Pressure-induced Phase Research Articles

Observation of non-superconducting phase changes in nitrogen doped lutetium hydrides

The recent report of near-ambient superconductivity and associated color changes in pressurized nitrogen doped lutetium hydride has triggered worldwide interest and raised major questions about the nature and underlying physics of these latest claims. Here we report synthesis and characterization of high-purity nitrogen doped lutetium hydride LuH2±xNy. We find that pressure conditions have notable effects on Lu-N and Lu-NH chemical bonding and the color changes likely stem from pressure-induced electron redistribution of nitrogen/vacancies and interaction with the LuH2 framework. No superconducting transition is found in all the phases at temperatures 1.8-300 K and pressures 0-38 GPa. Instead, we identify a notable temperature-induced resistance anomaly of electronic origin in LuH2±xNy, which is most pronounced in the pink phase and may have been erroneously interpreted as a sign of superconducting transition. This work establishes key benchmarks for nitrogen doped lutetium hydrides, allowing an in-depth understanding of its novel pressure-induced phase changes.

Drastic enhancement of magnetic critical temperature and amorphization in topological magnet EuSn2P2 under pressure

High pressure is an effective tool to induce exotic quantum phenomena in magnetic topological insulators by controlling the interplay of magnetic order and topological state. This work presents a comprehensive high-pressure study of the crystal structure and magnetic ground state up to 62 GPa in an intrinsic topological magnet EuSn2P2. With a combination of high resolution X-ray diffraction, 151Eu synchrotron Mössbauer spectroscopy, X-ray absorption spectroscopy, molecular orbital calculations, and electronic band structure calculations, it has been revealed that pressure drives EuSn2P2 from a rhombohedral crystal to an amorphous phase at 36 GPa accompanied by a fourfold enhancement of magnetic ordering temperature. In the pressure-induced amorphous phase, Eu ions take an intermediate valence state. The drastic enhancement of magnetic ordering temperature from 30 K at ambient pressure to 130 K at 41.2 GPa resulting from Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions likely attributes to the stronger Eu–Sn interaction at high pressure. These rich results demonstrate that EuSn2P2 is an ideal platform to study the correlation of the enhanced RKKY interactions, disordered lattice, intermediate valence, and topological state.

Pressure-induced concomitant topological and metal-insulator quantum phase transitions in Ce3Pd3Bi4

The electronic property and magnetic susceptibility of Ce3Pd3Bi4 were systemically investigated from 18 to 290 K for varying values of cell volume using dynamic mean-field theory coupled with density functional theory. By extrapolating to zero temperature, the ground state of Ce3Pd3Bi4 at ambient pressure is found to be a correlated semimetal due to insufficient hybridization. Upon applying pressure, the hybridization strength increases and a crossover to the Kondo insulator is observed at finite temperatures. The characteristic temperature signaling the formation of Kondo singlet, as well as the characteristic temperature associated with f-electron delocalization–localization change, simultaneously vanishes around a critical volume of 0.992 V0, suggesting that such metal–insulator transition is possibly associated with a quantum critical point. Finally, Wilson’s loop calculations indicate that the Kondo insulating side is topologically trivial, thus a topological transition also occurs across the quantum critical point.

Pressure-induced structural transition and antiferromagnetism in elemental terbium

Structural and magnetic properties of rare-earth Tb metal have been studied by means of neutron powder diffraction at pressures up to 9 GPa in the temperature range 7--290 K. A structural phase transition from the initial hexagonal close-packed (hcp) to the Sm-type rhombohedral phase develops gradually at high pressures above 4 GPa. The initial ferromagnetic state in the hcp phase is suppressed and an antiferromagnetic state is developed in the pressure-induced phase. In the Sm-type structure and the temperature range below ${T}_{\mathrm{MO}}=110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (at 9 GPa) down to 50 K, long-range order of Tb magnetic moments located in the layers resembling hexagonal close-packing type is formed with a propagation vector ${k}_{\mathrm{AF}1}=(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}\frac{1}{2})$, while the layers resembling cubic close-packing type remain disordered. This partial disorder disappears at temperatures below 50 K when magnetic order, including the moments in the latter layers, develops with a propagation vector ${k}_{\mathrm{AF}2}=(\frac{1}{2}\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}\frac{1}{2})$. The relative stability of the hcp and Sm-type structures under pressure was examined by density functional theory calculations, providing significant support to the experimental findings. The calculated bulk moduli of the hcp and Sm-type phases are close to the experimentally determined ones and the estimate ${P}_{0}\ensuremath{\approx}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ obtained for the equilibrium transition pressure is close to the onset pressure found in real material. The volume collapse at the hcp to Sm-type transition was evaluated to amount to $0.4\phantom{\rule{0.16em}{0ex}}{\AA{}}^{3}$ per atom.

Pressure-induced reconstructive phase transitions, polarization with metallicity, and enhanced hardness in antiperovskite MgCNi3.

In general, hydrostatic pressure can suppress electrical polarization, instead of creating and/or enhancing polarization like strain engineering. Here, a combination of first-principles calculations and CALYPSO crystal structures prediction is used to point out that hydrostatic pressure applied on antiperovskite MgCNi3 can stabilize polarization with metallicity, and thus a polar metal can exist under high pressure. Strikingly, the metallic polar phase of MgCNi3 exhibits an original linear-cubic coupling between polar and nonpolar modes, resulting in an asymmetrical double-well when the polarization is switched. Moreover, another novel phase of MgCNi3 under high pressure possesses an enhanced hardness stemming from a robust s-s electrons interaction of an unexpected C-C bond, rather than typical sp3 orbital hybridization. These discoveries open new routes to design superhard materials and polar metals.

Experimental equation of state of 11 lanthanide nitrides (NdN to LuN) and pressure induced phase transitions in NdN, SmN, EuN, and GdN

Through an extensive data analysis of powder X-ray diffraction data obtained at pressures up to at least 78 GPa, we report the experimental equations of state for all lanthanide nitrides between NdN and LuN, excluding the radioactive Pm. By fitting the obtained unit cell volumes as a function of pressure with the third order Birch–Murnaghan equation of state, we find that the bulk modulus increases with an increasing lanthanide number from K0 = 146(12) GPa for NdN to 182(7) GPa in EuN. Hereafter, the bulk modulus reaches a plateau for the rest of the series except for TmN which has a lower bulk modulus. We find that the first derivative of the bulk modulus is around 4 for all compounds except TbN, which displays a significantly different compression behavior. In addition, we find a B1 to B10 pressure-induced phase transition in NdN, SmN, EuN, and GdN at increasingly higher pressures. In fact, we observe that the onset pressure of the phase transition increases linearly with Ln atomic number. From TbN and onwards, we do not observe any sign of a B1 to B10 transition indicating that the transition pressure exceeds the studied pressure range. Therefore, we believe that, for the heavier lanthanides, the linear relationship between the onset pressure and the lanthanide number does not hold and even higher pressures are needed to observe the transition. This coherent study of the series of lanthanide nitrides offers a unique opportunity for benchmark studies of computational methods applied to compounds with 4f electrons.

Ab Initio High-Pressure Study of Semiconductor-Metal Phase Transition of the Chalcogenide Compound KPSe6

We report the results of pressure-induced semiconductor-metal phase transition of the semiconducting chalcogenide compound KPSe6 under high pressure using the ab initio methods. The ground-state energy calculations were performed within density functional theory and the generalized gradient approximation using the pseudopotential method with plane-wave basis sets. The projector augmented-wave (PAW) pseudopotentials were used in our calculation. The optimized lattice parameters were found from total energy calculations as 13 Bohr, 1.6 Bohr, and 1.8 Bohr for cell dimensions one, two, and three, respectively, which are in good agreement with experimental calculations. At zero pressure, the material portrayed a semiconducting property with a direct bandgap of ≈1.7 eV. As we subjected the material to pressure, the band gap was observed to reduce until it disappeared. The phase transition from the semiconductor to metal was found to occur at ∼45 GPa, implying that the material underwent metallization as pressure was increased further.

Amorphous silicon hexaboride at high pressure

ABSTRACT We investigate the pressure-induced structural phase transformation of amorphous silicon hexaboride (a-SiB6) using a constant pressure first principles approach. a-SiB6 is found to undergo a gradual phase transformation to a high-density amorphous phase (HDA) in which the average coordination number of both B and Si atoms is about 6. The HDA phase consists of differently coordinated motifs ranging from 4 to 8. B12 icosahedra are found to persist during compression of a-SiB6 and the structural modifications primarily occur around Si atoms and in the regions linking pentagonal pyramid-like configurations to each other. Upon pressure release, an amorphous structure, similar to the uncompressed one, is recovered, indicating a reversible amorphous-to-amorphous phase change in a-SiB6. When the electronic structure is considered, the HDA phase is perceived to have a wider forbidden band gap than the uncompressed one.

Pressure-Induced Phase Changes in Cesium Lead Bromide Perovskite Nanocrystals with and without Ruddlesden–Popper Faults

Lead halide perovskites have a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. Despite several reports on high-pressure...

Revisiting Pressure-Induced Transitions in Mesoporous Anatase TiO2

Mesoporous materials may be considered as the negative image of a collection of nanoparticles. In both cases, the large surface area induces an increase of the system energy. Pressure-induced phase...

Magnetic and electronic properties of magnetite across the high pressure anomaly

The magnetite Fe3O4, being anciently known magnetic material to human kind and remaining in leading positions for development of advanced technologies presently, demonstrates a number of puzzling physical phenomena, being at focus of extensive research for more than century. Recently the pressure-induced anomalous behavior of physical properties of magnetite in vicinity of the structural phase transition, occurring at P ~ 25–30 GPa, has attracted particular attention, and its nature remains unclear. Here we study the magnetic and electronic properties of magnetite across high pressure anomaly and in the pressure-induced phase by means of 57Fe synchrotron Moessbauer spectroscopy and neutron diffraction. The hyperfine interaction parameters behavior was systematically analysed over pressure 0–40 GPa and temperature 10–290 K ranges. In the high pressure phase the ferrimagnetic order formation below TNP ~ 420 K was observed and spin arrangement symmetry was deduced. The structural, magnetic and electronic phase diagram of magnetite in the discussed pressure range is established.

Pressure dependence of excited-state charge-carrier dynamics in organolead tribromide perovskites

Excited-state charge-carrier dynamics governs the performance of organometal trihalide perovskites (OTPs) and is strongly influenced by the crystal structure. Characterizing the excited-state charge-carrier dynamics in OTPs under high pressure is imperative for providing crucial insights into structure-property relations. Here, we conduct in situ high-pressure femtosecond transient absorption spectroscopy experiments to study the excited-state carrier dynamics of CH3NH3PbBr3 (MAPbBr3) under hydrostatic pressure. The results indicate that compression is an effective approach to modulate the carrier dynamics of MAPbBr3. Across each pressure-induced phase, carrier relaxation, phonon scattering, and Auger recombination present different pressure-dependent properties under compression. Responsiveness is attributed to the pressure-induced variation in the lattice structure, which also changes the electronic band structure. Specifically, simultaneous prolongation of carrier relaxation and Auger recombination is achieved in the ambient phase, which is very valuable for excess energy harvesting. Our discussion provides clues for optimizing the photovoltaic performance of OTPs.

Quasiparticle energies and dielectric functions of diamond polytypes

We perform ab initio many-body Green's function calculations to investigate the quasiparticle energies and optical properties of diamond polytypes that have been predicted to be producible via a pressure-induced structural phase transition from carbon nanotube solids. We find, through quasiparticle band-structure calculations within the $GW$ approximation, that the band gaps of two hexagonal $(2H$- and $4H$-type) polytypes of diamond differ significantly from that of cubic diamond as well as from that of the crystalline $s{p}^{3}$ carbon phase with a body-centered-tetragonal structure, called bct ${\mathrm{C}}_{4}$. We also examine the dielectric functions of three polytypes of diamond (cubic, $2H$, and $4H)$ by employing the $GW$ plus Bethe-Salpeter equation approach. The calculated optical absorption spectra are found to be distinct from each other. The lattice mismatches of carbon layers of these diamond polytypes are very small and the total-energy differences are also small. Our work opens up the possibility of fabricating diamond superlattices with various electronic and optoelectronic properties by utilizing and controlling the different stacking sequences of carbon layers.

Prediction of the critical point of pressure-induced deformation-related phase transitions in aligned single-walled carbon nanotubes on the basis of extreme-low-frequency-shift Raman spectroscopy

Experimental evidence investigating the high-pressure response (0--9 GPa) of aligned single-walled carbon nanotube (SWNT) arrays in the extreme low Raman shift region (10--100 ${\mathrm{cm}}^{\ensuremath{-}1}$, squash mode region) is provided to verify a predictive model for deformation-related phase transitions. In addition to the well-known radial breathing mode (RBM) and despite the technical challenges associated with the detection of Raman signals very close to the exciting laser frequency, clear SWNT squash mode peaks were identified and used to refine the predictive model. Furthermore, this paper investigates and proposes explanations for the detailed behavior of the pressure dependent cross-sectional transition. The results demonstrate experimentally, and confirm earlier theoretical models, that the critical pressure scales $\ensuremath{\propto}1/\mathcal{O}({d}_{t}^{3})$ against the chirality dependent nanotube diameter ${d}_{t}$. Finally, the pressure and chirality dependent Raman upshifts of the squash mode, characterizing the phase transition, are found to be larger than those of the RBM, respectively, confirming the general theoretical prediction of greater environmental sensitivity of squash modes.

Pressure-induced disordering and phase transformations in Eu2Zr2O7 pyrochlore

ABSTRACT The structural properties of pyrochlore Eu2Zr2O7 under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu2Zr2O7 undergoes a reversible structural change around 21.2 GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5 GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the <Zr–O> bonds should play a more important role than the <Eu–O> bonds in the process of the phase transformation.

Experimental and DFT high pressure study of fluorinated graphite (C2F)n

We studied the high pressure behavior of poly(dicarbon monofluoride) (C2F)n up to 37 GPa both theoretically, within the first-principle framework of the density functional theory, taking into account van der Waals corrections to the total energy, and experimentally, combining in situ Raman spectroscopy, XRD and X-ray photoelectron spectroscopy (XPS) measurements. The (C2F)n structure has a complex mixed sp2 sp3 character where graphene layers are intercalated among sp3 carbon hybridized structural domains. We observed three phase transitions in the high pressure range explored. The first transition is observed around 12 GPa and is dominated by strain effects. Above 16 GPa the intercalated graphene layers corrugate and change from the sp2 to sp3 hybridization state. Above 25–27 GPa the new pressure-induced phase becomes unstable. We find that, by varying applied pressure or by changing the hybridization due to the fluorination of graphite, the band structure of (C2F)n can be modified, resulting in valence and conduction bands touching at the K point, as in graphene, thus turning (C2F)n into a semimetal. This could provide a new pathway to reversibly engineer the band structure and conductivity for applications in high energy density batteries.

Theoretical Analysis of the Structural Phase Transformation and Elastic Properties in the ZrN under High Pressure

We report a phenomenological model based calculation of pressure-induced structural phase transition and elastic properties of ZrN compound. Gibb’s free energy is obtained as a function of pressure by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. From the present study, we predict a structural phase transition from NaCl structure (B1) to the CsCl structure (B2). The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.

Comment on “Pressure Induced Phase Transition, Elastic and Thermal Properties of Rare Earth Tellurides”

In the recent article ‘Pressure Induced Phase Transition, Elastic and Thermal Properties of Rare Earth Tellurides’ published by Rukmangad et al. (Trans Indian Inst Met 67(2):185–192, 2014), the high pressure phase transition, elastic and thermal properties of rare earth tellurides (RETe; RE = La, Ce, Pr, Sm and Eu) have been investigated by using a suitable inter-ionic potential with modified ionic charge (Zme) to include Coulomb screening effect. After studying their article, we have found some discrepancies in predicted crystal density, longitudinal, transverse and average elastic wave velocities, and Debye temperature data. The crystal density has been found multiplied by 2. Also the longitudinal, transverse and average elastic wave velocities and Debye temperature are different from our reexamined values. Although these discrepancies have no influence on the authors’ conclusion.

Pressure-induced phase and chemical transformations of lithium peroxide (Li2O2).

We present the pressure-induced phase/chemical changes of lithium peroxide (Li2O2) to 63 GPa using diamond anvil cells, confocal micro-Raman spectroscopy, and synchrotron x-ray diffraction. The Raman data show the emergence of the major vibrational peaks associated with O2 above 30 GPa, indicating the subsequent pressure-induced reversible chemical decomposition (disassociation) in dense Li2O2. The x-ray diffraction data of Li2O2, on the other hand, show no dramatic structural change but remain well within a P63/mmc structure to 63 GPa. Nevertheless, the Rietveld refinement indicates a subtle change in the structural order parameter z of the oxygen position O (13, 23, z) at around 35 GPa, which can be considered as a second-order, isostructural phase transition. The nearest oxygen-oxygen distance collapses from 1.56 Å at ambient condition to 1.48 Å at 63 GPa, resulting in a more ionic character of this layered crystal lattice, 3Li(+)+(LiO2)3 (3-). This structural change in turn advocates that Li2O2 decomposes to 2Li and O2, further augmented by the densification in specific molar volumes.

A detailed first principle study on the structural, elastic, and electronic properties of indium arsenide (InAs) under induced pressure

An ab initio calculation of the structural, elastic, and electronic properties of indium arsenide (InAs) under induced pressure is investigated using density functional theory with modified Becke–Johnson potential within the generalised gradient approximation of the Perdew–Burke–Ernzerhof scheme. The lattice parameters are found to be in good agreement with experimental and other theoretical data. The pressure-induced structural phase transition of InAs zinc blende to rock salt structure is found to occur at 4.7 GPa pressure with a 17.2% of volume collapse. The elastic properties of both the zinc blende and rock salt structures at different pressures are studied. The electronic band structures at different pressures for both the structures are investigated using the total and partial density of states. The energy band gap of the InAs zinc blende phase is increased with increasing pressure while in rock salt the phase the conduction band crosses towards the valence band and thus shows metallic behaviour.