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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Phase Transitions in Orthoenstatite and Subduction Zone Dynamics: Effects of Water and Transition Metal Ions

Synchrotron–based high–pressure and temperature single–crystal X–ray diffraction experiments were conducted on two hydrous orthoenstatite samples (oEn#1: Mg 1.004 Si 0.996 O 3 , ~619 ppm water; oEn#2: Mg 0.947 Ni 0.055 Si 0.998 O 3 , ~696 ppm water) to ~34 GPa and 700 K, using resistively heated diamond anvil cells. The α–opx (Pbca space group)→β–opx (P2 1 /c space group) phase transition of oEn#1 occurs at 12.90(2) GPa, and the β–opx phase persists to 34.25(1) GPa. The α–β transition of oEn#2 occurs at 13.50(1) GPa, and a new isosymmetric β–opx→β–opxII transition takes place at 29.80(4) GPa. The β–opxII phase is preserved down to 24.53(3) GPa during decompression. The transition to the monoclinic β–opxII phase is interpreted as a result of incorporation of Ni 2+ into the orthoenstatite structure. Fitting the third–order Birch–Murnaghan thermal equation of state to the single–crystal P–V–T data yields the thermoelastic parameters of the α– and β–opx phases for both orthoenstatite samples. This study is the first attempt to determine the thermal equation of state of the β–opx phase. Our results suggest that several hundred ppm of water has negligible effects on the bulk modulus of orthoenstatite but notably enhances the thermal expansion. The potential effects of metastable orthoenstatite on subduction zone dynamics are discussed, and the possible contributions of displacive phase transitions to enhancement of the transformational faulting mechanism of the deep–focus earthquakes in subducted slabs are considered. In conclusion, the presence of metastable orthoenstatite within cold slabs could promote slab stagnation above the 660–km discontinuity.

58 GEOSCIENCES↗
DOE OSTI2025

Isostructural Phase Transition of Fe 2 ⁢O 3 under Laser Shock Compression

We present in situ x-ray diffraction and velocity measurements of Fe 2⁢ O 3 under laser shock compression at pressures between 38–122 GPa. None of the high-pressure phases reported by static compression studies were observed. Instead, we observed an isostructural phase transition from 𝛼−Fe 2⁢ O 3 to a new 𝛼′−Fe 2 ⁢O 3 phase at a pressure of 50–62 GPa. The 𝛼′−Fe 2⁢ O 3 phase differs from 𝛼−Fe 2 ⁢O 3 by an 11% volume drop and a different unit cell compressibility. We further observed a two-wave structure in the velocity profile, which can be related to an intermediate regime where both 𝛼 and 𝛼′ phases coexist. Furthermore, density functional theory calculations with a Hubbard parameter indicate that the observed unit cell volume drop can be associated with a spin transition following a magnetic collapse.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Geometric pumping and dephasing at topological phase transition

A measure-preserving formalism (MPF) is constructed and applied to spin/band models, which yield observations about pumping. It occurs at the topological phase transition, i.e., a Z 2 flip, suggesting that Z 2 can imply bulk effects. The model's asymptotic behavior is analytically solved via MPF. The pumping probability is geometric, fractional, and has a ceiling of $\frac{1}{2}$. Intriguingly, theorems are proved about occurrence conditions, which are linked to the system's dimension and the distinction between rational and irrational numbers. Finally, experimental detection is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Borderline first-order phase transition and large cryogenic magnetocaloric effect in PrNdIn

We report a large cryogenic magnetocaloric effect stemming from an unconventional borderline first-order magnetic phase transition with negligibly small thermomagnetic hysteresis in a rare-earth-based intermetallic compound PrNdIn. The sample exhibits maximum magnetic field-induced entropy change as large as –10 J/Kg K (for 20 kOe magnetic field change) near the boiling point of oxygen. Magnetocaloric properties of PrNdIn are comparable to those of other known potential magnetocaloric materials with operating temperatures ranging between 50 and 125 K. Here, the magnetic properties of the present sample are qualitatively reminiscent of those of the binary Pr 2 In and Nd 2 In, including the emergence of a second low-temperature anomaly in the temperature dependence of magnetization.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Phase transitions in high-purity zirconium under dynamic compression

In this work, we present results from ramp compression experiments on high-purity Zr that show the α → ω, ω → β, as well as reverse β → ω phase transitions. Simulations with a multi-phase equation of state and phenomenological kinetic model match the experimental wave profiles well. While the dynamic α ! ! transition occurs ~ 9 GPa above the equilibrium phase boundary, the ω → β transition occurs within 0.9 GPa of equilibrium. We estimate that the dynamic compression path intersects the equilibrium ω – β line at P = 29:2 GPa, and T = 490 K. The thermodynamic path in the interior of the sample lies ~ 100 K above the isentrope at the point of the ω → β transition. Approximately half of this dissipative temperature rise is due to plastic work, and half is due to the non-equilibrium α → ω transition. The inferred rate of the α → ω transition is several orders of magnitude higher than that measured in dynamic diamond anvil cell (DDAC) experiments in an overlapping pressure range. We discuss a model for the influence of shear stress on the nucleation rate. We find that the shear stress s ji has the same effect on the nucleation rate as a pressure increase δP = cϵ ij s ji /(ΔV/V); where c is a geometric constant ~ 1 and, ϵ ij are the transformation shear strains. The small fractional volume change ΔV/V ≈ 0:1 at the α → ω transition amplifies the effect of shear stress, and we estimate that for this case δP is in the range of several GPa. Correcting our transition rate to a hydrostatic rate brings it approximately into line with the DDAC results, suggesting that shear stress plays a significant role in the transformation rate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The Boson peak in confined water: An experimental investigation of the liquid-liquid phase transition hypothesis

The Boson peak (BP) of deeply cooled confined water is studied by using inelastic neutron scattering (INS) in a large interval of the (P, T) phase plane. By taking into account the different behavior of such a collective vibrational mode in both strong and fragile glasses as well as in glass-forming materials, we were able to determine the Widom line that characterizes supercooled bulk water within the frame of the liquid-liquid phase transition (LLPT) hypothesis. The peak frequency and width of the BP correlated with the water polymorphism of the LLPT scenario, allowing us to distinguish the “low-density liquid” (LDL) and “high-density liquid” (HDL) phases in deeply cooled bulk water.Moreover, the BP properties afford a further confirmation of theWidom line temperature T W as the (P, T) locus in which the local structure of water transforms from a predominately LDL form to a predominately HDL form.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Phase transitions in compact stars. Problem of micro and macro stability

Problem of microscopic stability of dense matter in the case of a phase transition to the new, dense phase (for example quark matter) is discussed. An analysis of the constraints resulting from the observations of 2M{sub ⊙} pulsars and the consequences for a stiffness of a dense phase are presented. This may lead to the instability with respect to reconversion to the basic nuclear phase before reaching maximum mass. Macroscopic stability is defined as a dynamical stability with respect to radial oscillations (in the case of rotating star—axi-symmetric pulsations). The conditions for the stability of the star with a small, dense core of new phase are discussed and the universality of the stability properties of the families of rotating neutron stars is presented.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Heat Effect Due to Monoclinic-to-Orthorhombic Phase Transition in Ultrahigh Molecular Weight Polyethylene

We analyze heat effects associated with the solid-state phase transition in ultrahigh molecular weight polyethylene from the unstable monoclinic phase into thermodynamically stable orthorhombic phase. The model proposed here for the structure of supramolecular formations of monoclinic phase does not contradict to earlier X-ray diffraction data for this polymer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Phase Transitions in Lead Hafnate under High Pressure

The effect of hydrostatic pressure on phase transitions in lead hafnate (PbHfO{sub 3}) has been studied by the methods of X-ray diffraction and X-ray diffuse scattering. The measurements were performed in the temperature range of 182 < T < 316°C and the pressure range of 1.6 < P < 2.6 GPa. Four regions characterized by different X-ray diffraction patterns and corresponding to an antiferroelectric phase, phases with a long-wavelength modulation of the lead sublattice, and phases with different distortions of the oxygen sublattice are identified. In the temperature region immediately above the region of phases with a long-wavelength modulation, a temperature-dependent maximum in the distribution of diffuse scattering was detected, which means that such a modulation is formed due to condensation of an incommensurate soft mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Quantum phase transition and protected ideal transport in a Kondo chain

We study the low energy physics of a Kondo chain where electrons from a one-dimensional band interact with magnetic moments via an anisotropic exchange interaction. It is demonstrated that the anisotropy gives rise to two different phases which are separated by a quantum phase transition. In the phase with easy plane anisotropy, Z2 symmetry between sectors with different helicity of the electrons is broken. As a result, localization effects are suppressed and the dc transport acquires (partial) symmetry protection. This effect is similar to the protection of the edge transport in time-reversal invariant topological insulators. The phase with easy axis anisotropy corresponds to the Tomonaga-Luttinger liquid with a pronounced spin-charge separation. The slow charge density wave modes have no protection against localizatioin.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Phase transition studies of BiMnO{sub 3}: Mean field theory approximations

We studied the phase transition and magneto-electric coupling effect of BiMnO{sub 3} by employing mean field theory approximations. To capture the ferromagnetic and ferroelectric transitions of BiMnO{sub 3}, we construct an extended Ising model in a 2D square lattice, wherein, the magnetic (electric) interactions are described in terms of the direct interactions between the localized magnetic (electric dipole) moments of Mn ions with their nearest neighbors. To evaluate our model, we obtain magnetization, magnetic susceptibility and electric polarization using mean field approximation calculations. Our results reproduce both the ferromagnetic and the ferroelectric transitions, matching very well with the experimental reports. Furthermore, consistent with experimental observations, our mean field results suggest that there is indeed a coupling between the magnetic and electric ordering in BiMnO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phase Transition for Infinite Systems of Spiking Neurons

We prove the existence of a phase transition for a stochastic model of interacting neurons. The spiking activity of each neuron is represented by a point process having rate 1 whenever its membrane potential is larger than a threshold value. This membrane potential evolves in time and integrates the spikes of all presynaptic neurons since the last spiking time of the neuron. When a neuron spikes, its membrane potential is reset to 0 and simultaneously, a constant value is added to the membrane potentials of its postsynaptic neurons. Moreover, each neuron is exposed to a leakage effect leading to an abrupt loss of potential occurring at random times driven by an independent Poisson point process of rate γ>0. For this process we prove the existence of a value γ{sub c} such that the system has one or two extremal invariant measures according to whether γ>γ{sub c} or not.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Disorder-induced topological phase transition in a driven Majorana chain

Here, we study a periodically driven one-dimensional Kitaev model in the presence of disorder. In the clean limit our model exhibits four topological phases corresponding to the existence or nonexistence of edge modes at zero and π quasienergy. When potential disorder is added, the system parameters get renormalized and the system may exhibit a topological phase transition. When starting from the Majorana π mode (MPM) phase, which hosts only edge Majoranas with quasienergy π, disorder induces a transition into a neighboring phase with both π and zero modes on the edges. We characterize the disordered system using (i) exact diagonalization, (ii) Arnoldi mapping onto an effective tight-binding chain, and (iii) topological entanglement entropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Antiferroelectric Phase Transition in a Proton-Transfer Salt of Squaric Acid and 2,3-Dimethylpyrazine

A proton-transfer reaction between squaric acid (H 2 sq) and 2,3-dimethylpyrazine (2,3-Me 2 pyz) results in crystallization of a new organic antiferroelectric (AFE), (2,3-Me 2 pyzH + )(Hsq – )·H 2 O ( 1 ), which possesses a layered structure. The structure of each layer can be described as partitioned into strips lined with methyl groups of the Me 2 pyzH + cations and strips featuring extensive hydrogen bonding between the Hsq – anions and water molecules. Variable-temperature dielectric measurements and crystal structures determined through a combination of single-crystal X-ray and neutron diffraction reveal an AFE ordering at 104 K. The phase transition is driven by ordering of protons within the hydrogen-bonded strips. Considering the extent of proton transfer, the paraelectric (PE) state can be formulated as (2,3-Me 2 pyzH + ) 2 (Hsq 2 3– )(H 5 O 2 + ), whereas the AFE phase can be described as (2,3-Me 2 pyzH + )(Hsq – )(H 2 O). The structural transition caused by the localization of protons results in the change in color from yellow in the PE state to colorless in the AFE state. The occurrence and mechanism of the AFE phase transition have been also confirmed by heat capacity measurements and variable-temperature infrared and Raman spectroscopy. This work demonstrates a potentially promising approach to the design of new electrically ordered materials by engineering molecule-based crystal structures in which hydrogen-bonding interactions are intentionally partitioned into quasi-one-dimensional regions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Correction: CO 2 induced phase transitions in diamine-appended metal–organic frameworks

Correction for ‘CO 2 induced phase transitions in diamine-appended metal–organic frameworks’ by Bess Vlaisavljevich et al. , Chem. Sci. , 2015, 6 , 5177–5185. The authors regret that there are some discrepancies reproducing the data in the original article due to the determined coordinates not being the fully optimised geometries. The authors have provided more information.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗