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Novel bimetallic MOF phosphors with an imidazolium cation: structure, phonons, high- pressure phase transitions and optical response
摘要： We report the synthesis, crystal structure, phonons and luminescence properties of three novel heterometallic metal organic frameworks (MOFs) with perovskite-like topology of the following formulas: [C3H5N2]Na0.5Cr0.5(HCOO)3 (ImNaCr), [C3H5N2]Na0.5Al0.5(HCOO)3 (ImNaAl) and [C3H5N2]Na0.5Al0.475Cr0.025(HCOO)3 (ImNaAlCr with 5 mol% of Cr3+). ImNaCr crystallizes in a monoclinic system (P2/n space group) with one imidazolium cation (Im+) in an asymmetric unit forming six N–H?O and four C–H?O hydrogen bonds. In contrast to other known heterometallic MOFs, the complete substitution of Cr3+ ions with smaller Al3+ ions leads to a change of the crystal symmetry. ImNaAl adopts a monoclinic P21/n space group with two distinct Im+ cations and di?erent H-bonding patterns. The DSC measurements and XRD single-crystal studies show that the studied crystals do not undergo structural phase transitions in the 80–400 K range. The high-pressure Raman studies of ImNaCr reveal the presence of two reversible structural instabilities, ?rst in the 0.4–1.1 GPa range and second near 4 GPa. The ?rst pressure-induced phase transition involves weak distortion of the metal-formate framework, while the second one is associated with partial and reversible amorphization of the sample. We discuss the stability of heterometallic formate MOFs depending on their building blocks. The luminescence measurements show that both the fully concentrated crystal (ImNaCr) and the diluted one (ImNaAlCr) exhibit a Cr3+-based emission characteristic of intermediate ligand ?eld strength. We also show that the spectroscopic properties of heterometallic MOFs depend strongly on the templated cation, i.e. the decreasing size of the organic cation leads to an increase in the crystal ?eld.
关键词： high-pressure phase transitions,phonons,optical response,structure,imidazolium cation,bimetallic MOF phosphors
更新于2025-09-10 09:29:36
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Static and dynamic phases of a Tonks–Girardeau gas in an optical lattice
摘要： We investigate the properties of a Tonks–Girardeau gas in the presence of a one-dimensional lattice potential. Such a system is known to exhibit a pinning transition when the lattice is commensurate with the particle density, leading to the formation of an insulating state even at in?nitesimally small lattice depths. Here we examine the properties of the gas at all lattices depths and, in addition to the static properties, also consider the non-adiabatic dynamics induced by the sudden motion of the lattice potential with a constant speed. Our work provides a continuum counterpart to the work done in discrete lattice models.
关键词： Tonks–Girardeau gas,phase transitions,non-equilibrium dynamics,optical lattices
更新于2025-09-09 09:28:46
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Mn doped BiNbO4 ceramics: Thermal stability, phase transitions, magnetic properties, NEXAFS and ESR spectroscopy
摘要： Thermal stability, magnetic properties, ESR (Electron Spin Resonance) and NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectra of the orthorhombic and triclinic modi?cations of solid solutions BiNb1-xMnxO4-d have been studied. X-ray diffraction determined the reversibility of the phase transition from the high-temperature triclinic modi?cation to the orthorhombic modi?cation at 950 (cid:1)C in the BiNb1-xMnxO4-d solid solutions. The ESR spectra of the triclinic BiNb1-xMnxO4-d revealed the sextet structure of Mn(II) ions with 8.4 mT splitting and some features at g ? 3.80 and 1.47, and a broad diffuse band with g ~ 2.2 having a sextet with 8e9 mT splitting and g ? 2.0 against its background. The measurements of magnetic susceptibility allowed us to calculate the parameters of exchange interactions in dimers and the distribution of manganese atoms (II), (III) and (IV) in dependence on concentrations of the solid solutions in the orthorhombic and triclinic modi?cations. It was established that in the manganese-doped solid solutions of triclinic modi?cation, the degree of aggregation and the nuclearity of the clusters were higher than in the solid solutions of orthorhombic modi?cation. The clusters were presented in the forms of dimers and trimers of Mn(II), Mn(III) and Mn(IV) atoms with different valence atoms with antiferro- and ferromagnetic types of exchange. The solid solutions BiNb1-xMnxO4-d as well as manganese oxides MnO, Mn2O3 and MnO2 were studied by the NEXAFS spectroscopy in order to determine the degrees of oxidation of manganese atoms. The DSC (Differential Scanning Calorimetry) curves of the samples BiNb1-xMnxO4-d revealed the endothermic effects associated with the phase transition from the orthorhombic modi?cation to the triclinic one at 1050 (cid:1)C (x ? 0.03) and 1048 (cid:1)C (x ? 0.06). The melting peaks were observed at 1231 (cid:1)C (x ? 0.03) and 1220 (cid:1)C (x ? 0.06), respectively. The differences in the magnetic characteristics and in the results of ESR and NEXAFs spectroscopy of the BiNb1-xMnxO4-d of orthorhombic and triclinic modi?cations were due to the relative content of Mn(II), Mn(III), Mn(IV) ions and the degree of distortion of their local surroundings, composition and nature of exchange interactions in the clusters of manganese atoms.
关键词： ESR,Phase transitions,NEXAFS,Thermal stability,Magnetic properties
更新于2025-09-09 09:28:46
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Ionic modulation and ionic coupling effects in MoS2 devices for neuromorphic computing
摘要： Coupled ionic–electronic effects present intriguing opportunities for device and circuit development. In particular, layered two-dimensional materials such as MoS2 offer highly anisotropic ionic transport properties, facilitating controlled ion migration and efficient ionic coupling among devices. Here, we report reversible modulation of MoS2 films that is consistent with local 2H–1T′ phase transitions by controlling the migration of Li+ ions with an electric field, where an increase/decrease in the local Li+ ion concentration leads to the transition between the 2H (semiconductor) and 1T′ (metal) phases. The resulting devices show excellent memristive behaviour and can be directly coupled with each other through local ionic exchange, naturally leading to synaptic competition and synaptic cooperation effects observed in biology. These results demonstrate the potential of direct modulation of two-dimensional materials through field-driven ionic processes, and can lead to future electronic and energy devices based on coupled ionic–electronic effects and biorealistic implementation of artificial neural networks.
关键词： 2H–1T′ phase transitions,ionic modulation,MoS2 devices,synaptic cooperation,neuromorphic computing,ionic coupling,synaptic competition,memristive behaviour
更新于2025-09-09 09:28:46
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Orientation dependence of electric field induced phase transitions in lead-free (Na <sub/>0.5</sub> Bi <sub/>0.5</sub> )TiO <sub/>3</sub> -based single crystals
摘要： The orientation dependence of the electric field induced strain and phase transitions in 0.92(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.02(K0.5Na0.5)NbO3 (NBT-6BT-2KNN) single crystals has been investigated. The evolution of Raman spectra with electric field reveals that a tetragonal ferroelectric phase is initially induced at E=14kV/cm and completed above E=25kV/cm for [001] oriented single crystals. When the electric field is applied along [111] direction, a partial phase transition from pseudocubic to rhombohedral structure is triggered at E=19kV/cm, which is higher than that for inducing tetragonal ferroelectric phase along [001] direction. Both field-induced phase structures and stability of NBT-6BT-2KNN single crystal are strongly associated with the crystallographic orientations. These results provide a better understanding to the field-induced macroscopic strain in lead-free NBT-based ferroelectrics.
关键词： X-ray diffraction,lead-free ferroelectrics,electric field induced phase transitions,Raman spectroscopy,NBT-based single crystals
更新于2025-09-09 09:28:46
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Structural Aspects at Phase Transitions in Ferroelectric Copolymers of Vinylidenefluoride and Tetrafluoroethylene
摘要： low-voltage Permittivity and conductivity temperature dependences of a ferroelectric copolymer of vinylidenefluoride VDF with tetrafluoroethylene TFE in the ferroelectric to paraelectric phase transition range were investigated. The as received films obtained by crystallization from a solution had the metastable structure shown in particular in coexistence of ferroelectric and paraelectric phases below a Curie point. Temperature dependences of electrical characteristics in the first and second cycles of heating differ. Low glass transition (?40 oC) facilitate intensive micro-Brownian chains mobility in the amorphous phase at ambient temperatures and above. As per the x-ray and spectroscopic data, the first heating cycle will result in a recrystallization process for molecular structure improvement. Asymmetry of temperature dependence of a dielectric susceptibility in a vicinity of “order-disorder” phase change is found out. For the first time paid attention to the fact that for crystalline polymer ferroelectrics interpretation of the parameters of the temperature dependences of the dielectric susceptibility in the cycle of heating near the phase transition point should take into account the possibility of the premelt processes. First the method of IR spectroscopy shows that the melting of ordered structures, emerging secondary crystallization is accompanied by a change of the basic parameters (frequency position, the integral intensity and half-width) component of the doublet of valence vibrations of С-Н groups. Such structural changes lead to the emergence of “anomalies” on the temperature dependences of conductivity.
关键词： conductivity,IR-spectroscopy,phase transitions,Ferroelectric polymers,structure
更新于2025-09-04 15:30:14
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[IEEE 2018 19th International Conference of Young Specialists on Micro/Nanotechnologies and Electron Devices (EDM) - Erlagol (Altai Republic), Russia (2018.6.29-2018.7.3)] 2018 19th International Conference of Young Specialists on Micro/Nanotechnologies and Electron Devices (EDM) - Multifunctional Magnetophotonic Structure Creation Based on Magnetic Properties of Micro and Nanoscale Clusters
摘要： The optical and magnetic properties, phenomena and effects of nano- and microparticles of clusters and other formations can potentially influence the propagation and processing of optical signals in magnetophotonic structures. The analysis of existing devices and the ones proposed for interpretation of their implementation allows the correct operation algorithm in correlation to structural characteristics and to multi-functional magnetophotonic structures of micro- and nanoscale. The magnetophotonic structure models are considered and analyzed, which makes it possible to find the correct approaches for their implementation. The choice of the specific magnetophotonic medium for a functional simulator is determined by the requirements of functional orientation, simplicity and efficiency, manufacturability and other factors.
关键词： nanoclusters,magnetic phase transitions,superparamagnetism,Photonics,resonance
更新于2025-09-04 15:30:14
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Phase and Defect Engineering of MoS <sub/>2</sub> Stabilized in Periodic TiO <sub/>2</sub> Nanoporous Film for Enhanced Solar Water Splitting
摘要： Phase and defect engineering of the heterostructured MoS2@TiO2 nanoporous film is investigated to achieve a broad solar spectrum light absorption and high solar water splitting efficiency. The phase transition from the semiconducting 2H-MoS2 to the metallic 1T-MoS2 is achieved by a hydrothermal exfoliation treatment. Experimental studies elucidate that the solar water splitting activity is greatly improved by forming 1T-MoS2 along with increasing S-vacancies because of the significantly enhanced surface plasmon resonance. The mixed-phase MoS2@TiO2 film shows a high H2 yield rate of 308 μmol h?1 cm?2 and long-term durability for 30 h, which is superior to the state-of-the-art catalysts for solar water splitting. This study offers a universal and efficient avenue to rationalize the plasmonic catalysts for solar water splitting and other energy and environmental applications.
关键词： solar water splitting,defect engineering,MoS2,heterostructures,phase transitions
更新于2025-09-04 15:30:14
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Pressure-Induced Phase Transitions in TiO <sub/>2</sub> Rutile Nanorods
摘要： TiO2 nanorods (diameter of 6-8 nm) with the rutile-type structure have been investigated by Raman spectroscopy under high pressure. Additional bands in the Raman spectrum at ambient pressure are interpreted as resulting from defects that break the Raman selection rules as the simulated vibrational density of states (vDOS) is in good agreement with the experimental spectrum. Under pressure, the rutile structure transforms to the baddeleyite one at P~16 GPa. After pressure cycle, the α-PbO2 phase is recovered. The sequence of phase is discussed in terms of thermodynamics and kinetics based on crystallographic relationships. Contrary to other 1D TiO2 nanomaterials that adopt a metastable structure, the nanorods studied in this work show little impact of the morphology on the phase transitions sequence compared to the bulk TiO2.
关键词： phase transitions,vibrational density of states,TiO2 nanorods,high pressure,Raman spectroscopy
更新于2025-09-04 15:30:14