Pressure-induced Phase Transition Research Articles

Accelerating ultrafast processes in hydrogen-bonded complexes under pressure

Acceleration of ultrafast processes is vital in hydrogen-bonded coumarin–methanol complexes for improving the photoelectric conversion efficiency of dye-sensitized solar cells (DSSCs). The traditional methods expedite ultrafast processes individually related to electron injection in DSSCs, namely, internal conversion (IC) or intermolecular charge transfer (inter-CT), by adjusting molecular topologies. We introduce pressure as an external drive to realize the acceleration of both processes simultaneously without changing the configuration. In the definite hydrogen-bonded complexes, the acceleration of IC and inter-CT processes is visualized by in situ high-pressure femtosecond transient absorption spectroscopy. In liquid-phase methanol, the IC and inter-CT processes are actuated effectively from 150.20 to 59.21 fs and 93.95 to 29.05 ps, respectively. The quickening of both processes is attributed to the enhancement of intermolecular hydrogen bonds under pressure. After the pressure-induced methanol phase transition, the rates of IC and inter-CT processes at 3.67 GPa are increased by 36.42% and 80.55% compared to at 1.00 atm. Our study results open an enlightening avenue for boosting the photoelectric conversion efficiency of DSSCs.

Pressure-Induced Structural Phase Transitions in the Chromium Spinel LiInCr4O8 with Breathing Pyrochlore Lattice

This study reports high-pressure structural and spectroscopic studies on polycrystalline cubic chromium spinel compound LiInCr4O8. According to pressure-dependent X-ray diffraction measurements, three structural phase transitions occur at ∼14 GPa, ∼19 GPa, and ∼36 GPa. The first high-pressure phase is indexed to the low-temperature tetragonal phase of the system which coexists with the ambient phase before transforming to the second high-pressure phase at ∼19 GPa. The pressure-dependent Raman and infrared spectroscopic measurements show a blue-shift of the phonon modes and the crystal field excitations and an increase in the bandgap under compression. During pressure release, the sample reverts to its ambient cubic phase, even after undergoing multiple structural transitions at high pressures. The experimental findings are compared to the results of first principles based structural and phonon calculations.

Pressure-Induced Phase Transitions of Nonionic Polymer Brushes

While the temperature responsive behavior of nonionic polymers has been extensively studied and is nowadays one of the key mechanisms of smart materials, the pressure response of thin films remains basically unexplored. We investigate the conformational transition of nonionic brushes and semidilute solutions induced by hydrostatic pressure and temperature variations. Interestingly, the pressure–temperature phase diagram for the coil-to-globule transition of brushes, probed by neutron reflectometry, nearly coincides with that in semidilute solutions. We also show that the phase behavior can be understood and predicted with simple thermodynamic concepts employed so far for the denaturation of proteins. Fully atomistic molecular dynamics simulations provide molecular insight into the pressure-responsive behavior. Combining all three approaches allows us to demonstrate that pressure-induced hydration of nonionic polymers at low pressure is universal as it is dictated by water and is polymer-independent. In contrast, the pressure-induced dehydration at high pressure is strongly polymer-specific. The outcomes apply to a wide class of nonionic polymers and can aid the design of responsive coatings with the desired pressure-responsive behavior.

The pressure and temperature evolution of the Ca3V2O8 crystal structure using powder X-ray diffraction

The independently measured pressure and temperature evolution of Ca3V2O8 is characterized by X-ray diffraction and DFT calculations. A pressure-induced phase transition is discovered and the crystal structure for the HP phase reported.

Pressure-induced phase transition toward high symmetry in zero-strain Li2TiO3.

As a typical layered oxide material, Li2TiO3 has attracted considerable attention in the energy revolution and military industries, owing to its lithium-rich and "zero-strain" characteristics. However, its phase-transition behavior under high pressure remains unclear. Herein, we report a second-order phase transition from the monoclinic phase to a higher-symmetry phase in nano-polycrystalline Li2TiO3 at 43 GPa by in situ high-pressure Raman experiments and first-principles calculations at 300 K. As validated by the experiments and calculations, the distortion of layered oxide-TiO6 in Li2TiO3 is crucial in the phase transition. By modulating the gap between octahedral-TiO6 layers, we propose a potential Li2TiO3 structural model for improving the electrochemical performance of lithium-ion batteries. Our findings suggest that, based on its high-pressure phase, Li2TiO3 is a promising candidate for layered cathode materials and solid tritium breeding materials for lithium-ion batteries.

Creation of Fibrous Polymer Materials with Low-Temperature Flow Upon Pressure-Induced Phase Transition

Creation of Fibrous Polymer Materials with Low-Temperature Flow Upon Pressure-Induced Phase Transition

Study on pressure-induced isostructural phase transition and electrical transport properties of BiOBr.

In this work, we prepared a BiOBr powder sample by the coprecipitation method for in situ high-pressure AC impedance spectroscopy tests, in situ high-pressure Raman measurements and in situ high-pressure X-ray diffraction experiments to explore its structural properties and electrical transport processes under compression. Two pressure-driven isostructural phase transitions, T-T' and T'-T'' (T - tetragonal, T' - tetragonal 1 and T'' - tetragonal 2), were discovered at around 10.0 and 15.0 GPa, respectively. The pressure-induced changes in the crystal structure and electrical transport of BiOBr can provide a reference for explaining the mechanism of the isostructural phase transition of other similar compounds after compression.

Pressure-induced second-order phase transition in fluorine.

The relative stability between the crystal structure of α-F2, space group C2/c, and a hypothesized high-pressure phase, space group Cmce, was explored using Density Functional Theory at the PBE0+D3(ABC)/TVZP level of theory and further assessed by Quantum Monte Carlo calculations. The analysis of the phonon dispersion spectra reveals that, at ambient pressure, besides the energy difference favoring the C2/c structure, the Cmce phase also presents a dynamical instability near the Γ-point, which disappears with increasing pressure. The unstable vibrational mode can be related to the absence of σ-holes in the fluorine molecule, which renders a repulsive head-to-head interaction between molecules, as opposed to heavier halogens, in which the presence of σ-holes stabilizes the orthogonal Cmce structure. The results show that the pressure-induced phase transition C2/c → Cmce is of second-order.

Phase transition and bond symmetrization associated with noble gas bond in XeO3

The pressure-induced structural phase transition of XeO3 is studied by first principle calculations. The transition from P212121 to Pnma accompanied by a drastic reduction of volume is found at 2.18 GPa. The symmetrilized Xe−O2 bonds give rise to the better symmetry of high pressure phase. O-hopping between different possible local minima and the motion of Xe along the y axis may be responsible for phase transition. Results of electron localization function indicate that three ipsilateral Xe−O bonds lead to a lone-pair contour of Xe6+.

Pressure-modulated lattice structural evolution in TiS2.

Titanium disulfide (TiS2) has drawn considerable attention in materials, physics, and chemistry thanks to its potential applications in batteries, supercapatteries and thermoelectric devices. However, the simplified and controlled synthesis of high-quality TiS2 remains a great challenge. In this study, a straightforward widely accessible approach to the one-step chemical vapor transport (CVT) process is presented. Meanwhile, combining high-pressure (HP) Raman spectroscopy measurements and first-principles calculations, the pressure-induced phase transition of TiS2 from P3̄m1 phase (phase I) to C2/m phase (phase II) at 16.0 GPa and then to P6̄2m phase (phase III) at 32.4 GPa was disclosed. The discovery of HP being within the Weyl semi-metallic phase represents a significant advancement towards understanding the electronic topological states, discovering new physical phenomena, developing new electronic devices, and gaining insight into the properties of elementary particles.

Structural stability, dihydrogen bonding, and pressure-induced polymorphic transformations in hydrazine borane.

Hydrazine borane (N2H4BH3) has attracted considerable interest as a promising solid-state hydrogen storage material owing to its high hydrogen content and easy preparation. In this work, pressure-induced phase transitions of N2H4BH3 were investigated using a combination of vibrational spectroscopy, X-ray diffraction, and density functional theory (DFT) up to 30 GPa. Our results showed that N2H4BH3 exhibits remarkable structural stability in a very broad pressure region up to 15 GPa, and then two phase transitions were identified: the first one is from the ambient-pressure Pbcn phase to a Pbca phase near 15 GPa; the second is from the Pbca phase to a Pccn phase near 25 GPa. As revealed by DFT calculations, the unusual stability of N2H4BH3 and the late phase transformations were attributed to the pressure-mediated evolutions of dihydrogen bonding frameworks, the compressibility and the enthalpies of the high-pressure polymorphs. Our findings provide new insight into the structures and bonding properties of N2H4BH3 that are important for hydrogen storage applications.

Rediscovering the intrinsic mechanical properties of bulk nanocrystalline indium arsenide.

Is the inverse Hall-Petch relation in ceramic systems the same as that in metal systems? The premise to explore this subject is the synthesis of a dense bulk nanocrystalline material with clean grain boundaries. By using the reciprocating pressure-induced phase transition (RPPT) technique, compact bulk nanocrystalline indium arsenide (InAs) has been synthesized from a single crystal in a single step, while its grain size is controlled by thermal annealing. The influence of macroscopic stress or surface states on the mechanical characterization has been successfully excluded by combining first-principles calculations and experiments. Unexpectedly, nanoindentation tests show a potential inverse Hall-Petch relation in the bulk InAs with a critical grain size (Dcri) of 35.93 nm in the experimental scope. Further molecular dynamics investigation confirms the existence of the inverse Hall-Petch relation in the bulk nanocrystalline InAs with Dcri = 20.14 nm for the defective polycrystalline structure, with its Dcri significantly affected by the intragranular-defect density. The experimental and theoretical conclusions comprehensively reveal the great potential of RPPT in the synthesis and characterization of compact bulk nanocrystalline materials, which provides a novel window to rediscover their intrinsic mechanical properties, for instance, the inverse Hall-Petch relation of bulk nanocrystalline InAs.

Electronic, elastic and pressure-induced phase transitions of tetragonal NaBH4 under high pressure

In this study; the structural, electronic and mechanical properties of sodium borohydride (NaBH4) compound were studied under high hydrostatic pressure from 0 to 300 GPa using the generalized gradient approach (GGA) within the framework of density functional theory (DFT). In the study, the NaBH4 compound transformed from the tetragonal structure with the space group P42 /nmc to the triclinic structure with the space group with the effect of gradually increasing pressure. Enthalpy and total energy calculations were performed to observe the agreement of the study with the experimental results. As a result of the calculations, it was concluded that a phase transition of the NaBH4 compound occurred at approximately 22.6 GPa from the tetragonal structure to the triclinic structure. In addition, the electronic properties of the NaBH4 compound were investigated and 6.22 and 4.95 eV band gaps were obtained for the P42 /nmc and phases, respectively.

Biaxial Hard Compression, Anisotropic Elastic Property, and Pressure-Induced Isosymmetric Phase Transition in Ammonium Bicarbonate

The elastic property, exceptional anisotropic compression, and pressure-induced isosymmetric phase transition in ammonium bicarbonate (AB) are studied by in situ synchrotron X-ray diffraction, Raman spectroscopy, and computational methods. The exceptional anisotropic compression as a result of biaxial hard compression is induced by stiff hydrogen-bonding “double-wine-rack” geometric motifs. The large values of elastic anisotropy such as Young’s moduli, Shear moduli, and Poisson’s ratio verify the anisotropic nature in AB. The biaxial hard compression further induces an isosymmetric phase transition at 2 GPa through the generation of new N–H···O hydrogen bonds. Our work first demonstrates the stiff mechanical study of “double-wine-rack” geometric hydrogen-bonding networks and its influence on the phase transition, which can be used as a model to investigate the robust nature of hydrogen bonds and the structural origin of isosymmetric phase transition. Moreover, the hydrogen-bonding “double-wine-rack” geometric mechanism also provides an inspiration to construct a covalent-bonding “double-wine-rack” geometric model favoring anomalous biaxial mechanical/thermal responses. The determination of the high-pressure phase of AB provides new information to understand the composition of the ternary system H2O–CO2–NH3.

First-principles study of topological surface states and pressure induced phase transitions in a novel noncentrosymmetric superconductor PbTiSe2

Noncentrosymmetric superconductors have attracted much interest in exploring exotic superconducting excitations and probing Majorana zero modes, which are deemed as the building blocks for realizing the fault-tolerant quantum computation. Here, based on a systematic first principles study, we unveil a noncentrosymmetric PbTiSe2 superconductor with rosy stabilities. We demonstrate that the band structure of this PbTiSe2 compound is topologically nontrivial and depict the topological surface states around the Fermi surface. We further investigate the phase transitions tuned by pressure in PbTiSe2 and find a stable tetragonal superconducting compound which is also demonstrated as topological phase. Our calculations reveal that noncentrosymmetric PbTiSe2 is a promising candidate to explore topological superconductivity. The simulation results in this work illustrate the abundant phases and exotic electronic states in PbTiSe2, which may help with the future design and synthesis of new topological phases.

Pressure-induced phase transition of CO32−-bearing scapolite by in situ X-ray diffraction and vibrational spectroscopy

Pressure-induced phase transition of CO32−-bearing scapolite by in situ X-ray diffraction and vibrational spectroscopy

Tracking structural phase transitions via single crystal x-ray diffraction at extreme conditions: advantages of extremely brilliant source

Highly brilliant synchrotron source is indispensable to track pressure-induced phenomena in confined crystalline samples in megabar range. In this article, a number of experimental variables affecting the quality high-pressure single-crystal x-ray diffraction data is discussed. An overview of the recent advancements in x-ray diffraction techniques at extreme conditions, in the frame of European Synchrotron Radiation Facility (ESRF)- Extremely Bright Source (EBS), is presented. Particularly, ID15b and ID27 beamlines have profited from the source upgrade, allowing for measurements of a few-micron crystals in megabar range. In case of ID27, a whole new beamline has been devised, including installation of double-multilayer mirrors and double crystal monochromator and construction of custom-made experimental stations. Two case studies from ID27 and ID15b are presented. Hypervalent CsI3 crystals, studied up to 24 GPa, have shown a series of phase transitions: Pnma → P-3c1→ Pm-3 n. First transition leads to formation of orthogonal linear iodine chains made of I3 -. Transformation to the cubic phase at around 21.7 GPa leads to equalization of interatomic I–I distances and formation of homoleptic In m- chains. The second study investigates elastic properties and structure of jadarite, which undergoes isosymmetric phase transition around 16.6 GPa. Despite a few-micron crystal size, twinning and dramatic loss of crystal quality, associated with pressure-induced phase transitions, crystal structures of both compounds have been determined in a straightforward matter, thanks to the recent developments within ESRF-EBS.

Pressure-Induced Abnormal Electrical Transport Transition from Pure Electronic to Mixed Ionic–Electronic in Multiferroic BiFeO3 Ceramics

Pressure-induced structural changes could induce changes in transport properties and lead to a better understanding of the structure–property relationship. The evolution of the carrier transport properties of BiFeO3 (BFO) ceramics under a high pressure was investigated through impedance spectroscopy measurements at room temperature combined with first-principles calculations. A pressure-induced abnormal transition from pure electronic to mixed ionic–electronic was found in the BFO ceramics Cmmm and Pnma phases at 7.67 and 11.01 GPa, respectively. The pressure-induced structural phase transition from the R3c phase to Cmmm and then to Pnma was responsible for the change in electrical transport behavior from pure electronic to mixed ionic–electronic conduction, accompanied by a decrease in ionic resistance. The calculations of electronic structures and electron localization function from 1 atm to 40 GPa indicated that the ionic conduction in the Pnma phase resulted from the weakened Coulomb screen of the localized electron background to O2– between Bi3+ and O2–. This work provides a critical insight into the understanding of the relationship between structure and conduction and facilitates the application of ferroelectric materials in photoelectric fields.

Coherent Control and Magnetic Detection of Divacancy Spins in Silicon Carbide at High Pressures.

Spin defects in silicon carbide appear to be a promising tool for various quantum technologies, especially for quantum sensing. However, this technique has been used only at ambient pressure until now. Here, by combining this technique with diamond anvil cell, we systematically study the optical and spin properties of divacancy defects created at the surface of SiC at pressures up to 40 GPa. The zero-field-splitting of the divacancy spins increases linearly with pressure with a slope of 25.1 MHz/GPa, which is almost two-times larger than that of nitrogen-vacancy centers in diamond. The corresponding pressure sensing sensitivity is about 0.28 MPa/Hz-1/2. The coherent control of divacancy demonstrates that coherence time decreases as pressure increases. Based on these, the pressure-induced magnetic phase transition of Nd2Fe14B sample at high pressures was detected. These experiments pave the way to use divacancy in quantum technologies such as pressure sensing and magnetic detection at high pressures.

Pressure-induced band-gap energy increase in a metal iodate

A wide band gap is one of the essential requirements for metal iodates to be used as nonlinear optical materials. Usually, the band gap of these materials decreases under the application of pressure. Herein, we introduce a case in which the band-gap energy of a hydrated metal iodate, namely $\mathrm{Ca}{({\mathrm{IO}}_{3})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$, has been successfully increased, from 4.52 to 4.92 eV, by applying external pressure without showing signs of saturation upon increasing pressure. The pressure-induced nonlinear band-gap opening correlates with the pressure-induced shortening of the I-O bond distances, as obtained from x-ray diffraction measurements. In addition, two pressure-induced isostructural phase transitions are observed in the pressure regions of 6.6--8.0 and 13.0--15.5 GPa. These two isostructural phase transitions cause a nonlinear pressure-induced evolution of the band-gap energy and crystal lattice parameters, as well as the occurrence of several extra peaks and peak splitting in Raman spectra.