Pressure-induced Structural Phase Transition Research Articles

Pressure-Induced Structural Phase Transition and Superconductivity in NaSn5

The structural and electronic properties of the tin-rich compound NaSn5 were investigated under pressures of up to 10 GPa on the basis of the evolutionary algorithm (EA) technique coupled with first-principles total energy calculations. Upon compression, the known metallic tetragonal P4̅21m phase transforms into a metallic hexagonal P6/mmm phase at 1.85 GPa accompanied by an unusual change in the existing form of Sn atoms. The P6/mmm phase can be interpreted as a quasi-layered sandwich structure with two Sn layers and one sodium layer. The presence of softening phonon modes and the existence of Fermi pockets together with the obvious Fermi surface nesting indicate a strong electron-phonon coupling (EPC) and thus potential superconductivity in the P6/mmm phase. The strong EPC in the P6/mmm phase is mainly attributed to the phonons from Sn1 atoms together with electrons from the Sn1 py and Sn1 pz states. The calculated superconducting critical temperature Tc of the P6/mmm phase is 5.91 K at 1.85 GPa. This study provides a new clue for designing intercalated compounds with superconductivity.

Pressure-induced structural phase transition and vacancy filling in titanium monoxide TiO up to 50 GPa

Vacancy engineering can effectively modulate the optical and electronic properties of metal oxides. Here, we demonstrate that high-pressure could be a clean strategy to tune the vacancies in oxides with a high cationic vacancy content. By combining in situ synchrotron x-ray diffraction, Raman scattering, and charge transport measurements in a diamond anvil cell, we systematically study the structure and electrical properties of TiO with ∼16% ordered vacancies up to 50.2 GPa at room temperature. The monoclinic TiO transforms to the cubic phase at ∼37.8 GPa. After decompression to ambient conditions, the cubic phase survives. The vacancies are partially filled and become disordered with a concentration of approximately 12.5%. The charge transport of TiO at high pressure exhibits a metal-insulator transition, which originates from the ordered to disordered transition of vacancies under pressure. Molecular dynamics simulations suggest that the vacancies enhance the mobility of atoms in the lattice under pressure and lead to the pressure-induced amorphization and recrystallization.

Structural stability and ionic transport property of NaMPO4 (M = V, Cr, Mn, Fe, Co, Ni) as cathode material for Na-ion batteries

Sodium transition-metal phosphate NaMPO4 is known to mainly present two polymorphs: the thermodynamically metastable and electrochemically active olivine and the thermodynamically stable and electrochemically inactive maricite. The direct synthesis of the olivine and the activation of the maricite thus become the focuses of preparation of high-performance phosphate cathode materials for Na-ion batteries. To obtain better understanding of these two issues, the structural stabilities and the ionic transport properties of NaMPO4 polymorphs are investigated using atomistic simulation method. Free energy calculations predict pressure-induced structural phase transitions from maricite to olivine at low or negative pressures, and the critical pressures shift towards the negative pressure region as the applied temperature increases. It suggests that the introduction of negative pressure or lattice expansion will increase the structural stability and accordingly facilitate the direct synthesis of olivine NaMPO4. Migration energy calculations reveal a less direction-dependent ion migration for maricite lattice, which may contribute to the activation of nano-sized maricite NaMPO4. Interestingly, the calculated energy barriers decrease with the increase of the ionic radius of M2+ ion. Chemical substitution with larger divalent cations to increase the average ionic size of M2+ ions is thus expected to further improve the rate capability of maricite Na-phosphate.

Magnetocaloric Mn5Si3 and MnFe4Si3 at variable pressure and temperature

The influence of hydrostatic high pressure on the crystal structures and magnetic properties of magnetocaloric Mn5Si3 and MnFe4Si3 was studied with temperature dependent synchrotron powder diffraction, neutron single-crystal diffraction and magnetization measurements. Mn5Si3 shows no indication for any pressure-induced structural phase transition up to 24.2 GPa at room temperature. MnFe4Si3 exhibits no clear indication for any phase transition at high temperatures (296 K–373 K) and high pressures. Anomalies in the lattice parameter at low temperatures indicate a structural response to magnetic ordering. The gradient of decreasing magnetic transition temperature with increasing pressure is dTC/dP ≈ −15 K GPa−1. The transition temperature in MnFe4Si3 can be tuned by pressure in the temperature range relevant for applications, while pressure has hardly any detrimental influence on other key features relevant to magnetocaloric applications (the width of hysteresis, saturation magnetization, magnetocrystalline anisotropy).

Structural, elastic, magnetic and electronic properties of TiX2(X = Cr, Mn) alloys

The structural, elastic, magnetic and electronic properties of titanium-based alloys TiX2(X = Cr, Mn) are investigated by the first-principles calculations based on density functional theory using the Vienna ab-initio simulation code. The lattice constants of TiX2(X = Cr, Mn) alloys are optimized for various possible structures such as hexagonal, tetragonal and orthorhombic. TiX2(X = Cr, Mn) alloys are highly stable in hexagonal structure with the space group P63/mmc at ambient pressure. A pressure-induced structural phase transition from hexagonal structure to the tetragonal structure is observed in TiCr2at 443.3 GPa and in TiMn2hexagonal structure to orthorhombic structure is at 295.05 GPa. The electronic structure shows that TiX2(X = Cr, Mn) alloys are metallic in nature at all pressures. The magnetic property of nonmagnetic TiX2(X = Cr, Mn) alloys are analyzed by doping with ferromagnetic materials (Fe, Co and Ni) using the stoichiometries of TiX[Formula: see text]Y[Formula: see text] (X = Cr, Mn; Y = Fe, Co and Ni; z = 0.5,1,1.5). It is seen that the magnetic moment is induced by the substitution of ferromagnetic materials with TiX2alloys.

Pressure-induced structural phase transition and electronic structure ofIn1

We present the density functional calculation to study the pressure-induced structural phase transition and electronic structure of In$_{1-x}$Ga$_{x}$P ($x =$ 0.0, 0.25, 0.5, 0.75, 1) alloys. We report the optimized structural parameters such as lattice constant, bulk modulus, and derivative of the bulk modulus of the alloys. Our calculation confirms that the alloys in the zinc-blende structure are more stable than in the rock-salt structure. The phase transition study from zinc-blende to rock-salt under pressure shows that the transition pressure increases with an increase in doping concentration. The energy band diagram of the alloys in the zinc-blende phase reveals a direct band gap. However, the alloy at $x =$ 1 (GaP) is an indirect band gap compound in agreement with the reported experimental and theoretical results. The alloys that are in the rock-salt structure exhibit a metallic nature.

Investigation of the crystal structure of polymorphic KNbO3 nanowires by pressure

We have investigated the pressure-induced structural phase transition of orthorhombic and monoclinic KNbO3 nanowires by using x-ray diffraction (XRD) and Raman spectroscopy. Monoclinic KNbO3 nanowires underwent phase transitions three times: monoclinic to tetragonal, tetragonal to cubic, and cubic to orthorhombic-2 phase near 2.2 GPa, 8.2 GPa, and 40.0 GPa, respectively. Meanwhile, orthorhombic KNbO3 nanowires showed a phase transition once, orthorhombic to cubic phase near 8.5 GPa. Their exact structures were determined by the Rietveld method and confirmed by Raman spectroscopy. P-V equations of state for two kinds of KNbO3 nanowires were also derived from XRD data. Moreover, the elastic properties of the two nanowires in various structural phases were calculated. It was observed that two polymorphic nanowires, one from the monoclinic phase and the other from the orthorhombic phase, might have different isothermal bulk moduli even in the same crystal phase under high pressure.

Pressure-induced structural phase transition in multiferroic KBiFe2O5

Multiferroics materials have been intensively investigated for applications into photovoltaic cells. KBiFe2O5 (KBFO) emerged as a strong candidate for such applications since it exhibits low bandgap energy. It is usually orthorhombic at room temperature and pressure, however, phase diagrams investigations determined that it transforms into a monoclinic structure at high temperatures and pressures. This monoclinic structure has more interesting multiferroic and electronic properties for photovoltaic applications. In this paper, we obtained such monoclinic phase at room temperature and pressure and investigated its stability under pressure changes. We showed KBFO undergoes a new reversible first-order structural phase transition into an orthorhombic structure at high pressures as determined by synchrotron X-ray diffraction and Raman spectroscopy measurements.

Dynamic magnetic response across the pressure-induced structural phase transition in CeNi

Dynamic magnetic response across the pressure-induced structural phase transition in CeNi

Electronic structure and x-ray magnetic circular dichroism in the hyperhoneycomb iridate β−Li2IrO3

We studied the electronic, magnetic, and optical properties of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ insulator within the density-functional theory using the generalized gradient approximation taking into account strong Coulomb correlations (GGA+$U$) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The x-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Ir ${L}_{2,3}$ edges were investigated theoretically from first principles. The calculated results are in good agreement with experimental data. We found that the GGA+$U$ approach with Hubbard ${U}_{\text{eff}}=1.5\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ well describes the optical spectra and XMCD spectra at the Ir ${L}_{2,3}$ edges. We investigated the electronic structure of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ under pressure from first principles. The hyperhoneycomb iridate $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ is on the border of the magnetic ordering with relatively weak Coulomb electron-electron correlations. The $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ undergoes a pressure-induced structural and magnetic phase transitions at ${P}_{c}=4.4$ GPa with symmetry lowering to the monoclinic $C2/c$. The structural phase transition is accompanied by a dimerization of the previously equally long $x/y$ Ir-Ir bonds. We find remarkable nonmagnetic ground states of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ at ${P}_{c}$, with a concomitant electronic phase transition from a Mott insulator to band insulators. We found that ${J}_{\text{eff}}=1/2$ states are still dominant in the low-energy region in high-pressure monoclinic $C2/c$ structure.

The ideal commensurate value of Sc and the superconducting phase under high pressure

The host–guest structure of scandium is described as being built of two penetrating substructures with the incommensurate periods of the channels along the c axis. We present the ideal commensurate value of 4/3 in Sc-II using ab initio calculations. We reveal that the 3cH and 4cG structures do interpenetrate and combine to the commensurate value of 4/3 of Sc-II at a pressure of 70 GPa. Ab initio molecular dynamics confirms the stability of the commensurate value 4/3 of the host–guest structure at 300 K and 72 GPa. The pressure-induced structural phase transitions in scandium under high pressure up to 200 GPa are investigated. We use ab initio random structure searching to predict the crystal structure of Sc-III: it is the tetragonal structure with space group P41212. Our calculations show that superconductivity arises in the P41212 structure. This high pressure structure is not only a superconducting phase but also has been reported for the first time in this group of elements.

First principles studies of the structural stability and lattice dynamics for rare earth hydridies REHx (RE = Sm, Gd, Tb, Dy, Ho, Tm, x = 2, 3) under pressure

In the paper, the structures, pressure-induced structural transitions (from cubic to hexagonal), lattice dynamics and the possible transition mechanisms for rare earth dihydrides REH2 and trihydrides REH3 (RE = Sm, Gd, Tb, Dy, Ho, and Tm) are investigated comparatively and systematically using first-principles calculations. The transition pressures from the cubic CaF2-type structure to the hexagonal REB2-type structure for REH2 (RE = Sm, Gd, Tb, Dy, Ho, Tm) and the transition pressures from the cubic BiF3-type structure to the hexagonal LaF3-type structure for REH3 (RE = Sm, Gd, Tb, Dy, Tm) are predicted reasonably. It is concluded that the two kinds of structural phase transitions are not sensitive to RE 4f electrons. In the two kinds of structural phase transitions under pressure, we note that the ‘‘2.1 Å rule’’ (the purposed minimum HH distance for the stable metal hydirde at 0 K and 0 GPa) doesn't work. While lattice dynamics show that the dynamical stable pressure ranges for the cubic REH2 (RE = Sm Gd, Tb, Dy, Ho, Tm) are about in 0–52, 0–60, 0–70,0-87, 0–94 and 0–108 GPa, respectively. Ones of the cubic REH3 (RE = Sm, Gd, Tb, Dy, Tm) are around 17–147, 31–182, 13–200, 16–224 and 14–280 GPa, respectively. In contrast to the corresponding structural transition pressure as well as the lattice dynamics of the hexagonal structure under high pressure, the phonon modes softening behaviors under pressure might be responsible for the pressure-induced structural phase transition in the cubic REH2, but not in the cubic REH3.

Coexistence of pressure-induced structural phases in bulk black phosphorus: a combined x-ray diffraction and Raman study up to 18 GPa

We report a study of the structural phase transitions induced by pressure in bulk black phosphorus by using both synchrotron x-ray diffraction for pressures up to 12.2 GPa and Raman spectroscopy up to 18.2 GPa. Very recently black phosphorus attracted large attention because of the unique properties of few-layers samples (phosphorene), but some basic questions are still open in the case of the bulk system. As concerning the presence of a Raman spectrum above 10 GPa, which should not be observed in an elemental simple cubic system, we propose a new explanation by attributing a key role to the non-hydrostatic conditions occurring in Raman experiments. Finally, a combined analysis of Raman and XRD data allowed us to obtain quantitative information on presence and extent of coexistences between different structural phases from ~5 up to ~15 GPa. This information can have an important role in theoretical studies on pressure-induced structural and electronic phase transitions in black phosphorus.

Effect of dispersion corrections on ab initio predictions of graphite and diamond properties under pressure

There are several approaches to the description of van der Waals (vdW) forces within density functional theory. While they are generally found to improve the structural and energetic properties of those materials dominated by weak dispersion forces, it is not known how they behave when the material is subject to an external pressure. This could be an issue when considering the pressure-induced structural phase transitions, which are currently attracting great attention following the discovery of an ultrahard phase formed by the compression of graphite at room temperature. In order to model this transition, the functional must be capable of simultaneously describing both strong covalent bonds and weak dispersion interactions as an isotropic pressure is applied. Here, we report on the ability of several dispersion-correction functionals to describe the energetic, structural, and elastic properties of graphite and diamond, when subjected to an isotropic pressure. Almost all of the tested vdW corrections provide an improved description of both graphite and diamond compared to the local density approximation. The relative error does not change significantly as pressure is applied, and in some cases even decreases. We therefore conclude that the use of dispersion-corrected exchange-correlation functionals, which have been neglected to date, will improve the accuracy and reliability of theoretical investigations into the pressure-induced phase transition of graphite.

Enclathration of Ethane, Propane, and Propylene into Urea Clathrates and Roles of Methanol on Urea Clathrate Formation.

As a guest molecule of urea clathrate, a long-chain normal alkane and its derivative with low substituents in methanol solutions have been reported. To investigate the role of methanol in the urea clathrate formation, in the present study, we used propane (C3H8), propylene (C3H6), ethane (C2H6), and methane (CH4) as guest molecules. Raman spectra and powder X-ray diffraction profiles revealed that, regardless of the existence of methanol, the C3H8, C3H6, and C2H6 molecules are enclathrated into urea clathrates with a hexagonal structure, whereas there is no urea clathrate formation enclathrating CH4. The pressurization of the urea clathrates including C2H6 and C3H8 reveals that no pressure-induced structural phase transition occurs at pressures up to 200 MPa. In spite of the guest molecule much shorter than the lattice constant of the c-axis of the hexagonal channel structure, the urea clathrates have a fairly rigid structure against the compression. Methanol as an auxiliary solution is not always necessary for the urea clathrate formation. Methanol plays a role in decreasing the activation energy of the urea clathrate formation, although it makes urea clathrate thermodynamically unstable due to the high solubility of urea in methanol.

Pressure-induced isostructural phase transition and charge transfer in superconducting FeSe

We present extensive investigations of the crystallographic phase diagram and electronic properties of the Fe-based superconductor FeSe under extreme conditions (high pressure (HP) and low temperature (LT)) by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). An isostructural phase transition (Tetragonal (T) →high-pressure Tetragonal (Tʹ)) is discovered in FeSe at ∼2.8 GPa based on the axial ratio c/a with finer pressure step as observed in Fe-As-based superconductor such as EuFe2As2. We also find a pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe, which is consistent with the documented pressure-induced high-spin → low-spin transition (∼6–7 GPa). These results reveal the pressure-induced structural phase transition sequence in FeSe at room temperature to be T → Tʹ → Tʹ+MnP-type at pressures of 0–10.6 GPa, enriching the crystallographic phase diagram. The HPLT XRD data also indicate that a sluggish structural phase transition (Cmma → Pnma) begins at 7.5 GPa, and these two phases coexist up to 26.5 GPa. The HP X-ray absorption near-edge spectroscopy (XANES) measurement shows that Eo of Se experiences a pressure-induced shift to high energy, evidencing strongly charge transfer between Fe and Se under high pressure. Our results shed lights on the correlation between crystallographic/electronic structure and superconductivity in this material.

Pressure-induced structural phase transition and electrical properties of Cu2S

The structural phase transition and electrical properties of Cu2S have been investigated by X-ray diffraction and electrical resistance measurements at pressures up to 41.3 GPa in diamond anvil cell. A new high pressure phase is found above 22.3 GPa, which probably has a tetragonal structure. It is observed that the electrical resistance exhibits an insulating behavior at high pressures and low temperature. After the pressure is released, the high pressure phase transforms back to original monoclinic phase. This study sheds light on the electrical transport properties of Cu2S, as well as to improve our understanding of its high pressure phase transition behavior.

A Raman study of the pressure-induced densification of SiO2-based glass-ceramics

Here we report a structural characterization study of glass-ceramics as a function of pressure by Raman spectroscopy. We selected a glass-ceramics of Li2O–Al2O3–nSiO2 (n ≈ 8) doped with ZrO2 and TiO2. This composition induces slight structural modification in Si–O and Si–O–Si bonds in the glass matrix that are crucial to stabilize metastable hexagonal SiO2 (β-quartz) solid-solution nano crystals corresponding to γ-LiAlSi2O6. This structure yields a more compact β-quartz-type phase that eventually stabilizes it in wider pressure range than pure SiO2. Raman spectroscopy allows to unravel at least two pressure-induced structural phase transitions at about 6 and 15 GPa that could not be previously revealed by x-ray diffraction. We show that the phase-transition sequence can tentatively be described in terms of SiO2-type structure as: β-quartz → Coesite I → Coesite II. The measured transition pressures are consistent with the larger cell volume attained in the γ-LiAlSi2O6 yielding a wider metastability pressure range of the β-quartz-type phase.

Pressure-induced structural phase transition and dehydration for gypsum investigated by Raman spectroscopy and electrical conductivity

The structural phase transition and dehydration for gypsum were investigated by Raman spectroscopy and electrical conductivity up to ∼12.2 GPa and ∼723 K. A ∼5.8 GPa of phase transition was verified at atmospheric temperature. The variations in the sulfate and hydroxyl Raman shifts and a change in electrical conductivity marked the onset of dehydration. At ∼2.5 GPa, gypsum transformed into bassanite at ∼523 K, and then completely dehydrated to γ-anhydrite at ∼673 K. And furthermore, the gypsum-bassanite and bassanite-anhydrite dehydration boundaries were obtained: P (GPa) = −23.708 + 0.050 T (K) and P (GPa) = −4.907 + 0.011 T (K), respectively.

Facet-dependent electrical and mechanical properties of polyhedral Cu2O under compression

In this paper, both the facet-dependent electrical properties and mechanical stability of submicron-sized polyhedral Cu2O are investigated with in-situ resistivity measurements with pressure up to 25.0 GPa. The pressure-induced morphology changes are characterized by scanning/transmission electron microscopy. X-ray photoelectron spectroscopy measurements are also adopted to rationalize the diversity of the electrical resistivity of polyhedral Cu2O under compression. The electrical properties of cubic, truncated octahedral, and octahedral Cu2O show extremely different pressure dependence, which can be attributed to the preferential adsorption of oxygen on Cu2O (111) facet. The anomalous changes in electrical resistivity at 0.7–2.2, 8.5, 10.3, and 21.6 GPa are due to pressure-induced structural phase transitions of the Cu2O crystals. The dramatic reductions of resistivity in cubes and octahedras at 15.0 GPa and of truncated octahedras at 21.2 GPa are driven by pressure-induced crush and nanocrystallization of Cu2O crystals. The mechanical stability of truncated octahedral Cu2O is better than those of both cubic and octahedral Cu2O. Our results not only elucidate the underlying mechanisms for understanding the two previously reported pressure-dependent electrical resistance change trends difference, but also give a reasonably explanation of the facet-dependent mechanical property of Cu2O.