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Reorientational dynamics of organic cations in perovskite-like coordination polymers
摘要： Here we report the dynamics of organic cations as guest molecules in a perovskite host-framework. The molecular motion of CH3NH3+ (MAFe), (CH3)2NH2+ (DMAFe) and (CH3)3NH+ (TrMAFe) in the cage formed by KFe(CN)63? units was studied using a combination of experimental methods: (i) thermal analysis, (ii) dielectric and electric studies, (iii) optical observations, (iv) EPR and 1H NMR spectroscopy and (v) quasielastic neutron scattering (QENS). In the case of MAFe and TrMAFe, the thermal analysis reveals one solid-to-solid phase transition (PT) and two PTs for the DMAFe crystal. A markedly temperature-dependent dielectric constant indicates the tunable and switchable properties of the complexes. Also, their semiconducting properties are confirmed by a dc conductivity measurement. The broadband dielectric relaxation is analyzed for the TrMAFe sample in the frequency range of 100 Hz–1 GHz. QENS shows that we deal rather with the localized motion of the cation than a diffusive one. Three models, which concern the simultaneous rotation of the CH3 and/or NH3 group, π-flips and free rotations of the organic cation, are used to fit the elastic incoherent structure factor. The 1H NMR spin–lattice relaxation time for all compounds under study, as well as the second moments, has been measured in a wide temperature range. In all studied samples, the temperature dependence of the second moment of the proton NMR line indicated the gradual evolution of the molecular movements from the rigid state up to a highly disordered one.
关键词： quasielastic neutron scattering,phase transitions,perovskite,coordination polymers,dielectric properties,NMR spectroscopy,dynamics,organic cations
更新于2025-11-14 17:28:48
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Significantly enhanced room temperature electrocaloric response with superior thermal stability in sodium niobate-based bulk ceramics
摘要： Converting electrical into thermal energy in ferroelectrics path the way for a novel cooling technology, be able to open up new fields of applications. For electrocaloric refrigeration, large temperature change (ΔT) and excellent temperature stability are highly desired. Unfortunately, a large room temperature ΔT in lead free bulk ceramics was usually obtained at the expense of temperature stability, and vice versa, limiting their practical applications. In this work, composition engineering is carried out as an important strategy to tune phase transition to room temperature and simultaneously induce diffuse phase transition to achieve excellent temperature stability. A large room temperature ΔT is realized by the application of large electric fields, possibly due to the optimization of the sintering process in sodium niobate based ceramics. The 0.78NaNbO3-0.22BaTiO3 ceramic is found to exhibit both large room temperature ΔT (~ 0.70 K) and superior temperature stability (±1.4% variation in ΔT from 300 K to 380 K). Those properties are superior to previously reported lead-free bulk ceramics. This work provides a guideline for the development of high-performance ceramic materials such as NaNbO3-ABO3 (A=Ba, Sr and Ca; and B=Ti and Zr) for electrocaloric refrigeration. Most importantly, this work expands the applications of NN-based ceramics from electric energy storage, electrostrictive and piezoelectric applications, to a new field, i.e. solid-state refrigeration. This makes NN-based ceramics very promising multifunctional materials for device integration.
关键词： Sodium niobate,relaxor ferroelectrics,electrocaloric effect,adiabatic temperature change,diffuse phase transitions
更新于2025-11-14 17:28:48
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Nano-BaTiO3 phase transition behavior in coated BaTiO3-based dielectric ceramics
摘要： In this work, the phase structure of BaTiO3 nanopowders produced by the alkoxide-hydroxide and the hydrothermal method, respectively, was systemically investigated. BaTiO3 nanopowders with cubic phase produced by the alkoxide-hydroxide method could transform to tetragonal phase when heated to about 1100oC. Cubic to tetragonal phase transformation behavior of BaTiO3 nanopowders coated with 0.3BZT-0.7BT or 0.03Nb2O5-0.01Co2O3 was studied. The internal stress within core-shell structure was proposed to explain the BaTiO3 phase transformation behaviors. The mismatch of thermal expansion coefficient between core and shell plays a crucial role in cubic to tetragonal phase transformation of BaTiO3. By tuning the composition of shell and the ratio of the shell to the core, the cubic BaTiO3 core can transform to tetragonal phase successfully after sintering at 1100oC in BaTiO3 based ceramics with core-shell structure and it is mainly resulted by the reduced internal stress between the shell and core.
关键词： Phase transitions,BaTiO3 nanopowders,Internal stress.,Core-shell structure
更新于2025-09-23 15:23:52
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Crystal structure, optical and electrical properties of metal-halide compound [C7H16N2][ZnCl4]
摘要： A new organic–inorganic hybrid compound [C7H16N2][ZnCl4] was synthesized by hydrothermal method and characterized by single-crystal X-ray diffraction, IR and Raman spectroscopy, optical absorption, differential scanning calorimetry and dielectric measurements. Single crystal diffraction results showed that [C7H16N2][ZnCl4] crystallizes in the monoclinic system, space group P21/c at room temperature. In the molecular arrangement, the tetrachlorozincate anions are connected to organic cations through N-H...Cl hydrogen bonds. The Raman and IR analyses confirm the presence of the organic groups and the anionic entities. UV–Visible absorption spectrum revealed the energy of the optical band gap. DSC measurements indicated that [C7H16N2][ZnCl4] undergoes three sequential phase transitions at 287, 338 and 356 K. The dielectric study proved the ferroelectric properties below the 338K and indicates their classical character for this compound. The analysis of Nyquist plots revealed the contribution of the bulk mechanism and the grain boundaries.
关键词： Phase transitions,Hybrid material,Crystal structure,Optical absorption,Ferroelectric properties,Conduction mechanism
更新于2025-09-23 15:23:52
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Synthesis, crystal structures, second harmonic generation response and temperature phase transitions of two noncentrosymmetric Cu(II)-hybrid halides compounds: [(R)-C7H16N2][CuX4] (X?= Cl or Br)
摘要： (R)-(+)-3-aminoquinuclidine was used in the synthesis of [(R)-C7H16N2][CuCl4] (1) and [(R)-C7H16N2][CuBr4] (2), which both contain similar [CuX4]2- anions (X = Cl or Br). The structures of the two compounds were determined using single-crystal X-ray diffraction. The use of enantiomerically pure sources of (R)-C7H14N2 forces crystallographic noncentrosymmetry. These materials crystallize in the chiral space group P212121 (No. 19), which exhibits the enantiomorphic crystal class 222 (D2). In the molecular arrangement, the [CuX4]2- anions are linked to the organic cations through N-H...X and C-H...X hydrogen bonds to form cation-anion-cation molecular units, which are held together by means of offset face-to-face interactions giving a three-dimensional network. Thermal stability of the crystals was ascertained by TG measurement. Compounds (1) and (2) display several phases transition with higher transition temperature at T = 100°C. The Kurtz and Perry powder method using Nd:YAG laser shows that their second harmonic generation (SHG) efficiencies are about 0.81 and 0.82 times as large as that of KH2PO4 (KDP), respectively. Such a chiral hybrid metal halides skeleton could provide a new platform for future engineering in the areas including information storage, light modulators and optoelectronic functionalities.
关键词： nonlinear optics,second harmonic generation,crystal structure,chiral hybrid halide,phase transitions
更新于2025-09-23 15:23:52
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Detecting the Burns temperature in Na <sub/>0.5</sub> Bi <sub/>0.5</sub> TiO <sub/>3</sub> -xBaTiO <sub/>3</sub> lead-free relaxor ferroelectrics by means of acoustic emission
摘要： Na0.5Bi0.5TiO3-xBaTiO3 ceramics samples (0 < x < 0.8) were studied by means of an acoustic emission method. The characteristic temperatures such as the Burns temperature, TB, the transition to paraelectric phase, Tp, the temperature maximum of the dielectric constant, Tm, and the intermediate temperature, T ? were detected and their dependences on x were plotted. Based on the comparison of the obtained data with the previously published ones, it is shown that the Burns temperature, TB, cannot be reliably determined by the deviation of the reverse dielectric constant, 1/ε(cid:3), from a straight line.
关键词： phase transitions,relaxor ferroelectrics,Na0.5Bi0.5TiO3-xBaTiO3,Burns temperature,acoustic emission
更新于2025-09-23 15:23:52
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Phase formations and red-luminescence enhancement of Eu3+-activated NaBaY(BO3)2 phosphors
摘要： Eu3+-activated Na1-xKxBaY(BO3)2 (x = 0.1–1.0) phosphor was prepared via facile solid-state reaction ceramic method. The structural Rietveld refinements were completed to confirm the phase formations. The luminescence properties were reported such as photoluminescence excitation (PLE) and emission (PL) spectra, decay lifetime, experimental quantum efficiency (QE) and thermal stabilities. The phase formations and the luminescence properties of the phosphors have a dependence on the K+-substituting for Na+ in the lattices. The results showed that Na1-xKxBaY0.5Eu0.5(BO3)2 (x = 0.1–1.0) phosphors formed the trigonal NaBaY(BO3)2 single phase (Na-phase, space-group of R-3, No.148). Increasing the K+-substituting for Na+ (x ≤ 0.3) induced the appearance of KBaY(BO3)2 phase (K-phase, space-group of R-3 m, No. 166) together with the Na-phase (0.3 < x < 0.6). Heavily K-substituted samples crystallized into a single K-phase. Luminescence characteristics of Eu3+ ions were investigated on the evolution of the substituting-level. It was observed that the phosphors presented the improved emission intensity and thermal stability in the mixed phases.
关键词： Phase transitions,Photoluminescence,Optical materials and properties,Europium,Borates
更新于2025-09-23 15:22:29
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Theoretical and Experimental Features of Nano-Crystals Rochelle Salt: Piezoelectric Resonance and Phase Transition
摘要： A new methodology based on Three-Electrode System (TES) technique was used to measure both, elastic (C44) and piezoelectric (d14) constants of Rochelle salt (RS) crystal. This technique was also used to observe the phase transitions at ?18 °C and 24 °C, manifested as an increase of the output signal at these temperatures. Mechanical wave attenuation coefficient (α) into the crystal was found. Also, first principle methodology in the framework of the density functional theory was utilized to confirm the electronic structure of Rochelle Salt. The results present high accuracy theoretical/experimental as well as a complete agreement with those known in the literature done by different methodologies.
关键词： Phase Transitions,Rochelle Salt Crystal,Piezoelectric Device,Ab Initio Simulation
更新于2025-09-23 15:22:29
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Pressure-induced phase transitions, electronic, elastic and vibrational properties of zinc oxide under high pressure
摘要： In this work, the crystal structure of the ZnO was studied under high hydrostatic pressure using ab initio calculations. Pressure–volume relationships and structural transitions in ZnO were investigated using Siesta method. A first-order transition from the hexagonal wurtzite (B4) structure with space group P63mc to the cubic NaCl (B1) structure with space group Fm(cid:2)3m was successfully observed. A transition was also observed from Fm(cid:2)3m to another cubic CsCl (B2) structure with space group Pm(cid:2)3m for ZnO. These phase transitions which occur around 9 and 119.5 GPa were also analyzed from the total energy and enthalpy calculations. In addition, electronic, elastic and vibrational properties of ZnO were analyzed based on the high pressure.
关键词： Bulk crystals,Electronic structure calculations,Pressure,Phase transitions
更新于2025-09-23 15:22:29
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Structural investigation of oxygen stoichiometry during thermocycles in PMN-28PT
摘要： The current research focuses on the role of oxygen partial pressure on structure and phase transitions of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 with x ? 0.28 (PMN-28PT). The changes in oxygen content have been traced through weight gain/loss during 160 cycles between 250 and 300 (cid:2)C. The structural phase transitions have been investigated by in-situ high temperature powder X-ray diffraction and high temperature Raman spectroscopy. The variations in oxygen content in PMN-28PT result in structural changes and have a signi?cant effect on electrical impedance. It is suggested that bonding between oxygen and Ti and Pb atoms in the perovskite structure of PMN-28PT can be controlled by low oxygen partial pressure heat treatment.
关键词： Crystal structure,Phase transitions,Oxide materials,Thermodynamic properties,Thermal analysis
更新于2025-09-23 15:22:29