Search

Abstract We report pressure-induced structural instability and amorphization in compressed orthorhombic α -V 2 O 5 by in situ synchrotron x-ray diffraction observations and Raman spectroscopy measurements in diamond-anvil cells up to 20.2 GPa and 21.9 GPa, respectively. In situ x-ray lattice parameters (a , b , c) and d -spacings at high pressure show that the orthorhombic α -V 2 O 5 exhibits an apparent discontinuity at ∼4.3 GPa, indicating the occurrence of structural distortion upon compression. Upon further pressurization, the distorted α -V 2 O 5 starts to be amorphizing at ∼12.6 GPa and becomes fully amorphous at ∼20.2 GPa, which is again confirmed by our high-pressure Raman spectroscopy measurements. This obtained amorphous V 2 O 5 polymorph can be recovered at ambient conditions, showing that the pressure-driven distorted orthorhombic-to-amorphous phase transformation is irreversible. Moreover, Grüneisen parameters of various Raman modes for α -V 2 O 5 are derived, which are in agreement with the previously experiential study by Arora et al. Our results might be important to understand the high-pressure behaviors in M 2 O 5 polymorphs for their uses at extreme conditions. Highlights • Exploration of phase instability and structural evolution in α-V 2 O 5 upon compression. • Preparation of the amorphous V 2 O 5 polymorph at high pressure. • Pressure-induced amorphization phase transition in V 2 O 5 is irreversible. • Understanding of high-pressure behaviors in M 2 O 5 polymorphs for their uses at extreme conditions. [ABSTRACT FROM AUTHOR]

Compressional and shear wave velocities of polycrystalline niobium have been measured at simultaneously high pressures and temperatures up to 5.8 GPa and 1073 K, respectively, using ultrasonic interferometry in conjunction with synchrotron x-ray techniques. An anomalous pressure-induced softening behavior in the phonon velocities, probably owing to the topological change in the Fermi surface, has been observed at ~4.8 GPa during cold compression, which is supported by the elasticity data from our first-principles calculations. In contrast, both the bulk (BS) and shear (G) moduli increase with pressures but decrease with temperatures upon compression at extreme P-T up to 5.8 GPa and 1073 K. Using finite strain equation-of-state approaches, the elasticity of bulk and shear moduli and their pressure and temperature dependences are derived from the directly measured velocities and densities, yielding BS0 = 174.9(3.2) GPa, G0 = 37.1(3) GPa, ∂BS/∂P = 3.97(9), ∂G/∂P = 0.83(5), ∂BS/∂T = -0.064(7) GPa/K, and ∂G/∂T = -0.012(3) GPa/K. On the basis of the current thermoelasticity data, Debye temperature and the high-pressure melting curve of Nb are derived. The origin of the anomalies in shear behavior at high pressure might be attributed to the progressive s-d electron-transfer-induced topological changes of the Fermi surface upon compression. [ABSTRACT FROM AUTHOR]

The equation of state of aragonite was determined by in situ synchrotron X-ray diffraction experiments on a hot-pressed polycrystalline specimen of aragonite within its stability field up to 5.8 GPa and 1173 K. As a complement to this experimental study, first-principle density functional theory calculations were performed up to 20 GPa at 0 K. Thermoelastic parameters for aragonite (CaCO3) were derived by a least-squares fit of the experimental P-V-T data to the third-order Birch-Murnaghan equation of state (EOS), yielding the bulk modulus and its pressure and temperature derivatives K0 = 65.24 ± 0.24 GPa, Kʹ0 = 4.95 ± 0.12, (∂KT/∂T)P = -0.024 ± 0.002 GPa/K and volume thermal expansion α300 = (6.1 ± 0.7) × 10-5 K-1. The analyses of the axial compressibility at ambient temperature show that the c-axis is much more compressible than the a- and b-axes. Based on first-principle calculations, the anisotropic compression behavior of aragonite structure is explained by the heterogeneous shortening of and bond lengths and the rotation of angles along the a-, b-, and c-axes, whereas the unit-cell volume change of aragonite under compression is accommodated by comparable compression rate of the CaO9 polyhedra and the voids in the crystal lattice. The results attained from this study provide important thermoelastic parameters for understanding the thermodynamic behavior and chemical reactions involving aragonite at subduction zone conditions. [ABSTRACT FROM AUTHOR]

Thermoelastic properties of synthetic MgAlSiO pyrope garnet have been measured at high pressure and high temperature by using in situ energy-dispersive X-ray diffraction, using a Kawai-type multi-anvil apparatus. Measurements have been conducted up to 19 GPa and 1,700 K, equivalent to the P- T conditions of the middle part of mantle transition zone. Analyses of the room-temperature P- V data to a third-order Birch-Murnaghan (BM) equation of state (EoS) yields: V = 1,500 ± 1 Å, K = 167 ± 6 GPa and $$ K_{{_{0} }}^{'} $$ = 4.6 ± 0.3. When fitting the entire P- V- T data using a high-temperature Birch-Murnaghan (HTBM) EoS at a fixed $$ K_{{_{T0} }}^{'} $$ = 4.6, we obtain V = 1,500 ± 2 Å, K = 167 ± 3 GPa, (∂ K/∂ T) = −0.021 ± 0.009 GPa K and α = (2.89 ± 0.33) × 10 K. Fitting the present data to the Mie-Grüneisen-Debye (MGD) EoS with Debye temperature Θ = 806 K gives γ = 1.19 and 1.15 at fixed q = 1.0 and 1.5, respectively. Comparison of these fittings with two different approaches, we propose to constrain the bulk modulus and its pressure derivative to K = 167 GPa and $$ K_{{_{0} }}^{'} $$ = 4.4-4.6, as well as the Grüneisen parameter to γ = 1.15-1.19. [ABSTRACT FROM AUTHOR]

Abstract: Nanosintering behaviour of MgAl2O4 ultra-fine powder was investigated under high pressure. We found that the nanosintering mechanism under high pressure was quite different from that under ambient pressure. To shed light on the mechanism, the residual stress, grain size, and boundary situation were investigated using HRTEM and XRD, respectively. Our results show high pressure can restrain grain growth and initiate plastic deformation to eliminate pores and/or additional phases existing in triple junctions of the grains. However, the conventional sintering process is controlled by grain growth to avoid imperfections between grain boundaries when preparing transparent ceramics. [Copyright &y& Elsevier]

For the first time, the acoustic velocities and elasticity of double hexagonal close-packed neodymium (Nd) were measured at high pressure using ultrasonic interferometry in conjunction with synchrotron X-ray techniques. Both the compressional and shear wave velocities increased with an increase in pressure of up to ~5.4 GPa before they exhibited almost constant values upon further compression. This pressure-induced elastic wave velocity softening can be attributed to the structural instability in dhcp -Nd. From the current sound velocities and density data, we obtained new bulk and shear moduli as well as their pressure dependence for polycrystalline dhcp -Nd, yielding B 0 = 33.2(9) GPa, G 0 = 14.1(5) GPa, ∂ B / ∂ P = 3.6 (4) , and ∂ G / ∂ P = 1.3 (2) , while other elasticity-related/mechanical properties, including Young's modulus, Debye temperature, Poisson's ratio, and Grüneisen parameter, which are important parameters for the application of Nd in extreme environments, were also derived. Image 1 • Measuring sound velocities and elasticity of dhcp- Nd at high pressure by ultrasonic measurements and in situ x-ray techniques. • Exploring pressure-induced elastic softening in compressed polycrystalline dhcp -Nd. • Reporting new data on the acoustic velocities, elastic and mechanical properties of dhcp -Nd at high pressure. [ABSTRACT FROM AUTHOR]

Acoustic velocities and elasticity of high P-T synthesized new (VNbTaTi)C high-entropy carbides (HECs) are reported at high pressure using large volume press (LVP)-based ultrasonic interferometry technique. Using the finite-strain equation approach, the bulk modulus and shear rigidity, and their pressure dependences, are derived from the current experimental data, yielding B 0 = 261.6(2) GPa, G 0 = 191.4(3) GPa, ∂B/∂P = 4.34(18), and ∂G/∂P = 1.83(6). Interestingly, the (VNbTaTi)C HECs is more incompressible than the superhard γ-B (∼213.9 GPa), and almost as incompressible as superhard PcBN (∼254 GPa). Meanwhile, its shear rigidity (∼191.4 GPa) is comparable to the superhard B 6 O-B 4 C composite (G = 202 GPa), and as stiff as the (TiZrNbTa)C (193 ∼ 195 GPa) HECs. More strikingly, the fracture toughness is enhanced up to ∼4.6(3) MPa·m1/2, and the Vickers hardness of (VNbTaTi)C HECs is ∼22.6(2) GPa, which are significantly stronger than other high-entropy carbides. These new findings and results may be very important for its applications in extreme environments. [ABSTRACT FROM AUTHOR]

Elastic wave velocities of synthetic polycrystalline Mg3Al2(SiO4)3 garnet have been successfully measured to 20 GPa and temperatures up to 1700 K by ultrasonic interferometry combined with energy-dispersive synchrotron x-ray diffraction in a Kawai-type multi-anvil apparatus. Compressional (Vp) and shear (Vs) wave velocities as well as the adiabatic bulk (Ks) and shear (G) moduli exhibit monotonic increase with increasing pressure and decrease with increasing temperature, respectively. Two-dimensional (P-T) linear fittings of the present data yield the following parameters: KS0 = 170.0(2) GPa, [schwa]Ks/[schwa]P = 4.51(2), [schwa]Ks/[schwa]T = -0.0170(1) GPa/K, G0 = 93.2(1) GPa, [schwa]G/[schwa]P = 1.51(2), and [schwa]G/[schwa]T = -0.0107(1) GPa/K, which is in good agreement with the earlier results by Brillouin scattering and/or ultrasonic measurements at relatively low P-T conditions. The observed linear pressure and temperature dependence in both Vp and Vs is in contrast to the non-linear behavior of Vp and Vs for majorite garnet with the pyrolite composition, in particular for Vs. [ABSTRACT FROM AUTHOR]

Exploring the structural stability and elasticity of hexagonal ε-NbN helps discover correlations among its physical properties for scientific and technological applications. Here, for the first time, we measured the ultra-incompressibility and high shear rigidity of polycrystalline hexagonal ε-NbN using ultrasonic interferometry and in situ X-ray diffraction, complemented with first-principles density-functional theory calculations up to 30 GPa in pressure. Using a finite strain equation of state approach, the elastic bulk and shear moduli, as well as their pressure dependences are derived from the measured velocities and densities, yielding BS0 = 373.3(15) GPa, G0 = 200.5(8) GPa, ∂BS/∂P = 3.81(3) and ∂G/∂P = 1.67(1). The hexagonal ε-NbN possesses a very high bulk modulus, rivaling that of superhard material cBN (B0 = 381.1 GPa). The high shear rigidity is comparable to that for superhard γ-B (G0 = 227.2 GPa). We found that the crystal structure of transition-metal nitrides and the outmost electrons of the corresponding metals may dominate their pressure dependences in bulk and shear moduli. In addition, the elastic moduli, Vickers hardness, Debye temperature, melting temperature and a possible superconductivity of hexagonal ε-NbN all increase with pressures, suggesting its exceptional suitability for applications under extreme conditions. [ABSTRACT FROM AUTHOR]

The compressional and shear wave velocities for coesite have been measured simultaneously up to 5.8 GPa and 1073 K by ultrasonic interferometry for the first time. The shear wave velocity decreases with pressure along all isotherms. The resulting contrasts between coesite and stishovite reach ~34% and ~45% for P and S wave velocities, respectively, and ~64% and ~75% for their impedance at mantle conditions. The large velocity and impedance contrasts across coesite-stishovite transition imply that to generate the velocity and impedance contrasts observed at the X-discontinuity, only a small amount of silica would be required. The velocity jump dependences on silica, d(ln V P)/ d(SiO2) = 0.38 (wt %)−1 and d(ln V S)/ d(SiO2) = 0.52 (wt %)−1, are utilized to place constraints on the amount of silica in the upper mantle and provide a geophysical approach to track mantle eclogite materials and ancient subducted oceanic slabs. [ABSTRACT FROM AUTHOR]

Hafnium carbide (HfC) and tantalum carbide (TaC), as potential ultra-high temperature ceramics (UHTCs), have attracted widespread attention due to their unique physical and chemical properties. Here, dense HfC and TaC ceramics are synthesized under high pressure and high temperature 15 GPa and 1700 °C, respectively. The sound velocities and elasticity behavior of sintered HfC and TaC under high pressure have been experimentally investigated by using ultrasonic interferometry and validated by first-principles study. The bulk and shear moduli with their pressure dependence have been determined as B 0 = 272.6(2.7) GPa, G 0 = 215.8(2.2) GPa, ∂ B / ∂ P = 3.44 (5) , and ∂ G / ∂ P = 1.74 (3) for HfC, B 0 = 355.9(3.6) GPa, G 0 = 236.6(2.4) GPa, ∂ B / ∂ P = 3.98 (5) , and ∂ G / ∂ P = 1.65 (2) for TaC. The Vickers hardness and fracture toughness of HfC and TaC are generated from empirical models. The present work provides deep insights into the engineering application of UHTCs HfC and TaC under high-pressure extreme conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] Dislocations have been thought to generate significant effects on the performance of GaN-based devices. The intensive studies have usually focused on threading dislocations on the prismatic or pyramidal planes of wurtzite GaN. Here, we alternatively investigate the partial dislocations on the basal plane of GaN. By a combination of the fully discrete Peierls theory and first-principles calculations, the core structures and electronic properties of 90°/30° partial dislocations with glide-sets have been obtained. From the analytical displacement fields, we show that the single-period and double-period reconstructions of the partial dislocations are present. Importantly, due to the intrinsic polarity of dislocation cores, the introduction of excess charge is uncovered to strongly affect the reconstructions, exhibiting unique charge-induced structural transitions. Furthermore, the electronic density of states indicate that the stable reconstructed structures possess the defect levels inside the bulk gaps, which are localized along the dislocation lines at core regions. Our work not only extends the understanding of dislocations and their related properties in GaN but also offers fascinating guidance for designing GaN-based devices. [ABSTRACT FROM AUTHOR]

Abstract: This work presents a comprehensive study on phase transitions in LiAlO2 system at high pressures and temperatures (0.5–5.0GPa and 300–1873K, respectively), as well as the phase stability for polymeric phases of LiAlO2 in the studied P–T space by X-ray diffraction (XRD). Besides the previously described polymorphic hexagonal α-phase, orthorhombic β-phase and tetragonal δ-phase, a possible new phase of LiAlO2 was observed after the tetragonal γ-LiAlO2 sample was treated at 5.0GPa and 389K. The stable regimes of these high-pressure phases were defined through the observation of coexistence points of the polymeric phases. Our results revealed that LiAlO2 could experience structural phase transitions from γ-LiAlO2 to its polymorphs at lower pressures and temperatures compared to the reported results. Hexagonal α-LiAlO2 with highly (003) preferential orientation was prepared at 5.0GPa and 1873K. [Copyright &y& Elsevier]

• Sound velocities and elasticity of submicron h -AlN are explored at high pressure using LVP-based ultrasonic interferometry technique. • Mechanical properties of submicron h -AlN are measured, as compared with the micro-grained counterparts. • Elastic-related properties of acoustic thermal conductivity for submicrion h -AlN, for the first time, was reported. Compressional (V P) and shear (V S) wave velocities of high pressure (P) and temperature (T) sintered submicron hexagonal aluminum nitride (h -AlN) have been measured up to 15 GPa using ultrasonic interferometry. Using finite strain equation-of-state approaches, yielding K S0 = 293.1 ± 2.9 GPa, G 0 = 141.9 ± 1.4 GPa and K′ S = 3.72 ± 0.04, G′ = 0.52 ± 0.02. It is found that the Vickers hardness of submicron h -AlN is as high as 11.7 GPa, and its fracture toughness is up to ∼4.25 MPa m1/2. Meanwhile, based on the elastic wave velocities and density data, the elastic-related properties of h -AlN are derived as Debye temperature Θ 0 = 1006 K, Grüneisen parameter γ = 1.72 and acoustic thermal conductivity λ acoustic = 196 W/mK. These results may provide a comprehensive elasticity and elastic-related properties of submicron h -AlN under high pressure, which may be of great importance for its uses at extreme conditions. [ABSTRACT FROM AUTHOR]

LaGdZr 2 O 7 transparent ceramics were fabricated by the vacuum sintering method using the precursor made from coprecipitation synthesis. The effects of calcination temperature on cation ordering and the morphology of LaGdZr 2 O 7 powders, as well as the microstructure and transmittance of the LaGdZr 2 O 7 ceramics, were investigated. As the calcination temperature for LaGdZr 2 O 7 powder preparation increased from 1000 °C to 1300 °C, the domain and particle size increased, and the cations [(La 1/2 Gd 1/2 ) 3+ -Zr 4+ ] became more ordered. The LaGdZr 2 O 7 powders calcined from 1150 °C to 1200 °C showed weak agglomeration and gave the best sintering manner for fabricating transparent ceramics. LaGdZr 2 O 7 transparent ceramics with an in-line transmittance of 73–75% (at 1200 nm) were obtained from the LaGdZr 2 O 7 powder sintered at 1800 °C in vacuum for 10 h. The scattering of pores should be responsible for the low transmittance of LaGdZr 2 O 7 ceramics in the current study. [ABSTRACT FROM AUTHOR]

Compressional and shear wave velocities of coesite have been measured using ultrasonic interferometry in a multi-anvil apparatus up to 12.6 GPa at room temperature for the first time. While the P wave velocity increases continuously with pressure, the S wave exhibits an anomalous softening and the velocity decreases continuously with pressure. Finite strain analysis of the data yielded K S 0 = 103.6 ( 4 ) GPa , G 0 = 61.6 ( 2 ) GPa and K 0 ′ = 2.9 ( 1 ) , G 0 ′ = 0.3 ( 1 ) for the bulk and shear moduli and their pressure derivatives, respectively. The anomalous elastic behavior of coesite results in large velocity and impedance contrasts across the coesite–stishovite transition, reaching ∼39% and ∼48% for P and S wave velocity contrasts, and ∼70% and 78% for P and S wave impedance contrasts, respectively, at pressure ∼8 GPa, with P and S wave velocity perturbations showing no apparent dependence on depths (i.e., d ln ⁡ V ( P or S ) / d h ∼ 0 ) within 8–12 GPa. These unusually large contrasts and depth independent characteristics render the transition between the two silica polymorphs one of the most plausible candidates for the cause of the seismically observed X-discontinuity. The current P and S wave velocity perturbation dependences on the SiO 2 content, d ( ln ⁡ V P ) / d ( SiO 2 ) ∼ 0.43 (wt%) − 1 and d ( ln ⁡ V S ) / d ( SiO 2 ) ∼ 0.60 (wt%) − 1 , can serve as a geophysical probe to track ancient subducted eclogite materials to gain insights on the geodynamics of the mantle. [ABSTRACT FROM AUTHOR]

Abstract: In general, the high-density state is a preferred state of materials under high pressures. In this study, however, when the LiBO2–Al2O3 system quenched from melts at the cooling rate of 50–60 °C/s under high pressures, low density - Al2O3 was synthesized from high density - Al2O3, with a 7.1% volume expansion of Al2O3. While those samples which are quenched at the same cooling rate under a lower pressure appeared to be glassy. The minimal pressure synthesized the low-density - Al2O3 or the maximal glass forming pressure was decreased with increasing content of Al2O3 below its saturation point in LiBO2 melt. [ABSTRACT FROM AUTHOR]