I15-Extreme Conditions
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Diamond Proposal Number(s):
[19776]
Abstract: The vast compositional space of Prussian blue analogues (PBAs), formula AxM[M′(CN)6]y·nH2O, allows for a diverse range of functionality. Yet, the interplay between composition and physical properties—e.g., flexibility and propensity for phase transitions—is still largely unknown, despite its fundamental and industrial relevance. Here we use variable-pressure X-ray and neutron diffraction to explore how key structural features, i.e., defects, hydration, and composition, influence the compressibility and phase behavior of PBAs. Defects enhance the flexibility, manifesting as a remarkably low bulk modulus (B0 ≈ 6 GPa) for defective PBAs. Interstitial water increases B0 and enables a pressure-induced phase transition in defective systems. Conversely, hydration does not alter the compressibility of stoichiometric MnPt(CN)6, but changes the high-pressure phase transitions, suggesting an interplay between low-energy distortions. AMnCo(CN)6 (AI = Rb, Cs) transition from F4̅3m to P4̅n2 upon compression due to octahedral tilting, and the critical pressure can be tuned by the A-site cation. At 1 GPa, the symmetry of Rb0.87Mn[Co(CN)6]0.91 is further lowered to the polar space group Pn by an improper ferroelectric mechanism. These fundamental insights aim to facilitate the rational design of PBAs for applications within a wide range of fields.
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Feb 2021
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I15-Extreme Conditions
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Monika
Gamza
,
Paolo
Abrami
,
Lawrence V. D.
Gammond
,
Jake
Ayres
,
Israel
Osmond
,
Takaki
Muramatsu
,
Robert
Armstrong
,
Hugh
Perryman
,
Dominik
Daisenberger
,
Sitikantha
Das
,
Sven
Friedemann
Diamond Proposal Number(s):
[19319]
Abstract: Cadmium arsenide
(
Cd
3
As
2
)
hosts massless Dirac electrons in its ambient-condition tetragonal phase. We report x-ray diffraction and electrical resistivity measurements of
Cd
3
As
2
upon cycling pressure beyond the critical pressure of the tetragonal phase and back to ambient conditions. We find that, at room temperature, the transition between the low- and high-pressure phases results in large microstrain and reduced crystallite size, both on rising and falling pressure. This leads to nonreversible electronic properties, including self-doping associated with defects and a reduction of the electron mobility by an order of magnitude due to increased scattering. This paper indicates that the structural transformation is sluggish and shows a sizable hysteresis of over 1 GPa. Therefore, we conclude that the transition is first-order reconstructive, with chemical bonds being broken and rearranged in the high-pressure phase. Using the diffraction measurements, we demonstrate that annealing at
∼
200
∘
C
greatly improves the crystallinity of the high-pressure phase. We show that its Bragg peaks can be indexed as a primitive orthorhombic lattice with
a
HP
≈
8.68
Å
,
b
HP
≈
17.15
Å
,
and
c
HP
≈
18.58
Å
. The diffraction study indicates that, during the structural transformation, a new phase with another primitive orthorhombic structure may also be stabilized by deviatoric stress, providing an additional venue for tuning the unconventional electronic states in
Cd
3
As
2
.
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Feb 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[8176, 9366]
Open Access
Abstract: We report results from a series of diamond-anvil-cell synchrotron X-ray diffraction and largevolume- press experiments, and calculations, to investigate the phase diagram of commercial polycrystalline high-strength Ti-6Al-4V alloy in pressure-temperature space. Up to ~30 GPa and 886 K, Ti- 6Al-4V is found to be stable in the hexagonal-close-packed, or alpha phase. The effect of temperature on the volume expansion and compressibility of alpha-Ti-6Al-4V is modest. The martensitic alpha→omega (hexagonal) transition occurs at ~30 GPa, with both phases coexisting until further compression to ~38-40 GPa completes the transition to the omega phase. Between 300 K and 844 K the alpha→omega transition appears to be independent of temperature. Omega-Ti-6Al-4V is stable to ~91 GPa and 844 K, the highest combined pressure and temperature reached in these experiments. Pressure-volume-temperature equations-of-state for the alpha and omega phases of Ti- 6Al-4V are generated and found to be similar to pure Ti. A pronounced hysteresis is observed in the omega-Ti-6Al-4V on decompression, with the hexagonal structure reverting back to the alpha phase at pressures below ~9 GPa at room temperature, and at a higher pressure at elevated temperatures. Based on our data, we estimate the Ti-6Al-4V alpha-beta-omega triple point to occur at ~900 K and 30 GPa, in good agreement with our calculations.
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Jan 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[22477]
Open Access
Abstract: Defects are emerging as a key tool for fine-tuning the stimuli-responsive behavior of coordination polymers and metal–organic frameworks. Here, we study the ramifications of defects on the mechanical properties of the molecular perovskite [C(NH2)3]MnII(HCOO)3 and its defective analogue [C(NH2)3]Fe2/3III□1/3(HCOO)3, where □ = vacancy. Defects reduce the bulk modulus by 30% and give rise to a temperature-driven phase transition not observed in the nondefective system. The results highlight the opportunities that come with defect-engineering approaches to alter the mechanical properties and underlying thermodynamics, with important implications for the research on stimuli-responsive materials.
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Jan 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[16168]
Abstract: Local distortions in perovskite-like A-site-deficient (Sr,La)TiO3 solid solutions have been determined by refining large-scale atomic configurations against neutron/X-ray total-scattering and extended-X-ray-absorption-fine-structure data. Structural relaxations in this system are driven by the competing bonding requirements of Sr, La, and the undercoordinated oxygen atoms that surround vacant A-sites, which form upon substitution of La for Sr. La cations exhibit significant, disordered off-center displacements within their oversized oxygen cages required by the larger Sr cations. The resulting split-site probability density distributions of La vary with the Sr/La ratio and the state of the A-site ordering, which together modify the structure's ability to relieve the tensile bond strain around La through octahedral rotation and displacements of oxygens surrounding the vacancies. The displacive disorder of La can provide a hitherto overlooked mechanism for reducing the thermal conductivity, which is relevant to thermoelectric properties of this system. A comparison of the local structural behaviors in (Sr,La)TiO3 and the previously studied (Na,Bi)NbO3 solid solutions permits generalizations about A-site deficient perovskites. We find that A-site vacancies provide the nearest-neighbor oxygens with a degree of freedom to mediate the strain in the system, and their effects on local structural relaxations are determined by cation chemistry and stoichiometry.
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Jan 2021
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I15-Extreme Conditions
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Hongfen
Ji
,
Dawei
Wang
,
Weichao
Bao
,
Zhilun
Lu
,
Ge
Wang
,
Huijing
Yang
,
Ali
Mostaed
,
Linhao
Li
,
Antonio
Feteira
,
Shikuan
Sun
,
Fangfang
Xu
,
Dejun
Li
,
Chao-jie
Ma
,
Shi-yu
Liu
,
Ian M.
Reaney
Diamond Proposal Number(s):
[21714]
Abstract: Dense pseudocubic 0.62Na0.5Bi0.5TiO3-0.3Sr0.7Bi0.2TiO3-0.08BiMg2/3Nb1/3O3 (NBT-SBT-0.08BMN) ceramics with excellent recoverable energy density, Wrec = 7.5 J/cm3, and conversion efficiency, η = 92%, were synthesized. Large electric breakdown strength was facilitated by electrical homogeneity, high resistivity and large activation energy (1.86 eV). Transmission electron microscopy identified the presence of polar nano-regions (PNRs) in a matrix of short coherence in-phase and antiphase octahedral tilting. Combining polar and tilt order restricted the crystal classes of PNRs to tetragonal, orthorhombic and monoclinic. Using these symmetries, the enhancement of polarization was explained using Landau-Devonshire phenomenology and percolation theory. Octahedral tilting and introduction of larger B-site ions (Mg2+, Nb5+) inhibited long range polar coupling, minimizing strain and maximizing η. Wrec was further improved to 18 J/cm3 (>1000 kV/cm) in multilayers whose properties were stable from 0.01–100 Hz, from 20°C–160°C and up to 106 cycles, attractive for pulsed power applications and power electronics.
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Jan 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[4631, 6707]
Abstract: High-pressure studies have been performed on the ε-form of the powerful explosive CL-20. Hydrostatic compression over the pressure range 0–12 GPa has been monitored using synchrotron X-ray powder diffraction. The potential effects of X-ray radiation damage were observed and circumvented through a follow-up compression study over the pressure range 0–7 GPa using neutron powder diffraction. This second study revealed smooth compression behavior, and the absence of any phase transitions. Intermolecular interaction energies as obtained using PIXEL calculations did not show any discontinuity upon the application of pressure. An isothermal equation of state has been determined, and the high-pressure response is supported by dispersion-corrected density functional theory calculations. Inelastic neutron scattering (experimental and simulated) spectra for the ε-form are in excellent agreement.
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Dec 2020
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[17412]
Abstract: A melt-extracted HoErCo medium-entropy alloy (MEA) with large magnetic entropy change and excellent magnetic refrigerant capacity is successfully designed in this study. The microstructure evolution of these microwires near room temperature is studied by an in-situ high energy synchrotron X-ray diffraction (HEXRD). The wires show typical amorphous characteristics during room temperature to 250 K. For a field change of 5 T, the maximum magnetic entropy change (-ΔSMmax) of the microwires reaches a maximum value of 15.0 J•kg−1 K−1. Magnetization measurements revealed a paramagnetic to ferromagnetic phase transition at TC~16 K. The refrigerant capacity (RC) and relative cooling power (RCP) are 527 J•kg−1 and 600 J•kg−1, respectively. This investigation highlights the potential of HoErCo MEA microwires as promising magnetocaloric effect materials for cryogenic applications of magnetic cooling.
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Dec 2020
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[20613]
Abstract: The mechanical behavior of different microstructural constituents in SAE 52100 bearing steel has been studied at room temperature in relation to quenching and partitioning (QP) and bainitization (B) process parameters, and compared to the standard quenched and tempered (QT) microstructure using high-energy synchrotron X-ray diffraction in situ during tensile loading. Owing to a larger degree of carbon entrapment in its body-centered cubic lattice and associated lattice distortion, martensite in the QT microstructure showed a larger lattice parameter and broadened diffraction peaks as compared to lower bainitic ferrite or partitioned martensite. A reduction in diffraction peak broadness in tempered martensite occurs at a true stress value of ∼1800 MPa, and preserves its peak broadness even after subsequent unloading. In contrast, an equivalent effect in peak broadness is detected at ∼1500 MPa in the lower bainitic ferrite or mixed bainitic/martensitic matrix characteristic of the B and QP microstructures. In all studied microstructures, the metastable austenite phase transforms when a critical stress is reached, the value of which increases with the bainitic ferrite/martensite fraction and with the carbon content in austenite, but remains lower in the QP and B microstructures compared to the standard QT steel. These results suggest that the carbon solid solution strengthening and associated lattice distortion in bainitic ferrite or martensite are key in determining the mechanical performance of the constituent phases in the steel, with the phase fraction and local carbon content playing an additional role on the austenite mechanical stability.
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Nov 2020
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[26782]
Abstract: X-ray diffraction measurements performed in a diamond anvil cell under quasihydrostatic conditions up to 142 GPa at 300 K evidence an
α
-Zr
→
(17 GPa)
ω
-Zr
→
(35 GPa)
β
-Zr phase transitions sequence. Ab initio molecular dynamics calculations performed on the body-centered cubic
β
-Zr at 300 and 1000 K and between 0 and 100 GPa produced an equation of state in excellent agreement with the experiments. The stability of
β
-Zr under pressure has been verified by numerical heating-quenching experiments, and the anharmonicity of the thermal vibrations has been evaluated. No dynamical instability due to a soft mode is evidenced between 25 and 100 GPa, in line with the experimental finding of a wide stability range for
β
-Zr.
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Nov 2020
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