High-pressure Induced Phase Transitions Research Articles

High pressure-induced phase transition in nanocrystalline iron confined by single-walled carbon nanohorns

This study examines the high-pressure structural properties of nanocrystalline Fe confined by single-walled carbon nanohorns (SWCNHs) and SWCNHs (filled with Fe nanoparticles) named Fe-filled SWCNHs up to a pressure of 21.8 GPa. In detail, the Fe-filled SWCNHs did not undergo a special structural transformation at up to 21.8 GPa, except the partial irreversible amorphization of graphene sheets forming the core part above ∼18 GPa. Cubic Fe (bcc Fe) and Fe3O4 coexisted simultaneously in the Fe-filled SWCNHs. The Fe encapsulated by the SWCNH underwent a reversible transformation from bcc Fe to a hexagonal Fe (hcp Fe) at 11.7 GPa and reversed to bcc Fe at 6.2 GPa with a width of the domain of coexistence of bcc hcp of the order of 5 GPa. Due to the effect of the special tubular confinement of the SWCNHs, the high-pressure behavior of Fe confined by them exhibited a sharp contrast to that of Fe confined by carbon nanotubes. In comparison to the bulk bcc Fe, hcp Fe, and Fe3O4, these nanocrystallines confined by SWCNHs had higher compressibility than their bulk phases. Fe3O4 did not exhibit any phase transformation until 21.8 GPa, where this was similar to the results of bulk Fe3O4.

High pressure-induced phase transitions in AgI semiconducting compound up to 1 Mbar

In this work, we have resorted to a recent version of the full potential linear muffin-tin orbitals (FP-LMTO) method based on the density functional theory (DFT), within both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the calculation of the equilibrium structural parameters, and phase transition under high pressure of AgI semiconducting compound. Results are given for lattice parameters, bulk modulus and its first pressure derivative in different structures. In general, the use of the GGA approach in this work appears more appropriate than the LDA, and it is most correctly predicts the majority of the ground state properties for AgI material. The energy differences of NiAs (B81)/NaCl (B1) phases are around 1.76 and 1.15 meV, using the LDA and the GGA, respectively. Importantly, the phase transitions for this material from sixfolds coordinated NaCl-type structure (B1) to the eightfold coordinated CsCl-type structure (B2), and then to the hexagonal close packed HCP-type structure (A3) are possible under high pressure. Our calculations show that AgI semiconducting compound transforms firstly from B1 to B2 at pressure of around 55.58 GPa, and then from B2 to A3 at pressure of around 95.03 GPa, respectively.

High-Pressure Induced Phase Transitions in High-Entropy Alloys: A Review

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show potential in widespread applications. However, their phase stability/transition, which is of great scientific and technical importance for materials, has been mainly explored by varying temperature. Recently, pressure as another fundamental and powerful parameter has been introduced to the experimental study of HEAs. Many interesting reversible/irreversible phase transitions that were not expected or otherwise invisible before have been observed by applying high pressure. These recent findings bring new insight into the stability of HEAs, deepens our understanding of HEAs, and open up new avenues towards developing new HEAs. In this paper, we review recent results in various HEAs obtained using in situ static high-pressure synchrotron radiation x-ray techniques and provide some perspectives for future research.

High-pressure induced phase transition of FeS2: Electronic, mechanical and thermoelectric properties

Abstract Being as the most abundant natural mineral among the iron-based sulphides, iron pyrite (FeS2) has been extensively investigated. These investigations mainly focused on the known marcasite (Pnnm) and pyrite ( P a 3 ¯ ) phases, due to unknown of high pressure (HP) structural phases of FeS2. Here, we report a theoretical study up to 300 GPa to uncover a brand new monoclinic HP phase of C2/m structure at above 192 GPa. The stable of this structure is evaluated by the thermodynamical, dynamical and mechanical calculations. The C2/m structure is characterized by distorted FeS6 octahedron together with two connected 1D zigzag-like S chains. Analysis of the thermoelectric calculations indicates that the electronic thermal conductivity and electrical conductivity of C2/m structure linearly increasing with increasing temperature. Meanwhile, it is found that the C2/m phase behaves in a ductile manner through the elastic parameters calculations.

High pressure induced phase transition and superdiffusion in anomalous fluid confined in flexible nanopores.

The behavior of a confined spherical symmetric anomalous fluid under high external pressure was studied with Molecular Dynamics simulations. The fluid is modeled by a core-softened potential with two characteristic length scales, which in bulk reproduces the dynamical, thermodynamical, and structural anomalous behavior observed for water and other anomalous fluids. Our findings show that this system has a superdiffusion regime for sufficient high pressure and low temperature. As well, our results indicate that this superdiffusive regime is strongly related with the fluid structural properties and the superdiffusion to diffusion transition is a first order phase transition. We show how the simulation time and statistics are important to obtain the correct dynamical behavior of the confined fluid. Our results are discussed on the basis of the two length scales.

Pressure-induced structural phase transition in bulk Zn0.98Mn0.02O by angular dispersive X-ray diffraction

The high pressure induced phase transition in bulk Zn0.98Mn0.02O at ambient temperature have been investigated using angular-dispersive X-ray diffraction (ADXRD) under high pressure up to around 13.80GPa. For loading run, in situ ADXRD measurements found that a würtzite-to-rocksalt structural phase transition pressure of bulk Zn0.98Mn0.02O began at 7.35GPa. The fitting of volume compression data to the third-order Birch–Murnaghan equation of state yielded that the zero-pressure isothermal bulk moduli and the first-pressure derivatives were 157(8) GPa and 8(3) for the B4 phase, respectively. When decompress bulk Zn0.98Mn0.02O to ambient pressure a large part of the bulk Zn0.98Mn0.02O reverted to the B4 phase and only a small amount of the metastable B1 phase remained. We have exhibits the pressure dependence of lattice constant a and c axis, and the normalized ratio c/a of the bulk Zn0.98Mn0.02O. Possible pressure-induced phase transition mechanisms were explored by examining the cell parameters and the internal structural parameter (u) with pressures. The effect of the 3d electrons of manganese to increase the nearest-neighbor distance of O to Zn (Mn) parallel to the c axis may be the main reason for the phase transition of bulk Zn0.98Mn0.02O. An increase in the u value with pressure indicates that the B4-to-B1 phase transformation in bulk Zn0.98Mn0.02O is likely via the hexagonal path.

Pressure-induced changes in electronic and vibrational properties of LaS

The high-pressure induced phase transition in LaS has been investigated by means of first-principles calculations, and density-functional linear-response theory. A pressure-induced soft-acoustic phonon mode is identified at 30 GPa, which is in favourable agreement with the measured pressure-induced phase transition in LaS from the rocksalt to the CsCl structure. Phonon calculations reveal that pressure-induced instabilities of the transverse acoustic modes at the [ϵ00] and [ϵϵ0] directions are responsible for the phase transition. Furthermore, it is found that a decrease of bonding for S-p bonds and an increase of anti-bonding for La-d bonds significantly weakens the stability of the rocksalt phase of LaS under pressure, and hence inducing the structural phase transformation.

High pressure-induced phase transitions in CdS up to 1 Mbar

The pressure-induced structural transformations of CdS have been investigated using synchrotron x-ray diffraction in a diamond anvil cell up to 104 GPa and the density functional theory calculations. The x-ray diffraction experiments show that CdS is stable with the wurtzite-type structure at ambient conditions. The wurtzite-type phase transforms to NaCl-type structure at 3 GPa, which is followed by a second phase transition at 52.3 GPa. In the diffraction patterns, the peak-splitting is observed, indicating that the high-pressure phase appearing at 52.3 GPa is the Pmmn structure, rather than the Cmcm phase reported earlier. With increasing pressure, the lattice parameter a of the Pmmn phase increases abnormally, contrary to decrease of b and c axes. Our calculations reveal that the abnormal change of the a-axis could be related to the pressure-induced crystal structural change. The bulk modulus (B0), is 64.3(9) GPa for wurzite-type phase, 105(2) GPa for NaCl-type phase, and 54(4) GPa for the Pmmn phase, respectively.

Elastic behavior and high pressure-induced phase transition in chabazite: New data from a natural sample from Nova Scotia

The high pressure behavior of a natural chabazite from Nova Scotia (Canada) [(Ca1.32K0.45Na0.13Sr0.10)[Si8.55Al3.45O24]·11.30 H2O, s.g. R-3m, a=9.38749(8) and α=94.379(1)] was investigated by means of in situ synchrotron X-ray powder diffraction and silicone oil as non-penetrating P-transmitting medium, in the P-range from Pamb to 7.2GPa. Above 2.1GPa, a phase transition from rhombohedral to triclinic (pseudo-rhombohedral) symmetry is observed. The phase transition is accompanied by an abrupt decrease of most of the cell parameters and of the cell volume. Above the transition – between 2.5 and 5.9GPa – the triclinic/pseudo-rhombohedral cell parameters decrease regularly, with a compressibility decrease relative to the trend observed before the transition. A further increase in compressibility occurs in the highest P regime, between 5.9 and 7.2GPa. Overall, the cell volume decrease in the investigated P-range is 12.6%. Notwithstanding the abrupt and marked cell volume decrease accompanying the phase transition, this is reversible upon decompression. The refined elastic parameters were V0=826(1)Å3, K0=54(3)GPa and V0=784(2)Å3, K0=91(5)GPa, for the rhombohedral and triclinic phase, respectively. The results of the complete structural refinements performed up to 1.1GPa indicated that: (i) the double-six-membered ring present in chabazite framework undergoes flattening as a result of a HP-induced cooperative tetrahedral anti-rotation; (ii) the 8-membered rings, limiting the channel apertures, become more circular and their crystallographic free area decreases; (iii) the extraframework content does not exhibit relevant modifications under pressure – as concerns the site occupancies – while some significant changes are observed in the bond distances. The elastic behavior and deformation mechanism of chabazite from Nova Scotia are compared with those of another chabazite sample from Vallerano and with those of other porous materials belonging to the 6-ring zeolite family.

Anomalous structural phase transition properties in ReSe2 and Au-doped ReSe2

The high pressure induced phase transition in rhenium diselenides (ReSe(2)) and gold-doped rhenium diselenides (Au-ReSe(2)) at ambient temperature have been investigated using angular-dispersive x-ray diffraction (ADXRD) under high pressure up to around 10.50 and 9.98 GPa, respectively. In situ ADXRD measurements found that the phase transition pressures of ReSe(2) and Au-ReSe(2) began at 9.98 and 8.52 GPa, respectively. Compressibilities analysis shows the relationship of along c-axis > along a-axis > along b-axis. The linear compressibility of the pressure dependence of α, β, and γ of ReSe(2) shows that a phase transition can be related to a counterclockwise rotational trend of the selenium atoms around the chain of Re(4) atoms during the decrease of the c-axis distance by a combination of stresses due to the bending effect of α and stretching effect of β. The cause of the reduction of the phase transition pressure of Au-ReSe(2) is attributed mainly to a structural distortion as evidenced by the observation of a weak clockwise rotational trend of Se atoms around the chain of Re(4) atoms in the pressure range 3.99-4.99 GPa which subsequently reversed to counterclockwise rotation under higher pressure.

Phenomenological theory of high-pressure induced phase transitions in Ga-, In-, and Al-mononitrides

Phase transitions in GaN, InN, and AlN are analyzed within the context of the Fermi model. The moduli of the volume rigidity of phases stable at low and high pressure are obtained. The obtained results are compared with computational results yielded by quantum chemistry models.

Spin crossovers in Mott–Hubbard insulators at high pressures

The high-pressure induced phase transitions initiated by electronic transition in 3d ions from the high-spin (HS) to the low-spin (LS) state (HS-LS spin-crossover) are considered. Behavior of the system with d6 electronic configuration is investigated in the ground state of zero temperature and critical pressure Pc. Magnetic properties of the Mott–Hubbard insulator (Mg1−xFex)O are studied in the vicinity of the quantum critical point (T=0, Pc). At the critical pressure of spin crossover Pc, the spin gap energy εS between HS and LS states is zero. The quantum spins fluctuations HS⇔LS do not require any energy, and the antiferromagnetism is destroyed in the quantum critical point by the first order transition.

An experimental exploration of chemical bond characteristic, bulk modulus and phase stability in ZnO:Cu nanocrystals under high pressure

The high pressure induced phase transitions in Zn1−xCuxO (x=0.005 and 0.011) are investigated by angle-dispersive synchrotron radiation X-ray diffraction. As the pressure increases, phase transformations from the wurtzite structure to the rocksalt structure are observed in both samples, with the transition pressures at 9.8 GPa and 7.9 GPa, respectively. With the increasing of the Cu-doping concentration in ZnO, crystalline parameters, the bulk moduli, and the Zn–O bond lengths all increased, meanwhile, the transition pressures decreased. The results could be explained in terms of the reduction of phase transformation barriers and the lowering of bond energy.

The first-order structural phase transition of YSb

The high-pressure induced phase transition of YSb has been studied using the density functional theory method within the generalized gradient approximation. It was found that the first-order structural phase transition began to occur at 30.8 GPa, agreeing well with available experiments and theoretical calculations. In this phase transition, we do not find changes in the heat capacity and thermal expansion coefficients at lower and higher temperature, but the transition pressures decrease with temperature. The bulk modulus and Debye temperature decrease with increasing temperature, while they increase with increasing pressure. Also, the density of states and band structure of these two compounds with B1 and B2 structures have been presented and analyzed.

High-Pressure Study of Gd2(MoO4)3by Raman Scattering and Ab Initio Calculations

High-pressure induced phase transitions in a single crystal of gadolinium molybdate, Gd-2(MoO4)(3) were studied by the Raman spectroscopy and ab initio calculations. The amorphization of the sample takes place at about 6 GPa in a mixture of alcohol as a pressure transmitting medium and begins as soon as 3 GPa in argon. In both media, the amorphization is irreversible in the 0-9 GPa investigated pressure range. The joint ab initio and Raman results allowed us to conclude that rotations of MoO4 tetrahedra axe the primary structural changes involved in the first phase transition (at about 2 GPa) explaining the softening of the low frequency modes at about 50 cm(-1). In addition, a progressive distortion of tetrahedra followed by a coordination change (IV-VI) of Mo atoms is observed through the five structural transitions including amorphization. This mechanism based on the steric hindrance of polyhedra is believed to be the most relevant for explaining the amorphization of Gd-2(MoO4)(3).

High-pressure induced phase transitions of Y2O3 and Y2O3:Eu3+

We investigated high-pressure induced phase transitions in Y2O3 and Eu-doped Y2O3 (Y2O:Eu3+) using angular dispersive synchrotron x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). With increasing pressure, we observed a series of phase transformations in Y2O3:Eu3+, which followed a structure sequence of cubic→monoclinic→hexagonal, while Y2O3 followed a sequence of cubic→hexagonal. During decompression, both hexagonal structured Y2O3 and Y2O3:Eu3+ transformed into monoclinic phases which were quenchable back to ambient pressure. Raman and PL measurements shed additional light on the different phase transition behavior in these two samples.

High pressure induced phase transition in sulfur doped indium phosphide: An angular-dispersive X-ray diffraction and Raman study

The high pressure induced phase transitions in sulfur doped indium phosphide (InP:S) at ambient temperature has been investigated using angular-dispersive X-ray diffraction (ADXRD) and Raman scattering under high pressure up to around 44.6 and 37.4 GPa, respectively. In situ ADXRD measurements found that the transition of InP:S to a rock-salt phase began at 10.4 GPa and completed at 13.3 GPa with a 15.7% volume decrease. Another transition to the orthorhombic structure with space group Cmcm (the Cmcm phase) was found to occur at 35.8 GPa with a 4.1% volume decrease. The fitting of volume compression data to the third-order Birch–Murnaghan equation of state yielded that the zero-pressure isothermal bulk moduli ( B 0 ) and the first-pressure derivatives ( B 0 ′ ) were 74 GPa and 3.9 for the zinc-blende phase, respectively. The rock-salt to Cmcm phase transition pressure increases relative to that of undoped InP of 32 GPa, which may be attributable to the increase of the ionicity of InP:S by S doping favorable to the ionic rock salt structure. In situ Raman measurements have similar findings.

First principles study of structural phase transition of YSb and ScSb compounds

High pressure induced phase transition of YSb and ScSb compounds have been studied using Density Functional Theory method within Generalized Gradient Approximation. It was found that the phase transition from the NaCl-type (B1) to a CsCl-type structure (B2) began to occur at around 29 GPa for YSb compound, agreeing well with available experiments and theoretical calculations. For ScSb compound it was suggested that structural phase transition from B1 to B2 will occur at about 40 GPa, differing greatly with experimental and theoretical results. The finding that the transition pressures increase with decreasing lattice constant in the NaCl-type structure for YSb and ScSb compounds was found to be similar to the phenomena observed for LnSb (Ln: lanthanide) compounds. Mulliken charge and overlap population analysis revealed that YSb and ScSb compounds in B1 structure show similar interaction between anion and cation, while in B2 structure a higher degree of covalency was found for ScSb than that in YSb. Also, DOS and band structure of these two compounds in B1 and B2 structures were presented and analyzed.

Raman study of high pressure induced phase transition in sodium nitrite

The effect of pressure up to 200 kbars on the Raman active internal and external modes in NaNO 2 has been measured using a diamond anvil cell. The appearance of a new low frequency Raman active lattice mode at 50 kbars indicates the existence of a high pressure phase not previously observed. Slightly above this pressure a transfer of intensity is observed between two lattice modes whose frequencies have been shifted close to each other by pressure, indicative of a pressure-induced coupling between the modes.

The phase transitions in cesium azide: Temperature and pressure dependent Raman studies

The high temperature and the high pressure induced phase transitions in polycrystalline cesium azide (CsN3) have been studied by Raman spectroscopy. The remarkable broadening of the Eg rotatory mode with increasing temperature and the observation of quasielastic low frequency scattering in the high temperature phase (Phase I) support the order–disorder character of the temperature induced transition. In the high pressure phase (Phase III), the Raman spectrum shows a decrease in the rotatory mode linewidth and splittings of the lines due to internal modes, suggesting that the new structure is more compact and involves at least two crystallographically nonequivalent azide ion sites. Phase III could not be obtained by lowering the temperature to 20 °K at 10−6 bar.