Lattice Contribution Research Articles

Quantum versus classical nature of the low-temperature magnetic phase transition in TbAl3(BO3)4

Specific heat, $C_B$, of a TbAl$_3$(BO$_3$)$_4$ crystal was studied for 50 mK $<T<$ 300 K, with emphasis on $T<1$ K, where a phase transition was found at $T_c =0.68$ K. Nuclear, non-phonon ($C_m$), and lattice contributions to $C_B$ were separated. Lowering of $T_c$ with ncrease of magnetic field parallel to the easy magnetization axis, $B_{||}$, was found. It was established that $C_m$ and a Gr\"uneisen ratio depend on $B_{||}$ and $T$ in a way characteristic of systems, in which a classical transition is driven by quantum fluctuations, QF, to a quantum critical point at $T=0$, by tuning a control parameter ($B_{||}$). The $B_{||} - T$ phase diagram was constructed and the dynamical critical exponent $0.82 \le z \le 0.96$ was assessed. Nature of the transition was not established explicitly. Magnetization studies point at the ferromagnetic ordering of Tb$^{3+}$ magnetic moments, however, lowering of $T_c$ with increase in $B_{||}$ is opposite to the classical behavior. Hence, a dominant role of QF was supposed.

Synergistic Optimization of the Thermoelectric and Mechanical Properties of Large-Size Homogeneous Bi0.5Sb1.5Te3 Bulk Samples via Carrier Engineering for Efficient Energy Harvesting.

Manufacturing an economically viable, efficient commercial thermoelectric (TE) module is essential for power generation and refrigeration. However, mediocre TE properties, lack of good mechanical stability of the material, and significant difficulties involved in the manufacturing of large-scale powder as well as bulk samples hinder the potential applications of the modules. Herein, an economically feasible single-step water atomization (WA) is employed to synthesize BST powder (2 kg) by Cu doping within a short time and consolidated into large-scale bulk samples (500 g) for the first time with a diameter of 50 mm and a thickness of about 40 mm using spark plasma sintering (SPS). The incorporation of Cu into BST greatly boosts the carrier concentration, leading to a significant increase in electrical conductivity, and inhibits the bipolar thermal conductivity by 73%. Synchronously, the lattice contribution (κL) is greatly reduced by the effective scattering of phonons by comprising fine-grain boundaries and point defects. Therefore, the peak ZT is shifted to the mid-temperature range and obtained a maximum of ∼1.31 at 425 K and a ZTave of 1.24 from 300 to 500 K for the BSTCu0.05 sample, which are considerably greater than those of the bare BST sample. Moreover, the maximum compressive mechanical strength of large-size samples manufactured by the WA-SPS process is measured as 102 MPa, which is significantly higher than commercial zone melting samples. The thermoelectric module assembled with WA-SPS-synthesized BSTCu0.05 and commercial n-type BTS material manifests an outstanding cooling performance (-19.4 °C), and a maximum output power of 6.91 W is generated at ΔT ∼ 200 K. These results prove that the BSTCux samples are eminently suitable for the fabrication of industrial thermoelectric modules.

Modification of electronic and thermoelectric properties of InSe/GaSe superlattices by strain engineering

In recent years, superlattices and layered materials have been highlighted as potential candidates for thermoelectric applications, this thanks to their low thermal conductivity. Moreover, external applied pressure and biaxial strain can be used to enhance their properties by achieving a band engineering and electronic tuning. With this in mind, we performed an $ab$ initio based study on InSe/GaSe superlattices under biaxial strain along the layer planes. Layers of InSe and GaSe with ${D}_{3h}$ symmetry were stacked along the $c$ axis to create the superlattice. The atomic stacking along the $c$ axis is Se-Ga-Ga-Se-Se-In-In-Se, which corresponds to the space group #187. Our ab initio calculations predict the superlattice to be a semiconductor with an electronic band gap of ${E}_{g}$ = 0.54 eV. With the aim to increase thermoelectric performance, we apply positive and negative biaxial strain on the ab plane. Under compressive strain, the electronic structure evolves to an insulating behavior by increasing the band gap. When tensile strain is applied, we observe a transition towards a metallic character with a systematic reduction of the band gap. Interestingly, the semiconductor-metal transition only occurs when spin-orbit coupling (SOC) is switched off. With the inclusion of SOC, the system experiences an electronic topological transition around 3% tensile strain, with a double band gap along the $K\text{\ensuremath{-}}\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}M$ high-symmetry paths. We have found that for both, $n$-type and $p$-type doping, compressive strain improves the electronic figure of merit ($Z{T}_{e}$ under constant relaxation time approximation). Not only the electronic part increases thermoelectric performance, but also the lattice contribution to the thermal conductivity decreases with compressive strain.

Origin of high-power drive stability in (Na1/2Bi1/2)TiO3-BaTiO3 based piezoceramics

Hard-type ferroelectrics are indispensable and increasingly used for numerous high-power piezoelectric resonance applications, ranging from ultrasonic welding and cleaning, to voltage transformers and miniaturized ultrasonic motors in robotics. While the state-of-the-art lead-based ferroelectrics are hitting their operational limits, several new and environmentally-friendly lead-free alternative materials have emerged recently. Particularly compositions of the (Na1/2Bi1/2)TiO3 family have demonstrated promising high-power properties and increased depolarization temperatures. However, the underlying mechanisms remain unclear and hamper further development of these materials. Here, we investigate the high-power electromechanical behavior of several (Na1/2Bi1/2)TiO3-based compositions and reveal that these exhibit an inherent intrinsic stability, which is retained at large vibration velocity and broad temperature range also after acceptor doping or composite formation. This results in excellent high-power figures of merit even at vibration velocities beyond 1 m/s, at which the lead-based compositions already fail. Moreover, the critical vibration velocity for fracture was found to be almost two-times higher for (Na1/2Bi1/2)TiO3-based materials, demonstrating their high dynamic fracture strength. The microscopic origin of this behavior was identified using synchrotron radiation, revealing that the strain is dominated by lattice contributions, induced by the emerging mechanical stress. Due to the large coercive stress, the domain wall contributions are in the range of only 3‒5% and, unlike in PZT, remain relatively constant also at higher vibration velocities. The high-power properties and stability are thus determined by the intrinsic loss instead of the usually-dominant extrinsic loss.

In-situ reactive synthesis of dense nanostructured β-FeSi2 by Spark Plasma Sintering

β-FeSi2 is a promising material for thermoelectric application, especially if it can be nanostructured to decrease the lattice contribution to the thermal conductivity. It can be even more interesting if the entire chain of processes implemented to obtain it is not too time and energy consuming. In this paper we report a simple route to synthesize nano-β-FeSi2 by combining mechanical milling and reactive spark plasma sintering of Fe-Si alloys. A map of the pressure-temperature conditions using a spark plasma sintering setup was performed and shows that it is possible to obtain nano-β-FeSi2 pellets with a density above 90% using dwell times as short as 5 min. The impact, of both, the density and nanostructuring on the hardness of the β-FeSi2 pellets obtained by this process are presented as well as the impact of the thermoelectric properties of the materials.

Effect of Irradiation with Low-Energy He2+ Ions on Degradation of Structural, Strength and Heat-Conducting Properties of BeO Ceramics

The paper is devoted to the study of radiation-induced damage kinetics in beryllium oxide ceramics under irradiation with low-energy helium ions with fluences of 1015–1018 ion/cm2. It was revealed that at irradiation fluences above 1017 ion/cm2, a decrease in radiation-induced damage formation and accumulation rate is observed, which indicates the saturation effect. At the same time, the main mechanisms of structural changes caused by irradiation at these fluences are amorphization processes and dislocation density increase, while at fluences of 1015–1016 ion/cm2, the main mechanisms of structural changes are due to the reorientation of crystallites and a change in texture, with a small contribution of crystal lattice distorting factors. It was discovered that the radiation-induced damage accumulation as well as an implanted helium concentration increase leads to the surface layer destruction, which is expressed in the ceramic surface hardness and wear resistance deterioration. It was determined that with irradiation fluences of 1015–1016 ion/cm2, the decrease in thermal conductivity is minimal and is within the measurement error, while an increase in the irradiation fluence above 1017 ion/cm2 leads to an increase in heat losses by more than 10%.

Heat Capacity of Scandoborate NdSc-=SUB=-3-=/SUB=-(BO_3)-=SUB=-4-=/SUB=-

Single crystals of trigonal neodymium scandoborate NdSc3(BO_3)4 were grown by the group method from a solution-melt based on bismuth trimolybdate. The molar heat capacity C(T) was studied in the temperature range 2-300 K and magnetic fields up to 9 T. The experimental curve was approximated by the combined Debye-Einstein model. The lattice contribution was determined from ab-initio calculations. Schottky anomaly was observed in the low-temperature region C(T) with the applied magnetic field. Keywords: crystal growth, magnetoelectrics, heat capacity, Schottky anomaly.

A review on the effects of various elemental doping and nanostructuring of [formula omitted]-[formula omitted]/[formula omitted] composites on the thermoelectric performance enhancement

Thermoelectric device is a transformative technology for renewable energy generation. It is designed in a small compact feature with a quiet mechanism and has no gas emission, enabling it to conserve energy and preserve the global environment. Iron disilicide (β-FeSi2/Si) composite materials prepared by eutectoid reaction is found promising for thermoelectric applications. The strategies to enhance the thermoelectric properties of these composites are mainly by conducting band structure engineering such as elemental doping and nanostructure engineering. This article reviews the effects of elemental doping and nanostructuring of β-FeSi2/Si composite materials. Mn and Co were found to be the most common dopants due to their contribution to carrier mobility and carrier concentration. Other dopant elements such as Al, introduced point defects on the composite structure that is effective in lowering thermal conductivity. P was also found to be effective on both electronic and lattice contribution of thermal conductivity. Furthermore, in thin films structure, flatter surface is highly recommended rather than the crystallinity of films to enhance carrier mobility and suppress thermal conductivity. Thus, the way to optimize β-FeSi2/Si composite materials were discovered by the significant number of studies on refinement of composite structure into nanoscale. From these studies, electrical conductivity value was significantly enhanced while reducing its thermal conductivity and Seebeck coefficient value. The deterioration of Seebeck coefficient is yet to be improved hence was estimated possible by introducing doping mechanisms. Therefore, this review concludes some potential strategies for improving Seebeck coefficient while simultaneously increasing electrical conductivity and decreasing thermal conductivity towards enhancing its thermoelectric performance.

Ab initio thermodynamics of one-component plasma for astrophysics of white dwarfs and neutron stars

ABSTRACT Using path-integral Monte Carlo (PIMC) simulations, we have calculated energy of a crystal composed of atomic nuclei and uniform incompressible electron background in the temperature and density range, covering fully ionized layers of compact stellar objects, white dwarfs, and neutron stars, including the high-density regime, where ion quantization is important. We have approximated the results by convenient analytic formulae, which allowed us to integrate and differentiate the energy with respect to temperature and density to obtain various thermodynamic functions such as Helmholtz free energy, specific heat, pressure, entropy etc. In particular, we have demonstrated, that the total crystal specific heat can exceed the well-known harmonic lattice contribution by a factor of 1.5 due to anharmonic effects. By combining our results with the PIMC thermodynamics of a quantum Coulomb liquid, updated in the present work, we were able to determine density dependences of such melting parameters as the Coulomb coupling strength at melting, latent heat, and a specific heat jump. Our results are necessary for realistic modelling of thermal evolution of compact degenerate stars.

Long-Range Cationic Order Collapse Triggered by S/Cl Mixed-Anion Occupancy Yields Enhanced Thermoelectric Properties in Cu5Sn2S7

To investigate pathways to adjust the charge carrier concentration and optimize the thermoelectric properties, we characterized structural properties, thermal stability, and thermoelectric performance of pristine and Cl-doped Cu5+εSn2−εS7. We demonstrate that Cl doping in Cu5Sn2S7-type monoclinic compounds induces a collapse of the long-range cationic ordering, ultimately leading to a sphalerite-type cubic phase characterized by ordered [Sn(S,Cl)4]x clusters. The change in crystal structure symmetry upon Cl doping is analyzed by Rietveld refinements against X-ray powder diffraction data, transmission electron microscopy, Mössbauer and X-ray absorption spectroscopy, and low- and high-temperature transport property measurements. The thermoelectric properties of the so-obtained cubic sphalerite Cu5+εSn2−εS7–yCly (0 ≤ ε ≤ 0.133, y = 0.35, 0.70) are strongly enhanced with respect to the undoped Cu5Sn2S7: the power factor improves slightly while both electronic and lattice contributions to the thermal conductivity are reduced. Overall, single-phase Cl-doped Cu5.133Sn1.866S7–yCly (y = 0.35, 0.70) compounds exhibit high thermoelectric performance, reaching a maximum ZT of 0.45 at 670 K.

The challenge of tuning the ratio of lattice/total thermal conductivity toward conversion efficiency vs power density

Minimizing the lattice thermal conductivity of thermoelectric materials is essential for preserving the temperature difference during the operation of thermoelectric devices incorporating these materials. During the past two decades, there has been substantial improvement in the thermoelectric figure of merit (zT) due to reduced lattice thermal conductivity. Employing alloying effects in solid-solution compounds is the most common and practical approach for inhibiting lattice thermal conductivity. This Perspective takes the n-type Mg3Sb2−xBix thermoelectric alloys as examples, addressing their lattice thermal conductivity and corresponding zT as functions of their Bi concentration. Additionally, we seek to understand the effect of the lattice contribution to total thermal conductivity for most thermoelectric materials currently being researched. The lattice/total thermal conductivity ratio at the temperature corresponding to the peak zT shows weak material dependence, widely ranging from 0.5 to 0.75, which implies that the lattice thermal conductivity of most thermoelectric materials can be decreased further to improve thermoelectric performance. On the other hand, thermoelectric materials with relatively low ratios exhibit high power factors in their operating temperature ranges, which is ascribed to their excellent electrical performance. These observations provide guidelines to tune transport properties for future applications in thermoelectric power generation.

Abundant Solvatomorphism-Tuned Spin Crossover in a Dinuclear Fe(II) Compound: Computational Insights on Molecular Distortion and Packing Effects

A series of solvates based on a dinuclear Fe(II) complex [(TPA)2Fe2(μ-DHBQ)][BF4]2·S [S = CH3OH (1·CH3OH), 2CH3OH (2·2CH3OH), 4DMF (3·4DMF), 2Et2O (4·2Et2O), Et2O·MeCN (5·Et2O·MeCN), and 2CH2Cl2 (6·2CH2Cl2)] were synthesized. Upon solvent removal, single-crystal-to-single-crystal (SC-to-SC) transitions could occur for the first four solvates to give 1, 2, 3, and 4·0.5Et2O. Within the temperature range of 400–10 K, these compounds exhibited abundant variations in their spin crossover (SCO) properties. 2,5·Et2O·MeCN and 6·2CH2Cl2 displayed half high-spin (HS) to low-spin (LS) transitions, and 4·2Et2O underwent incomplete LS-to-HS conversion, whereas other solvatomorphs showed complete SCO. As all these solvatomorphs possessed the identical [(TPA)2Fe2(μ-DHBQ)][BF4]2 core, these variations in SCO behavior emphasized the critical role of the crystal lattice contributions. With the aim of deciphering their origin, periodic DFT+U+D3 computations were performed on these solvatomorphs to quantify the importance of all possible intramolecular and intermolecular effects on their spin-state energetics. Computationally, the isolated [(TPA)2Fe2(μ-DHBQ)]2– molecule in the gas phase would undergo SCO around 350 K in one step intrinsically. However, distinctive roles of the crystal-lattice effects in the solid state resulted in varying SCO behaviors. Different reasons were discovered for the incomplete spin transitions of different solvatomorphs. For 2, although serious volume shrinkage of the LS state caused efficient packing of the overall crystal-lattice organization, the originally close-packing SCO cations got loose and thus strongly destabilized its LS state. For 5·Et2O·MeCN and 6·2CH2Cl2, severe molecular distortions kinetically trapped their LS state. These computationally magneto-structural studies on dinuclear solvatomorphs have great importance for designing SCO compounds with selected properties, which is critical for their actual application.

Expand band gap and suppress bipolar excitation to optimize thermoelectric performance of Bi0.35Sb1.65Te3 sintered materials

Bismuth-telluride-based alloys have been the state-of-art thermoelectric candidates near room temperature. However, sintered samples with higher ZT values in a wide temperature range are still urgently needed for further applications. Especially owing to the small band gap, the minority carriers are boosted at higher temperature and the thermoelectric properties are severely deteriorated. Here, we prepared p-type Bi0.35Sb1.65Te3-xSex samples by hot pressing to enlarge the band gap and suppress the bipolar excitation. Although the carrier concentrations are slightly increased by the tiny Se alloying, the Seebeck coefficients above 400 K are significantly improved, and the bipolar and lattice contributions to thermal transport are obviously reduced. The x = 0.04 sample thus achieves an average value of 1.1 between 300 and 500 K, and the constructed thermoelectric device produces an output power density of 0.36 W cm−2 and conversion efficiency of 5.24% at a temperature difference of 200 K. These results suggest that Se-doped Bi0.35Sb1.65Te3 is a robust candidate for low-grade heat harvest near room temperature.

Determination of magnetic, electronic and lattice contributions to low-temperature specific heat: Procedure and its application to metamagnetic alloys

Procedure of determination of magnetic part of the specific heat of ferromagnetic (FM) solid from the results of the low-temperature measurements is proposed and applied to the number of non-stoichiometric Ni50Mn50-xInx alloys. It is shown that the disregard of magnetic contribution to the specific heat of FM solid results in the noticeable underestimation of Debye temperature and overestimation of the specific heat of electron subsystem of FM solid. The magnetic contribution to the low-temperature specific heat of antiferromagnetic (AFM) solid is negligibly small if Néel temperature exceeds the Debye temperature. In the opposite case the magnetic part of the specific heat of AFM solid may be comparable in value with non-magnetic part, but separation of these parts is an open problem.

Heat capacity of a MnFe(P,Si,B) compound with first-order magnetic transition

The heat capacity of a MnFe(P,Si,B) giant magnetocaloric material is investigated by several calorimetric techniques and first-principles calculations. Semi-adiabatic relaxation calorimetry, commercial heat flow and home-made Peltier cells Differential Scanning Calorimeters (DSC) are used to record the heat capacity both around the magnetic transition at room temperature and at low temperatures. The ferromagnetic first-order transition is characterized by a sharp and intense latent heat peak corresponding to a transition entropy change of ∼2.23 J mol−1 K−1, while the thermal hysteresis of ∼2 K remains moderate. The analysis of the heat capacity over a broad temperature range provides an estimate of several important parameters, such as the electronic density of states (N(EF) ≈ 7.2 states eV−1f.u.−1) at low temperatures or an estimate of the Debye temperature θD ≈ 455 K. Different methods (non-magnetic reference, Debye function, phonons calculations) were used to model the lattice contribution to the heat capacity. While this study highlights the difficulty to disentangle lattice, electronic and magnetic contributions from a single quantity such as the heat capacity, it reveals that the excess entropy (magnetic contribution plus latent heat) is considerably larger than in the parent Fe2P.

Remarkable suppression of lattice thermal conductivity by electron-phonon scattering in iridium dioxide nanowires

The well-known electrocatalyst iridium dioxide (IrO2) has recently shown interesting topological and spin transport properties with potential applications as electrodes and spin injector/detector in oxide-based electronic and spintronic devices. So far, no study has been reported on its thermal properties that could disclose important aspects of electron and phonon interactions. Through combined experimental and modeling efforts, here we show rather high thermal conductivity of IrO2 that is mainly attributed to phonon transport. Analysis indicates that the large lattice contribution results from the strong interatomic bonding and large difference in the atomic mass between iridium and oxygen. Interestingly, it is found that electron-phonon scattering plays a significant role and leads to a remarkable reduction in the lattice thermal conductivity.

Lattice contributions to the anisotropic dielectric response of rare-earth langasites

We investigate the phonon spectrum of pure and rare-earth substituted langasites. For electric fields oriented along the $c$ axis, a strong low-frequency phonon is observed, softening at lower temperature in all samples studied. The dielectric contribution and temperature dependence of this mode is the primary source of the large $[\ensuremath{\varepsilon}(0)\ensuremath{\sim}100]$ static dielectric permittivity observed in these systems. Softening of this mode with increasing mass of the rare-earth substitutes further suggests that langasites are close to a structural instability at low temperatures. The application of a magnetic field to a diluted Ho-langasite sample reveals a shift in the strong absorption band at THz frequencies associated with this soft lattice mode, suggesting spin-lattice effects may also be at play in these systems.

Giant reversible barocaloric effect with low hysteresis in antiperovskite PdNMn3 compound

Barocaloric (BC) refrigeration is a new type of clean and energy-efficiency refrigeration technology. However, the giant and colossal BC effects are mainly based on the first-order phase transitions, leading to the unfavorable hysteresis. Here we report the giant BC effect along with weak hysteresis at the antiferromagnetic (AFM) to paramagnetic (PM) phase transition (TN = 283 K) in antiperovskite PdNMn3 compound. The reversible isothermal entropy change reaches 28.3 J kg−1 K−1 under 290 MPa. The hysteresis is only about 2 K, which is associated with a small volumetric change ΔV/V at TN (~0.2%). The magnetic entropy is proposed to mainly contribute to the entropy change at TN, which compensates for the reduced lattice contribution due to the small ΔV/V. Our result demonstrates the possibility of realizing giant BC effect together with weak hysteresis in materials where both crystallographic and non-crystallographic entropy changes cooperate at the first-order phase transition (FOPT).

Specific Heat on Single-crystalline YVO3

The specific heat of single-crystalline YVO3 was measured from 2 K up to 250 K at zero field. The results reveal three transitions, at around 75, 115, and 200 K. The transitions at around 115 K and 200 K show that the phase transition is of the second-order type, whereas at around 75 K, unusual features of the specific heat are found. These unusual features are attributed to the effect of a large change in the volume. The specific heat data were analyzed in terms of a lattice contribution, a Schottky contribution and an excess magnetic contribution at high temperature. The magnetic contribution well above the magnetic ordering temperature is ascribed to short-range interactions due to the presence of strong magnetocrystalline anisotropy. The magnetic entropy considered by using this approach is 9.13 J/mole K which is close to the theoretical estimate for the S = 1 system.

Enabling Real-Time Quantification of Electrochemical Desorptionphenomena with Sub-Monolayer Sensitivity

Across many scientific and industrial fields, it is a challenge to analyze chemical reaction products with high sensitivity and fast time-response. This is especially true in systems related to electrochemical energy conversion and storage, such as batteries and fuel cells, where accurate monitoring of fast and transient reaction phenomena often reveals key insights.Here, we present a unique analysis instrument using a microchip-based inlet system to couple a mass spectrometer directly to any liquid test environment, aqueous or non-aqueous, and exhibits a sensitivity up to 2-3 orders of magnitude than conventional differential electrochemical mass spectrometry (DEMS).(1–3) The instrument allows to measure the evolution of volatile products and consumption of reactants at electrode surfaces. Due to its extraordinary sensitivity, the system can measure all the individual volatile molecules desorbing from an electrode surface during a single electrochemical turnover. Product formation can be measured from total Faradaic currents of 1 mA all the way down to 200 nA, corresponding to approximately 0.2% of a monolayer (ML) desorbing from the electrode surface in 1s. These features enable time-resolved, fully quantitative measurements of transient phenomena during electrochemistry, providing fundamental insight in the electrochemical reaction mechanisms. Furthermore, an accurately calibrated on-chip gas system allows rapid gas switching between different gases, both inert and reactive. Due to the near-instantaneous equilibration between gas and electrolyte, the transient response of electrodes to rapid gas exposure changes can be measured.As direct evidence of the ultrahigh sensitivity, i n situ quantification the anodic desorption of less than ~1% of a ML of hydrogen on a Cu electrode in alkaline electrolyte (Fig. 1) is shown.(4) Finally, several examples of quantitative electrochemical measurements will be presented, including lattice oxygen contribution during OER on NiFeOxHy,(2) and hydrocarbon oxidation of propene on palladium. (3, 5)Fig.1 H2 desorption on Cu(pc) in 0.1 M KOH. (4) D. B. Trimarco et al., Electrochim. Acta. 268, 520–530 (2018).C. Roy et al., Nat. Catal. 1, 820–829 (2018).A. Winiwarter et al., Energy Environ. Sci. 12, 1055–1067 (2019).S. B. Scott et al., Catal. Sci. Technol. (2020).A. Winiwarter et al., ChemElectroChem, in press. Figure 1