Enthalpy Difference Research Articles

Evidence of conformational changes and self-aggregation of 1,2-Bis(4-pyridyl)ethylene in solutions with ethanol

In this work, we report on a concentration and temperature dependent study of the 1,2-Bis(4-pyridyl)ethylene (BPE) – ethanol solutions by means of ultrasonic relaxation spectroscopy. The results revealed two distinct relaxation processes that follow Debye-type frequency dependence. Despite the presence of both processes in the full concentration range studied, the low-frequency relaxation process, related to conformational change between the trans- and gauche-BPE conformers, dominates the acoustic spectra in the low-concentration range, while diminishes at higher concentrations with a parallel enhancement of the high-frequency relaxation process, which is attributed to the self-association of BPE molecules. Quantum mechanical calculations were performed to investigate the energetics of the trans- and gauche-conformers. The trans-species was found more thermodynamically stable than the gauche-conformer. By applying the Synchronous Transit-Guided Quasi-Newton (STQN) methodology, we confirmed the presence of a single transition structure. From the temperature dependence of the acoustic properties, we estimated the activation enthalpy and the enthalpy difference between the two conformers. Density Functional Theory (DFT) calculations have been applied to attain the corresponding enthalpies that were found close to the experimental values. Molecular docking calculations established the self-aggregation mechanism between BPE monomers forming three types of dimeric units, namely the trans-trans, the gauche-gauche and the trans-gauche dimer species with the latter found to be the most thermodynamically favorable. The concentration dependence of the sound velocity, mass density and shear viscosity evidenced the formation of BPE aggregates. From the temperature dependence of the acoustic spectra, the thermodynamic properties of the self-aggregation mechanism of BPE were also determined. Based on the vibrational spectroscopic data, the population of the gauche conformers was found to increase with concentration at the expense of the population of the trans conformers. From the study of the vibrational properties of the system in the short-rage order, we cannot exclude the presence of a specific dimer type in the overall structure. Nevertheless, regarding to the binding score of the trans-gauche dimer (-1.25 kcal/mol), this species is the most thermodynamically stable and most likely its population is higher in dense solutions relative to the other two dimer species.

Experimental Study on the Performance of a Novel Data Center Air Conditioner Combining Air Cooling and Evaporative Cooling

Addressing challenges of high-temperature shutdowns, low energy efficiency, and limited natural cooling utilization in data center air conditioning, a novel hybrid air cooling and evaporative cooling air-conditioning(ACEC-AC) system has been developed. The system is well-suited for application in data centers located in regions with high outdoor temperatures and large dry-bulb to wet-bulb temperature differences during summer. Experimental evaluations in an enthalpy difference laboratory revealed that the hybrid system's performance in wet mode (combining air cooling and evaporative cooling) significantly outperforms its dry mode (solely air cooling) and traditional cooling units. At an outdoor temperature of 35.60°C dry-bulb and 29.95°C wet-bulb, the system achieved a COP of 3.85, a 21.45% improvement over dry mode and 16.16% higher than conventional Packaged computer room air conditioning (CRAC). Notably, the system maximizes its benefits in regions like Guangzhou, where the temperature difference between the dry bulb and wet bulb is significant, resulting in an annual operational cost savings of over 34.16%. In conclusion, the hybrid air cooling and evaporative cooling system represents an effective energy-saving technology with great potential to enhance data center performance and reduce energy consumption.

Quantitative detection of refrigerant charge faults in multi-unit air conditioning systems based on machine learning algorithms

Refrigerant charging discrepancies constitute the predominant malfunctions in air conditioning systems. Achieving the optimal charging level is crucial for system performance, underscoring the importance of precise refrigerant level prediction. This study introduces an algorithm designed for the quantitative detection of refrigerant charging errors by integrating the Markov Transition Field (MTF), Convolutional Neural Networks (CNN), and Multi-head Self-Attention (MSA) mechanisms. A high-precision enthalpy difference chamber was employed to establish a Variable Refrigerant Flow (VRF) refrigerant charging test bench. This setup facilitated the analysis of system parameter sensitivity to charging faults and aided in the creation of a training dataset for the algorithm. Comparative analysis was conducted against Support Vector Machines (SVM), Random Forests (RF), CNN with Self-Attention (AT), and MTF-CNN-MSA. The findings reveal that our method adeptly captures temporal dependencies and dynamic shifts in time series as visual representations, offering novel insights for discerning fault patterns within such data. Notably, the maximum pressure variations at high-pressure and low-pressure points were 0.25 MPa and 0.07 MPa, respectively, with temperature shifts of 12 °C and 3.5 °C at the high and low-temperature points. The high-pressure and high-temperature points are particularly sensitive to changes in refrigerant charging, and parameters from these sections were utilized to construct the dataset. The CNN-MSA algorithm demonstrates consistent performance across various fault types, effectively delineating fault characteristics. The accuracies achieved by SVM, RF, CNN-AT, and MTF-CNN-MSA were 84.38 %, 73.75 %, 88.13 %, and 93.75 %, respectively. In comparison, the CNN-MSA algorithm was able to more accurately detect refrigerant charge faults at different levels.

Numerical Study on the Variable-Temperature Drying and Rehydration of Shiitake.

Variable-temperature convective drying (VTCD) is a promising technology for obtaining high-quality dried mushrooms, particularly when considering rehydration capacity. However, accurate numerical models for variable-temperature convective drying and rehydration of shiitake mushrooms are lacking. This study addresses this gap by employing a model with thermo-hydro and mechanical bidirectional coupling to investigate five dehydration characteristics (moisture ratio, drying rate, temperature, evaporation rate, and volume shrinkage ratio) and a drying load characteristic (enthalpy difference) during VTCD. Additionally, a mathematical model combining drying and rehydration is proposed to analyze the effect of VTCD processes on the rehydration performance of shiitake mushrooms. The results demonstrate that, compared to constant-temperature drying, VTCD-dried mushrooms exhibit moderate shrinkage rates and drying time (16.89 h), along with reduced temperature variation and evaporation rate gradient (Max. 1.50 mol/(m3·s)). VTCD also improves enthalpy stability, reducing the maximum drying load by 58.84% compared to 338.15 K constant-temperature drying. Furthermore, drying time at medium temperatures (318.15-328.15 K) greatly influences rehydration performance. This study quantitatively highlights the superiority of variable-temperature convective drying, offering valuable insights for optimizing the shiitake mushroom drying processes.

Evaporation temperature prediction of the refrigerant-direct convective-radiant cooling system based on LSTM neural network

The existing radiant cooling systems are effective at handling indoor sensible heat load. However, the indoor latent load needs to be treated with an additional dehumidification system, which will increase the complexity and initial investment of the system. To settle this dilemma, a novel aluminum column-wing type refrigerant-direct convective-radiant cooling (ACT-RCC) system was proposed, which adopted refrigerant as direct medium and provided cooling and independent dehumidification simultaneously. Experiments were conducted in an enthalpy difference laboratory with the indoor air temperature of 22.0 °C − 30.0 °C and relative humidity of 60.0%. A numerical model of the ACT-RCC terminal was established and validated with the experimental data. To meet the indoor heat and latent load of residential buildings, a heuristic approach based on Long Short-Term Memory (LSTM) neural network for predicting the evaporation temperature of this system was proposed. Results showed that the predicted values were accurate enough by measured data and the maximum deviation of the evaporation temperature was 0.7 °C. The relationship between the evaporation temperature, indoor air temperature and relative humidity, and heat moisture ratio was derived. Besides, the indoor thermal comfort of the selected bedroom was also evaluated. Results showed that when the indoor air temperature and relative humidity remained 26.0 °C and 40.0% − 70.0%, the thermal comfort of this bedroom can achieve Level Ⅰ by adjusting the evaporation temperature of the ACT-RCC system of 2.0 °C − 19.8 °C.

On the size- and shape-dependence of integral and partial molar Gibbs energies, entropies, enthalpies and inner energies of solid and liquid nano-particles

In this paper the size- and shape dependences of 8 different integral and partial molar thermodynamic quantities are derived for solid and liquid nano-phases, starting from the fundamental equation of Gibbs: i) The integral molar Gibbs energies of nano-phases and the partial molar Gibbs energies of components in those nano-phases, ii) The integral molar enthalpies of nano-phases and the partial molar enthalpies of components in those nano-phases, iii) The integral molar entropies of nano-phases and the partial molar entropies of components in those nano-phases, and iv). The integral molar inner energies of nano-phases and the partial molar inner energies of components in those nano-phases. All these 8 functions are found proportional to the specific surface area of the phase, defined as the ratio of its surface area to its volume. The equations for specific surface areas of phases of different shapes are different, but all of them are inversely proportional to the characteristic size of the phase, such as the diameter of a nano-sphere, the side-length of a nano-cube or the thickness of a thin film. Therefore, the deviations of all properties discussed here from their macroscopic values are inversely proportional to their characteristic sizes. The 8 equations derived in this paper follow strict derivations from the fundamental equation of Gibbs. Only the temperature dependent surface energy of solids and surface tension of liquids will be considered as model equations to simplify the final resulting equations. The theoretical equations are validated for the molar Gibbs energy against the experimental values of liquidus temperatures of pure lead. The theoretical equations for the molar enthalpy are validated i). Against the experimental values of dissolution enthalpy differences between nano- and macro cobalt particles in the same liquid alloy and ii). Against the size dependent melting enthalpy of nano-indium particles. In this way, also the theoretical equations for the molar entropy and molar inner energy are validated as they are closely related to the validated equations for the molar Gibbs energy and molar enthalpy.

Strain effects on the structural stability and defect properties of γ-CsPbI3

Black γ phase CsPbI3 exhibits better structure stability at room temperature, making it a compelling candidate for perovskite solar cells. However, the long-term stability remains a challenge due to lattice strain. Here, the lattice structure stability and defect properties of γ-CsPbI3 under biaxial strains on the different planes were investigated by first-principles method. The results show that the tensile strain effectively reduces the PbI6 octahedral tilt to maintain the structural stability and improves the defect formation energies of VI, VPb, VCs and ICs, while the compressive strain can increase the formation energies of IPb and CsPb, hinders the diffusion of VI along the octahedral path and makes the defect level of VI shallower. The biaxial strain on the (001) plane is favourable for decreasing the formation enthalpy difference and maintaining the shallow level of VI to preserve the black phase structure, whereas that on the (100) plane is more beneficial for modulating these properties. This work offers theoretical guidance for applying strain to influence structure stability and suppress defect formation in γ-CsPbI3. Furthermore, it proposes a strategy utilizing lattice strain to modulate the energy levels of defects, thereby facilitating their passivation and enhancing device performance of perovskite photovoltaics.

Strain Engineered Bridged Bicyclic Diene Photoswitches in the Race of Next‐Generation Molecular Solar Thermal Energy Storage

AbstractNorbornadiene/Quadricyclane (NBD/QC) is a prototypical bridged bicyclic diene (BBD)‐based photoswitch that has been well‐studied for molecular solar thermal energy storage (MOST). Inspired by the recent synthetically accessed BBDs, herein several photoswitches are rationally designed with modulated ring strain energies (RSE) in photoisomers to incorporate high energy storage density (ESD) and storage time in a single couple. The storage energy ( ) calculated at DLPNO‐CCSD(T)/Def2TZVP level is correlated with difference in RSE of two isomers (ΔRSE) whereas thermal back reaction (TBR) barrier calculated at (8,8)‐CASPT2/6‐311++G** shows correlation with RSE in metastable photoproduct. On the basis of these structure‐property‐RSE relationships, we recognized that two photoisomers need not to be highly strained. Instead, the RSE in the photoproduct and diene should be minimized while maintaining a large enthalpy difference between them to increase ESD and extend energy storage times in a single photoswitch. TBR barrier is governed by RSE in photoproduct and increasing strain in photoproduct may improve the but at the cost of the TBR barrier. Herein, the structural skeletons are explored that holds promise to remarkably improve thermochemical properties relative to the unsubstituted BBD‐based photoswitches reported so far. The BBD molecules with short saturated bridge length but elongated unsaturated bridges could bestow desirable thermochemical parameters and can be regarded as excellent candidates for MOST application. The work lays a theoretical foundation that guides to improve thermochemical properties via strain engineering of BBD‐based photoswitches and opens a new avenue for designing principles and future experimental investigations of MOST systems.

Cation interception and permeability characteristics of bentonite barriers exposed to NaCl and NH4Cl solutions.

High concentrations of Na+ and NH4+ in landfill leachate lead to deterioration of bentonite barrier and pose a threat to the environment. This study focused on the pollution interception and permeability characteristics of the bentonite barrier exposed to NaCl and NH4Cl solutions. Based on previous findings, salt solution concentrations were established at 74.80, 37.40, 18.70, and 9.4 mmol/L. The bentonite contents in the mixture were set at 0, 5, 10, and 15%. The results indicate that the samples exhibit better interception of NH4+ compared to Na+. This difference arises from the cation exchange sequence, the size of the hydration radius, and the hydrogen bonding of the two cations. Additionally, the difference in hydration enthalpy between the two cations leads to variations in the swelling of bentonite, resulting in a higher hydraulic conductivity coefficient in NH4Cl solution. This study shows that although bentonite barriers have better interception for NH4+, they exhibit greater hydraulic conductivity in NH4Cl solution, increasing the risk of leachate carrying other contaminants.

Unveiling the mechanism of deformation-induced supersaturation

A Cu-6at%Ag cast alloy was deformed by means of high-pressure torsion to different applied strain levels until a steady-state regime is reached. The continuous structural refinement is attended by the successive dissolution of the Ag precipitates in the Cu matrix. The results show that the Ag regions need to fall below a phase size of ~ 5 nm to fully dissolve. Atomistic calculations indicate that the final dissolution can be explained based on the enthalpy difference between the solid solution and layered systems which are in between the coherent and semi-coherent structure. These findings are supported by detailed microstructural investigations.

Structural characterization and keto-enol tautomerization of 4-substituted pyrazolone derivatives with DFT approach

The synthesis of two pyrazolone derivative compounds, PYR-I(4-Acetyl-1-(4-chlorophenyl)-3-isopropyl-1H-pyrazol-5(4H)-one) and PYR-II1-(4-Chlorophenyl))-3-isopropyl-5-oxo-4,5-5-dihydro-1H-pyrazole-4-carbaldehyde, their characterization by FT-IR, NMR, UV–Vis and GC-MS techniques, and the evaluation of the keto-enol tautomerization process of the structures along with the DFT approach and spectral data were reported in this paper. Spectral findings indicated that PYR-I was stable at the keto state. The IR spectrum recorded in solid form showed that the PYR-II structure was stable in the enol state, while the NMR spectrum in the solution medium showed that it was stable in the keto state. DFT-based analyses were realized with the B3LYP hybrid functional and the 6–311++G(d,p) basis set. The modelled keto, transition and enol state molecular geometries of structures were optimized in the gas phase and different solvent media and the total energy and dipole moment values were investigated at the specified theoretical level. The possible keto-enol tautomerism mechanism of the structures was evaluated through some thermodynamic parameters such as the difference in free Gibbs energy (ΔG), enthalpy (ΔH), entropy (ΔS), and predictive tautomeric equilibrium constants (Keq), acidity constants (pKa) and percentages of tautomers at 298.15 K and 1 atm pressure. The results of these analyses based on the DFT approach indicated that the keto-enol tautomer equilibrium heavily favours the keto form for PYR-I and the enol form for PYR-II in all cases. Moreover, natural bond orbital (NBO) analysis was performed for the tautomers, and the chemical reactivity profiles of the most stable tautomers were examined with the values of frontier molecular orbital energy and some reactivity descriptors.

Equivalent Enthalpy Model and Log-Mean Enthalpy Difference Method for Heat-Mass Transfer in Cooling Towers

Abstract In industrial processes involving both heat and mass transfer, the enthalpy or total energy difference, not the temperature difference, is a more comprehensive driving-force potential for heat exchanger systems, such as a cooling tower. However, the common practice of current enthalpy methods based on Merkel integral requires numerical integration with implied linear distributions of air enthalpy and water temperature. If the cooling range is relatively small, the outcome of the integration may carry little error. But for a large cooling range and a substantially increased temperature, the error involved may be significant. Like log-mean temperature difference, a log-mean enthalpy difference method may require only the inlet and outlet conditions without prior knowledge of the enthalpy distributions of the fluids within the heat exchanger. The known Berman's enthalpy difference method requires correction factors, which was derived based on the linearization of the interfacial enthalpy and the enthalpy difference between the interfacial moist-air enthalpy and the bulk air enthalpy without underlying interpretation to demonstrate that the method could be broadly used. In this paper, an equivalent enthalpy model is proposed, and the log-mean enthalpy difference is derived without any constraints for broad uses. The derived method is compared with the results of Merkel's method from literature with excellent agreement. Thus, the work in this paper would open the possibility for broad uses of the log-mean enthalpy difference method for many industrial processes involving mass transfer.

Study on personal comfort heating system and human thermal comfort in extremely low-temperature building environments

Exposure to extremely low-temperature building environments around −20 °C causes various hazards to human health. However, few studies have studied human thermal comfort and physiological response in such environments, nor the personal comfort heating system. Thus, this study investigated the heating characteristics of personal comfort heating system and their effects on thermal responses and thermophysiological parameters in extremely low-temperature building environments. We conducted experimental tests and questionnaires in a cryogenic enthalpy difference laboratory with air temperature ranging from −21.2 to −15.6 °C and a relative humidity of 55 %. Eight experimental conditions were designed with several heating measures (heating mat, heating gloves, heating cushion and airwarmer). The heating parameters of the personal comfort heating systems as well as the subjective evaluations and skin temperature of the subjects were recorded and analyzed. The results showed that the airwarmer confinement box condition had a corrective power of up to 38.3 K. The overall thermal sensation vote and overall thermal comfort vote were maintained at 0.25 and −0.25. Further, we found that the face, hands, calves and feet got cold easier with the largest skin temperature difference of −4.9, −8.1, −6.7 and −7.6 °C, respectively. Besides, the relationships between thermal responses and between thermal responses and skin temperature were examined, as well as a thermal comfort assessment methodology was established for extremely low-temperature environments.

EXPERIMENTAL STUDY OF THERMODYNAMIC PROPERTIES OF BIOFUEL COMPONENTS

The density () and speed of sound (u) of two main components of biofuels, methyl laurate and methyl stearate, were measured at temperatures from (283 to 353) K and from (313 to 353) K at atmospheric pressure, respectively. An Anton Paar DSA 5000 М sound-speed analyzer, has been used to simultaneously measurements of the density and speed of sound of the methyl laurate and methyl stearate as the primary components of biodiesel fuel. The measured values of density and speed of sound were used to calculate other derived thermodynamic properties such as adiabatic coefficient of bulk compressibility, coefficient of thermal expansion, isothermal coefficient of bulk compressibility, isochoric and isobaric heat capacities, enthalpy and entropy difference, partial temperature derivative of enthalpy, entropy, and the partial specific volume derivatives of internal energy (internal pressure), of the methyl laurate and methyl stearate as a function of temperature. The overall uncertainties (at the 95 % confidence level) of the reference correlations of the density and speed of sound of methyl laurate and methyl stearate are 0.025 % and 0.045 %, respectively.

Long-range magnetic order in CePdAl3 enabled by orthorhombic deformation

We investigate the effect of structural deformation on the magnetic properties of orthorhombic CePdAl3 in relation to its tetragonal polymorph. Utilizing x-ray and neutron diffraction, we establish that the crystal structure has the Cmcm space-group symmetry and exhibits pseudotetragonal twinning. According to density functional calculations, the tetragonal-orthorhombic deformation mechanism has its grounds in the relatively small free enthalpy difference between the polymorphs, allowing either phase to be quenched, and fully accounts for the twinned microstructure of the orthorhombic phase. Neutron diffraction measurements show that orthorhombic CePdAl3 establishes long-range magnetic order below TN=5.29(5) K characterized by a collinear, antiferromagnetic arrangement of magnetic moments. Magnetic anisotropies of orthorhombic CePdAl3 arise from strong spin-orbit coupling as evidenced by the crystal-field splitting of the 4f multiplet, fully characterised with neutron spectroscopy. We discuss the potential mechanism of frustration posed by antiferromagnetic interactions between nearest neighbors in the tetragonal phase, which hinders the formation of long-range magnetic order in tetragonal CePdAl3. We propose that orthorhombic deformation releases the frustration and allows for long-range magnetic order. Published by the American Physical Society 2024

Design of nonflammable mixed refrigerants based on the partial molar enthalpy difference in mixed-refrigerant Joule-Thomson refrigerators with/no pre-cooling stage

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) is an important cooling method at temperatures from 80 to 230 K. It can be used in cryosurgery, high-temperature superconductivity and sensor cooling etc. However, most of the studies are on nitrogen-hydrocarbon refrigerants, which are not allowed in applications where there is a special need to avoid flammability risks. Therefore, non-flammable mixed refrigerants were investigated in this study.The composition of the mixed refrigerant is a key factor in the system performance (refrigeration temperature, refrigeration capacity, etc). However, the purely mathematical optimization methods lack system knowledge in the optimization of the process, and may have the disadvantages of being a time-consuming process. In this study, the isothermal throttling effect of mixed refrigerants is optimized, and the optimization process is based on the partial molar enthalpy difference of each component. The method is based on thermodynamic properties and is time-saving relative to purely mathematical optimization techniques.Non-flammable mixed refrigerants (NFMR) with refrigeration temperatures from 100 K to 140 K were designed in this study. The results show that the designed mixed refrigerant has a higher COP compared to the reference. Argon has an advantage at refrigeration temperatures from 120 K to 140 K, while nitrogen has an advantage from 100 K to 120 K. In addition, mixed refrigerants for systems with pre-cooling stage were optimized and the results showed that the highest exergy efficiency is achieved at a pre-cooling temperature of 250 K. The exergy efficiencies with pre-cooling stage are nearly twice as high as those without that. Therefore, a pre-cooling stage for nonflammable mixed refrigerants is necessary where there is no requirement for system size.

Body-centered cubic phase stability in cobalt-free refractory high-entropy alloys

Cobalt-free refractory high-entropy alloys (RHEAs) are strong contenders for structural materials in nuclear reactors because they do not exhibit cobalt activity under irradiation. The mechanical properties and thermal stability of RHEAs are primarily attributed to the solid solution phase, which is essentially the body-centered cubic (BCC) phase. The BCC phase formation rules thus became the basic criterion in the compositional design of RHEAs. In this paper, the BCC phase formation rules in cobalt-free RHEAs were determined via the calculation of six semiempirical parameters, namely, the entropy of mixing, enthalpy of mixing, atomic size difference, Ω-parameter, d-orbital energy level and valance electron concentration. The mixing enthalpy and atomic size differences are more effective than other semiempirical parameters for predicting BCC phase stability in cobalt-free RHEAs. The presence of aluminum is found to cause a notable alteration in the range of phase stability in cobalt-free RHEAs.

Heat-Transfer Properties of Additively Manufactured Aluminum Lattice Structures in Combination with Phase Change Material.

Compared to sensible heat storage, latent heat storage provides higher energy density due to the enthalpy difference of the storage medium undergoing a phase change. However, the heat storage capability of phase change materials is opposed by low thermal conductivity. To enable sufficient heat transfer within a latent heat storage unit, phase change materials can be used in combination with a metallic matrix. One approach is the infiltration of phase change materials into additively manufactured metallic lattice structures. In this work, the fabrication of aluminum lattice structures through laser powder bed fusion is described. During fabrication, the cell size and the strut diameter were varied to obtain specimens of different geometries. To obtain the thermal conductivity of the fabricated lattices, measurements were conducted based on the transient plane source method. Additionally, finite element simulations were carried out to evaluate the effect of fabrication and measurement uncertainties. The thermal conductivity of the fabricated lattices was found to be between 3 W/(m·K) and 130 W/(m·K). The numerically and analytically performed calculations provide good estimations of the experimentally obtained data.

Melting point of iron at high pressure: An assessment of uncertainties and effect of electronic temperature

An accurate calculation of the melting point of iron at various pressures in the Earth's core is important for understanding the core structure, geodynamo, and the Earth's history. Previous studies have assessed the melt line of iron at these extreme conditions using various experimental measurement techniques as well as both ab initio and classic molecular dynamics simulations. However, experimental measurements have uncertainties up to several hundred Kelvin, and inconsistencies remain among simulation results. In this work, we propose an iterative framework that couples density functional theory (DFT) calculations and molecular dynamics simulations performed using an ensemble of interatomic potentials to assess the effect of electronic temperature on the melting point. We systematically validate the potentials by comparing lattice constants and phonon dispersion curves at 0 K and enthalpy differences between liquid and HCP, FCC, BCC phases of iron close to the melt line at 300 GPa with DFT. Our results show that HCP iron melts at 6144 K (at 300 GPa), BCC phase is thermodynamically unstable, and FCC is metastable at this temperature. The melting points of FCC and BCC phases at 300 GPa are 5858 and 5647 K, respectively.

Structural phase transition, s±-wave pairing, and magnetic stripe order in bilayered superconductor La3Ni2O7 under pressure

Motivated by the recently discovered high-Tc superconductor La3Ni2O7, we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y2− mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y2− mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector (π, 0) was stable at the intermediate region. Pairing is induced in the s±-wave channel due to partial nesting between the M = (π, π) centered pockets and portions of the Fermi surface centered at the X = (π, 0) and Y = (0, π) points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest La3Ni2O7 is qualitatively different from infinite-layer nickelates and cuprate superconductors.