Structural Phase Transition Research Articles

IR magnetotransmission in double NdBaMn<sub>2</sub>O<sub>6</sub> manganite with different degrees of ordering in A-position

The structural, magnetic, and optical properties of double manganites NdBaMn2O6 have been studied depending on the degree of ordering of Nd and Ba atoms in the A-position. Analysis of temperature dependences of light transmission shows the changes of charge carrier subsystem in vicinity of structural and magnetic phase transitions. When the metal-insulator transition occurs the effect of light magnetotransmission near Curie temperature was found.

Surface Phonons and Possible Structural Phase Transition in a Topological Semimetal PbTaSe2

AbstractTopological insulators are characterized by protected gapless surface or edge states but insulating bulk states which is due to the presence of spin‐orbit interactions and time‐reversal symmetry. Here, an in‐depth investigation of a topological nodal line semimetal PbTaSe2 via temperature, polarization dependent Raman spectroscopy, and temperature dependent single crystal X‐ray diffraction (SC‐XRD) measurements is reported. The analysis shows signature of electron‐phonon coupling as reflected in the Fano asymmetry in line shape of M1‐M4 modes and anomalous temperature variation of line‐width of P3‐P4 modes. Further polarization dependent phonon symmetry changes at different temperature (6K and 300K), discontinuities in bulk phonon dynamics for P2‐P5 modes, and disappearance of phonon modes, i.e., M1‐M5, on decreasing temperature indicates toward a thermally induced structural phase transition which is also supported by the SC‐XRD results. Hence based on the findings, it is proposed that M1‐M4 modes are surface phonon modes, the material undergoes a thermally induced structural phase transition from α to β phase at Tα→β ≈ 150 K or is in close proximity to the β phase and another transition below TCDW+β ≈ 100K which is possibly due to the interplay of remanent completely commensurate charge density wave (CCDW) of 1H‐TaSe2 and β phase.

Engineering the Coherent Phonon Transport in Polar Ferromagnetic Oxide Superlattices.

Artificial superlattices composed of perovskite oxides serves as an essential platform for engineering coherent phonon transport by redefining the lattice periodicity, which strongly influences the lattice-coupled phase transitions in charge and spin degrees of freedom. However, previous methods of manipulating phonons have been limited to controlling the periodicity of superlattice, rather than utilizing complex mutual interactions that are prominent in transition metal oxides. In this study onoxide superlattices composed of ferromagnetic metallic SrRuO3 and quantum paraelectric SrTiO3, phonon modulation by controlling the geometry of superlattice in atomic-scale precision is realized, demonstrating the coherent phonon engineering using structural and magnetic phase transitions. By modulating the interface density, coherent-incoherent crossover of the phonon transport at room temperature is observed, which is coupled with a change in interfacial structural continuity. Upon cooling, the close relation between phonon transport and multiple phase transitions is identified. In particular, the enhancement of the polar state in SrTiO3 layer at ≈200K leads to the weakening of phonon coherence and a further reduction of thermal conductivity in superlattices compared to the bulk limit. These findings provide a guide to developing future thermoelectric nanodevices by engineering the coherence of phonons via the design of complex oxide heterostructures.

Pressure-Induced Structural Phase Transition and Fluorescence Enhancement of Double Perovskite Material Cs2NaHoCl6

Cs2NaHoCl6, a double perovskite material, has demonstrated extensive application potential in the fields of anti-counterfeiting and optoelectronics. The synthesis of Cs2NaHoCl6 crystals was achieved using a hydrothermal method, followed by the determination of their crystal structures through single crystal X-ray diffraction techniques. The material exhibits bright red fluorescence when exposed to ultraviolet light, confirming its excellent optical properties. An in situ high-pressure fluorescence experiment was conducted on Cs2NaHoCl6 up to 10 GPa at room temperature. The results indicate that the material possibly undergoes a structural phase transition within the pressure range of 6.9–7.9 GPa, which is accompanied by a significant enhancement in fluorescence. Geometric optimization based on density functional theory (DFT) revealed a significant decrease in the bond lengths and crystal volumes of Ho-Cl and Na-Cl across the predicted phase transition range. Furthermore, it was observed that the bond lengths of Na-Cl and Ho-Cl reach an equivalent state within this phase transition interval. The alteration in bond length may modify the local crystal field strength surrounding Ho3+, consequently affecting its electronic transition energy levels. This could be the primary factor contributing to the structural phase transition.

Separate magnetic and structural phase transitions in Mn50−xFexRh50 films grown on MgO

Abstract Investigations of the magnetic and structural characteristics of Mn50−x Fe x Rh50 alloys are important due to their notable phase transition behavior. In this study, a series of highly ordered epitaxial films with varying Fe concentrations are grown on MgO (001) substrate. At low Fe concentrations (x = 0, 2, 6), a separation between the structural phase transition and the magnetic phase transition is observed. Unlike structural phase transitions, temperature-dependent magnetization exhibits fairly large temperature hysteresis. In addition, the structural transition induces further tetragonal distortion, resulting in an intermediate phase between the B2 and L10 structures. This separated magnetic and structural phase transitions have been further validated through x-ray diffraction, anisotropic magnetoresistance and spin-pumping measurements. Moreover, as the Fe concentration is increased, the Mn50−x Fe x Rh50 films exhibit ferromagnetic behavior due to competitive magnetic exchange interactions, while the structural phase transition is suppressed.

Pressure induced semiconductor-like to metal transition and linear magnetoresistance in Cr2S2.88 single crystal

The transition metal chalcogenide Cr2S3-xhas unique properties, such as a lower antiferromagnetic transition temperature, semiconducting behavior, and thermoelectric properties. We focus on the effects of high pressure on the properties of electrical transport and structure in the single crystal Cr2S2.88. It is observed that the resistance drops abruptly by approximately two orders of magnitude and the temperature derivative of the resistance changes from negative to positive after 15.7 GPa. The Cr2S2.88crystal has undergone transitions from a semiconductor-like phase to a metal I phase and then to another metal II phase. Simultaneously, a structural phase transition after 16.1 GPa is confirmed by synchrotron angle dispersive x-ray diffraction. After the structural phase transition, the negative magnetoresistance becomes positive with increasing pressure and shows a linear relationship in the metal II phase. Electron-type carriers dominate in the semiconductor-like phase, but hole-type carriers dominate after the structural phase transition. Our work provides an example of the effective modulation of semiconductor-like properties by pressure, which is meaningful for the innovation and development of semiconductor technology.

The stannides SrPdSn and m-BaPtSn

Abstract The stannides SrPdSn (TiNiSi type, orthorhombic space group Pnma) and m-BaPtSn (EuNiGe type, monoclinic space group P21/c) were synthesized from the elements in sealed tantalum ampoules in a high-frequency furnace. Their structures were refined from single-crystal X-ray diffraction data. SrPdSn crystallizes directly from the melt and is stable upon annealing at T = 1073 K. A BaPtSn sample quenched from the melt adopts the cubic LaIrSi-type structure, cubic space group P213 (c-BaPtSn) and shows a temperature induced (annealing at 1070 K) structural phase transition leading to a EuNiGe-type low-temperature modification m-BaPtSn. The phase transition leads to a reconstruction within the polyanionic [PtSn] δ− network. The latter is three-dimensional and composed of ten-membered Pt5Sn5 rings in c-BaPtSn, while the polyanion is two-dimensional in m-BaPtSn and is composed of condensed Pt4Sn4 and Pt2Sn2 rings. The two-dimensional substructure leads to a strong moisture sensitivity for m-BaPtSn. The Pt–Sn distances in both modifications range from 257 to 267 pm, indicating substantial covalent bonding.

Active learning-based automated construction of Hamiltonian for structural phase transitions: a case study on BaTiO3

The effective Hamiltonians have been widely applied to simulate the phase transitions in polarizable materials, with coefficients obtained by fitting to accurate first-principles calculations. However, it is tedious to generate distorted structures with symmetry constraints, in particular when high-ordered terms are considered. In this work, we implement and apply a Bayesian optimization-based approach to sample potential energy surfaces, automating the effective Hamiltonian construction by selecting distorted structures via active learning. Taking BaTiO3(BTO) as an example, we demonstrate that the effective Hamiltonian can be obtained using fewer than 30 distorted structures. Follow-up Monte Carlo simulations can reproduce the structural phase transition temperatures of BTO, comparable to experimental values with an error<10%. Our approach can be straightforwardly applied on other polarizable materials and paves the way for quantitative atomistic modelling of diffusionless phase transitions.

Strain-Induced Antiferroelectric-Ferroelectric-Antiferroelectric Phase Transitions in Epitaxial LaTaO4 Films.

We employed first-principles to delve into the strain-induced structural phase transitions in epitaxial LaTaO4 films. The ground state of bulk LaTaO4 adopts a monoclinic antiferroelectric phase, characterized by the antiphase rotation of two adjacent oxygen octahedra layers. Under epitaxial tensile strain, LaTaO4 thin films undergo a consecutive phase transition, namely, antiferroelectric-ferroelectric-antiferroelectric phase transitions. The ferroelectricity in the orthorhombic phase under tensile strain originates from the in-phase rotation of two adjacent oxygen octahedra layers, in contrast to the case of perovskite systems, in which octahedra rotation suppresses the conventional proper ferroelectricity. With spin-orbit coupling effects, a pronounced Rashba effect at the Brillouin zone center naturally leads to the formation of opposite directions of spin texture. Moreover, the tensile strain also synergistically enhances the corresponding Rashba parameters. Our findings may provide novel insights for controlling oxygen octahedra rotation to achieve control over the ferroelectricity and Rashba effect, which holds promising implications for applications in electronics and spintronics.

Pattern formations and their active manipulation in a Rydberg noisy-dressed Bose-Einstein condensate.

We investigate the formation and manipulation of spatial patterns and their structural phase transitions by examining the effects of laser linewidth on the dynamics of Rydberg noise-dressed Bose-Einstein condensates (RnD BECs). We discover that the homogeneous matter wave can transition into various types of self-organized patterns, which are manipulated by tuning the laser linewidth, control field intensity, and atomic density. Remarkably, the system exhibits stable nonlocal matter-wave solitons, vortices, and soliton molecules under specific laser linewidths.

Structural Transition Analysis of Bilayer Black Phosphorus under High Pressure via Ultralow-Frequency Raman Spectroscopy.

The unique anisotropic electron-photon and electron-phonon interactions of black phosphorus (BP) set it apart from other isotropic 2D materials. These anisotropic properties can be adjusted by varying the stacking thickness and sequence as well as by applying external pressure and strain. In contrast to multilayer or bulk BP, the effects of pressure on bilayer BP are still not fully elucidated. This study presents experimental evidence that explores the high-pressure response (0-20 GPa) of bilayer BP within the extreme low Raman shift region (5-150 cm-1), aiming at verifying structural phase transitions and stacking sequence variations. Notably, phase transitions were observed at ∼7 GPa for the orthorhombic to rhombohedral transition and at ∼14-16 GPa for a further transition to a simple cubic structure. The effects of pressure on bilayer BP with three distinct stacking sequences (AA, AB, and AC) were analyzed using angular-resolved polarized Raman spectroscopy (ARPR). The recovery of all characteristic Raman modes after the application of pressure substantiates the reversibility of the structural phase transitions in bilayer BP across all three stacking sequences. However, significant alterations in the stacking sequences were observed before and after the application of hydrostatic pressure. Specifically, AA and AC stacking sequences showed a tendency to relax into an optimized interlayer AB stacking structure upon the release of pressure from 20 to 0 GPa. This observed change in the stacking sequences of bilayer BP under pressure is detected for the first time in this work. The pressure-induced modifications in the stacking sequences, and consequently in the anisotropic properties of bilayer BP, highlight its potential as a promising material for pressure- (or strain-) sensitive optoelectronic nanodevices. Additionally, this study offers novel insights into the analysis of anisotropic interlayer interactions in other van der Waals-stacked 2D materials.

Bonding states underpinning structural transitions in IrTe2 observed with micro-ARPES

Competing interactions in low-dimensional materials can produce nearly degenerate electronic and structural phases. We investigate structural phase transitions in layered IrTe2 for which a number of potential transition mechanisms have been postulated. The spatial coexistence of multiple phases on the micron scale has prevented a detailed analysis of the electronic structure. By exploiting micro-angle-resolved photoemission spectroscopy obtained with synchrotron radiation we extract the electronic structure of the multiple structural phases in IrTe2 in order to address the mechanism underlying the phase transitions. We find direct evidence of lowered energy states that appear in the low-temperature phases, states previously predicted by calculations and extended here. Our results validate a proposed scenario of bonding and antibonding states as the driver of the phase transitions. Published by the American Physical Society 2024

Charge transfer induced phase transition in Li2MnO3 at high pressure

Efficient and better energy storage materials are of utmost technological importance to reduce energy dependence on the fossil fuels. Li2MnO3is one such material having potential to meet most of the requirements for energy storage. This material has been synthesized using solid state synthesis route. High pressure structural and vibrational studies on this material have been carried out upto ∼22 and 26 GPa respectively. These investigations show occurrence of a hitherto unknown second order phase transition to a new low symmetry phase whose symmetry is constrained to be monoclinic with space group P21/n at pressure of ∼2.3 GPa in Li2MnO3. The bulk modulus and its derivative determined by fitting theP-Vdata with third order Birch-Murnaghan equation of state are 113.3 ± 13.1 GPa and 4.1 ± 1.2 respectively. Mode Grüneisen parameter calculated for all the Raman modes show positive values which indicates the absence of any soft mode in this material. A microscopic mechanism based on bond-charge transfer is invoked and applied to understand the spectroscopic changes occurring in this material which also manifests second order structural phase transition. Enhancement in covalent character of Li-O bonds in the Li-O polyhedra is inferred based on the spectroscopic observation and above mechanism.

Reversible Single-Crystal to Single-Crystal Transformation and Associated Magnetism of a Cyanide-Bridged Chiral-Structured Magnet.

A chiral molecule-based magnet [MnII (S-pnH) (H2O)][MnIII(CN)6]·H2O (S-pn = S-1,2-diaminopropane), 1S·2H2O (P212121), has been obtained, which has a two-dimensional (2D) square network of cyanide-bridged MnII-MnIII ions. The crystallographic research on this coordination polymer indicates that it is robust enough to transform the single-crystal structure upon dehydration as well as rehydration. Meanwhile, the magnetic property changes are reversibly associated with the structural phase transitions. The complete reversibility upon dehydration/rehydration was demonstrated using in situ X-ray diffraction from the as-prepared 1S·2H2O to the dehydrated [MnII (S-pnH)][MnIII(CN)6], 1S, then rehydrated to [MnII (S-pnH) (H2O)][MnIII(CN)6]·H2O, 1S-HP, and dehydrated again to [MnII (S-pnH)][MnIII(CN)6], 1S-DeHP. Neither the space groups nor the crystal axes changed during the dehydration/rehydration process. The dehydration is considered to be associated with a change in the coordination of the S-pnH from one 2D layer to the neighboring one involving a proton transfer and is accompanied by new cyanide bridging of MnII and MnIII ions formed between the 2D sheet. Corresponding to the 2D to three-dimensional (3D) structural transition, the study of magnetic properties reveals that the Curie temperature of long-range magnetic ordering for 1S (TC = 45 K) is more than double that for 1S·2H2O (TC = 21 K).

Room Temperature Phase Transition and Luminescence Behavior of a Novel Metal‐Free Ionic Crystal [C7H16NO][ClO4

AbstractMetal‐free ionic crystals have attracted tremendous attention for their environmental friendliness, biocompatibility, easy fabrication and structural variability. Herein, a metal‐free ionic crystal [C7H16NO][ClO4] ([C7H16NO]=1‐methyl‐4‐piperidinemethanol) with a dielectric transition at room temperature was designed and constructed. Differential scanning calorimetry (DSC) proved that there exists a reversible phase transition near room temperature at 304 K, accompanied with switchable dielectric behaviors between the low and high dielectric states. The single crystal structure show that the mechanism of such structural phase transition mainly results from the dynamic motion of perchlorate anions. The compound is further shown to exhibit remarkable luminescent properties, characterized by a prolonged lifetime of 18.6 ns. This work offers valuable insights into the construction of room temperature phase‐transition ionic crystals involved with dielectric switching and blue luminescence properties.

Polymorphism in Type-II Dirac Semimetal WSi2 under Pressure: Structural, Mechanical, and Electronic Insights.

The type-II Dirac candidate semimetal WSi2 is a promising candidate for electronic devices, quantum computing, and topological materials research, owing its distinct electronic structure and superior mechanical properties. Here, we synthesized high-quality WSi2 materials and systematically investigated their compressive behavior, and structural and electronic properties under high pressure using in-situ high pressure experiments, complemented by first-principles calculations. The results confirms that WSi2 has the properties of a type-II Dirac semimetal. Our results demonstrate that WSi2 maintains structural stability under high pressure but undergoes an electronic phase transition from a semimetal to a metal around 40 GPa. Additionally, the mechanical hardness softens discontinuously at this pressure. The structural stability of WSi2 under high pressure is attributed to the strong hybridization of Si-3p and W-5d electrons, the rigid crystal lattice, and the adaptable electronic structure. The pressure-induced electronic phase transition and softening are primarily governed by the energy band reconstruction and W-5d orbitals. This study provides valuable insights into the high-pressure behavior of type-II Dirac semimetal, highlighting their potential for advanced applications in electronic devices and topological quantum computing under extreme conditions by elucidating their structural stability and electronic phase transition mechanisms.

Three‐Dimensional Imaging of the Structural Phase Transition in a Single Vanadium Dioxide Nanocrystal

Vanadium dioxide (VO2) is a strongly correlated material that exhibits a number of structural phase transitions (SPT) near to room temperature of considerable utility for various technological applications. When reduced to the nanoscale, a foreknowledge of surface and interface properties of VO2 during the SPT can facilitate the development of devices based on VO2. Herein, it is shown that Bragg coherent X‐ray diffractive imaging (BCDI) combined with machine learning is an effective means to recover three‐dimensional images of a single VO2 nanocrystal during a temperature‐induced SPT from a room‐temperature monoclinic phase to a high‐temperature rutile phase. The findings reveal the coexistence of multiple phases within the nanocrystal throughout the transition, along with missing density which indicates the presence of a newly formed rutile phase.

Strain Programming of Oxygen Octahedral Symmetry in Perovskite Oxide Thin Films

AbstractThe collective rotations of oxygen octahedra play an important role in determining the physical properties of functional perovskite oxides. The epitaxial strain can serve as an effective means to modify the oxygen octahedral symmetry (OOS), i.e., oxygen octahedral rotation or tilt (OOR/OOT). However, the strain‐OOS coupling that may alter the details of the OOS, thereby the physical properties, has not been fully understood. In this work, it is demonstrated that epitaxial strain can not only induce a structural phase transition but also precisely tune the degree of OOR. The correlated metal CaNbO3, which is orthorhombic, is studied by growing as epitaxial thin films. By imposing epitaxial strain, it is found that the film undergoes a structural phase transition from orthorhombic to tetragonal upon fully straining (i.e., from a+b−b− to a0a0c−). In unstrained films, the octahedral rotation along the c‐axis is as large as 15.7° that can be tuned to 6.6° by strain. This finding offers a general approach to manipulating OOR/OOT via strain engineering in complex oxide heterostructures.

Theoretical study on the lattice distortion and electronic structure in the Al–Co–Cr–Fe–Ni multicomponent alloys

At present, the phase structure design for multicomponent alloys (MCAs) is mainly based on the valence electron concentration (VEC). In this work, based on the classification of valence electrons by the empirical electron theory of solid and molecule (EET), the VEC is further dissected and found that the covalent electron concentration (CEC) can be used as a criterion for determining the phase structure of MCAs. The Alx(CoCrFeNi)1-x (x = 0, 1/24, 1/12 and 1/8) alloys are designed on the basis of the CEC. The effect of Al content on the lattice distortion in the FCC phase of the alloys by combining density functional theory (DFT) and EET. The results show that the degree of lattice distortion in the FCC phase gradually increases with the increase of Al content and the interatomic bonding decreases before the phase structure transition. The alloy reaches the criticality of the phase structure transition when the Al content is 9.48 at.%.

Identifying and controlling the order parameter for ultrafast photoinduced phase transitions in thermosalient materials

The drastic shape deformation that accompanies the structural phase transition in thermosalient materials offers great potential for their applications as actuators and sensors. The microscopic origin of this fascinating effect has so far remained obscure, while for technological applications, it is important to learn how to drive transitions from one phase to another. Here, we present a combined computational and experimental study, in which we have successfully identified the order parameter for the thermosalient phase transition in the molecular crystal 2,7-di([1,1'-biphenyl]-4-yl)-fluorenone. Molecular dynamics simulations reveal that the transition barrier vanishes at the transition temperature. The simulations further show that two low-frequency vibrational-librational modes are directly related to the order parameter that describes this phase transition, which is supported by experimental Raman spectroscopy studies. By applying a computational THz pulse with the proper frequency and amplitude we predict that we can photoinduce this phase transition on a picosecond timescale.