Pressure-induced Structural Phase Transformation Research Articles

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 Structural Phase Transformation and Yield Strength of AlN

Aluminum nitride, a significant ceramic material used in electronic technological applications, was investigated by in situ synchrotron radiation X-ray diffraction in a diamond anvil cell at ambient temperature. The starting sample of AlN powder with a 1–2 μm average grain size was compressed to 36.9 GPa under the hydrostatic condition and 36.2 GPa under the nonhydrostatic condition, respectively. The phase transformation from hexagonal wurtzite to cubic rock salt (B4-to-B1) for the hydrostatic condition 20.5 and 20.4 GPa for the nonhydrostatic condition. We found that the phase transition was irreversible and that pure cubic phase AlN was obtained after pressure relief to reestablish ambient pressure. The experimental results reveal that the bulk modulus of hexagonal AlN is B0 = 287.1 GPa at a fixed B0′ = 4 under hydrostatic compression. For the recompressed pure cubic sample in the diamond anvil, the equation of state (EOS) of cubic AlN is B0 = 357.0 GPa when fit from ambient pressure to 39.5 GPa. The h...

Pressure-induced structural phase transformation of yttrium diborocarbide at high pressure

Using an efficient structure search method combined with first-principles calculations, the structural behavior of YB2C2 has been extensively investigated in a wide pressure range of 0–200 GPa. A structural phase transition from the ambient tetragonal phase to a new high-pressure tetragonal phase was uncovered above 110 GPa. By total-energy calculations, this pressure-induced structural phase transition was determined to be first order with a volume drop of 9.1%. Inspections of crystal and electronic structures indicated that this high-pressure phase exhibits a 3D sp3 B-C network built up from the sharply distortions and reconstructions of the 2D B-C layers in the ambient-pressure phase under compression, leading to an improved mechanical ductility and weakened elastic anisotropy.

Pressure-induced structural phase transformation and superconducting properties of titanium mononitride

In this work, we have systematically performed the first-principles structure search on titanium mononitride (TiN) within Crystal Structure AnaLYsis by Particle Swarm Optimization (CALYPSO) methodology at high pressures. Here, we have confirmed a phase transition from cubic rock-salt (fcc) phase to CsCl (bcc) phase of TiN at ∼348 GPa. Further simulations reveal that the bcc phase is dynamically stable, and could be synthesized experimentally in principle. The calculated elastic anisotropy decreases with the phase transformation from fcc to bcc structure under high pressures, and the material changes from ductile to brittle simultaneously. Moreover, we found that both structures are superconductive with the superconducting critical temperature of 2–12 K.

Pressure-induced structural phase transformation in cobalt(II) dicyanamide.

In situ synchrotron powder diffraction has been used to probe the pressure-dependent structural properties of the magnetic molecular framework material Co(dca)2 [dca = dicyanamide or N(CN)2(-)]. An orthorhombic (Pmnn) to monoclinic (P2₁/n) transformation to a high-pressure phase, namely γ-Co(dca)2, occurs at 1.1 GPa. Structural determination of γ-Co(dca)2 shows that the rutile-like topology of the pristine material is retained at high pressures, with the lower symmetry allowing a progression of volume-reducing structural distortions. γ-Co(dca)2 was stable at the maximum pressure measured of 4.2 GPa. Both phases were soft, with bulk moduli (B0) for α-Co(dca)2 and γ-Co(dca)2 of 13.15 (18) and 9.0 (6) GPa, respectively. Modest uniaxial negative linear compressibility (K) of the order of -4 TPa(-1) was observed over the entire measured pressure range.

Pressure induced structural phase transformation in TiN: A first-principles study

Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at ∼7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-à-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally.

Pressure induced increase in Tc for the organic-based magnet FeII(TCNE)2 (TCNE = tetracyanoethylene)

Pressure dependent magnetization and 57Fe Mössbauer studies were performed on Fe(TCNE)[C4(CN)8]1/2·zCH2Cl2 (TCNE=tetracyanoethylene). Pressure did not influence the Mössbauer parameters in paramagnetic state. Mössbauer data reveals the onset of magnetic ordering at 130K and significant enhancement of the magnetic ordering temperature from 100 to 150K accompanied by an increase of the spontaneous magnetization, which is higher than reported from the magnetic data, and application of pressure induces the reversible formation of a new, metastable magnetic species. These changes suggest an increase of the dimensionality of magnetic interaction, i.e., stronger interlayer coupling. Application of 9.3kbar increases Tc to 160K, and the moderate dTc/dP of ∼1K/kbar in the pressure-induced metastable state suggests a pressure-induced structural phase transformation that increases the number of next–nearest neighbors of iron.

High pressure behavior of AlAs nanocrystals: the first-principle study

In this study, the first-principle density functional approach has been used to analyze the pressure-induced structural stability and phase transformation in AlAs nanocrystals. This study includes the stability analysis of AlAs nanocrystals in their B4-, B3-, B1- and B2-type phases, and we observed that the B3-type phase is the most stable. We also observed the structural transformations in AlAs nanocrystals from B3→B1 at around 8.9 GPa, B3→B2 at 7.12 GPa and B3→B4 at 3.88 GPa. The stability of the materials has been analyzed using local density approximation with the Perdew–Zunger parameterization and also with the Perdew–Burke–Ernzerhof (PBE) and revised PBE parameterizations of the generalized gradient approximation potential.

High pressure powder X-ray diffraction study of Cr2As and pressure-induced structural phase transition

Cr2As, which is an intermetallic compound of Cu2Sb-type with Strukturbericht designation C38 was studied under a pressure of up to 32.5 GPa at room temperature using in situ synchrotron X-ray powder diffraction with a diamond anvil cell. From the X-ray diffraction analysis, our results showed that the Cu2Sb (C38)-type phase of Cr2As undergoes a pressure-induced structural phase transition near 15.4 GPa. The high-pressure (HP) phase of Cr2As is suggested as an orthorhombic structure. No additional structural phase transition was observed up to 32.5 GPa, and the initial low-pressure (LP) Cu2Sb (C38)-type structure was recovered as the pressure was released, implying that the observed pressure-induced structural phase transformation was reversible. The pressure–volume data of Cr2As was fitted to a second-order Birch–Murnaghan equation of state, which yielded a bulk modulus of B0=125(3) and 340 (12) GPa for the LP and the HP phases, respectively. Furthermore, the a axis is more compressible than the c axis for the LP phase of Cr2As. The anisotropic compressibility of the studied crystal is discussed in terms of the crystallography stacking.

Nanoindentation of semiconductors: experiment and atomistic simulations

With recent developments in advanced materials and nanostructures, particularly those designed for electronics, it is evident that their successful application will depend not only on their electrical properties, but also on their mechanical characteristics. Nanoindentation is a unique method for examining nanostructured materials, as it requires a small volume of the solid and probes the surface layers of particular interest. Nanoindentation of bulk semiconductor crystal structure has been frequently used to study the onset of irreversible deformation – incipient plasticity. Here we present recent experiments supported by molecular dynamics simulations that allow determination of the origin of incipient plasticity in GaAs crystals. It will be demonstrated that, as in case of silicon, plastic deformation of GaAs starts from a pressure-induced structural phase transformation.

Phase stability, electronic and elastic properties of ScN

The structural stability, electronic and elastic properties of ScN under high pressure were investigated by using full-potential linearized augmented plane-wave (FP-LAPW) method. Our calculations indicate that there exists a pressure-induced structural phase transformation from the ambient rock-salt (B1) phase to CsCl-type (B2) phase above 335 GPa. The origin of moderate high bulk modulus, mechanical and high melting point was analyzed by electronic properties. The variations of elastic constants ( C ij ) with volume and pressure have been investigated. Moreover, the volume dependencies of Debye temperature, the longitudinal and transverse elastic wave velocities have been presented.

High-pressure stability and compressibility ofAPO4(A=La, Nd, Eu, Gd, Er, and Y) orthophosphates: An x-ray diffraction study using synchrotron radiation

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type YPO4 and ErPO4, and monazite-type GdPO4, EuPO4, NdPO4, and LaPO4 were performed in a diamond-anvil cell up to 27 GPa using neon as pressure-transmitting medium. In the zircon-structured oxides we found evidence of a reversible pressure-induced structural phase transformation from zircon to a monazite-type structure. The onset of the transition is near 17-20 GPa. In LaPO4 a non-reversible transition is found around 26 GPa, being a barite-type structure proposed for the high-pressure phase. In the other three monazites, this structure is found to be stable up to highest pressure reached in the experiments. No additional phase transitions or evidences of chemical decomposition are found in the experiments. The equations of state and axial compressibility for the different phases are also determined. In particular, we found that in a given compound the monazite structure is less compressible than zircon structure, being this fact related to the larger packing efficiency of monazite compared with zircon. The differential bond compressibility of different polyhedra is also reported and related the anisotropic compressibility of both structures. Finally, the sequence of structural transitions and compressibilities are discussed in comparison with other orhtophosphates.

Structural stability and optical properties of AlN explored by ab initio calculations

Pressure-induced structural phase transformations, electronic and optical properties of AlN are investigated by first-principles method based on the plane-wave basis set. The wurtzite (B4), zinc-blende (B3), rocksalt (B1), b- β - Sn, NiAs, anti-NiAs, cinnabar, and simple cubic with 16-atom basis (SC16) phases of AlN have been considered. The calculations demonstrate that there exists a phase transition from B4 structure to B1 phase at the transition pressure of 12.7 GPa. Analysis of band structures suggests that the B4-AlN has a direct gap of 4.13 eV, while B1 phase become indirect under high pressure. The mechanism of these changes of band structures is analyzed. The positive pressure derivative of band gap energies for B1 phase might be due to the absence of d occupations in the valence bands. In addition, the imaginary parts of dielectric function for the polarization in the xy plane and average of the imaginary parts of dielectric function over three Cartesian directions were calculated. The origin of the spectral peaks was interpreted based on the electronic structure.

High-pressure structural investigation of several zircon-type orthovanadates

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type EuVO4, LuVO4, and ScVO4 were performed up to 27 GPa. In the three compounds we found evidence of a pressure-induced structural phase transformation from zircon to a scheelite-type structure. The onset of the transition is near 8 GPa, but the transition is sluggish and the low- and high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO4 and LuVO4 a second transition to a M-fergusonite-type phase was found near 21 GPa. The equations of state for the zircon and scheelite phases are also determined. Among the three studied compounds, we found that ScVO4 is less compressible than EuVO4 and LuVO4, being the most incompressible orthovanadate studied to date. The sequence of structural transitions and compressibilities are discussed in comparison with other zircon-type oxides.

Pressure-induced electronic and structural phase transformation properties in half-metallic PmN: A first-principles approach

Abstract The electronic, structural and magnetic properties of half-metallic promethium nitride (PmN) have been studied using first-principles tight-binding linear muffin-tin orbital (TB-LMTO) method within the local-density approximation (LDA). We calculate the total energy, lattice parameter, bulk modulus, magnetic moment, density of states and energy band structures of half-metallic PmN at ambient as well as at high pressure. Both spin-polarised and non-spin-polarised electronic calculations are performed to check the magnetic stability of PmN. It is found that PmN is ferromagnetically stable. From the total energy calculations it is also found that PmN is stable in the NaCl-type structure under ambient conditions. The structural stability of PmN changes under the application of pressure. We predict a structural phase transformation from NaCl- to CsCl-type structure at the pressure of 3.4 GPa. The high-pressure phase of PmN remains half-metallic and ferromagnetic. The calculated equilibrium lattice parameter and bulk modulus are in good agreement with the experimental and theoretical work.

Pressure-induced structural phase transformation and elastic properties of transition metal mononitrides

The high-pressure structural phase transition in six transition metal mononitrides (TMNs) (M=Ti, Mo, V, Nb, Hf, and Zr), have been studied using a two-body interionic potential theory which includes the effect of Coulomb screening due to the semi-metallic nature of these compounds. The present theoretical results have been compared with the corresponding experimental and predictions of LDA theory. These TMN compounds have been found to undergo NaCl ( B 1) to CsCl ( B 2) phase transition, at a pressure quite high as compared to other binary systems. We have also predicted the elastic constants. It is shown that these binary materials are partially ionic in nature and the structural transformation is analogous to several other ionic binary systems.

Observation of complete regular trivalent rare earth sequence in heavy lanthanide metal holmium under high pressure

Pressure-induced structural phase transformations in heavy rare earth holmium have been studied in a diamond anvil cell using energy dispersive X-ray diffraction at a synchrotron source to pressures of 62 GPa at ambient temperature. The complete sequence of hcp→Sm-type→dhcp→fcc→dfcc (hR24) is observed for the first time with transitions at 10, 19, 54, and 58 GPa, respectively. The fcc phase is not observed as a pure phase. Each phase transition is correlated to predicted changes in d-band occupation at high pressure. As with many other rare earths, the regions of stability of the several phases are observed to overlap, and significant hysteresis is observed. The measured volume compression for Ho is V/V 0=0.526 at 62 GPa.

Pressure-Induced Structural Phase Transformations in Silicon Nanowires

High-pressure structural behavior of silicon nanowires is investigated up to approximately 22 GPa using angle dispersive X-ray diffraction measurements. Silicon nanowires transform from the cubic to the beta-tin phase at 7.5-10.5 GPa, to the Imma phase at approximately 14 GPa, and to the primitive hexagonal structure at approximately 16.2 GPa. On complete release of pressure, it transforms to the metastable R8 phase. The observed sequence of phase transitions is the same as that of bulk silicon. Though the X-ray diffraction experiments do not reveal any size effect, the pressure dependence of Raman modes shows that the behavior of nanowires is in between that of the bulk crystal and porous Si.

Stability of the high-pressure monoclinic phases in Ce and Pr metals: Comparative diffraction study and phenomenological theory

Pressure-induced structural phase transformations in rare-earth metals $\mathrm{Ce}$ and $\mathrm{Pr}$ were studied at different temperatures with an externally heated diamond anvil cell by angle dispersive x-ray diffraction using a synchrotron source. Evidence for the presence of the monoclinic $C2∕m$ $(Z=8)$ phase, $\mathrm{Pr}\text{\ensuremath{-}}\mathrm{VII}$, in the pressure range $10--25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ has been observed. Theoretical models are proposed and the corresponding phase diagrams are worked out. They are compared to the experimental phase diagrams of cerium and praseodymium. The theory explains consistently the contradictory data on the stability of monoclinic and orthorhombic phases of $\mathrm{Ce}$.

Pressure induced phase transitions in Ga1 − x In x P*

We have theoretically investigated the effect of pressure on the structural stability of GaP : InP mixed system. The three-body-potential (TBP) model has been used. The TBP model consists of long-range as well as short-range interactions; the long-range part includes the modified Coulomb force as well as a three-body term; the short-range part in TBP defines the van der Waals and overlap repulsive interactions. We observe a pressure-induced structural phase transformation from ZnS (B3) to NaCl (B1) type phase in Ga 1−x In x P. Our calculated transition pressures for the initial GaP and final InP compound semiconductors are in good agreement with other reported data.