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Effect on Local Structure and Phase Transition of Perovskite-Type [N(CH3)4]2Zn1-xCuxBr4 (x?=?0, 0.5, 0.7, and 1) Crystals with the Various Doping of Cu2+ Ions
摘要： This study focused on how the local structures in pure [N(CH3)4]2ZnBr4 crystal are affected by the partial replacement of Zn2+ ions with Cu2+ ions. The structures and phase transition temperatures TC of perovskite-type [N(CH3)4]2Zn1-xCuxBr4 (x = 0, 0.5, 0.7, and 1) mixed crystals were almost unchanged by the partial doping of Cu2+ ions. The environments for the local structures of [N(CH3)4]2Zn1-xCuxBr4 mixed systems were studied according to differences in the chemical shifts of the 1H magic angle spinning (MAS) NMR, 13C cross-polarization (CP)/MAS NMR, and 14N NMR spectra. The 1H and 13C NMR results showed that the local environments of 1H and 13C nuclei near TC are not affected by substituting Zn2+ ions with Cu2+ ions, whereas the 14N NMR results showed that the local environment is affected near TC. Consequently, the main indicators of the phase transition in [N(CH3)4]2Zn1-xCuxBr4 are related to the ferroelastic characteristics with different orientations.
关键词： phase transition,ferroelastic characteristics,NMR spectroscopy,perovskite structures,Metal-organic hybrids
更新于2025-09-23 15:21:01
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density functional
摘要： It was recently reported in experiments that at temperatures below 2500 K liquid nitrogen (N) remains molecular up to 120 GPa [Phys. Rev. Lett. 119, 235701 (2017)], which contradicts a liquid-liquid transition at 88 GPa and 2000 K predicted by PBE-GGA density functional. To clarify this, we perform extensive ?rst principles molecular dynamics using SCAN meta-GGA density functional, which captures the intermediate-range part of the van der Waals interaction better than PBE-GGA. It is found that SCAN gives more accurate bond energy and length of an isolated N2 molecule than PBE. SCAN, as well as PBE, is capable of reproducing the ?rst-order molecular-to-polymeric phase transition, but in contrast to PBE, it predicts a wider stability range for ?uid N2. The boundary of this range is 15 GPa higher than the one predicted with PBE, which is in closer agreement with experiments. In addition, SCAN predicts a higher amount of threefold coordinated atoms in the polymeric phase than PBE, which is expected from experiments in amorphous N. These improvements indicate that SCAN is more accurate than PBE in predicting the phase transition from molecular to polymeric ?uid N.
关键词： phase transition,density functional,molecular dynamics,SCAN meta-GGA,liquid nitrogen
更新于2025-09-23 15:21:01
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Temperature Dependence of the Lattice Parameters of Cu2?–?xSe (0.03 ≤ x ≤ 0.23) Powders Fabricated by Mechanochemical Synthesis
摘要： The Cu2 – xSe (0.03 ≤ x ≤ 0.23) powders fabricated by mechanochemical synthesis have been studied by X-ray diffraction. The in situ study has been carried out for the temperature dependences of the lattice parameters, the structures, and the phase compositions of the powders in the temperature range 25–350°C. The powder compositions are shown to differ from the charge compositions and are shifted to lower copper concentrations. The estimation of peak half-widths of the cubic β phase indicates an increase in the structure imperfection after the phase transition from the α phase to the β phase of Cu2 – xSe at ~140°C. It is shown that the superpositions of the subtraction solutions (copper vacancies) and interstitials solutions (copper atoms in interstitial sites), whose proportion is changed as a function of temperature and the deviation from stoichiometry, are in the thermodynamic equilibrium in the copper selenide solid solution at room temperature. The change in the slope of the dependence of the lattice parameter of the powder Cu2 – xSe samples on the composition (0.03 ≤ x ≤ 0.23) in the temperature range 25–350°C enables the suggestion that interstitial copper atom concentration increases with temperature and deviation from stoichiometry.
关键词： phase transition,lattice parameters,mechanochemical synthesis,Cu2 – xSe,X-ray diffraction,thermodynamic equilibrium
更新于2025-09-23 15:21:01
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Improved piezoelectric and strain performance of Na2B4O7-doped (Li,K,Na)NbO3 lead-free piezoceramics
摘要： A polymorphic phase transition effect has been employed to regulate the piezoelectric property of lead-free (K,Na)NbO3-based materials, which intrinsically bear the disadvantage of the evident temperature sensitivity. In this research, Li0.04[(K0.49Na0.51)]0.96NbO3–xNa2B4O7 piezoceramics were prepared through a conventional solid-state reaction. A piezoelectric coefficient d33 of about 285 pC/N and a unipolar strain of 0.14%@5 kV/mm were achieved in the ceramics with x = 0.5%. These improved properties can be ascribed to the coexistence of orthorhombic and tetragonal phase structure around room temperature. Besides, the field-induced unipolar strain varied less than 15% in the temperature region from room temperature to 180 °C. The stability of strain property could be rationally interpreted by a quantitative approach of electrostrictive coupling to the polarization amplitude.
关键词： Piezoelectric coefficient,Lead-free piezoceramics,Temperature stability,Na2B4O7 doping,Polymorphic phase transition
更新于2025-09-23 15:21:01
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Effect of phase transition on SiO2-coated CsPbBr3/Cs4PbBr6 quantum dots: Air-stability and quantum efficiency improvement
摘要： To develop white light-emitting diodes (WLEDs) with wide color gamut for displays, compared with nitride-based phosphors and traditional core-shell quantum dots (QDs) such as CdSe, InP, CuInS2, all-inorganic perovskite QDs CsPbX3 (X = Cl, Br, I) were more promising luminescent materials due to tunable wavelength, narrow emission spectrum and high quantum efficiency. However, when QDs were made into solid form (powders or films), poor air-stability and drastic decrease of quantum efficiency would be observed in CsPbBr3. These drawbacks would restrict their practical applications. To resolve these issues, in this paper, we proposed a new concept that zero-dimensional perovskite QDs powders Cs4PbBr6 with outstanding quantum efficiency and long lifetime up to three months could be successfully prepared via silica-coated method and crystal phase transition in low-temperature synthesis. This phenomenon of phase transition would be discussed in detail and the quantum efficiency could be improved from 31.41% to 45.87%. Moreover, green LEDs with high color purity of 92% and luminous efficiency of 88.59 lm/W could also be achieved by using this material. Therefore, our proposed perovskite QDs powders Cs4PbBr6 had extreme potential for displays applications.
关键词： Zero-dimensional perovskite quantum dots,Phase transition mechanisms
更新于2025-09-23 15:19:57
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Lead Free, Aqua Stable A3Bi2I9 Perovskites: Crystal Growth and Blue Emitting Quantum dots [A = CH3NH3+, Cs+ and (Rb0.05Cs2.95)+]
摘要： Despite a great success in the rise in the power conversion efficiency of lead-halide perovskite solar cells, the toxicity of lead and the unstable nature of materials are the major concerns for their wide range of implementation at the industrial level. Herein, we have developed large size single crystals (SCs) in HI solution using temperature lowering method and nanocrystals (NCs) in ethanol (EtOH) and toluene (TOL) solvents of A3Bi2I9 perovskites [where, A = CH3NH3+ (MA)+, Cs+ and (Rb0.05Cs2.95)+]. The stability of A3Bi2I9 perovskite is investigated by immersing the SCs for 24 h and pellet for 12 h into water. Moreover, the A3Bi2I9 perovskite NCs displays a promising photoluminescence quantum yield of 17.63% and a longer lifetime of 8.20 ns.
关键词： Pb-Free Perovskites,Crystal Growth,High PLQY,Water Stability,Phase Transition,Nanocrystals,Low Bandgap
更新于2025-09-23 15:19:57
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Phasea??Controlled Synthesis of Monolayer W <sub/>1a??</sub><i> <sub/>x</sub></i> Re <i> <sub/>x</sub></i> S <sub/>2</sub> Alloy with Improved Photoresponse Performance
摘要： Tuning bandgap and phases in the ternary 2D transition metal dichalcogenides (TMDs) alloys has opened up unexpected opportunities to engineer optoelectronic properties and explore potential applications. In this work, a salt-assisted chemical deposition vapor (CVD) growth strategy is reported for the creation of high-quality monolayer W1?xRexS2 alloys to fulfill a readily phase control from 1H to DT by changing the ratio of Re and W precursors. The structures and chemical compositions of doping alloys are confirmed by combining atomic resolution scanning transmission electron microscopy-annular dark field imaging with energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy, matching well with the calculated results. The field-effect transistors (FETs) devices fabricated based on 1H-W0.9Re0.1S2 monolayer exhibit a n-type semiconducting behavior with the mobility of 0.4 cm2 V?1 s?1. More importantly, the FETs show high-performance responsivity with a value of 17 μA W?1 in air, which is superior to that of monolayer CVD-grown WS2. This work paves the way toward synthesizing monolayer ternary alloys with controlled phases for potential optoelectronic applications.
关键词： W1?xRexS2 alloys,phase transition,ternary TMDs,chemical vapor deposition,photoresponsivity
更新于2025-09-23 15:19:57
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Masked states of an atom coupled to a standing-wave cavity mode
摘要： The form of the eigenstates of an atom coupled to a cavity mode displaying a three-dimensional periodic profile are obtained. It is shown that the quantized motion leads to degenerate states where the atomic degrees of freedom are masked, that is, upon detection of one component of this composite system the others remain in an entangled state. When the system is extended to include drive and dissipation it is found to undergo a dissipative quantum phase transition at a critical drive amplitude. Unlike other phase transitions reported in the literature, the degeneracy prepares the system in a superposition of incompatible states upon detection of the electromagnetic field. Probing the field hints at an order above the transition point that, due to state masking, allows for atomic coherence to survive at long times.
关键词： atom-cavity coupling,atomic coherence,entangled state,dissipative quantum phase transition,state masking,degenerate states,quantized motion
更新于2025-09-23 15:19:57
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Temperature modulating rectifier behavior and photovoltaic response of VO<sub>x</sub> /Nb:SrTiO<sub>3</sub> heterojunctions
摘要： Vanadium oxide (VOx) is a promising candidate to be used for smart window and thermochromic photovoltaic modulating devices. The crystalline structure and electrical characteristics of VOx thin film deposited on NSTO were investigated. It was found that asymmetric hysteresis current between increasing and decreasing voltages occurs at 80, 300 and 360K. The photovoltaic effect was observed under irradiation by a pulse laser.
关键词： semiconductor-metal phase transition,photovoltaic modulating devices,heterojunction,thermochromism,resistance switch phenomena
更新于2025-09-23 15:19:57
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revealed by high-energy x-ray diffraction
摘要： We explore the evolution of the structural phase transition of (Sr, Ca)3Ir4Sn13, a model system to study the interplay between structural quantum criticality and superconductivity, by means of high-energy x-ray diffraction measurements at high pressures and low temperatures. Our results confirm a rapid suppression of the superlattice transition temperature T ? against pressure, which extrapolates to zero at a critical pressure of ≈1.79(4) GPa. The temperature evolution of the superlattice Bragg peak in Ca3Ir4Sn13 reveals a drastic decrease of the intensity and an increase of the linewidth when T → 0 K and p → pc. Such anomaly is likely associated with the emergence of quantum fluctuations that disrupt the formation of long-range superlattice modulation. The revisited temperature-pressure phase diagram of (Sr, Ca)3Ir4Sn13 thus highlights the intertwined nature of the distinct order parameters present in this system and demonstrates some similarities between this family and the unconventional superconductors.
关键词： high-energy x-ray diffraction,structural phase transition,superconductivity,quantum criticality,quantum fluctuations
更新于2025-09-23 15:19:57