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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Enabling resonant ultrasound spectroscopy in high magnetic fields

Resonant ultrasound spectroscopy (RUS) is a powerful method to determine elastic constants with high accuracy and precision from a single measurement of the mechanical resonances of a sample. Conventionally, the quantitative extraction of elastic moduli with RUS assumes free boundary conditions which can often lead to the adoption of unstable sample positioning between ultrasonic transducers that is incompatible with extreme environments like high magnetic fields. We show that, under specific conditions, introducing a small amount of adhesive between a RUS sample and ultrasonic transducers introduces a perturbation to the free resonance condition which can be accounted for by a simple model. This means elastic constants can be determined to within the uncertainty of conventional RUS, but with significant improvements including sample stability and control of sample orientation. We demonstrate the efficacy of this approach with measurements on a range of materials including room temperature measurements on polycrystalline metals, temperature-dependent measurements of the structural phase transition in strontium titanate single crystals, and magnetic field-dependent measurements of magnetic phase transitions in gadolinium polycrystals up to 14 T.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Assessment of Physics Models for Phase Transition Kinetics

The time dependence of phase diagrams and how to model rate dependent transitions remains one of the key unanswered questions in physics. When a material is loaded dynamically through equilibrium phase boundaries, it is the kinetics that determines the real time expression of a phase transition. Here we report the atomic and nanosecond-scale quantification of kinetics of shock-driven phase transition in multiple materials. We uniquely make use of a both a simple shock as well as shock-and-hold loading pathways compress different crystalline solids and induce structural phase transitions below melt. Coupling shock loading with time-resolved synchrotron x-ray diffraction (DXRD), we probe the structural transformations of these solids in the short-lived high pressure and temperature states generated. The novelty and power of using DXRD for the assessment of kinetics of phase transitions lies in the ability to discover and identify new phases and to examine kinetics without prior knowledge of a material's phase diagram. Our results provide a quantified expression and a physics model of kinetics of formation of high-pressure phases under shock loading: transition incubation time, evolution, completion time and crystallization rate.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Decoupling the metal insulator transition and crystal field effects of VO 2

VO 2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the T c can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO 2 (Cr-VO 2 and Co-VO 2 ) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO 2 and Co-VO 2 films showed sharp MIT features. The T c values of the Cr-VO 2 and Co-VO 2 films first decreased and then increased relative to that of pristine VO 2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO 2 films with a Cr ion flux ≥ 10 13 ions/cm 2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t 2g and e g orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO 2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO 6 octahedrons or the V 3d t 2g and eg states. This indicates that the MIT of VO 2 occurs with no changes of the crystal fields.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Emergence of the isotropic Kitaev honeycomb lattice $α$–RuCl 3 and its magnetic properties

Here, we present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnet $α$–RuCl 3 using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic (C2/m). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral ($R\bar3$) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops below $T_{\textrm{N}} = 6\sim6.6$ K. The critical exponent of the magnetic order parameter was determined to be $\beta = 0.11(1)$, which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within the ab–plane. These findings reflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Microscopic Characterization of Pb 10−x Cu x (PO 4 ) 6 O by 31 P and 63/65 Cu NMR Measurements

The report of the first room-temperature, ambient-pressure superconductivity in copper-doped lead apatite Pb 10−x Cu x (PO 4 ) 6 O has attracted lots of attention. However, subsequent studies revealed the presence of numerous impurity phases in the polycrystalline sample, and the sharp superconducting-like transition is not due to a superconducting transition but most likely due to a reduction in resistivity caused by the first-order structural phase transition of Cu 2 S at around 385 K from the β phase at high temperature to the γ phase at low temperature. Before now, only bulk measurements have been performed on a Pb 10−x Cu x (PO 4 ) 6 O powder sample, which could be affected by the impurity phases, masking the intrinsic properties of Pb 10−x Cu x (PO 4 ) 6 O. In this study, 31 P and 63/65 Cu nuclear magnetic resonance (NMR) measurements have been performed on a Pb 10−x Cu x (PO 4 ) 6 O powder sample to investigate its physical properties from a microscopic point of view. Our NMR data evidence the non-magnetic insulating nature of Pb 10−x Cu x (PO 4 ) 6 O without any trace of electron correlation effects. Furthermore, the 63/65 Cu NMR results suggest that no copper or very little copper is substituted for Pb in Pb 10 (PO 4 ) 6 O prepared by sintering Pb 2 SO 5 and Cu 3 P.

lead apatite↗
DOE OSTI2021

Temperature Dependent Local Atomic Structure and Vibrational Dynamics of Barium Hydride and Calcium Hydride

Solid-state ionic conductors that exhibit pure ionic transport of hydride anions are rare. Here, we investigate two alkaline earth metal hydrides, barium hydride and calcium hydride, using neutron scattering techniques to understand how the local atomic environment plays a role in the diffusion of hydride ions. At high temperatures, barium hydride exhibits exceptional transport properties with ionic conductivities that are higher than those of many of the typical proton and oxide ion conductors in use today. Total neutron scattering and pair distribution function analysis reveal how a structural phase transition converts barium hydride from a modest ionic conductor into a fast ionic conductor through the introduction of disorder, deuterium site splitting, and dynamic structural fluctuations. Furthermore, neutron vibrational spectroscopy is employed to probe changes in the temperature evolution of the lattice dynamics and local energy landscape. These results improve our fundamental knowledge of the interplay between structure and dynamics governing a rare conduction process of hydride ions in solid-state materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Negative linear compressibility in Se at ultra-high pressure above 120 GPa

A series of in situ synchrotron X-ray diffraction (XRD) measurements were carried out, combined with first-principles calculations, to study structural phase transitions of selenium at high pressures and room temperature. Several phase transitions were observed, among which an isostructural phase transition was found at around 120 GPa for the first time. Evolved from the rhombohedral (space group R$\bar{3}$m) structure (Se-V), the new phase (Se-V') exhibited an interesting increase of lattice parameter a at pressures from 120 to 148 GPa, known as negative linear compressibility (NLC). The discovery of NLC behavior observed in this work is mainly attributed to the accuracy and fine steps controlled by the membrane system for in situ XRD data collected with an exposure time of 0.5 s. After 140 GPa, a body-centered cubic (b.c.c.) structure Se-VI (space group Im$\bar{3}$m) was formed, which remains stable up to 210 GPa, the highest pressure achieved in this study. The bulk moduli of phases Se-V, Se-V' and Se-VI were estimated to be 83 ± 2, 321 ± 2 and 266 ± 7 GPa, respectively, according to the P–V curve fit by the third-order Birch–Murnaghan equation of state. The Se-V' phase shows a bulk modulus almost 4 times larger than that of the Se-V phase, which is mainly due to the effect of its NLC. NLC in a higher pressure range is always more significant in terms of fundamental mechanism and new materials discovery, yet it has barely been reported at pressures above 100 GPa. This will hopefully inspire future studies on potential NLC behaviors in other materials at ultra-high pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Kinetic Understanding of Field-Induced Phase Transition from Tetragonal to Ferroelectric Orthorhombic Phase in Ferroelectric CeO 2 –HfO 2 –ZrO 2 Films

The ferroelectric properties and structural phase transition behaviors of fluorite-type CeO 2 −HfO 2 −ZrO 2 films were investigated. The epitaxial films on indium tin oxide (ITO) (111)/yttria-stabilized zirconia (YSZ) (111) substrates were grown through pulsed laser deposition at room temperature and subsequently heat-treated at 1000 °C under a N 2 gas flow. The crystalline phases and Curie temperatures of the films were investigated by X-ray diffraction. An increase in the Ce or Zr content in the films led to a higher crystallographic symmetry, such as orthorhombic or tetragonal. In addition, electrical characterization revealed that the orthorhombic films and some of the tetragonal films displayed ferroelectricity. This was due to the field-induced phase transition from the tetragonal to ferroelectric orthorhombic phase in the films, where the Curie temperatures were relatively low. The tetragonal metastable phase was kinetically frozen and could not change into the stable orthorhombic phase at such a low temperature. The critical electric field where the field-induced phase transition occurred was below 0.8 MV/cm, which was sufficiently small compared to the coercive field. These results evidence the kinetic driving force that causes a field-induced phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase in HfO 2 -based ferroelectrics. They also enhance our understanding of the thermodynamic phase stabilities of HfO 2 -based material polymorphs.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Structural transition and possible pressure-induced superconductivity in a suboxide La 5 Pb 3 O

Here, in this study, we report a structural phase transition and its interplay with superconductivity in the suboxide La 5⁢ Pb 3 ⁢O. Upon cooling through 𝑇 𝑡 = 225K, La 5⁢ Pb 3⁢ O transforms from a high-temperature 𝐼⁢4/mcm to a low-temperature 𝑃⁢4/ncc structure in which La-Pb dimerization along the 𝑐 axis occurs. This transition is accompanied by anomalies in the temperature dependence of electrical resistivity and specific heat. High-pressure electrical transport measurements reveal that hydrostatic pressure suppresses the structural transition and possibly induces superconductivity with a maximum superconducting temperature of 10 K. Density functional theory calculations show minimal changes in the electronic density of states and no gap opening at 𝐸 𝐹 across 𝑇 𝑡 , suggesting that the transition is driven by bonding effects rather than Fermi surface instability. These findings establish La 5⁢ Pb 3 ⁢O as a promising platform for exploring the interplay between weak structural transitions and superconductivity.

Yan, Jiaqiang [Oak Ridge National Laboratory (ORNL↗
DOE OSTI2026

Giant Magnetostriction in Ferrimagnetic SmFe 5 As 3

Magnetostrictive materials are of interest not only from a fundamental perspective but also for their potential applications, spanning spintronics to energy harvesting. A new magnetostrictive material—SmFe 5 As 3 —reveals a complex interplay between magnetostriction, magnetic properties, and the crystal structure behavior. The ground state of SmFe 5 As 3 is ferrimagnetic, as evidenced by magnetic susceptibility data, band-structure calculations, and XANES measurements. At T m1 = 28 ± 4 K, part of the Fe sublattice reorients, transitioning into a ferromagnetic state. This is followed by an entrance into the paramagnetic state at T m2 = 76 ± 4 K. The effects observed in the magnetic measurements are accompanied by structural phase transitions. All three phases—below T m1 , between T m1 and T m2 , and above T m2 —are described by the same structural motif of the UCr 5 P 3 type (monoclinic space group P2 1 /m), differing only in the degree of deformation of the Fe–As framework. The new material exhibits giant magnetostriction of 2500 x 10 -6 . Dilatometry measurements on a SmFe 5 As 3 single crystal indicate strongly diverse behavior, exhibiting not only negative, but also zero, as well as positive thermo-elastic effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1982

X-ray diffraction from high pressure Ge using synchrotron radiation

The high pressure structural phase transition in Ge has been studied using the energy dispersive X-ray diffraction technique and a synchrotron radiation source. Ge was observed to transform to the beta-Sn tetragonal structure in agreement with the earlier results of Jamieson, but the phase transition began at 80 + or - 5 kilobars, a somewhat lower value than generally reported. These experimental diffraction results are compared with the recent self-consistent pseudopotential calculations of Yin and Cohen (1981) and with the observed transition pressure for shock wave loaded Ge.

Baublitz, M., Jr.↗
DOE OSTI2021

Probing the lattice structure of dynamically compressed and released single crystal iron through the alpha to epsilon phase transition

In this work, experiments using broadband Laue x-ray diffraction (XRD) were used to examine the lattice structure of dynamically compressed [100]-oriented single crystal iron samples at the Dynamic Compression Sector at the Advanced Photon Source. These experiments used 1 μm thick iron single crystal samples sandwiched between a polyimide ablator and a polycarbonate window. A 100 J, 10 ns duration laser pulse incident on the polyimide ablator was used to shock compress the iron samples to initial stresses greater than 25 GPa, exceeding the ~13 GPa alpha (body-centered-cubic or bcc structure) to epsilon (hexagonal-close-packed or hcp structure) phase transition stress. XRD measurements were performed at various times relative to the shock wave entering the iron sample: early times, <~150 ps while the initial shock waves propagated through the iron; intermediate times, after the iron equilibrated with the ablator and window reaching a plateau stress state (12 or 17 GPa) lasting several nanoseconds; and late times, during uniaxial strain release. The early time measurements show that in <~150 ps, the high-pressure hcp phase is relaxed with a c/a ratio of 1.61, contrary to previous laser shock experiments where a c/a ratio of 1.7 was inferred. In the plateau stress state and partially released states, XRD measurements showed that the hcp structure retained a c/a ratio of 1.61 with no observable changes in the microstructure. Upon stress release at ~1 GPa/ns release rate, the reverse phase transition (hcp to bcc) to the original single crystal orientation (implying a transformation memory effect) was observed to reach completion somewhere between 13 and 11 GPa, indicating little stress hysteresis under rapid uniaxial strain release. A similar memory effect for the reverse hcp to bcc transformation has been previously observed under hydrostatic compression. However, the bcc/hcp orientation relationships differ somewhat between dynamic and static compression experiments, implying that the transformation pathway under uniaxial dynamic strain differs from the Burgers mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Water-Stable 1D Hybrid Tin(II) Iodide Emits Broad Light with 36% Photoluminescence Quantum Efficiency

The optical and light emission properties of tin and lead halide perovskites are remarkable because of the robust room-temperature (RT) performance, broad wavelength tunability, high efficiency, and good quenching resistance to defects. These highly desirable attributes promise to transform current light-emitting devices, phosphors, and lasers. One disadvantage in most of these materials is the sensitivity to moisture. Here, we report a new air-stable one-dimensional (1D) hybrid lead-free halide material (DAO)Sn 2 I 6 (DAO, 1,8-octyldiammonium) that is resistant to water for more than 15 h. The material exhibits a sharp optical absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half-maximum (fwhm) of 142 nm (0.44 eV). The emission has a long photoluminescence (PL) lifetime of 582 ns, while the intensity is constant over a very broad temperature range (145–415 K) with a photoluminescence quantum yield (PLQY) of at least 20.3% at RT. Above 415 K the material undergoes a structural phase transition from monoclinic (C2/c) to orthorhombic (Ibam) accompanied by a red shift in the band gap and a quench in the photoluminescence emission. Density functional theory calculations support the trend in the optical properties and the 1D electronic nature of the structure, where the calculated carrier effective masses along the inorganic chain are significantly lower than those perpendicular to the chain. As a result, thin films of the compound readily fabricated from solutions exhibit the same optical properties, but with improved PLQY of 36%, for a 60 nm thick film, among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Intercalation as a versatile tool for fabrication, property tuning, and phase transitions in 2D materials

Recent advances in two-dimensional (2D) materials have led to the renewed interest in intercalation as a powerful fabrication and processing tool. Intercalation is an effective method of modifying the interlayer interactions, doping 2D materials, modifying their electronic structure or even converting them into starkly different new structures or phases. Herein, we discuss different methods of intercalation and provide a comprehensive review of various roles and applications of intercalation in next-generation energy storage, optoelectronics, thermoelectrics, catalysis, etc. The recent progress in intercalation effects on crystal structure and structural phase transitions, including the emergence of quantum phases are also reviewed.

2D materials↗
DOE OSTI2026

Effects of heteroatom doping on hydrogen uptake in tungsten oxide

Redox-active transition metal oxides (TMOs) that can undergo proton-insertion coupled electron transfer (PICET) are promising candidates for catalyzing molecular conversion reactions which require the transfer of hydrogen atoms (or the thermochemical equivalent, H + , e − ). Herein, we studied the effects of isovalent (Mo 6+ ) and aliovalent (V 5+ and Nb 5+ ) heteroatom doping on the electrochemical PICET behavior of monoclinic tungsten oxide (WO 3 ). Cyclic voltammetry in aqueous acidic electrolytes shows that the addition of redox-active heteroatoms (Mo 6+ and V 5+ ) leads to systematic shifts in redox couple half-wave potentials (E 1/2 ), broadening, and an overall decrease in the current response. Conversely, the non-redox active heteroatom (Nb 5+ ) only reduces the current response with no observable peak-current broadening. This broadening is attributed to changes in the proton binding affinities of oxygen in different chemical environments, i.e., bridging different pairs of redox-active transition metal cations. We determined the hydrogen bond dissociation free energy (H BDFE) values to elucidate the thermodynamic effect of heteroatom substitution. Density functional theory calculations reveal a differentiation in the hydrogen binding and oxygen vacancy formation energies between heteroatom doped structures. The PICET-induced structural phase transitions of the pristine and doped samples were further probed with operando electrochemical X-ray diffraction (EC-XRD) and with ex situ chemical reduction. The broadening of the potential-dependent current response with increased heteroatom doping manifests in the operando EC-XRD results as prolonged structural regions where multiple hydrogen bronze phases exist and the appearance of cubic bronze phases at lower degrees of reduction compared to pristine WO 3 .

Holzapfel, Noah P. [North Carolina State Universit↗
DOE OSTI2021

Revealing pressure-driven structural transitions in the hybrid improper ferroelectric Sr 3 Sn 2 O 7

In this work, we combine diamond anvil cell techniques and synchrotron-based infrared spectroscopy with a detailed symmetry analysis and lattice dynamics calculations to uncover a series of pressure induced structural phase transitions in the hybrid improper ferroelectric Sr 3 Sn 2 O 7 . The microscopic character of each high pressure phase is determined by comparing the measured spectrum with the predicted vibrational patterns of several related but distinct candidate space groups. Our analysis reveals a sequence of compression-induced transitions from A2 1 am ↔ Pnab ↔ Acaa ↔ I4/mmm at room temperature. Remarkably, this space group progression matches the sequence of temperature-dependent structural transitions observed in Sr 3 Sn 2 O 7 between 77 and 1000 K. Other hybrid improper ferroelectrics display a similar set of transitions, suggesting that pressure and probably strain will be very effective tuning parameters for this entire class of materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Complex structure due to As bonding and interplay with electronic structure in superconducting BaNi 2 As 2

BaNi 2 As 2 is a superconductor chemically related to the Fe-based superconductors, with a complex and poorly understood structural phase transition. We show, based on first-principles calculations, that, in fact, there are two distinct competing structures. These structures are different from the electronic, transport, and bonding points of view but are close in energy. These arise due to complex As bonding patterns and drive distortions of the Ni layers. This is supported by photoemission experiments. This is very distinct from views of the distortion as being primarily driven by electrons at the Fermi surface as in a classical charge density wave, or by correlated electron physics associated with the Ni d electrons and the Ni d orbitals. The structural distortion, although connected with As does lead to an interplay of electronic and structural behavior including induced anisotropic electronic transport. Here, the local bonding nature of the instabilities and the competition between distortions is discussed in the context of the complex behavior observed in BaNi 2 As 2 samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Structural Distortions and Uniaxial Negative Thermal Expansion in the Polar Dion–Jacobson Oxide RbNdTa 2 O 7

We provide deeper insight into the crystal structures, sequential structural phase transitions (I2cm → Cmce → I4/mcm → P4/mmm), thermal expansion, and electronic properties of the n = 2 Dion–Jacobson polar oxide RbNdTa 2 O 7 , through X-ray powder diffraction, neutron powder diffraction, Raman studies, and density functional theory calculations. We observed a uniaxial negative thermal expansion (NTE) across the first-order transition, I2cm → Cmce, where the unit cell contracts along the c-axis, which is driven by a contraction of the NdTa 2 O 6 layer. Here, this NTE occurs within the temperature range of the first-order phase transition and contrasts with the corkscrew mechanism typically observed in Ruddlesden–Popper phases. In RbNdTa 2 O 7 , the I2cm (hybrid improper ferroelectric) → Cmce (antipolar) transition involves crucial changes in the bond lengths of Nd and Ta polyhedra, coupled with polar to antipolar displacement of the Nd ions, leading to a net contraction in the NdTa 2 O 6 layer along the c-axis, while preserving the overall octahedral tilting magnitude. This transition highlights the intricate interplay between the Nd and Ta coordination and the associated TaO 6 distortions. Temperature-dependent Raman spectra analysis further confirms the first-order structural transition and associated NTE, providing evidence for increased bond stiffness across this transition. Additionally, using neutron powder diffraction, we have determined that the transition I4/mcm → P4/mmm occurs at approximately 1150 K. Finally, we have calculated from DFT + U, the partial density of states, the energy bandgaps, and effective masses of the charge carriers of the polar ground structure.

Chemical structure↗