Phase Transition Temperature Tc Research Articles

Size and surface-dependent phase transition temperature in Cu2Se nanobridges

The critical phase transition temperature (Tc) of Cu2Se thermoelectric nanomaterials has been a focal point of extensive research, yet the quantification of surface energy on Tc is frequently ignored. In this paper, we systematically investigate the impact of both the width/thickness and surface configuration of Cu2Se nanobridges (NBs) on Tc. We find that the Tc decreases with size reduction, which becomes particularly accelerated when the size decreases to a few nanometers. Additionally, the NBs with higher energy surfaces exhibit lower Tc. Then we propose an optimized thermodynamic model to quantify the combined effect of size and surface energy on Tc in Cu2Se NBs, which provides an approach to predict Tc in Cu2Se and other thermoelectric nanomaterials. Our study facilitates the understanding of the dependence of Tc on size and surface in Cu2Se, with an eye towards the stable room temperature thermoelectric applications of Cu2Se nanomaterials.

Magnetocaloric effect in kagome MgMn6Sn6

We have studied magnetocaloric effect of kagome MgMn6Sn6 single crystal. The MgMn6Sn6 single crystal exhibits a typical transition of ferromagnetic (FM) to paramagnetic (PM). Maximum magnetic entropy change (−ΔSMmax) and relative cooling power (RCP) for a field change of 0–50 kOe were calculated to be 3.05 J/kg K and 129 J/kg for H//ab, 2.76 J/kg K and 115 J/kg for H//c, respectively. Concurrently, around the phase transition temperature TC, the scaling approach for MgMn6Sn6 is used to study the magnetic entropy change [ΔSM(T,H)]. The outcome suggests that, in the presence of high field, the curves of ΔSM(T,H) can be effectively combined to form a single, universal curve that is unaffected by temperature or external fields. Nevertheless, under low field, ΔSM(T,H) curves diverge slightly below TC, indicating a first-order transition feature. According to the scaling analysis of ΔSM(T,H) curves, the phase transition of MgMn6Sn6 is forced into a second-order one under high field conditions, even if in the low field zone, it is a weak first-order type. Furthermore, it is discovered that the observed magnetocaloric effect (MCE) in kagome MgMn6Sn6 could not be satisfactorily explained by a basic theoretical model that simply took into consideration the magnetoelastic and magneto electron coupling.

Theoretical Study of the Phonon Energy and Specific Heat of Ion-Doped LiCsSO4-Bulk and Nanoparticles.

Using a microscopic model, the temperature dependence of two phonon modes, ω0 = 32 cm-1 and 72 cm-1, and their damping of the ferroelastic LiCsSO4 compound, are calculated within Green's function technique. It is observed that the first mode increases whereas the second one decreases with increasing temperature T. This different behavior is explained with different sign of the anharmonic spin-phonon interaction constant. At the ferroelastic phase transition temperature TC, there is a kink in both modes due to the spin-phonon interaction. The phonon damping increases with T, and again shows an anomaly at TC. The contributions of the spin-phonon and phonon-phonon interactions are discussed. TC is reduced by decreasing the nanoparticle size, and can be enhanced by doping with K, Rb and NH4 ions at the Cs site. TC decreases by doping with Na, K or Rb on the Li site. The specific heat Cp also shows a kink at TC. Cp decreases with decreasing nanoparticle size and the peak disappears, whereas Cp increases with increasing K ion doping concentration.

Enhancement of phase transition temperature through hydrogen bond modification in molecular ferroelectrics

Molecular ferroelectrics are attracting great interest due to their light weight, mechanical flexibility, low cost, ease of processing and environmental friendliness. These advantages make molecular ferroelectrics viable alternatives or supplements to inorganic ceramics and polymer ferroelectrics. It is expected that molecular ferroelectrics with good performance can be fabricated, which in turns calls for effective chemical design strategies in crystal engineering. To achieve so, we propose a hydrogen bond modification method by introducing the hydroxyl group, and successfully boost the phase transition temperature (Tc) by at least 336 K. As a result, the molecular ferroelectric 1-hydroxy-3-adamantanammonium tetrafluoroborate [(HaaOH)BF4] can maintain ferroelectricity until 528 K, a Tc value much larger than that of BTO (390 K). Meanwhile, micro-domain patterns, in stable state for 2 years, can be directly written on the film of (HaaOH)BF4. In this respect, hydrogen bond modification is a feasible and effective strategy for designing molecular ferroelectrics with high Tc and stable ferroelectric domains. Such an organic molecule with varied modification sites and the precise crystal engineering can provide an efficient route to enrich high-Tc ferroelectrics with various physical properties.

Double hysteresis (P-E) loops in the transparent composite containing BaTiO3 at room temperature

A transparent composite sample containing BaTiO3 was synthesized by using the melting quenching method. The presence of a glassy phase in this composite sample was detected through XRD analysis, and this was further confirmed by DSC study. TEM and SAED analyses provided evidence that the BaTiO3 nanoparticles/clusters are embedded within the borate glass matrix, establishing the composite nature of this sample. FTIR spectrum of the present sample revealed that the glass matrix is composed of two structural groups (BO3 with NBO’s, and BO4), along with the distinct groups for BaTiO3. XPS spectra of the present composite sample indicated the presence of more than one type of boron, barium, titanium and oxygen. DSC and dielectric studies of the present composite sample revealed the presence of the phase transition temperature (Tc). Dielectric constant (ɛr) and dielectric loss (tan δ) curves of the present composite sample displayed an anomaly peak in the vicinity of TC. The optical transmission spectrum of the present composite sample exhibit two transmission bands of Ti3+ (3d1) ions in tetragonal distorted sites. At room temperature, the present transparent composite sample exhibited double hysteresis loops for BaTiO3 at low electric fields. The results obtained can be used for the development of lead-free ferroelectric material.

Intermolecular Forces Regulating the Phase-Transition Temperatures in Organic-Inorganic Hybrid Materials.

Organic-inorganic hybrid phase-transition materials have attracted widespread attention in energy storage and sensor applications due to their structural adaptability and facile synthesis. However, increasing the phase-transition temperature (Tc) effectively remains a formidable challenge. In this study, we employed a strategy to regulate intermolecular interactions (different types of hydrogen bonds and other weak interactions), utilizing bismuth chloride as an inorganic framework and azetidine, 3,3-difluoro azetidine, and 3-carboxyl azetidine as organic components to synthesize three compounds with different Tc values: [C3H8N]2BiCl5 (1, 234 K), [C3H6NF2]3BiCl6 (2, 256 K), and [C4H8O2N]3BiCl6 (3, 350 K). 1 is a one-dimensional chain structure and 2 and 3 are zero-dimensional structures. Analysis of the crystal structure and the Hirshfeld surface and 2D fingerprints further suggests that the intermolecular forces are efficiently modulated. These findings emphasize the efficacy of our strategy in enhancing Tc and may facilitate further research in this area.

Electron paramagnetic resonance probing of the spin fluctuation transition in the conical spiral phase of MnSi

In the spiral conical phase of manganese monosilicide, MnSi, the EPR spectra are studied at a frequency of 9.3 GHz in the temperature range of 5–30 K for an external magnetic field oriented along the crystallographic direction [100]. It is found that in the temperature range T < T* = 25 K, located 4 K below the paramagnetic phase-conical phase transition temperature TC = 29 K, a complex EPR spectrum consisting of several lines appears. A procedure is proposed for decomposing experimental spectra into components, which makes it possible to find for each spectral line the temperature dependences of the resonance magnetic field, line width, and integrated intensity. At Tf ∼16 K, one of the main lines in the EPR spectrum with the highest integrated intensity demonstrates a maximum in the temperature dependence of the line width W(T), corresponding to a change in W by a factor of ∼2.8. We argue that this unusual behavior appears as a consequence of a spin fluctuation transition in the conical spiral magnetic phase with long range magnetic order.

Optical, electrical, and crystal structure study of BaZr0.01Ti0.99O3 ceramics modified with Gd3+ and Y3+ rare-earth ions

Ceramics with compositions Ba0.95Gd0.034Ti0.99Zr0.01O3 (BGdZT) and Ba0.95Y0.034Ti0.99Zr0.01O3 (BYZT) were prepared using the solid-state reaction method. The single tetragonal phase (space group P4mm) of BGdZT and BYZT ceramics is verified by analyzing X-ray spectra using the Rietveld method. The analysis of the tolerance factor (t) reveals an order-disorder phase transition in the tetragonal structure, resulting from the displacement of Ti4+/Zr4+ along the c-axis. A significant correlation is observed between the tolerance factor (t) and the phase transition temperature (Tc), as well as between the band gap of optical measurements and the degree of displacement of Ti4+/Zr4+ along the c-axis. The analysis of high-temperature electrical properties indicates that the conduction mechanism in both studied materials is a correlated barrier hopping (CBH) model, and electrical conductivity improves with increasing densification. The non-Debye behavior is confirmed by the curves of the M′′ vs.ƒ(Hz). The combination of M′′/M′′max and Z′′/Z′′max traces confirms the type of short-range charge carrier mobility.

Competition between Electron–Phonon and Spin–Phonon Interaction on the Band Gap and Phonon Spectrum in Magnetic Semiconductors

Using the microscopic s-f model and Green’s function theory, we study the temperature dependence of the band gap energy Eg and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of Eg and the phonon spectrum, there is a kink at the phase transition temperature TC or TN due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I&gt;0,R&gt;0, Eg decreases with increasing SPI and EPI, whereas for I&lt;0,R&gt;0, there is a competition; Eg increases with raising the EPI and decreases for enhanced SPI. For R&lt;0, in both cases, the SPI and EPI reduce Eg. The magnetic field dependence of Eg for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO (I&gt;0, R&lt;0), whereas ω = 151 cm−1 in EuSe (I&gt;0, R&gt;0) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data.

Preparation study and simulation of high transmittance W–Sn–Zr Co-doped VO2 thin films prepared by sol-gel method for energy-saving smart windows

The thermochromic properties of vanadium dioxide (VO2) hold great promise for applications in smart glass windows. VO2 films exhibit a high phase transition temperature (Tc), low visible transmission (Tlum), and low solar modulating ability (ΔTsol), making them inadequate for the requirements of smart glass windows. While some improvement in VO2 film performance can be achieved through single-element doping, these limited enhancements still fall short of ideal results. Therefore, there is a pressing need for the improvement of VO2 films. In this study, we utilized the sol-gel method to prepare VO2 samples simultaneously doped with three elements: W, Sn, and Zr. Microscopic characterization confirmed the successful incorporation of these elements. Optical measurements, including ultraviolet–visible–near-infrared spectroscopy, were conducted to assess the optical properties of the films. Ultimately, outstanding Tlum (69%) and ΔTsol (14%) were achieved, along with an appropriate Tc (27.6 °C). Energy consumption simulations were performed using the EnergyPlus model with experimental data, revealing that compared to conventional glass, VO2 glass reduces total cooling energy consumption by 9.8%. In comparison to previous work, the multi-element-doped films push the performance limits of VO2 films, offering a viable approach for fabricating superior VO2 smart glass windows.

Effect of progressive substitution of spin-3/2 by spin-7/2 on the magnetocaloric properties of a mixed spin binary alloy system: A Monte Carlo study

Using Monte Carlo simulation the concentration dependency of the cooling exponent (n) of a binary alloy AxB1-x mixed spin system (7/2, 3/2) was studied rigorously. For low magnetic field (h) the entropy change (ΔS), excellently follows the relation ΔS∝hn. The exponent n hardly changes with x within the region 0.5 < x < 0.75. But for further increment of x > 0.75, n sharply drops down. The exponent n is strongly affected by the exchange interaction energy (ε). Due to the decrement of the antiferromagnetic interactions n value rapidly decreases. For the increment of x or the number of A(spin-7/2) atom the magnetization M as well as the magnetic entropy change ΔS increases. The phase transition temperature (i.e. Curie temperature) tC linearly decreases for the increase of x.

Continuous modulation of VO2 film’s phase transition threshold and transmittance via n-AZO/n-VO2 heterojunction

To solve the irreconcilable contradiction between the phase transition threshold and the optical property of VO2 film, an n-AZO/n-VO2 heterojunction, in which AZO is Al-doped ZnO, was constructed with the radio frequency (RF) magnetron sputtering and the post annealing. The effects of annealing on the composition and properties of vanadium oxide films have been investigated, as well as the effects of annealing and AZO layer thickness on the phase transition performance of the n-AZO/n-VO2 heterojunction. The results indicate that the V2O5 film will gradually transform into VO2 (M) through V6O13 and VO2 (B) during the reduction annealing at 500 °C. The phase transition temperature (Tc) of VO2 (M) film increases with the annealing temperature. In the heterojunction formed by the ohmic contact between AZO and VO2 layers, the free electrons will be injected into the VO2 layer from AZO layer, which reduces the heterojunction’s Tc from 69.2 to 41.5 °C. The first-principles calculations were performed with the Vienna Ab initio Simulation Package (VASP) to assist in the influence mechanism explanation. In addition to modulating Tc, the AZO layer in the heterojunction also plays an antireflection role, increasing the maximum visible transmittance of VO2 film by ∼ 10 %.

Grand herringbone architecture securing the high thermoelectric performance of GeTe

This work demonstrates a unique two-stage cooling procedure for synthesizing high-performance thermoelectric (TE) GeTe alloys with a maximum zT value of 2.0 (720 K) without assistance of chemical doping. The efficiency breakthrough stems from the grand herringbone structure resulted from the annealing step intentionally inserted near the phase transition temperature Tc of GeTe when cooling. Compared with the samples prepared by conventional methods, the two-stage cooled GeTe processes not only lower carrier concentrations p (4.7–5.3 × 1020 cm−3) but also higher mobility (∼125 cm2 V−1 s−1) that prevent electrical conductivity and TE power factor from declining as p decreases. The twinned domains in the two-stage cooled samples are further analyzed and classified by the compatibility theory. Our calculations show that the twinning structures in the two-stage cooled samples fit well with the compatibility criterion for twin variant selections and are thermodynamically in a stable state. These results not only attest to the efficacy of using the two-stage cooling procedure to manipulate the microstructures of GeTe but also provides a new paradigm for optimizing the performance of GeTe-based TE materials.

Continuously tunable the phase transition behavior of VO2 by T-doping and the formation of T-T chain (T = Ta, Nb)

Vanadium dioxide (VO2), a typical strongly correlated transition metal oxide, undergoes a fascinating reversible metal-insulator transition (MIT) at 340 K, attracting increasing interest from both scientific and technological perspectives. Nevertheless, achieving a rational regulation of the phase transition temperature (Tc) and gaining a deeper understanding of the modulation mechanism, as well as clarifying the relationship between the dopants and Tc in VO2, remain significant ongoing challenges. In this work, we present a density functional theoretical study on the structural and electronic properties of T-doped (T = Ta, Nb) VO2. The results demonstrate that the introduction of T (T = Ta, Nb) atoms tends to drive the R phase structure towards a dimerized monoclinic-like structure, while the uniformly aligned V–V chains exhibit alternating short and long V–V dimerization feature. Notably, the R phase VO2 appears to be more susceptible to structural perturbations. Moreover, T (T = Ta, Nb) doping concentration, compressive strain and the numbers of T-T chain cause curious changes in the structural evolution and electronic structures of VO2, acting as effective modulation methods for tuning the phase transition temperature. In addition, the Tc of VO2 could be significantly reduced when the concentration of T (T = Ta, Nb) dopant is less than 6.3%. The current results offer valuable insights into the mechanism of the doping driven MIT in T-doped (T = Ta, Nb) VO2, and contribute to a deeper understanding of the relationship between the dopants and phase transition temperature, further providing valuable insights into its metal-insulator transition behavior.

Finite-temperature properties of PbTiO3 by molecular dynamics simulations

PbTiO 3 is a prototypical ferroelectric perovskite that is known to undergo a temperature driven ferroelectric tetragonal to paraelectric cubic phase transition, but the understanding of some key phenomena and associated mechanisms underlying this transition remains unclear. Here, using molecular dynamics simulations based on first-principles effective Hamiltonian, we show the behaviors of the phase transition temperature Tc and adiabatic temperature change ΔT of PbTiO3 under an external electric field and tensile stress along the [001] direction. Our results show that the electric field E induces rising Tc via a linear relationTc∝ 0.3083E, rendering the phase transition to go from first-order with thermal hysteresis to second-order without thermal hysteresis above ∼200 kV/cm; meanwhile, a maximum electrocaloric response ΔTmax∼34 K is obtained under E=500 kV/cm. Moreover, external stress (σz) causes rising Tc via a linear relationTc∝160σz and improves the electrocaloric response ΔTmax when combined with the electric field. The present results offer insights into the physical processes and mechanisms that dictate finite-temperature properties of ferroelectric perovskite oxides, laying a foundation for further exploration of this intriguing class of materials.

Engineering room temperature multiferroicity and temperature dependent dielectric properties of Cr doped BaTiO3 ceramics

The coexistence of ferroelectricity, ferromagnetism, and piezoelectricity at room temperature in BaTi1-xCrxO3 (0 ≤ x ≤ 0.06) nanostructures is interesting and leads to the multiferroic nature of system. Sol-gel auto-combustion technique has been used for the synthesis of nanostructures. The XRD patterns along with Rietveld refinement analysis, confirm that the samples possess parent tetragonal perovskite structure in the P4mm space group. The oxygen vacancies stoichiometry ratio was estimated through XPS studies. The scanning electron microscopy (SEM) confirms the large grain size in the doped sample. The study of leakage current density reveals an increase in its value with the increase in Cr content due to defects induced in the system. P-E measurements confirm the ferroelectric nature of the samples that diminishes in the doped samples due to the high leakage current. Moreover, the estimated value of energy efficiency (η) was found to be maximum for the undoped sample. The undoped BaTiO3 has a diamagnetic character that is partially altered into weak ferromagnetic under the Cr-doping at the expense of decreasing polarization parameters. The temperature-dependent dielectric constant shows all characteristics of phase transitions in the samples. The doping of Cr enhances the ferroelectric to paraelectric phase transition temperature (Tc). The piezoelectricity remains preserved in the doped samples. Thus, the coexistence of ferroelectricity, ferromagnetism, and piezoelectricity at room temperature finalizes the multiferroic nature of the synthesized samples.

Fully Discrete VO2 Particulate Film with Ultra‐High Transmittance and Excellent Thermochromic Performance

AbstractVanadium dioxide (VO2) has attracted widespread attention due to its extraordinary thermochromic properties. Visually transparent VO2 films hold great promise for applying in smart windows, smart radiative coolers, solar cells, and microwave absorbing windows. However, it remains a huge challenge to simultaneously achieve ultra‐high luminous transmittance (Tlum) and excellent thermochromic performance due to their contradictory relationship. Here, a fully discrete VO2 particulate (FDVP) film is developed. It shows an ultra‐high Tlum of 92.7% due to the reflectance being significantly reduced. Below the phase transition temperature (Tc, 69.1 °C), it is highly transparent to near‐infrared (NIR) light, while above Tc, it blocks NIR light through the strong localized surface plasmon resonance (LSPR) effect, resulting in an excellent solar energy modulation (ΔTsol) of 10.5% for smart windows. In addition, a 3.5 at.%W‐doped FDVP film exhibits a desired Tc of 25.8 °C, an extremely narrow hysteresis width (ΔT) of 4.6 °C, and good optical properties (Tlum = 92.0%, ΔTsol = 8.2%). The performance represents a new milestone for thermochromic VO2 films. This work will shed light on the structural design for high‐performance VO2 films.

Mn-doped 0.15Pb(Yb1/2Nb1/2)O3–0.48Pb(Mg1/3Nb2/3)O3–0.37PbTiO3 piezoelectric ceramics for high performance high-power transducers

For high-power transducer applications, the piezoelectric materials should fulfill specific requirements of a high mechanical quality factor (Qm), a high piezoelectric strain coefficient (dij), and a high Curie phase transition temperature (Tc). In this study, we present an investigation of the structural, piezoelectric, and high-power characteristics of Mn-doped 0.15Pb(Yb1/2Nb1/2)O3–0.48Pb(Mg1/3Nb2/3)O3–0.37PbTiO3 (0.15PYN–0.48PMN–0.37PT) ceramics. The introduction of Mn dopants led to an increase in the tetragonal phase fraction within the 0.15PYN–0.48PMN–0.37 PT ceramics and the formation of oxygen vacancies, consequently significantly enhancing Qm due to their synergistic effect. Despite the hardening effect, the high value of the piezoelectric coefficient (d33) was maintained due to the increase in grain size. Particularly, the specimen doped with 3 mol% of Mn exhibited excellent piezoelectric properties of d33 = 345 pC/N, Qm = 1159, and Tc = 207°C. This specimen displayed a high vibration velocity of 0.6 m/s under the condition of equilibrium ΔT < 20°C (temperature rise from initial temperature), thereby confirming its immense potential for high-power transducer applications.

Voltage, thermal and magnetic field fiber sensors based on magnetic nanoparticles-doped photonic liquid crystal fibers.

In this article, highly sensitive voltage, thermal and magnetic field fiber sensors were obtained in magnetic nanoparticles-doped E7 liquid crystals filled into photonic crystal fibers (PLCF). The voltage and temperature sensitivity reached at 12.598 nm/V and -3.874 nm/°C, respectively. The minimum voltage response time is 48.2 ms. The phase transition temperature Tc of liquid crystal with magnetic dopant was reduced from 60 °C to 46 °C. The magnetic field sensor based on magnetic nanoparticles-doped PLCF were obtained with sensitivity of 118.2 pm/mT from 400 to 460 mT.

Tuning electrical properties of BaTiO3 with iron modification

BaTiO3 plays an important role in advanced functional devices owing to its fascinating properties. Though it is an excellent ferroelectric material, its magnetism can be switchable via suitable modification. In this report, we have studied iron-modified BaTiO3, which has been prepared using cost-effective high-temperature solid-state reaction techniques. The Fe-modified BaTiO3 maintains its crystallinity in the tetragonal phase with P4mm space group with a slight variation of tetragonality ratio to 1.002 from 1.008 of BaTiO3 of the identical synthesis method. This is confirmed by Rietveld refinement. Field emission scanning electron microscopy (FESEM) and EDX (energy dispersive X-rays) spectrum along with an elemental mapping analysis has provided the feature of the Fe-modifed BaTiO3. Atomic force microscope (AFM) has been employed to get the detailed surface topology (2D and 3D) and surface roughness of the developed systems. This deformation caused by the incorporation of Fe significantly modifies the electrical properties of barium titanate. The ferroelectric-paraelectric phase transition temperature (Tc) of Fe-modified BaTiO3 has been diffused and shifted to 300 °C from 120 °C of pure BaTiO3, suggesting the materials for the high-temperature capacitive application. The room temperature M-H hysteresis of Fe-modified BaTiO3 indicates the ferromagnetism developed in the BaTiO3 with the introduction of Fe. The ac conductivity is in the order of 10−4 Ω−1cm−1, and its frequency response obeys Jonscher's power law. Different charge carriers are responsible for diverse conduction processes in different temperature ranges confirmed by the Arrhenius plot. Impedance spectroscopy studies reveal the existence of NTCR characteristics of the developed system. Well defined ferroelectric hysteresis loop indicates the existence of ferroelectricity in the developed system. The analysis of various electrical parameters has provided information on the material's dielectric relaxation and conduction mechanisms for possible application in functional devices.