Low-temperature Phase Transition Research Articles

Magnetocaloric Materials for Low-Temperature Magnetic Cooling

State of research in the study of magnetocaloric materials based on rare-earth metals that are promising for application in the technology of low-temperature magnetic cooling is reviewed. Physical principles and characteristics of the magnetocaloric effect in materials based on rare-earth metals with low-temperature magnetic phase transitions are presented.

Low Temperature Phase Transitions in Sucrose-Containing Solutions of Some Cryoprotectants

Low Temperature Phase Transitions in Sucrose-Containing Solutions of Some Cryoprotectants

Catalogue of phonon modes in several cuprate high-temperature superconductors from density functional theory

Cuprates are promising candidates for study in developing higher temperature superconductors. A thorough understanding of a material’s phonon modes enables further investigation of its emergent properties, however, no complete reference of the phonon modes exists. Here, using density functional theory, we evaluate the phonon frequencies and atomic displacements for La2CuO4, Bi2Sr2CuO6, and Bi2Sr2CaCu2O8, in their tetragonal structures. The phonon modes for all materials agree with those expected from space group symmetry and display instabilities corresponding to known low-temperature structural phase transitions.

Low-temperature crystal structures of the solvent dimethyl carbonate

Dimethyl carbonate (DMC) is an important industrial solvent but is additionally a common component of liquid lithium-ion battery electrolytes. Pure DMC has a melting point of 277 K, so encountering solidification under outdoor climatic conditions is very likely in many locations around the globe. Even eutectic, ethylene carbonate:dimethyl carbonate commercial LiPF6 salt electrolyte formulations can start to solidify at temperatures around 260 K with obvious consequences for their performance. No structures for crystalline DMC are currently available which could be a hindrance for in situ battery studies at reduced temperatures. A time-of-flight neutron powder diffraction study of the phase behavior and crystal structures of deuterated DMC was undertaken to help fill this knowledge gap. Three different orthorhombic crystalline phases were found with a previously unreported low-temperature phase transition around 50–55 K. The progression of Pbca → Pbcm → Ibam space groups follow a sequence of group–subgroup relationships with the final Ibam structure being disordered around the central carbon atom.

Thermochromic Ni(II) Organometallics With High Optical Transparency and Low Phase-Transition Temperature for Energy-Saving Smart Windows.

Thermochromic smart windows with rational modulation in indoor temperature and brightness draw considerable interest in reducing building energy consumption, which remains a huge challenge to meet the comfortable responsive temperature and the wide transmittance modulation range from visible to near-infrared (NIR) light for their practical application. Herein, a novel thermochromic Ni(II) organometallic of [(C2 H5 )2 NH2 ]2 NiCl4 for smart windows is rationally designed and synthesized via an inexpensive mechanochemistry method, which processes a low phase-transition temperature of 46.3 °C for the reversible color evolution from transparent to blue with a tunable visible transmittance from 90.5% to 72.1%. Furthermore, cesium tungsten bronze (CWO) and antimony tin oxide (ATO) with excellent NIR absorption in 750-1500 and 1500-2600nm are introduced in the [(C2 H5 )2 NH2 ]2 NiCl4 -based smart windows, realizing a broadband sunlight modulation of a 27% visible light modulation and more than 90% of NIR shielding ability. Impressively, these smart windows demonstrate stable and reversible thermochromic cycles at room temperature. Compared with the conventional windows in the field tests, these smart windows can significantly reduce the indoor temperature by 16.1 °C, which is promising for next-generation energy-saving buildings.

Thermochromic composite film of VO2 nanoparticles and [(C2H5)2NH2]2NiBr4@SiO2 nanospheres for smart window applications

Vanadium dioxide (VO2) has attracted consideration because of its thermochromic performance in smart windows. However, the practical applications of VO2-based smart windows are seriously hindered by their low luminous transmittance (Tlum), poor solar modulation efficiency (ΔTsol), and the monotonous “brown-yellowish” color. This study aims at developing a newly reported metal complex [(C2H5)2NH2]2NiBr4 and its core–shell structure of [(C2H5)2NH2]2NiBr4@SiO2, combined with the thermochromic VO2 to enhance optical performance (Tlum and ΔTsol). Density Functional Theory (DFT) calculations indicate that the as-obtained metal complex is thermodynamically stable, and the weaker NH…Br hydrogen bond makes the complex possesses a lower phase transition temperature (TC = 57.6 °C) than the reported [(C2H5)2NH2]2NiCl4 (TC = 75.4 °C). In addition, it is found that the SiO2 shell can effectively inhibit the deliquescence of the [(C2H5)2NH2]2NiBr4 complex, and DFT calculations reveal that the oxygen of SiO2 can bond with the hydrogen of ammonium, thus the water molecules in the air cannot continue to react with the complex. Interestingly, the combination of [(C2H5)2NH2]2NiBr4@SiO2/Polystyrene film with VO2/Polyvinylbutyral film demonstrates exemplary solar modulation abilities (Tlum,low = 52.9 %, Tlum,high = 37.3 %, ΔTsol = 25.7 %, Haze = 26.5 %), 2.1 times better than that of VO2/Polyvinylbutyral film (Tlum,low = 55.9 %, Tlum,high = 54.2 %, ΔTsol = 12.4 %, Haze = 17.2 %). Moreover, the color of VO2-based film changes from light-brown (at low temperature) to green (at high temperature).

The Evolution of the Magnetic Properties of Iron Borate Single Crystals Doped with Gallium

Highly perfect FeBO3 and Fe0.91Ga0.09BO3 single crystals were studied in a wide temperature range using SQUID magnetometry. A theoretical model describing the temperature and field dependences of the magnetization of single crystals has been developed. It is found that even a small concentration of gallium,which is a diamagnetic impurity, substantially affects the magnetic properties of single FeBO3 crystals. In par-ticular, the Fe0.91Ga0.09BO3 crystal differs from the pure FeBO3 phase in by a lower magnetic phase transition temperature and a higher antiferromagnetic susceptibility at low temperatures.

Process of Zanthoxylum armatum DC. oil by a novel low-temperature continuous phase transition extraction: Evaluation of aroma, pungent compounds and quality

Zanthoxylum armatum DC. (ZAD) oil is one of the most popular seasonings for its unique flavor and numbing taste. However, aromatic and pungent components are easily affected by extraction process. In this study, a novel low-temperature continuous phase transition process (CPTP) was applied to extract ZAD oil, and the flavor and quality of oil were compared with the those of products obtained from fried process (FP), fresh squeezed process (FSP) and supercritical carbon dioxide process (SCDP) using Gas Chromatography-Mass Spectrometry/Olfactometry and UPLC-DAD-ESI-Q-TOF-MS/MS. The outcome demonstrated that the oil preserved higher volatile aroma components from SCDP and CPTP (606.073 mg/g and 194.905 mg/g, respectively). It's interesting to note that the majority of the alkyl components were only present in the ZAD oil created using CPTP, which further preserved the original makeup of the fragrance components of ZAD. Four ZAD oil had considerably different pungent compounds, with SCDP having the greatest concentration (16697.78 g/mL), followed by CPTP (6425.66 g/mL), FP (2662.6 g/mL), and FSP (719.69 g/mL). Physicochemical properties analysis indicated that unsaturated fatty acid content was highest for FSP oil (91.97%). Overall, as a fresh and effective extraction technique, CPTP may be appropriate for the extraction of natural fragrance components.

Thermo-Responsive Poly(N-isopropylacrylamide)/Hydroxypropylmethyl Cellulose Hydrogel with High Luminous Transmittance and Solar Modulation for Smart Windows.

Thermochromic smart windows are considered to be promising energy-saving devices for reducing energy consumption in buildings. The ideal materials for thermochromic smart windows should have high transmittance, high solar modulation, low phase-transition temperature, and excellent high-temperature thermal stability, which are difficult to achieve simultaneously. This work reports a simple one-step low-temperature polymerization method to prepare a thermo-responsive poly(N-isopropylacrylamide)/hydroxypropylmethyl cellulose (PNIPAM/HPMC) hydrogel achieving the above performances simultaneously. The low-temperature polymerization environment endowed the hydrogel with a high luminous transmittance (Tlum) of 90.82%. HPMC as a functional material effectively enhanced the mechanical properties and thermal stability of the hydrogel. Meanwhile, the PNIPAM/HPMC hydrogel showed a low phase-transition temperature (∼32 °C) and high solar modulation (ΔTsol = 81.52%), which proved that it is an ideal material for thermochromic smart windows. Moreover, a PNIPAM/HPMC smart window exhibited high light transmittance (T380-760 = 86.27%), excellent light modulation (ΔT365 = 74.27%, ΔT380-760 = 86.17%, and ΔT940 = 63.93%), good indoor temperature regulation ability and stability, which indicated that it was an attractive candidate for application in reducing energy consumption in buildings. This work also provides an option and direction for modifying PNIPAM-based thermochromic smart windows.

Magnetic properties and cryogenic magnetocaloric effect in monoclinic RE8.66(BO3)2(B2O5)O8 (RE = Er, Tm) compounds

Researchers in cryogenic magnetic refrigeration have never stopped pursuing magnetic refrigerants with lower ordering temperatures and larger magnetocaloric effects. Rare earth borates have been found to be potential cryogenic magnetocaloric materials due to their low phase transition temperatures and considerable magnetic entropy changes. Here, two rare earth borates RE8.66(BO3)2(B2O5)O8 (RE = Er, Tm) were successfully fabricated, and the crystallographic and magnetic parameters as well as cryogenic magnetocaloric effects were systematically evaluated. The compounds are deemed to crystalize in a monoclinic structure belonging to space group C2/m. They are both second-order phase transition materials without any long-range order above 2 K. These monoclinic rare earth borates, especially for Er8.66(BO3)2(B2O5)O8, show more considerable low-field magnetocaloric effects than rare earth orthoborates. For ΔH = 1 and 2 T, the values of −ΔSMmax are 5.1 and 11.2 J/(kg K) for Er8.66(BO3)2(B2O5)O8, 3.0 and 5.3 J/(kg K) for Tm8.66(BO3)2(B2O5)O8, respectively. Therefore, they are regarded as potential candidates for cryogenic magnetic refrigeration.

Fluorinated linkers enable the synthesis of flexible MOFs with 1D alkaline earth SBUs and a temperature-induced phase transition.

Fluorination as a functionalization of organic linkers in MOFs has shown surprising effects both on the structure of the linker itself as well as on the topology and properties of the resulting framework materials. 4,4',4''-Benzene-1,3,5-triyl-tris-(benzoate), typically abbreviated to BTB, is a well-known linker in the construction of MOFs. It is expected to be planar due to a complete sp2 hybridisation of its carbon atoms. However, some flexibility is frequently observed by twists of the outer carboxylate groups as well as by the benzoate rings. The latter is mainly influenced by substituents of the inner benzene ring. Herein, we present two novel alkaline earth metal based MOFs [EA(II)5(3F-BTB)3OAc(DMF)5] (EA(II) = Ca, Sr) utilizing a fluorinated derivative of the BTB-linker (perfluorination of the inner benzene ring) with a unique topology, crystalline sponge behaviour and a low temperature induced phase transition.

Influence of Ga-substitution to the structural and magnetic properties of (Mn,Fe)-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- bixbyite

To study the dependence of the properties of ternary oxides (Mn,Fe,Ga)2O3 with the bixbyite structure on the composition, the temperature dependences of the magnetization and ac magnetic susceptibility of two single-crystal samples of different compositions obtained using the flux method were analyzed. A detailed study of the structure was carried out using single-crystal X-ray diffraction analysis, and the changes in structural parameters depending on the composition were analyzed. The dc magnetization and ac magnetic susceptibility of Fe1.1Mn0.76Ga0.14O3 and Fe0.65Mn1.1Ga0.26O3 bixbyites have been studied. Despite the qualitatively similar behavior of the magnetic properties, significant differences were also found, despite a small difference in the Mn/Fe/Ga ratio in the samples under study. It is shown that both compounds experience two successive low-temperature magnetic phase transitions from the paramagnetic phase at T=20-32 K as the temperature is lowered. Calculations of the Mydosh parameter for the detected phase transitions showed different degrees of ordering in the compounds under study. Keywords: transition metal oxides, magnetic phase transitions, spin glass state, bixbyite.

Влияние Ga-замещения на структурные и магнитные свойства биксбиита (Mn,Fe)-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

To study the dependence of the properties of ternary oxides (Mn,Fe,Ga)2O3 with the bixbyite structure on the composition, the temperature dependences of the magnetization and ac magnetic susceptibility of two single-crystal samples of different compositions obtained using the flux method were analyzed. A detailed study of the structure was carried out using single-crystal X-ray diffraction analysis, and the changes in structural parameters depending on the composition were analyzed. The dc magnetization and ac magnetic susceptibility of Fe1.1Mn0.76Ga0.14O3 and Fe0.65Mn1.1Ga0.26O3 bixbyites have been studied. Despite the qualitatively similar behavior of the magnetic properties, significant differences were also found, despite a small difference in the Mn/Fe/Ga ratio in the samples under study. It is shown that both compounds experience two successive low-temperature magnetic phase transitions from the paramagnetic phase at T = 20–32 K as the temperature is lowered. Calculations of the Mydosh parameter for the detected phase transitions showed different degrees of ordering in the compounds under study.

Effect of Pd(II) uptake on high-temperature phase transitions in a hybrid organic-inorganic perovskite semiconductor.

Hybrid organic-inorganic perovskites (HOIPs) have been widely studied for their interesting functions and potential applications. Here, we report a novel sulfur-containing hybrid organic-inorganic perovskite based on a one-dimensional ABX3-type compound: [C3H7N2S]PbI3 ([C3H7N2S]+ is 2-amino-2-thiazolinium) (1). Compound 1 undergoes two high-temperature phase transitions at 363 K and 401 K, respectively, showing a band gap of 2.33 eV, and has a narrower band gap compared to other one-dimensional materials. Moreover, by introducing thioether groups into the organic component, 1 has the ability to uptake Pd(II) ions. Compared with previously reported low-temperature isostructural phase transition sulfur-containing hybrids, the molecular motion of 1 becomes more intense under the stimulation of high temperature, leading to changes in the space group during the two phase transitions (Pbca → Pmcn → Cmcm), which are no longer the previous isostructural phase transitions. Significant changes in the phase transition behavior and semiconductor properties before and after metal absorption make it possible to monitor the absorption process of metal ions. The study of the effect of Pd(II) uptake on phase transitions may be helpful to reveal the mechanism of phase transitions more deeply. This work will broaden the hybrid organic-inorganic ABX3-type semiconductor family and pave the way for the development of organic-inorganic hybrid-based multifunctional phase transition materials.

Key phonon modes to determine the phase transition of two dimensional Janus transition metal dichalcogenides: a DFT and tight-binding study.

Phase stability and the phase transition of Janus transition metal chalcogenides (TMDs) have become interesting issues that have not been fully resolved since their successful synthesis. By fitting the results from first principles calculations, a tight-binding dynamics matrix of the 1T' phase is constructed and the eigenvectors are also obtained. We propose a method to project the atomic motion causing the phase transition from 2H to 1T' onto these eigenvectors, and identify four key phonon modes which are the major factors to trigger phase transition. Temperature excitation is used to excite the key modes and the free energy criterion is used to determine the phase stability. The relatively large enthalpy difference between the 2H and 1T' phases favours the 2H one as the stable phase at low temperature. While the 1T' phase has a quick increase in vibrational free energy with rising temperature, especially for 1T' Janus TMDs which have a quicker increase in the total free energy than that of 1T' non-Janus TMDs, making them show a lower phase transition temperature. Our work will deepen our understanding of the phase transition behavior of 2D Janus TMDs, and the tight-binding dynamics matrix and the method to obtain the key modes will be a useful tool for further study of the phase transitions of 2D Janus TMDs and other related materials.

Halogen‐Modulated Reversible Phase Transition in Organic‐Inorganic Hybrid Perovskites

AbstractThe tunable structure, abundant raw materials, and ease of preparation have made molecular dielectric crystals popular for use in device design. In spite of this, some known molecular switching materials have a low phase transition temperature and a low dielectric constant, which limit their applications. Therefore, designing and synthesizing molecular‐based phase transition compounds with high phase transition temperature and superior properties is especially important. In this work, we use 3‐chloropropan‐1‐ammonium hydrochloride and SbCl3/SbBr3inorganic salts as building blocks to synthesize compounds (CPA)2(BPA)2Sb2Br10(1) (CPA=3‐chloropropan‐1‐ammonium, BPA=3‐bromopropan‐1‐ammonium) and (CPA)2Sb2Cl8(2). Compound1has a high phase transition temperature (407.45 K). Dielectric measurements and differential scanning calorimetry (DSC) confirm the structural phase transition in compound1, and no fatigue decay is observed after several dielectric cycles. In addition, compounds1 and2possess semiconductor properties. The findings of this study provide new directions for the design and application of multifunctional molecular dielectric materials.

High-pressure investigations in CH3NH3PbX3 ( X = I, Br, and Cl): Suppression of ion migration and stabilization of low-temperature structure

Hybrid organic-inorganic halide perovskites represent a promising next-generation photovoltaic material with drawbacks for structure stability and composition concerns. Demonstrations of ion migration and molecular dynamics suggest room for structural contraction and subsequent property adjustments. Here, we deploy dielectric and infrared spectroscopy under external pressure to probe the full structural phase diagram and dielectric response of methylammonium lead halide perovskites ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{X}_{3}$ ($X\phantom{\rule{4pt}{0ex}}=$ I, Br, or Cl). Ion migration can be fully suppressed by pressure beyond 4 GPa. The low-temperature orthorhombic phase transition can be gradually enhanced and stabilized at ambient conditions with increasing pressure. A slow relaxation mode, presumably the motion of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cation, is observed at lower pressure and is absent in the orthorhombic phase for every halide.

Improving low temperature degradation of 3Y-TZP ceramics via high temperature carburizing

3Y-TZP ceramics are prepared by solid state method and surface carburization process, and the effect of surface carburization on its the low temperature degradation is studied. The conventional sintered samples completely lost its mechanical properties after aging for 15 h, while the failure time of the surface carburized samples are 300 h. In addition, the nuclear growth rate of the surface carburized samples (αd) and the nucleation rate (Nr) is lower than that of sintered samples, αd plays a dominant role in the degradation process at low temperature and is the key factor determining the aging rate. At the same time, it is found that carbon is dissolved in zirconia lattice in the form of electrically neutral atoms, which will not destroy the original charge balance and produce new oxygen vacancies when entering the interstitial site. More importantly, the precipitation rate of Y3+ from zirconia lattice is the key factor to determine the low-temperature phase transition of tetragonal-monoclinic(T-M). The treatment method of surface carburization has significantly improved the low-temperature degradation performance of 3Y-TZP ceramics, which provides a basis for the application of zirconia ceramics in low-temperature and humid environment.

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO2) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2 thin films remains a challenge. In this work, high-quality thermochromic VO2 thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO2 thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO2 thin films was studied. The shift of the VO2(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO2 thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO2 films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.

Order and disorder, crossovers, and phase transitions in dipolar artificial spin ice on the Cairo lattice.

We study the thermodynamic properties of the magnetic dipolar spin ice on a 2D pentagonal Cairo lattice by using the numerical Metropolis and the complete enumeration methods. We use the model of point Ising-like dipoles considering long-range interactions with up to 100 nearest neighbors and with periodic boundary conditions. There are two explicit peaks both in the temperature behavior of the heat capacity and in the magnetic susceptibility. The low-temperature peak is caused only by long-range interactions and is not present in the model where each dipole interacts only with four nearest neighbors. The height of the peak depends logarithmically on the quantity of dipoles, which indicates a phase transition. The nature of the low-temperature phase transition is related to the transformation from order to disorder in orthogonal sublattices while maintaining the spin ice state and the spin ice rule in the sublattice of crosses. The high-temperature heat capacity peak is associated with the melting of spin ice, i.e., with the crossover from spin ice to paramagnetic chaos. Its height is constant and does not depend on the quantity of dipoles. It is shown that the choice of the radius of the dipole-dipole interaction has a significant effect on the statistical properties of the model. The model may even show the appearance of the long-range order and the phase transition in the case of long-range interaction or its absence in the case of short-range interaction.