Low-temperature Monoclinic Phase Research Articles

Topotactic Reduction-Induced Stabilization of β-La2Mo2O8.68 Phase: Structure, Static Oxygen Disorder, and Electrical Properties.

La2Mo2O9 is acknowledged as an exceptional oxide ion conductor. It undergoes a reversible phase transition around 580 °C from the nonconductive low-temperature monoclinic α-La2Mo2O9 phase to the highly conductive high-temperature cubic β-La2Mo2O9 phase. In addition, La2Mo2O9 demonstrates complex chemistry under reducing conditions. This study reports, for the first time, the stabilization at ambient temperature of a novel cubic phase through a topotactic reduction of α-La2Mo2O9 employing CaH2. This phase contains approximately ∼3 atom % oxygen vacancies relative to the nominal composition (La2Mo2O8.68(1)). The cubic symmetry is associated with a static distribution of these vacancies, in contrast to the dynamic distribution observed in the high-temperature cubic β-La2Mo2O9 phase reported previously. Additionally, the material exhibits mixed-ion-electronic conduction, which expands its potential use in applications requiring both ionic and electronic transport.

Contribution of oxygen vacancies to phase transition and ferroelectricity of Al:HfO2 films

We investigate the effects of oxygen vacancies on the ferroelectric behavior of Al:HfO2 films annealed in O2 and N2 atmosphere. X-ray photoelectron spectroscopy results showed that the O/Hf atomic ratio was 1.88 for N2-annealed samples and 1.96 for O2-annealed samples, implying a neutralization of oxygen vacancies during O2 atmosphere annealing. The O2-annealed films exhibited an increasing remanent polarization from 23 μC cm−2 to 28 μC cm−2 after 104 cycles, with a negligible leakage current density of ∼2 μA cm−2, while the remanent polarization decreased from 29 μC cm−2 to 20 μC cm−2 after cycling in the N2-annealed films, with its severe leakage current density decreasing from ∼1200 μA cm−2 to ∼300 μA cm−2. A phase transition from the metastable tetragonal (t) phase to the low-temperature stable orthorhombic (o) phase and monoclinic (m) phase was observed during annealing. As a result of the fierce· competition between the t-to-o transition and the t-to-m transition, clear grain boundaries of several ruleless atomic layers were formed in the N2-annealed samples. On the other hand, the transition from the t-phase to the low-temperature stable phase was found to be hindered by the neutralization of oxygen vacancies, with almost continuous grain boundaries observed. The results elucidate the phase transformation caused by oxygen vacancies in the Al:HfO2 films, which may be helpful for the preparation of HfO2-based films with excellent ferroelectricity.

Nanostructured Vanadium Dioxide Materials for Optical Sensing Applications.

Vanadium dioxide (VO2) is one of the strongly correlated materials exhibiting a reversible insulator-metal phase transition accompanied by a structural transition from a low-temperature monoclinic phase to high-temperature rutile phase near room temperature. Due to the dramatic change in electrical resistance and optical transmittance of VO2, it has attracted considerable attention towards the electronic and optical device applications, such as switching devices, memory devices, memristors, smart windows, sensors, actuators, etc. The present review provides an overview of several methods for the synthesis of nanostructured VO2, such as solution-based chemical approaches (sol-gel process and hydrothermal synthesis) and gas or vapor phase synthesis techniques (pulsed laser deposition, sputtering method, and chemical vapor deposition). This review also presents stoichiometry, strain, and doping engineering as modulation strategies of physical properties for nanostructured VO2. In particular, this review describes ultraviolet-visible-near infrared photodetectors, optical switches, and color modulators as optical sensing applications associated with nanostructured VO2 materials. Finally, current research trends and perspectives are also discussed.

Shear strain stabilized high-temperature metallic monoclinic VO2 variants with symmetry permission

In this work, temperature-dependent in situ x-ray diffraction reciprocal space mappings are employed to reveal three-dimensional lattice shear deformation in epitaxial VO2 variants, and linear Lagrangian strain tensors of epitaxial lattices are deduced by metric tensors of the reciprocal space at various temperatures. An equilibrium modulated-monoclinic phase is identified above the critical temperature, which is permitted in a translationengleiche subgroup from high symmetry with an index of 4. Being different from the conventional low-temperature monoclinic phase, the high-temperature monoclinic phase presents a specific metallic feature, which is ascribed to the Mott–Hubbard and charge density wave mechanisms. Under the proposed general procedure, which precisely identifies the normal and shear strain status of deformed lattices, it is demonstrated that the structural symmetry reduction under shear deformation unambiguously acts as the origin of unexpected metallic modulated-monoclinic VO2 at high temperatures. Our results highlight the significance of precise detection and contribution of shear deformation in various fields of physics.

Structural phase transition and transport properties in topological material candidate NaZn4As3

We report a comprehensive study on a layered-structure compound of NaZn4As3, which has been predicted to be an ideal topological semimetal (TSM) candidate. It is found that NaZn4As3 undergoes a structural transformation from high temperature rhombohedral to a low temperature monoclinic phase. The electric resistivity exhibits a metal-to-insulator-like transition at around 100 K, and then develops a plateau at low temperature, which might be related to the protected topologically conducting surface states. Our first-principles calculation confirms further that NaZn4As3 is a topological insulator (TI) for both different phases rather than a previously proposed TSM. The Hall resistivity reveals that the hole carriers dominate the transport properties for the whole temperature range investigated. Furthermore, an obvious kink possibly associated to the structure transition has been detected in thermopower around ∼170 K. The large thermopower and moderate κ indicate that NaZn4As3 and /or its derivatives can provide a good platform for optimizing and studying the thermoelectric performance.

Facile novel acid coprecipitation synthesis of BiPO4 polymorphs: Enhanced photocatalytic degradation of the antibiotic AMX and the dyes RhB, MB and MO

BiPO4 photocatalyst was synthesized by an acid coprecipitation route followed by heat treatments. The effect of the annealing process was correlated to the formation of BiPO4 polymorphs and the enhancement of photocatalytic activities. Bismuth phosphate calcined at 500 °C (BiP-500) showed the highest photocatalytic performance in the presence of Rhodamine B dye, due to the formation of both low-temperature monoclinic phase (LTMP) and high-temperature monoclinic phase (HTMP) on the one hand and the developed particle size on the other. The efficient photocatalyst was also used to photodegrade a wider range of organic pollutants: methylene blue, methyl orange and amoxicillin. The pH environment concerning the degradation of the anionic dye was studied and revealed increased activities in basic media. The radical superoxide species were highlighted as the major role in the photodegradation mechanism, according to the active radical species trapping experiments. Photoluminescence measurements performed under UV excitation revealed green and orange emissions with the greatest intensity in BiPO4 annealed at 400 and 450 °C.

Methylhydrazinium]2PbCl4, a Two-Dimensional Perovskite with Polar and Modulated Phases

Two-dimensional (2D) lead halide perovskites are a family of materials at the heart of solar cell, light-emitting diode, and photodetector technologies. This perspective leads to a number of synthetic efforts toward materials of this class, including those with prescribed polar architectures. The methylhydrazinium (MHy+) cation was recently presumed to have an unusual capacity to generate non-centrosymmetric perovskite phases, despite its intrinsically nonchiral structure. Here, we witness this effect once again in the case of the Ruddlesden–Popper perovskite phase of formula MHy2PbCl4. MHy2PbCl4 features three temperature-dependent crystal phases, with two first-order phase transitions at T1 = 338.2 K (331.8 K) and T2 = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase I, with the centrosymmetric space group Pmmn, through the room-temperature modulated phase II, with the average structure being isostructural to I, to the low-temperature monoclinic phase III, with non-centrosymmetric space group P21. The intermediate phase II is a rare example of a modulated structure in 2D perovskites, with Pmmn(00γ)s00 superspace symmetry and modulation vector q ≅ 0.25c*. MHy2PbCl4 beats the previous record of MHy2PbBr4 in terms of the shortest inorganic interlayer distance in 2D perovskites (8.79 Å at 350 K vs 8.66 Å at 295 K, respectively). The characteristics of phase transitions are explored with differential scanning calorimetry, dielectric, and Raman spectroscopies. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy2PbCl4 also exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K. The demonstration of polar characteristics for one more member of the methylhydrazinium perovskites settles a debate about whether this approach can present value for the crystal engineering of acentric solids similar to that which was recently adopted by a so-called fluorine substitution effect.

Temporal interaction of hybrid signals in various phases of Eu3+: BiPO4 through photon–phonon dressing

Laser interaction with doped crystals exhibiting photon–photon and photon–phonon coupling has been focused on recently. In pretext, here we report the spectral and temporal profile interaction of two lasers excitation through various phases of Eu3+: BiPO4 crystals. We reveal that spectral-temporal profile interaction of hybrid signals (coexisting fluorescence and spontaneous four-wave mixing) are dressed by nested and cascade processes of two-photons (two-phonon). Such interaction comes from thermal phonon constructive and phase transition phonon destructive dressing. The spectral and temporal (profile) interactions are interrelated and reduced by about 2-times due to two-photon nested dressing in contrast to the interaction through the sum of each laser excitation. In contrast to a single laser, spectral (Fano)-dip interaction reduces by 2-times due to two-photon destructive dressing coupling. Moreover, thermal phonon dressing at 300 K exhibits 3-times more extensive temporal interaction than that at 77 K. The phase transition phonon dressing for a half hexagonal and half low-temperature monoclinic phase is about 1.5-times longer than that of the pure hexagonal phase of Eu3+: BiPO4. These results may help to understand the spectral-temporal relationship in the fields of nonlinear and quantum optics.

Thermal properties of single crystals of the low-positive thermal expansion material Al2W3O12

Investigations of the thermal conductivity and heat capacity of a monocrystalline sample Al2W3O12, along with computational investigations using density functional theory, reveal a complete picture of anisotropic thermoelastic properties of Al2W3O12, a low-positive thermal expansion material. The thermal conductivity at room temperature was essentially isotropic, with a directionally average value of 2.1 ± 0.2 W m−1 K−1, about twice the value reported for polycrystalline samples. The thermal conductivity shows a significant peak at about 20 K, and at 5 K corresponds to a phonon mean free path of about 2000 nm, much shorter than that of a typical high-quality oxide crystal with a more rigid framework. The flexible Al2W3O12 structure also gives rise to low-frequency modes, as evidenced by the high heat capacity in comparison with that of the constituent oxides. The thermal shock resistance figures of merit for moderate and severe thermal shock are both anisotropic, but high, with directionally average values of about 6500 W m−1 and 3300 K, respectively. An additional finding was that the phase transformation from the low-temperature monoclinic phase to the room-temperature orthorhombic phase shows considerable hysteresis and, evidenced by DSC on shards of single crystals, nucleation is a stochastic process.

Nanoscale self-organization and metastable non-thermal metallicity in Mott insulators

Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V2O3, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.

Effect of annealing temperatures on the structural and photoluminescence properties of Tb3+ ions doped BiPO4 nanophosphors

ABSTRACT In this paper the synthesis of BiPO4:Tb3+ phosphor by co-precipitation method and effects of annealing temperature on its structural, crystallinity and photoluminescence properties have been investigated. The crystallinity of the phosphor was found to increase up to 800°C of annealed temperature for 2 h without changing its phase. However, a change from low temperature monoclinic phase to high temperature monoclinic phase was observed when annealed at 1000°C for 2 h. Photoluminescence spectra showed strong and sharp emission peaks of Tb3+ upon excitation at 351 nm and the peaks intensities were found to increase with increase of annealing temperature up to 800°C. However intensities were found decreasing drastically for 1000°C annealed sample, which may be due to the change of phase upon annealing. Emission of cyan colour was observed from CIE study. These observations indicate that the prepared phosphors may have potential applications in solid-state lighting systems.

Ta2NiSe5 : A candidate topological excitonic insulator with multiple band inversions

The electronic structures and topological properties of the orthorhombic and monoclinic phases of the quasi-one-dimensional excitonic insulator Ta2NiSe5 are investigated based on density functional theory. In contrast to a single parity or band inversion across the Fermi level in many topological insulators studied previously, there are multiple parity and band inversions with or without spin-orbit coupling in both phases of Ta2NiSe5, resulting in more complex and topologically nontrivial electronic structures. The Dirac cone type surface states of the low-temperature monoclinic phase are also obtained. In this paper, we demonstrate that Ta2NiSe5 is a promising candidate as a three-dimensional topological excitonic insulator.

Thermal and Magnetic Field Switching in a Two-Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings.

A MnIII spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices.

Trimeron-phonon coupling in magnetite

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe$_3$O$_4$) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic $Cc$ structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation.

Reversible structural transition in monocrystalline Ni2FeGa microwires for shape-memory applications

We report on the production and characterization of Heusler-based Ni2FeGa microwires exhibiting two – way shape memory effect. The microwires are characterized by a monocrystalline structure with a strong preferred crystallographic orientation that shows [1 1 1] axis parallel to the wire’s axis for high-temperature L21 austenite phase, while the [0 1 7] axis is preferred for low-temperature monoclinic phase. Variation of crystallographic axis (and corresponding easy magnetization axis) leads to 1600% variation of magnetic permeability due to a 2% strain in axial direction. Such straining is reversible immediately after production without the necessity of further thermal treatment. These properties give the microwire function of very sensitive SMART actuators that can be easily produced in a large amount.

Structural Anharmonicity Explains Continuous Frequency Shifts of Intramolecular Ring Vibrations in a Hydrogen-Bonded Antiferroelectric Crystal

Rational principles from which one can design the energy storage capability of antiferroelectric crystalline materials composed of hydrogen-bonded molecular constituents remain deficient. In particular, there remains gaps in our understanding of the fundamental physical mechanisms by which microscopic electrostatics in this class of materials control macroscopic properties central to potential energy storage technologies. While molecular vibrations have been applied as probes of microscopic electrostatics in other areas of materials physical chemistry, their ability to report the electrostatics in antiferroelectrics remains an open question. In this study, we use temperature-dependent Raman spectroscopy to investigate the fundamental physical mechanism by which aromatic ring distortional vibrational peaks shift in the antiferroelectric phase of crystalline 2-trifluoromethylbenzimidazole (TFMBI). With density functional theory (DFT) calculations and a theoretical model of anharmonic contributions to vibrational frequencies, we propose the quartic anharmonic contribution to the interatomic potential energy of TFMBI explains the continuous vibrational peak shifts we observe in the temperature-dependent Raman spectra of this material. Specifically, we find the shifts likely result from thermally driven changes to the average occupation of other, lower frequency intramolecular vibrations interacting with the Raman-active ring distortion vibrations of crystalline TFMBI. To support this conclusion, we use explicit fits of our measured peak shifts to an equation for the fundamental vibrational transition we develop from a theoretical model of material anharmonicity and the harmonic frequencies found from DFT calculations. In addition to highlighting the importance of anharmonic contributions to the structural dynamics of molecular crystals, this analysis also allows us to assess the ability of DFT calculations to predict the harmonic frequency of the aromatic ring vibrations of TFMBI correctly. Furthermore, we characterize the correlation between the structure of a low-temperature monoclinic phase of TFMBI unreported previously and discontinuous shifts in the positions of three peaks in the Raman spectrum of TFMBI. This characterization indicates the quartic anharmonic coupling does not depend on the hydrogen-bonding structure of the material. Our results illustrate the care researchers must take to appropriately interpret the physical mechanism explaining vibrational peak shifts in hydrogen-bonded antiferroelectric materials.

Microstructural, crystallographic, and luminescent analysis after grinding high-temperature monoclinic bismuth phosphate

Phase transformation of inorganic materials is usually done via either the re-arrangement of structural units at high pressures and/or temperatures or bottom-up construction from cations and anions using solution chemistry-related approaches. The purpose of this study was to evaluate the microstructural, crystallographic, and luminescent changes after grinding a high-temperature monoclinic phase (H-BPO) of bismuth phosphate. Initially, the pre-ground H-BPO was formed by solid-state reaction of the starting materials. After mechanical grinding, the pre-ground H-BPO gradually transformed to another low-temperature monoclinic phase (L-BPO). Furthermore, a switchable phase transformation from L-BPO to H-BPO was observed for the sample after grinding followed by annealing at high temperature. For the samples obtained by grinding or grinding followed by annealing, the microstructure, crystallography, and luminescence properties were characterized by SEM, XRD with Rietveld analysis, FT-IR, Raman, and photoluminescence (PL) spectra in detail. The switchable phase transformation was elucidated by taking into account the effects of the particle size, and lattice strain as well as other structural factors such as the arrangements of BiO8 polyhedra and PO4 tetrahedra, unit lattice volume (V/Z), and the symmetry of PO4 groups. The phase transition from H-BPO to L-BPO occurred at a relatively low pressure generated under grinding, which is irreversible and 1st order nature. Moreover, with the switchable phase transformation, the luminescence properties including emission intensity, decay time, and quantum efficiency were assessed.

Sliding twin-domains in self-heated needle-like VO2 single crystals

The prototypical metal-insulator transition in VO2 at 340 K is from a high-temperature rutile phase to a low-temperature monoclinic phase. The lower symmetry of the monoclinic structure removes the degeneracy of the two equivalent directions of the tetragonal structure, giving rise to twin domains. Since formation of domain walls require energy most needle-like monoclinic single crystal are single-domain. The mixed metal-insulator state in self-heated needle-like single crystals exhibits various domain patterns, the most remarkable being static insulating triangular domains embedded in the metal and narrow insulating domains sliding along the metallic background in the direction of the electric current. Reported here are results obtained for some rare needle-like twinned VO2 single crystals. Such sample revealed a unique feature: joint static triangular twins emit sliding twin domains, first overlapping and later disjoining. Dark and bright twins and dim metallic background were seen for optimal orientation under a microscope, due to polarization by reflection.

Probing phase transition in VO2 with the novel observation of low-frequency collective spin excitation

VO2 is well known for its first order, reversible, metal-to-insulator transition (MIT) along with a simultaneous structural phase transition (SPT) from a high-temperature metallic rutile tetragonal (R) to an insulating low-temperature monoclinic (M1) phase via two other insulating metastable phases of monoclinic M2 and triclinic T. At the same time, VO2 gains tremendous attention because of the half-a-century-old controversy over its origin, whether electron-electron correlation or electron-phonon coupling trigger the phase transition. In this regard, V1-xMgxO2 samples were grown in stable phases of VO2 (M1, M2, and T) by controlled doping of Mg. We have observed a new collective mode in the low-frequency Raman spectra of all three insulating M1, M2 and T phases. We identify this mode with the breather (singlet spin excitation) mode about a spin-Pierls dimerized one dimensional spin ½ Heisenberg chain. The measured frequencies of these collective modes are phenomenologically consistent with the superexchange coupling strength between V spin ½ moments in all three phases. The significant deviation of Stokes to anti-Stokes intensity ratio of this low-frequency Raman mode from the usual thermal factor exp(hʋ/KBT) for phonons, and the orthogonal dependency of the phonon and spinon vibration in the polarized Raman study confirm its origin as spin excitations. The shift in the frequency of spin-wave and simultaneous increase in the transition temperature in the absence of any structural change confirms that SPT does not prompt MIT in VO2. On the other hand, the presence of spin-wave confirms the perturbation due to spin-Peierls dimerization leading to SPT. Thus, the observation of spin-excitations resulting from 1-D Heisenberg spin-½ chain can finally resolve the years-long debate in VO2 and can be extended to oxide-based multiferroics, which are useful for various potential device applications.