High-pressure Phase Transformations Research Articles

High pressure and high temperature phase transformations of covalent triazine-based frameworks

A monolithic two-dimensional (2D) biphenyl-linked triazine-based framework (CTF-2) showing merely slight crystallinity was prepared by Lewis acid-promoted polymerisation of 4,4′-biphenyldicarbonitrile in liquid TiCl4 at moderate temperature to avoid unwanted carbonization. The transformations of CTF-2 induced by high pressure (up to 60 GPa) and temperature (up to 2400 K) were studied by IR and in situ Raman spectroscopy as well as TEM microscopy. In contrast to graphitic C3N4 materials, CTF-2 exhibits high optical density and reduced fluorescence at high pressure, allowing laser-induced sample heating. Spectral data show the formation of sp3 hybridised carbon atoms and triazine ring distortion in CTF-2 material at pressures above 17 GPa. According to the pressure Raman effect, the bulk modulus of the high-pressure phase of CTF-2 (high-pressure phase CTF-2) was found to be 570 ± 10 GPa, which is a typical value for superhard carbon materials. Structural studies revealed nitrogen preservation in the specimen after high-pressure treatment and stability of the disordered sp3 states of the high-pressure phase CTF-2 in the quenched specimen. This behaviour is quite different from that of the quenched disordered sp3 states of high pressure pure carbon phases, which convert to the sp2 state at pressures below 3–7 GPa.

Electron Counting and High-Pressure Phase Transformations in Metal Hexaborides.

Pressure-induced structural transitions of the alkaline earth hexaborides, CaB6, SrB6, and BaB6, are studied theoretically using electron counting rules and density functional theory calculations. We demonstrate the applicability of gas-phase borane electron counting methods to solid-state metal borides under pressure and validate the assumptions of the rules by density functional theory (DFT) calculations. All three compounds share ambient-pressure and high-pressure structures, but BaB6 differs from CaB6 and SrB6 at intermediate pressures. The unique BaB6 phase is shown to break electron counting rules, while all other phases obey them. This anomaly is resolved by DFT, which reveals B-Ba covalency and unusual B-B π bonding under pressure. The relationships between structure and bonding can help us to understand the exotic behavior of lanthanide hexaborides and design new borides with desirable properties. Developing electron counting procedures for solids will enhance materials discovery efforts with chemical intuition.

The Phase Transformation of Silicon Assessed by an Unloading Contact Pressure Approach

Silicon is of great economic importance for the semiconductor industry as well as of academic interest because of its high-pressure phase transformations. These transformations also occur during the indentation of silicon. To further investigate these transformations, a modified method using the continuous stiffness measurement (CSM) during unloading is presented in this work. The use of the CSM signal allows directly calculating the mean contact pressure while unloading. The measurements will be compared to conventional indentation tests and data from high-pressure cell experiments reported in the literature. Furthermore, the influence of constant load holding segments on the phase transformation during unloading is investigated.

Multi-phase equation of state of ultrapure hafnium to 120 GPa

Hafnium (Hf) is an industrially important material due to its large neutron absorption cross-section and its high corrosion resistance. When subjected to high pressure, Hf phase transforms from its hexagonal close packed α-Hf phase to the hexagonal ω-Hf phase. Upon further compression, ω-Hf phase transforms to the body centered cubic β-Hf phase. In this study, the high pressure phase transformations of Hf are studied by compressing and decompressing a well-characterized Hf sample in diamond anvil cells up to 120 GPa while collecting x-ray diffraction data. The phase transformations of Hf were compared in both a He pressure transmitting medium (PTM) and no PTM over several experiments. It was found that the α-Hf to ω-Hf phase transition occurs at a higher pressure during compression and lower pressure during decompression with a helium (He) PTM compared to using no PTM. There was little difference in the ω-Hf to β-Hf phase transition pressure between the He PTM and no PTM. The equation of state was fit for all three phases of Hf and under both PTM and no-PTM.

Nonexistence of the s−f volume-collapse transition in solid gadolinium at pressure

Gadolinium has long been believed to undergo a high-pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended $s$-orbital to the compact $f$-orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the $f$-electrons [Fabbris et al., Phys. Rev. B 88, 245103 (2013)]. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high-pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using x-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes $hcp\ensuremath{\rightarrow}9R\ensuremath{\rightarrow}\mathit{dhcp}\ensuremath{\rightarrow}\mathit{fcc}\ensuremath{\rightarrow}\mathit{d}\ensuremath{-}\mathit{fcc}$ is reproduced. The so-called ``volume-collapsed'' high-pressure phase is shown to be an unusual stacking of close-packed planes, with $\mathit{Fddd}$ symmetry and a density change of less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the $f$-electrons. The key result is that there is no XRD evidence for volume collapse in gadolinium. The sequence of phase transitions is well described by standard density functional theory. There is no need for special treatment of the $f$-electrons or evidence of $f$-electron bonding. Noting that in previous spectroscopic evidence there is no change in the $f$-electrons we conclude that high-pressure gadolinium has no complicated $f$-electron physics such as Mott-Hubbard, Kondo, or valence transitions.

Stacking Fault Energy Reduction Phenomena Discovered in High-Pressure Phase Transformations of CrFeNi and CoCrFeMnNi Alloys

Stacking Fault Energy Reduction Phenomena Discovered in High-Pressure Phase Transformations of CrFeNi and CoCrFeMnNi Alloys

What Determines the fcc-bcc Structural Transformation in Shock Compressed Noble Metals?

High pressure structural transformations are typically characterized by the thermodynamic state (pressure-volume-temperature) of the material. We present insitu x-ray diffraction measurements on laser-shock compressed silver and platinum to determine the role of deformation-induced lattice defects on high pressure phase transformations in noble metals. Results for shocked Ag show a copious increase in stacking faults (SFs) before transformation to the body-centered-cubic (bcc) structure at 144-158GPa. In contrast, shock compressed Pt remains largely free of SFs and retains the fcc structure to over 380GPa. These findings, along with recent results for shock compressed gold, show that SF formation promotes high pressure structural transformations in shocked noble metals that are not observed under static compression. Potential SF-related mechanisms for fcc-bcc transformations are discussed.

High-Pressure Phase Transformations under Severe Plastic Deformation by Torsion in Rotational Anvils

High-Pressure Phase Transformations under Severe Plastic Deformation by Torsion in Rotational Anvils

High-pressure phase transformations in NdVO4 under hydrostatic, conditions: a structural powder x-ray diffraction study

Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO4 were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO4 transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO4 are reported. A direct comparison of our results with former experimental and theoretical studies on other rare-earth orthovanadates found in literature highlights the importance of the role played by non-hydrostatic stresses in their high-pressure structural behavior.

A2TiO5 (A = Dy, Gd, Er, Yb) at High Pressure.

The structural evolution of lanthanide A2TiO5 (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic Dy2TiO5 and Gd2TiO5 form P21am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy2TiO5 and Er2TiO5 as well as defect-fluorite-type Yb2TiO5 form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal Dy2TiO5 forms Pnma directly, although a small amount of remnants of hexagonal Dy2TiO5 is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy2TiO5 phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3̅m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er2TiO5. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A2BO5 composition at high pressure.

Deformation mechanisms at multiple pop-ins under spherical nanoindentation of (1 1 1) Si

The pop-in events and the deformation mechanisms of (1 1 1) Si under spherical nanoindentation were investigated using molecular dynamics simulations. The simulations result successfully reproduced the plastic deformation mediated by high-pressure phase transformations and dislocation burst, and the highly desirable brittle fracture of silicon. Quadruple occurrences of the pop-ins were observed in loading curves. The one-to-one relationship between the multiple pop-ins and the deformation mechanisms was established. Two fundamental processes, the asynchronous occurrences of high-pressure phase transformation and extrusion of α-Si, were identified from several deformation modes to be responsible for the multiple pop-ins. Moreover, the dislocation burst also contributes to the plastic deformation, and the cracks were observed to contribute to the brittle deformation, but contribute nothing to the pop-ins.

High-pressure torsion-induced phase transformations and grain refinement in Al/Ti composites

High-pressure torsion (HPT) deformation of multiphase metallic systems produces a high density of interfaces and leads to atomic mixing between the constituent phases. Here we present a study of the interphase boundary structure, grain size evolution and intermetallic phase formation during HPT deformation of a nano-crystalline Al/Ti composite. High-resolution transmission electron microscopy was used to study the structural features of the interphase boundaries. The Al/Ti interphase boundaries were found to significantly promote the generation of dislocations during deformation. After HPT deformation to a shear strain of 87, the average grain sizes of Al and Ti are 22 and 31 nm, respectively. The chemical mixing between the Al and Ti phases was enhanced by defect-mediated short circuit diffusion and dislocation shuffle-controlled plastic deformation at the interphase boundaries. The intermetallic phases formed during HPT deformation are associated with the strain energy stored by the high density of dislocations at the interphase boundaries.

Coupled elastoplasticity and plastic strain-induced phase transformation under high pressure and large strains: Formulation and application to BN sample compressed in a diamond anvil cell

In order to study high-pressure phase transformations (PTs), high static pressure is produced by compressing a thin sample within a high strength gasket in a diamond anvil cell (DAC). However, since a PT occurs during plastic flow, it is classified and treated here as a plastic strain-induced PT. A thermodynamically consistent system of equations for combined plastic flow and plastic strain-induced PTs is formulated for large elastic, plastic, and transformation strains. The Murnaghan elasticity law, pressure-dependent J2 plasticity (both dependent of the concentration of a high-pressure phase), and plastic strain-induced and pressure-dependent PT kinetics are utilized. A computational algorithm within finite element method (FEM) is presented and implemented in a user material subroutine (UMAT) in the FEM code ABAQUS. Combined plastic flow and strain-induced PT from the highly-disordered hexagonal boron nitride (hBN) sample to a superhard wurtzitic wBN is simulated within the rhenium gasket for pressures up to 50 GPa. The evolution of the fields of stresses and plastic strains, as well as the concentration of phases in a sample is obtained and discussed in detail. Stress-strain fields in a gasket and diamond are presented as well. An unexpected shape of the deformed sample with almost complete PT in the external part of the sample that penetrated the gasket was found. Obtained results demonstrated the difference between material and system behavior which are often confused by experimentalists. Thus, while plastic strain-induced PT may start (and end) at plastic straining slightly above 6.7 GPa, it is not visible below 12 GPa. It becomes detectable at 21 GPa and is not completed everywhere in a sample even at a maximum pressure of 50 GPa. Due to a strong gasket the gradient of pressure is much smaller than the gradient of plastic strain, and therefore the distribution of the high pressure phase is mostly determined by the plastic strain field instead of the pressure field. Possible misinterpretation of the experimental data and characterization of the PT is discussed. The developed model will allow computational design of experiments for synthesis of high-pressure phases.

High-Pressure Phase Transformations in TiPO4 : A Route to Pentacoordinated Phosphorus.

Titanium(III) phosphate, TiPO4 , is a typical example of an oxyphosphorus compound containing covalent P-O bonds. Single-crystal X-ray diffraction studies of TiPO4 reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high-pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO4 -V is the first inorganic phosphorus-containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO4 -V coexists with TiPO4 -IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO4 -II is found to be an incommensurately modulated phase whereas a lock-in transition at about 7 GPa leads to TiPO4 -III with a fourfold superstructure compared to the structure of TiPO4 -I at ambient conditions. TiPO4 -II and TiPO4 -III are similar to the corresponding low-temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin-Peierls interactions.

Plastic strain and grain size effect on high-pressure phase transformations in nanostructured TiO2 ceramics

Anatase TiO2 was severely deformed using high-pressure torsion and the effect of plastic strain and grain size on phase transformations was investigated. A high-pressure TiO2-II phase (columbite) with the orthorhombic structure was formed under pressures of 1 and 6GPa. Fraction of TiO2-II increased with increasing the plastic strain and remained stable at ambient pressure. Microstructural analysis showed that TiO2-II was stabilized in grains with sizes less than ~15nm because of high energy barrier for reverse phase transformation, while larger grains had the anatase structure. Large densities of oxygen vacancies and dislocations were also formed after severe plastic deformation.

Fluorine partitioning between eclogitic garnet, clinopyroxene, and melt at upper mantle conditions

In this experimental study we obtained new mineral/melt (DF=cmineral/cmelt) partitioning data for fluorine in a bimineralic hydrous eclogite under Earth's upper mantle conditions (4–6GPa, 1460–1550°C). Omphacitic clinopyroxene displays mineral/melt partition coefficients between DF=0.056±0.005 and DF=0.074±0.001. Garnet partition coefficients are consistently lower with an average partition coefficient of DF=0.016±0.003. We found that omphacitic clinopyroxene is the dominant nominally fluorine-free phase in subducted oceanic crust and hence omphacite is expected to be the major fluorine carrier during subduction of crust into the deeper mantle. Together with previously obtained partitioning data we propose that the oceanic crust can host more fluorine per mass unit than the underlying depleted oceanic mantle.If the majority of entrained fluorine is recycled into Earth's transition zone it is possible that the fluorine is either incorporated into high-pressure transition zone phases or released during high-pressure phase transformations and forming fluorine-rich small degree partial melts. Both scenarios are supported by elevated fluorine concentration in ocean island basalts, kimberlites, and lamproites.Combining the fluorine partitioning data with water partitioning data yields a plausible process to generate lamproitic magmas with a high F/H2O ratio. The enrichment of fluorine relative to H2O is triggered by multiple episodes of small degree melting that deplete the residual more in H2O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H2O.

Volumetric and timescale analysis of phase transformation in single-crystal silicon during nanoindentation

Clarifying the phase transformation process and mechanism of single-crystal silicon induced by high pressure is essential for preparation of new silicon phases. Although many previous researches have focused in this area, the volume of high-pressure phases and the duration of phase transformation are still unclear. In this paper, the volume change and the duration of phase transformation from Si-II phase into Si-XII/Si-III phases were investigated quantitatively by introducing a holding process in the unloading stage of a nanoindentation test. Experimental results indicate that the high-pressure phase volume is dependent strongly on the maximum indentation load and independent of the loading/unloading rate and the holding time at the maximum indentation load, while phase transformation duration is independent of the aforementioned experimental parameters. By analyzing the results, a critical volume of Si-XII/Si-III phases was identified which determines the occurrence of sudden phase transformation, and a modified nucleation and growth mechanism of high-pressure phases was proposed. These results provide new insights into high-pressure phase transformations in single-crystal silicon.

Electronic phases of substances. Phase transitions with change of electron and crystalline structure

There is plenty of experimental data on high-pressure phase transformations in various materials. Variations in materials characteristics (for example, equilibrium density and bulk modulus), while the crystalline structure remains unchanged, are indicative of energy variations in outer-shell electrons of solid atoms. In experiments with crystalline structure variations, the dependence of pressure on density in some cases can be described by the same curve, the parameters of which are independent of the crystalline structure. Examples of such transformations in some materials at static compression and in shock-wave experiments are given.

Phase transformations during high-pressure torsion (HPT) in titanium, cobalt and graphite

Several elements were processed by high-pressure torsion and phase transformations were investigated. Titanium exhibited a transition from hcp to an ω phase at pressures higher than 4 GPa, while the transition was facilitated with increasing the shear strain. The stability of ω phase appeared to decrease with decreasing the grain size to the nanometer level (e.g., by processing at cryogenic temperatures and/or by consolidation of powders). Cobalt exhibited a transition from metastable fcc to hcp until the grain size reached the submicrometer level, but the fcc phase appeared to be more stable with decreasing the grain size to the nanometer level. Graphite exhibited a transition to diamond-like carbon (DLC), while the formation of DLC was facilitated with increasing the pressure, temperature and strain.

Puzzling calcite-III dimorphism: crystallography, high-pressure behavior, and pathway of single-crystal transitions

High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO3 were investigated by analytical in situ high-pressure high-temperature experiments on oriented single-crystal samples. All experiments at non-ambient conditions were carried out by means of Raman scattering, X-ray, and synchrotron diffraction techniques using diamond-anvil cells in the pressure range up to 6.5 GPa. The composite-gasket resistive heating technique was applied for all high-pressure investigations at temperatures up to 550 K. High-pressure Raman spectra reveal distinguishable characteristic spectral differences located in the wave number range of external modes with the occurrence of band splitting and shoulders due to subtle symmetry changes. Constraints from in situ observations suggest a stability field of CaCO3-IIIb at relatively low temperatures adjacent to the calcite-II field. Isothermal compression of calcite provides the sequence from I to II, IIIb, and finally, III, with all transformations showing volume discontinuities. Re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to determine the transition sequence. Clausius–Clapeyron slopes of the phase boundary lines were determined as: ΔP/ΔT = −2.79 ± 0.28 × 10−3 GPa K−1 (I–II); +1.87 ± 0.31 × 10−3 GPa K−1 (II/III); +4.01 ± 0.5 × 10−3 GPa K−1 (II/IIIb); −33.9 ± 0.4 × 10−3 GPa K−1 (IIIb/III). The triple point between phases II, IIIb, and III was determined by intersection and is located at 2.01(7) GPa/338(5) K. The pathway of transition from I over II to IIIb can be interpreted by displacement with small shear involved (by 2.9° on I/II and by 8.2° on II/IIIb). The former triad of calcite-I corresponds to the [20-1] direction in the P21/c unit cell of phase II and to [101] in the pseudomonoclinic C $${\bar{1}}$$ setting of phase IIIb. Crystal structure investigations of triclinic CaCO3-III at non-ambient pressure–temperature conditions confirm the reported structure, and the small changes associated with the variation in P and T explain the broad stability of this structure with respect to variations in P and T. PVT equation of state parameters was determined from experimental data points in the range of 2.20–6.50 GPa at 298–405 K providing $$K_{{{\text{T}}_{0} }}$$ = 87.5(5.1) GPa, (δK T/δT) P = −0.21(0.23) GPa K−1, α 0 = 0.8(21.4) × 10−5 K−1, and α 1 = 1.0(3.7) × 10−7 K−1 using a second-order Birch–Murnaghan equation of state formalism.