue N = 1). This makes the two forms of Pb3Bi2S6 potential equivalents of high- and ultrahigh-pressure Mg silicates that could occur both in the deep earth and in other rocky extrasolar planetary bodies.

Strain induced electronic transition in 1T′ MoTe2: high pressure Raman, x-ray diffraction, resistivity measurements and first principles theoretical studies

A detailed high pressure study is carried out on 1T′ MoTe2 using x-ray diffraction (XRD) and Raman spectroscopy measurements up to about 30.5 GPa along with a room temperature resistivity measurement up to 14.3(4) GPa and density functional theory calculations. Though high-pressure XRD measurements show no structural transition, all the lattice parameters exhibit anomalous changes in the pressure region 8.4 to 12.7 GPa. The compressibility of the sample is found to be reduced by almost four times above 12.7 GPa with respect to that below 8.4 GPa. The anomalies in the Raman mode corresponding to the out of plane vibrations of Mo atoms sitting in the unit cell surface indicate a strong electron–phonon coupling possibly mediated by differential strain inside the unit cell. A rapid decrease in resistivity value up to about 7.0(2) GPa of pressure agrees well with the increase in the density of states (DOS) at the Fermi energy with pressure. Pressure evolution of band structure, as well as DOS at the Fermi level, shows an enhancement of the metallic character of the sample. First principle calculations show increased stress in the x and y directions compared to the z-direction with the application of pressure.

High-Pressure and High-Temperature Synthesis and In Situ High-Pressure Synchrotron X-ray Diffraction Study of HfSi2.

High-quality hafnium disilicide (HfSi2) has been successfully synthesized using a high-pressure and high-temperature (HPHT) method at 3 GPa and 1573 K in a DS6 × 10 MN cubic press. The modest synthesis temperature is aided by significant decreases in both liquidus and solidus temperatures at high pressure for the Si-rich portion of the Hf-Si binary system. The in situ high-pressure X-ray diffraction study yielded a bulk modulus of B0 = 124.4 ± 0.8 GPa with a fixed B0' = 4.0 for HfSi2, which exhibits a dramatically anisotropic compressibility, with a and c axes nearly twice as incompressible as the b axis. The bulk HfSi2 as synthesized has a Vickers hardness of 6.9 ± 0.1 GPa and high thermal stability of 1163 K in air, indicating its hard and refractory ceramic properties. The core-level XPS data of Hf 4f and Si 2p have been collected on the bulk samples of HfSi2, HfSi, and Hf, as well as Si powder to examine the Hf-Si bonding in hafnium silicides. The Hf 4f7/2 binding energies are 15.0 and 14.8 eV for bulk HfSi2 and HfSi, respectively.

Anomalous structural behavior and antiferroelectricity in BiGdO3: detailed temperature and high-pressure study

A comprehensive temperature and high-pressure investigation on BiGdO3 is carried out by means of dielectric constant, piezoelectric current, polarization-electric field loop, Raman scattering and x-ray diffraction measurements. Temperature dependent dielectric constant and dielectric loss show two anomalies at about 290 K (T r) and 720 K (T C). The latter anomaly is most likely due to antiferroelectric to paraelectric transition as hinted by piezoelectric current and polarization-electric field loop measurements at room temperature, while the former anomaly suggests reorientation of polarization. A small deviation from linear behaviour of both the Raman modes due to structural modification in the vicinity of T C; and sharp decrease in integrated intensities of these two modes above T C provide further proof for the above antiferroelectric to paraelectric transition. Cubic to monoclinic structural transition is observed at about 10 GPa in high-pressure x-ray diffraction studies accompanied by anisotropic lattice parameter changes and large unit cell volume collapse during the transition. This structural transition is corroborated by anomalous softening and large increase in full width half maximum of M2 (640 cm−1) Raman mode above 10 GPa. We speculate that enhancement of large structural distortion and large reduction in c/a ratio above 10 GPa might be associated with antiferroelectric to ferroelectric transition in the system.

Cover Feature: Stepwise Stress‐Induced Transformations of Metal‐Organic Polyhedral Cluster‐Based Assemblies: Where Conformational and Supramolecular Features Meet (Chem. Eur. J. 55/2021)

The understanding of multiple polymorphic phase transitions in molecular crystals is still in its infancy. With this in mind, single-crystal high-pressure X-ray diffraction studies were targeted at a polyhedral oxo cluster featuring phenyl rings in the outer sphere. Pressure evolution leads to a gradual relaxation of induced structural strains through stepwise conformational twisting of the phenyl rings with the reorganization of cooperative noncovalent interactions; this propagates the conformational isomerization. More information can be found in the Full Paper by A. Katrusiak, J. Lewiński et al. (DOI: 10.1002/chem.202101732).

Thermostructural and Elastic Properties of PbTe and Pb0.884Cd0.116Te: A Combined Low-Temperature and High-Pressure X-ray Diffraction Study of Cd-Substitution Effects

Rocksalt-type (Pb,Cd)Te belongs to IV–VI semiconductors exhibiting thermoelectric properties. With the aim of understanding of the influence of Cd substitution in PbTe on thermostructural and elastic properties, we studied PbTe and Pb0.884Cd0.116Te (i) at low temperatures (15 to 300 K) and (ii) at high pressures within the stability range of NaCl-type PbTe (up to 4.5 GPa). For crystal structure studies, powder and single crystal X-ray diffraction methods were used. Modeling of the data included the second-order Grüneisen approximation of the unit-cell-volume variation, V(T), the Debye expression describing the mean square atomic displacements (MSDs), <u2>(T), and Birch–Murnaghan equation of state (BMEOS). The fitting of the temperature-dependent diffraction data provided model variations of lattice parameter, the thermal expansion coefficient, and MSDs with temperature. A comparison of the MSD runs simulated for the PbTe and mixed (Pb,Cd)Te crystal leads to the confirmation of recent findings that the cation displacements are little affected by Cd substitution at the Pb site; whereas the Te displacements are markedly higher for the mixed crystal. Moreover, information about static disorder caused by Cd substitution is obtained. The calculations provided two independent ways to determine the values of the overall Debye temperature, θD. The resulting values differ only marginally, by no more than 1 K for PbTe and 7 K for Pb0.884Cd0.116Te crystals. The θD values for the cationic and anionic sublattices were determined. The Grüneisen parameter is found to be nearly independent of temperature. The variations of unit-cell size with rising pressure (the NaCl structure of Pb0.884Cd0.116Te sample was conserved), modeled with the BMEOS, provided the dependencies of the bulk modulus, K, on pressure for both crystals. The K0 value is 45.6(2.5) GPa for PbTe, whereas that for Pb0.884Cd0.116Te is significantly reduced, 33.5(2.8) GPa, showing that the lattice with fractional Cd substitution is less stiff than that of pure PbTe. The obtained experimental values of θD and K0 for Pb0.884Cd0.116Te are in line with the trends described in recently reported theoretical study for (Pb,Cd)Te mixed crystals.

Pressure-Driven Symmetry-Preserving Phase Transitions in Co(IO3)2

High-pressure synchrotron X-ray diffraction studies of cobalt iodate, Co(IO3)2, reveal a counterintuitive pressure-induced expansion along certain crystallographic directions. High-pressure Raman and infrared spectroscopy, combined with density-functional theory calculations, reveal that with increasing pressure, it becomes energetically favorable for certain I–O bonds to increase in length over the full range of pressure studied up to 28 GPa. This phenomenon is driven by the high-pressure behavior of iodate ion lone electron pairs. Two pressure-induced isosymmetric monoclinic–monoclinic phase transitions are observed at around 3.0 and 9.0 GPa, which are characterized by increasing oxygen coordination of the iodine atoms and the probable formation of pressure-induced metavalent bonds. Pressure–volume equations of state are presented, as well as a detailed discussion of the pressure dependences of the observed Raman- and infrared-active modes, which clarifies previous inconsistencies in the literature.