Pressure-induced Amorphization Research Articles

Soft modes and anharmonicity inH3[Co(CN)6]: Raman spectroscopy and first-principles calculations

In situ high-pressure Raman spectroscopy and ab initio calculations are carried out to investigate the phase stability and the thermal expansion behavior of ${\mathrm{H}}_{3}[\mathrm{Co}(\mathrm{CN}{)}_{6}]$. Raman studies at high pressures in a diamond anvil cell identify soft phonons and phase instability at 2.3 GPa. Evidence of pressure-induced amorphization is found at 11 GPa. The phonon frequencies and eigenvectors obtained from ab initio calculation are used to complement the observed phonon spectra and for assignment of Raman modes. The computed eigenvector displacement patterns indicate that the soft modes correspond to the CN-librational vibrations (${E}_{\mathrm{g}}$ mode) of the Co-CN-H-NC-Co linkages and their Gr\"uneisen parameters are found to be negative, in agreement with our measured values. The thermal expansion coefficient $(15.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ calculated using our computed mode Gr\"uneisen parameters is found to be in good agreement with the reported value $(20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$. Temperature-dependent phonon spectra down to 77 K are used to obtain the anharmonicities of different modes.

Pressure-Induced Amorphization and a New High Density Amorphous Metallic Phase in Matrix-Free Ge Nanoparticles.

Over the last two decades, it has been demonstrated that size effects have significant consequences for the atomic arrangements and phase behavior of matter under extreme pressure. Furthermore, it has been shown that an understanding of how size affects critical pressure-temperature conditions provides vital guidance in the search for materials with novel properties. Here, we report on the remarkable behavior of small (under ~5 nm) matrix-free Ge nanoparticles under hydrostatic compression that is drastically different from both larger nanoparticles and bulk Ge. We discover that the application of pressure drives surface-induced amorphization leading to Ge-Ge bond overcompression and eventually to a polyamorphic semiconductor-to-metal transformation. A combination of spectroscopic techniques together with ab initio simulations were employed to reveal the details of the transformation mechanism into a new high density phase-amorphous metallic Ge.

Pressure-induced structural transformations in In2-x Yx (MoO4 )3 systems

In this work, we report results of high-pressure Raman experiments (P < 8 GPa) on In2-xYxMo3O12 for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure-induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In2(MoO4)3 and In1.5Y0.5(MoO4)3, respectively, resulting in the change of structure from monoclinic P21/a to a more denser structure. The PIA started at 5 and 3.4 GPa for In2Mo3O12 and In1.5Y0.5Mo3O12, respectively. The amorphization process takes place in two stages in the case of In1.5Y0.5Mo3O12 phase, while for In2Mo3O12, it is not complete until the pressure is as high as 7 GPa. Our results also suggest that with increase of ionic size of the A3+ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A2(MoO4)3 system, where A is a trivalent ion (In, Y3+, Sc3+, Fe3+, etc.). Copyright © 2015 John Wiley & Sons, Ltd.

Pressure-Induced Amorphization of Small Pore Zeolites-the Role of Cation-H2O Topology and Anti-glass Formation.

Systematic studies of pressure-induced amorphization of natrolites (PIA) containing monovalent extra-framework cations (EFC) Li+, Na+, K+, Rb+, Cs+ allow us to assess the role of two different EFC-H2O configurations within the pores of a zeolite: one arrangement has H2O molecules (NATI) and the other the EFC (NATII) in closer proximity to the aluminosilicate framework. We show that NATI materials have a lower onset pressure of PIA than the NATII materials containing Rb and Cs as EFC. The onset pressure of amorphization (PA) of NATII materials increases linearly with the size of the EFC, whereas their initial bulk moduli (P1 phase) decrease linearly. Only Cs- and Rb-NAT reveal a phase separation into a dense form (P2 phase) under pressure. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) imaging shows that after recovery from pressures near 25 and 20 GPa long-range ordered Rb-Rb and Cs-Cs correlations continue to be present over length scales up to 100 nm while short-range ordering of the aluminosilicate framework is significantly reduced—this opens a new way to form anti-glass structures.

High pressure synthesis of amorphous TiO2 nanotubes

Amorphous TiO2 nanotubes with diameters of 8-10 nm and length of several nanometers were synthesized by high pressure treatment of anatase TiO2 nanotubes. The structural phase transitions of anatase TiO2 nanotubes were investigated by using in-situ high-pressure synchrotron X-ray diffraction (XRD) method. The starting anatase structure is stable up to ∼20GPa, and transforms into a high-density amorphous (HDA) form at higher pressure. Pressure-modified high- to low-density transition was observed in the amorphous form upon decompression. The pressure-induced amorphization and polyamorphism are in good agreement with the previous results in ultrafine TiO2 nanoparticles and nanoribbons. The relationship between the LDA form and α-PbO2 phase was revealed by high-resolution transmission electron microscopy (HRTEM) study. In addition, the bulk modulus (B0 = 158 GPa) of the anatase TiO2 nanotubes is smaller than those of the corresponding bulks and nanoparticles (180-240 GPa). We suggest that the unique open-ended nanotube morphology and nanosize play important roles in the high pressure phase transition of TiO2 nanotubes.

Pressure-Induced Phase Transformation, Reversible Amorphization, and Anomalous Visible Light Response in Organolead Bromide Perovskite.

Hydrostatic pressure, as an alternative of chemical pressure to tune the crystal structure and physical properties, is a significant technique for novel function material design and fundamental research. In this article, we report the phase stability and visible light response of the organolead bromide perovskite, CH3NH3PbBr3 (MAPbBr3), under hydrostatic pressure up to 34 GPa at room temperature. Two phase transformations below 2 GPa (from Pm3̅m to Im3̅, then to Pnma) and a reversible amorphization starting from about 2 GPa were observed, which could be attributed to the tilting of PbBr6 octahedra and destroying of long-range ordering of MA cations, respectively. The visible light response of MAPbBr3 to pressure was studied by in situ photoluminescence, electric resistance, photocurrent measurements and first-principle simulations. The anomalous band gap evolution during compression with red-shift followed by blue-shift is explained by the competition between compression effect and pressure-induced amorphization. Along with the amorphization process accomplished around 25 GPa, the resistance increased by 5 orders of magnitude while the system still maintains its semiconductor characteristics and considerable response to the visible light irradiation. Our results not only show that hydrostatic pressure may provide an applicable tool for the organohalide perovskites based photovoltaic device functioning as switcher or controller, but also shed light on the exploration of more amorphous organometal composites as potential light absorber.

Pressure-Induced Amorphization and Recrystallization of SnI2

The high-pressure behavior of SnI2 has been investigated in a combined experimental and theoretical study by angle-dispersive X-ray diffraction, Raman scattering measurements, and ab initio calculations. Both the Raman and XRD results confirm that the SnI2 crystal undergoes a gradual crystal to amorphous transition and subsequently recrystallizes to a new crystal structure upon compression. The intensity of the Sn–I symmetric stretching mode greatly decreases and manifests a red shift at 2.15 GPa in Raman spectra. The XRD patterns show a bonding break phenomenon before the pressure-induced amorphization. We propose that the bond breaking between neighboring layers leads to the formation of the amorphous phase under high pressure. The sample recrystallizes into a new high-pressure crystal phase at 33.18 GPa. The experimental and theoretical results provide a good candidate structure for the recrystallized phase with C2/m space group.

Pressure-induced amorphization of metavanadate crystals SrV2O6 and BaV2O6

Lattice vibrations and electrical transport properties of SrV2O6 (SVO) and BaV2O6 (BVO) under high pressure have been investigated by Raman spectra and alternating current (AC) impedance spectra measurements. A pressure-induced structural phase transition in SVO is observed at 3.9 GPa, and the phase transition in BVO happens at 4.3 GPa from their high-pressure Raman spectra. With further increasing pressures, amorphization is found in both SVO and BVO at 10.1 and 9.3 GPa, respectively. Pressure-induced amorphization of SVO and BVO is suggested to be associated with the breaking up of infinite chains of corner-linked tetrahedral VO4 into VO3−. The recovery of original crystalline phases along with the re-linking of VO3− chains is observed after heating the reclaimed samples. Furthermore, the in situ high pressure measurements of AC impedance spectra of BVO reveal two distinct changes in its resistance, which can correspond to the transitions as observed in the Raman spectra.

Universal elastic-hardening-driven mechanical instability in α-quartz and quartz homeotypes under pressure.

As a fundamental property of pressure-induced amorphization (PIA) in ice and ice-like materials (notably α-quartz), the occurrence of mechanical instability can be related to violation of Born criteria for elasticity. The most outstanding elastic feature of α-quartz before PIA has been experimentally reported to be the linear softening of shear modulus C44, which was proposed to trigger the transition through Born criteria B3. However, by using density-functional theory, we surprisingly found that both C44 and C66 in α-quartz exhibit strong nonlinearity under compression and the Born criteria B3 vanishes dominated by stiffening of C14, instead of by decreasing of C44. Further studies of archetypal quartz homeotypes (GeO2 and AlPO4) repeatedly reproduced the same elastic-hardening-driven mechanical instability, suggesting a universal feature of this family of crystals and challenging the long-standing idea that negative pressure derivatives of individual elastic moduli can be interpreted as the precursor effect to an intrinsic structural instability preceding PIA. The implications of this elastic anomaly in relation to the dispersive softening of the lowest acoustic branch and the possible transformation mechanism were also discussed.

Pressure-induced amorphization of ionic liquid [HMIM][PF6

Phase behavior of ionic liquid [HMIM][PF6] has been investigated under high pressure up to 5.6GPa at room temperature. The results indicated that [HMIM][PF6] might experience a phase transition at about 3.4GPa upon compression, which could be identified as solidification to superpressurized glass by ruby R1 line broadening measurement and synchrotron X-ray diffraction (XRD) patterns. For conformational equilibrium, the fraction of all-anti (AAAA) conformer increased upon compression, while the conformational change was independent of temperature. These facts indicated that there were large differences of the structure in response to the extreme conditions, especially in the structure of the cation.

Polymorphic phase transition mechanism of compressed coesite.

Silicon dioxide is one of the most abundant natural compounds. Polymorphs of SiO₂ and their phase transitions have long been a focus of great interest and intense theoretical and experimental pursuits. Here, compressing single-crystal coesite SiO₂ under hydrostatic pressures of 26-53 GPa at room temperature, we discover a new polymorphic phase transition mechanism of coesite to post-stishovite, by means of single-crystal synchrotron X-ray diffraction experiment and first-principles computational modelling. The transition features the formation of multiple previously unknown triclinic phases of SiO₂ on the transition pathway as structural intermediates. Coexistence of the low-symmetry phases results in extensive splitting of the original coesite X-ray diffraction peaks that appear as dramatic peak broadening and weakening, resembling an amorphous material. This work sheds light on the long-debated pressure-induced amorphization phenomenon of SiO₂, but also provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature.

Room-temperature vibrational properties of potassium gadolinium double tungstate under compression up to 32 GPa

KGd(WO4)2 has been studied by high-pressure Raman spectroscopy at room temperature up to 32.2GPa. Evidences of two pressure-driven phase transitions have been detected. The low-pressure monoclinic phase undergoes a phase transition at 7.2(±0.9)GPa. The second transition is found at 14.2(±1.6)GPa. Both transitions are reversible. No evidence of pressure-induced amorphization is found up to 32GPa. The pressure dependence of the Raman active modes of the low- and high-pressure phases is reported. A Raman mode is detected to exhibit a weak softening in the low-pressure phase. Three Raman modes are also observed to have a similar behavior in the second high-pressure phase. The reported results are discussed comparing with the studies available in related tungstates. Emphasis is placed on the physical implications of the results.

High-pressure phase behavior of the room temperature ionic liquid 1-ethyl-3-methylimidazolium nitrate

The phase behavior of 1-ethyl-3-methylimidazolium nitrate ([emim][NO3]) was investigated at 298K under high pressures, reaching 8.6GPa, by Raman spectroscopy. This compound was found to crystallize at 0.4GPa. The crystalline state of [emim][NO3] seems to undergo, at least, three more phase changes under the present experimental conditions. The conformational isomerism of the [emim]+ cation between planar and nonplanar forms was also investigated. Remarkably, these crystals showed drastic conformational changes upon phase changes, concomitant with the appearance of a new Raman peak attributable to the planar [emim]+ cation. This behavior contrasts markedly with previous results obtained for 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), which showed pressure-induced amorphization at 2.8GPa.

Size-dependent pressure-induced amorphization: a thermodynamic panorama.

Below a critical particle size, some pressurized compounds (e.g. TiO2, Y2O3, PbTe) undergo a crystal-to-amorphous transformation instead of a polymorphic transition. This effect reflects the greater propensity of nanomaterials for amorphization. In this work, a panorama of thermodynamic interpretations is given: first, a descriptive analysis based on the energy landscape concept gives a general comprehension of the balance between thermodynamics and kinetics to obtain an amorphous state. Then, a formal approach based on Gibbs energy to describe the thermodynamics and phase transitions in nanoparticles gives a basic explanation of size-dependent pressure-induced amorphization. The features of this transformation (amorphization occurs at pressures lower than the polymorphic transition pressure!) and the nanostructuration can be explained in an elaborated model based on the Ginzburg-Landau theory of phase transition and on percolation theory. It is shown that the crossover between polymorphic transition and amorphization is highly dependent on the defect density and interfacial energy, i.e., on the synthesis process. Their behavior at high pressure is a quality control test for the nanoparticles.

Origin of “memory glass” effect in pressure-amorphized rare-earth molybdate single crystals

The memory glass effect (MGE) describes the ability of some materials to recover the initial structure and crystallographic orientation after pressure-induced amorphization (PIA). In spite of numerous studies the nature and underlying mechanisms of this phenomenon are still not clear. Here we report investigations of MGE in β′-Eu2(MoO4)3 single crystal samples subjected to high pressure amorphization. Using the XRD and TEM techniques we carried out detailed analysis of the structural state of high pressure treated single crystal samples as well as structural transformations due to subsequent annealing at atmospheric pressure. The structure of the sample has been found to be complex, mainly amorphous, however, the amorphous medium contains evenly distributed nanosize inclusions of a paracrystalline phase. The inclusions are highly correlated in orientation and act as “memory units” in the MGE.

Pressure-enhanced decomposition of Ag3[Co(CN)6

The structural stability of silver hexacyanocobaltate, Ag3[Co(CN)6], was investigated at high pressure and moderate temperature using X-ray diffraction, Raman spectroscopy, and Fourier Transform Infrared spectroscopy. Pressure induced amorphization (PIA) or pressure enhanced decomposition were observed depending of the processing conditions. In situ X-ray diffraction analysis unequivocally confirmed the PIA at room temperature for pressures higher than 13GPa. The usual decomposition temperature at ambient pressure, 227°C, decreased to 180°C at 7.7GPa, and to 150°C at 14GPa. The decomposition products after moderate heating at high pressure are at least two phases: crystalline elemental silver and an amorphous phase containing the remaining elements (Co, C and N), which presents a Raman spectrum similar to that observed for amorphous graphite-like carbon nitride (CNx). Our findings confirm the previously reported pressure induced amorphization of Ag3[Co(CN)6] at room temperature, and indicate that this amorphization is the result of a kinetically hampered decomposition.

Pressure-induced amorphization of a dense coordination polymer and its impact on proton conductivity

The proton conductivity of a dense coordination polymer (CP) was investigated under high-pressure conditions. Impedance measurements under high pressures revealed that the proton conductivity of the CP decreased more than 1000-fold at pressures of 3–7 GPa and that the activation energy for proton conduction almost doubled compared with that at ambient pressure. A synchrotron X-ray study under high pressure identified the amorphization process of the CP during compression, which rationally explains the decrease in conductivity and increase in activation energy. This phenomenon is categorized as reversible pressure-induced amorphization of a dense CP and is regarded as a demonstration of the coupling of the mechanical and electrical properties of a CP.

Thermal dehydration of potash alum studied by Raman spectroscopy and X-ray diffraction analysis

The thermal dehydrations of potash alum caused by heating at various temperatures for at least 2 h were investigated by ex situ Raman spectroscopy and X-ray diffraction (XRD) analyses in air. With increasing the heating temperature, all Raman peaks were observed to broaden, while an additional broad peak appeared at approximately 1030 cm−1 and shifted toward higher wavenumbers. In addition, the Raman band assigned to the O–H stretching mode weakened. The orientational disorder (OD) of the sulfate ions, as indicated by the intensity ratio of doublet peaks at 989 and 974 cm−1, was found to increase with increasing the heating temperature. The XRD patterns demonstrated that a structural phase transition from crystalline KAl(SO4)2⋅12H2O to amorphous phases began at around 75 °C, while broadening of the Raman peaks and an increase in OD also suggested the onset of an amorphous phase. Raman peaks corresponding to anhydrous KAl(SO4)2 appeared at approximately 180 °C. It was concluded that the elimination of water molecules was responsible for increase in the extent of OD, and this in turn induced the observed phase transitions. The formation of the amorphous phases observed in this work was similar to the pressure-induced amorphization of KAl(SO4)2⋅12H2O.

Compression behavior of Sm2Ti2O7-pyrochlore up to 50 GPa: single-crystal X-ray diffraction and density functional theory calculations

Single-crystal X-ray diffraction at pressures up to 50 GPa has been employed to study the compression behavior of Sm2Ti2O7-pyrochlore. In contrast to earlier reports, we observed no pressure-induced amorphization or pressure-induced anion disorder up to 50 GPa. The experimental study has been complemented by density functional theory-based calculations. A combination of the theoretical and experimental data yields a bulk modulus of $$\approx $$ 185 GPa, significantly higher than a value which had been reported earlier. In comparison to earlier work, the current study provides more reliable data due to the use of neon as a pressure medium, which provides a more hydrostatic pressure than the aluminum, which had been employed as a pressure medium in the earlier studies. An analysis of the compressibility of Al $$_2$$ B $$_2$$ O $$_7$$ pyrochlores shows an approximately linear dependence of the bulk modulus on the unit cell volume.

High-pressure structural and elastic properties of Tl2O3

The structural properties of Thallium (III) oxide (Tl2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl2O3 has been determined and compared to related compounds. It has been found experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-induced amorphization process, we have studied theoretically the possible high-pressure phases of Tl2O3. Although a phase transition is theoretically predicted at 5.8 GPa to the orthorhombic Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic α-Gd2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused by the mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa.