Sb Sites Research Articles

Significantly Enhanced Thermoelectric Figure of Merit of N-Type Mg3sb2-Based Zintl Phase Compounds Via Co-Doped of Mg and Sb Site

Significantly Enhanced Thermoelectric Figure of Merit of N-Type Mg3sb2-Based Zintl Phase Compounds Via Co-Doped of Mg and Sb Site

Enhanced Thermoelectric Performance of Polycrystalline Insb1-Xbix by Using Isoelectronic Substitution on the Sb Site

Enhanced Thermoelectric Performance of Polycrystalline Insb1-Xbix by Using Isoelectronic Substitution on the Sb Site

Effect of Sb/Bi atom substitution site on electronic transport properties of Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.625</sub> alloy

Mg<sub>2</sub>(Si,Sn)-based thermoelectric materials, which are environmentally friendly and low-cost, have great development potential in a moderate temperature range. Electronic transport properties of Mg<sub>2</sub>Si<sub>1-<i>x</i></sub>Sn<sub><i>x</i></sub> alloys can be optimized by doping elements. Doping is still one of the most effective methods of optimizing electronic transport performance, such as carrier concentration, mobility, and effective mass. The most effective doping elements are Sb and Bi. Much attention has been paid to the influence of element type and doping content. Different substitution sites will also greatly affect the electronic transport parameters. In this work, the defect formation energy value of Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.625</sub> alloy for substituting Sb atoms and Bi atoms for Sn sties and Si sites, respectively, are calculated by first-principles calculations. The influence on electronic transport parameters is systematically analyzed by combining the calculated results of band structures and density of states. Corresponding component Sb and Bi atoms doped Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.625</sub> alloys are prepared by rapid solidification method, and microstructures, Seebeck coefficients, and electrical conductivities of the alloys are measured. Combined with the predicted results by solving the Boltzmann transport equation, electronic transport performances are compared and analyzed. The results indicate that both Sn and Si sites are equally susceptible to Sb and Bi doping, but the Si sites are preferentially substituted due to their lower ∆<i>E</i><sub>f</sub> values. Doped Bi atoms provide a higher electron concentration, and Sb atoms offer higher carrier effective mass. Thus, the maximum <i>σ</i> value of the Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.615</sub>Bi<sub>0.01</sub> alloy is 1620 S/cm. The maximum <i>S</i> value of the Mg<sub>2</sub>Si<sub>0.365</sub>Sn<sub>0.625</sub>Sb<sub>0.01</sub> alloy is –228 μV/K. Correspondingly, the highest <i>PF</i> value for this alloy is 4.49 mW/(m·K) at <i>T</i> = 800 K because of the dominant role of <i>S</i> values. Although its power factor is slightly lower, the Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.615</sub>Sb<sub>0.01</sub> alloy is expected to exhibit lower lattice thermal conductivity due to the lattice shrinkage caused by substituting Sb sites for Sn sites. The optimal doping concentration of the Bi-doped alloy is lower than that of the Sb-doped alloy. These results are expected to provide a significant reference for optimizing the experimental performance of Mg<sub>2</sub>(Si, Sn)-based alloys.

Enhancement Thermoelectric Properties of Cux-doped Nanostructure Bi-Sb-Te Thermoelectric Materials by Spark Plasma Sintering

Nanostructure Cux-doped Bi0.5Sb1.5-xTe3 thermoelectric materials was successfully prepared by Mechanical alloys and spark plasma sintering. In the reasearch, the crystallinity, particle size, and chemical composition were characterized by XRD, EDS, respectively. Thermoelectric properties with a maximum ZT value up to 1.17 has been obtained at 407 K in prepared Cu0.04-doped Bi0.5Sb1.496Te3 sample. The achieved higher ZT value is attributed that Cu as doping at the Sb sites introduced additional holes to enhance carrier mobility and Cu dopants interrupted the periodicity of lattice vibration to decrease lattice thermal conductivity. It is suggested that the as-prepared nanostructure Cux-doped Bi0.5Sb1.5-xTe3 thermoelectric materials has high potential for thermoelectric energy conversion application.

Synergistic Optimization of Electronic and Thermal Transport Properties for Achieving High ZT in Ni and Te Co-substituted CoSb3

The current work aims to explore the enhancement in thermoelectric performance of CoSb3-based materials via doping with less expensive rare-earth-free dopants. For this purpose, Te was chosen as a dopant on the Sb site in pre-optimized parent alloy Ni0.07Co0.93Sb3. The facile and fast synthesis route of arc melting and spark plasma sintering was utilized to prepare uniform and dense samples. The structural and transport properties were experimentally measured over a range of temperatures. As a result of suitable optimization of thermal and electronic transport properties, a state-of-the-art ZT value of ∼0.9 at 870 K was achieved for the composition Ni0.07Co0.93Sb2.94Te0.06. This acquired value of peak ZT is ∼50% enhanced from the parent compound, which is majorly attributed to the augmented power factor. In order to analyze this huge increment, first-principles-based density functional theory calculations were performed. Ni doping in CoSb3 is found to result in an n-type doped semimetal. The co-doping with Te at the Sb site was effective in opening a gap resulting in a heavily doped n-type semiconductor with a higher power factor than the Ni-doped one. Further, the theoretically calculated electronic transport properties were found in agreement with those observed experimentally for the synthesized samples.

Charge Transport and Thermoelectric Properties of Sn-Doped Tetrahedrites Cu12Sb4-ySnyS13

In this study, tetrahedrite compounds doped with Sn were prepared by mechanical alloying and hot pressing, and their charge transport and thermoelectric properties were analyzed. X-ray diffraction analysis revealed that both the synthetic powders and sintered bodies were synthesized as a single tetrahedrite phase without secondary phases. Densely sintered specimens were obtained with relatively high densities of 99.5%-100.0% of the theoretical density, and the component elements were distributed uniformly. Sn was successfully substituted at the Sb site, and the lattice constant increased from 1.0348 to 1.0364 nm. Positive signs of the Hall and Seebeck coefficients confirmed that the Sn-doped tetrahedrites were p-type semiconductors. The carrier concentration decreased from 1.28 × 1019 to 1.57 × 1018 cm-3 as the Sn content decreased because excess electrons were supplied by doping with Sn4+ at the Sb3+ site of the tetrahedrite. The Seebeck coefficient increased with increasing Sn content, and Cu12Sb3.6Sn0.4S13 exhibited maximum values of 238-270 µVK-1 at temperatures of 323-723 K. However, the electrical conductivity decreased as the amount of Sn doping increased. Thus, Cu12Sb3.9Sn0.1S13 exhibited the highest electrical conductivity of (2.24-2.40) × 104 Sm-1 at temperatures of 323-723 K. A maximum power factor of 0.73 mWm-1K-2 was achieved at 723 K for Cu12Sb3.9Sn0.1S13. Sn substitution reduced both the electronic and lattice thermal conductivities. The lowest thermal conductivity of 0.49-0.60 Wm-1K-1 was obtained at temperatures of 323-723 K for Cu12Sb3.6Sn0.4S13, where the lattice thermal conductivity was dominant at 0.49-0.57 Wm-1K-1. As a result, a maximum dimensionless figure of merit of 0.66 was achieved at 723 K for Cu12Sb3.9Sn0.1S13.

Charge Transport and Thermoelectric Properties of Sn-Doped Tetrahedrites Cu12Sb4-ySnyS13

In this study, tetrahedrite compounds doped with Sn were prepared by mechanical alloying and hot pressing, and their charge transport and thermoelectric properties were analyzed. X-ray diffraction analysis revealed that both the synthetic powders and sintered bodies were synthesized as a single tetrahedrite phase without secondary phases. Densely sintered specimens were obtained with relatively high densities of 99.5%-100.0% of the theoretical density, and the component elements were distributed uniformly. Sn was successfully substituted at the Sb site, and the lattice constant increased from 1.0348 to 1.0364 nm. Positive signs of the Hall and Seebeck coefficients confirmed that the Sn-doped tetrahedrites were p-type semiconductors. The carrier concentration decreased from 1.28 × 1019 to 1.57 × 1018 cm-3 as the Sn content decreased because excess electrons were supplied by doping with Sn4+ at the Sb3+ site of the tetrahedrite. The Seebeck coefficient increased with increasing Sn content, and Cu12Sb3.6Sn0.4S13 exhibited maximum values of 238-270 µVK-1 at temperatures of 323-723 K. However, the electrical conductivity decreased as the amount of Sn doping increased. Thus, Cu12Sb3.9Sn0.1S13 exhibited the highest electrical conductivity of (2.24-2.40) × 104 Sm-1 at temperatures of 323-723 K. A maximum power factor of 0.73 mWm-1K-2 was achieved at 723 K for Cu12Sb3.9Sn0.1S13. Sn substitution reduced both the electronic and lattice thermal conductivities. The lowest thermal conductivity of 0.49-0.60 Wm-1K-1 was obtained at temperatures of 323-723 K for Cu12Sb3.6Sn0.4S13, where the lattice thermal conductivity was dominant at 0.49-0.57 Wm-1K-1. As a result, a maximum dimensionless figure of merit of 0.66 was achieved at 723 K for Cu12Sb3.9Sn0.1S13.

Optimized electronic performance in half-Heusler Ti-doped NbFeSb materials by stoichiometric tuning at the Fe and Sb sites

Electronic structure is known to be highly influenced by the site occupancy and the stoichiometry of the material which in turn largely effects the thermoelectric properties. Herein, we present electronic calculations using density functional theory (DFT) of non-stoichiometric Ti doped NbFeSb configuration, showing the effect of the anti-site Fe atoms on the electronic properties, and supporting them with experimental results of the prepared Nb0.8Ti0.2Fe1+xSb1−x samples. The electronic structure of the non-stoichiometric sample shows the introduction of two distinct peaks near the Fermi level by the Fe atoms at the Sb sites. These resonance states are known to cause an increase in the density of states effective mass near the Fermi level, which explains the increase in the Seebeck coefficient in the sample x = 0.03 compared to the sample x = 0.00. In addition, a comparatively higher electrical conductivity is reported from sample x = 0.03, which is attributed to the aliovalent substitution of Sb atoms by Fe atoms. The simultaneous increase in the Seebeck coefficient and electrical conductivity culminates in an increased power factor of ∼50.3 µW/cmK2 at 373 K, which is ∼46% higher than that of samples x = 0.00 and x = 0.05, highlighting the possibility of increasing the power density by stoichiometric variation to achieve the high joule-per-dollar performance of NbFeSb-based TE devices, the relevance of which is also currently emphasized in the quest for commercial viability.

Band convergence and thermoelectric performance enhancement of InSb via Bi doping

The thermoelectric performance of InSb has been significantly improved via defect engineering or nanostructuring to reduce thermal conductivity in the past decade; however, less attention has been paid to improve its electrical properties via other key factors, e.g., carrier scattering mechanism, bipolar effect, etc. Here, we show that Bi doping on the Sb site not only significantly suppresses the high-temperature bipolar effect, but also leads to band convergence which enhances the Seebeck coefficient. As a result, a maximum zT of ∼0.43 @ 675 K is achieved in InSb0.97Bi0.03, which is increased by ∼43% as compared to that of pure InSb. In addition, the comparison of Bi and Ga doping (J. Mater. Chem., 2011, 21, 12398–12401) on the low-temperature electrical transport properties reveals that Bi doping is more beneficial for achieving high power factor due to the mixed acoustic phonon and ionized impurity scattering. Finally, it is suggested that Bi should be an effective dopant for electrical optimization of InSb and a further experiment on co-doping of Ga and Bi will be a considerable alternative for thermoelectric performance enhancement of InSb-based materials.

Glassy magnetic ground state in layered compound MnSb2Te4

As a sister compound of MnBi2Te4, the high-quality MnSb2Te4 single crystals are grown via solid-state reaction where prolonged annealing and narrow temperature window play critical roles on account of its thermal metastability. Single-crystal X-ray diffraction (SCXRD) analysis on MnSb2Te4 illustrates a crystal model that is isostructural to MnBi2Te4, consisting of Te-Sb-Te-Mn-Te-Sb-Te septuple layers (SLs) stacking in an ABC sequence. However, MnSb2Te4 reveals a more pronounced cation intermixing in comparison with MnBi2Te4, comprising 28.9(7)% Sb antisite defects on the Mn (3a) site and 19.3(6)% Mn antisite defects on the Sb (6c) site, which may give rise to novel magnetic properties in emerging layered MnBi2Te4-family materials. Unlike the antiferromagnetic (AFM) nature in MnBi2Te4, MnSb2Te4 exhibits a glassy magnetic ground state below 24 K and can be easily tuned to a ferromagnetic state under a weak applied magnetic field. Its magnetic hysteresis, anisotropy, and relaxation process are investigated in detail via static and dynamic magnetization measurements. Moreover, anomalous Hall effect as a p-type conductor is demonstrated with transport measurements. This work grants MnSb2Te4 a possible access to the future exploration of exotic quantum physics by removing the odd/even layer number restraint in realizing quantum transport phenomena in intrinsic AFM MnBi2Te4-family materials, as a result of the crossover between its magnetism and potential topology arising from the Sb-Te layer.

Mixed Valence and Superconductivity in Perovskite Antimonates

Herein, we present results of the 121Sb Mössbauer spectroscopy in perovskite antimonates Ba1–xKxSbO3, sibling compounds of the well-known high-Tc superconductors Ba1–xKxBiO3. Two Sb valence states SbIII and SbV, forming a charge-density-wave (CDW) order, are unambiguously revealed in un- and under-doped phases at x = 0–0.5. As the CDW order is suppressed at x = 0.65, the compound becomes superconducting and all of the Sb sites become equivalent with the intermediate valence of +4.65, consistent with that anticipated from the potassium content. These results provide the direct evidence of metal s electrons participating in the formation of the CDW order and superconductivity in the material, and thus they have important implications for clarifying the underlying mechanism related to mixed valence and potential valence fluctuations of metal ions. The use of Mössbauer active nuclei offers a deeper insight into the structural properties and superconductivity in mixed-valence main-group oxides.

Electronic Transport and Thermoelectric Properties of Te-Doped Tetrahedrites Cu12Sb4-yTeyS13

Tetrahedrite is a promising thermoelectric material mainly due to its low thermal conductivity, a consequence of its complicated crystal structure. However, tetrahedrite has a high hole concentration; therefore, optimizing carrier concentration through doping is required to maximize the power factor. In this study, Te-doped tetrahedrites Cu12Sb4-yTeyS13 (0.1 ≤ y ≤ 0.4) were prepared using mechanical alloying and hot pressing. The mechanical alloying successfully prepared the tetrahedrites doped with Te at the Sb sites without secondary phases, and the hot pressing produced densely sintered bodies with a relative density >99.7%. As the Te content increased, the lattice constant increased from 1.0334 to 1.0346 nm, confirming the successful substitution of Te at the Sb sites. Te-doped tetrahedrites exhibited p-type characteristics, which were confirmed by the positive signs of the Hall and Seebeck coefficients. The carrier concentration decreased but the mobility increased with Te content. The electrical conductivity was relatively constant at 323–723 K, and decreased with Te substitution from 2.6 × 104 to 1.6 × 104 Sm-1 at 723 K. The Seebeck coefficient increased with temperature and Te content, achieving values of 184–204 μVK-1 at 723 K. The thermal conductivity was <1.0 Wm-1K-1, and decreased with increasing Te content. Cu12Sb3.9Te0.1S13 exhibited the highest dimensionless figure of merit (ZT = 0.80) at 723 K, achieving a high power factor (0.91 mWm-1K-2) and a low thermal conductivity (0.80 Wm-1K-1).

Improving the thermoelectric properties of thick Sb2Te3 film via Cu doping and annealing deposited by DC magnetron sputtering using a mosaic target

Thick Cu-doped Sb2Te3 films were deposited on flexible substrate by DC magnetron sputtering from a mosaic Cu–Sb2Te3 target. The Cu-doped Sb2Te3 films were vacuum annealed to improve their thermoelectric properties. Density functional theory was used to clarify the internal mechanism of the Cu doped into the Sb2Te3 system. The results showed that Cu substitution on a Sb site induced electronic states or impurity peaks of Sb2Te3 at a valence band maximum. The carrier concentration of the Cu-doped Sb2Te3 films increased as the Cu-doped concentration increased. However, the crystallite size and Seebeck coefficient of the Cu-doped Sb2Te3 films decreased as the Cu-doped concentration increased. Post-annealing treatment improved the microstructure and thermoelectric properties of the Cu-doped Sb2Te3 films. The maximum electrical conductivity and power factor values of 754.20 S/cm at 50 °C and 1.56 10−3 W/mK2 at 100 °C were obtained in the annealed film with a Cu-doped concentration of 3 at%.

S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2

We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin-lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.

A COMBINATION OF POINT DEFECTS AND NANOSIZED GRAINS TO MINIMIZE LATTICE THERMAL CONDUCTIVITY OF Sn AND Se CO-DOPED CoSb3 VIA MIXED BALL MILLING AND SPARK PLASMA SINTERING

The efficient strategies to minimize thermal conductivity in skutterudite materials are creating point defects along with nanosized grains. In this report, Sn and Se co-doped CoSb3 materials were synthesized through mixed-ball milling and spark plasma sintering techniques to utilize this strategy. Their phases, microstructure and thermoelectric properties were investigated under the content variation of Sn and Se in CoSb3 samples. The experimental results revealed that the Sn and Se were substituted at Sb sites in CoSb3 crystal structure and grain sizes were restricted to a hundred nanometer. The lattice thermal conductivity was reduced to 2.4[Formula: see text]W/mK at 298K. Interestingly, increasing Sn and Se doped content could further minimize the lattice thermal conductivity. The lowest value at room temperature is 1.79[Formula: see text]W/mK for CoSb[Formula: see text]Sn[Formula: see text]Se[Formula: see text] which was dramatically lower than pure CoSb3. Moreover, the increment of Sn and Se content also increased the electrical conductivity of doped samples, while the negative Seebeck coefficient sign tended to decrease. As expected, low electrical conductivity and substantial reduction in the Seebeck coefficient of doped samples at high measurement temperature, resulting in low power factor and low ZT values. It was clearly seen that the highest power factor of 880[Formula: see text][Formula: see text]W/mK2 was found at 516[Formula: see text]K in CoSb[Formula: see text]Sn[Formula: see text]Se[Formula: see text]. Furthermore, it also dominated the highest ZT value of 0.29 at 565 K, compared to the other Sn and Se co-doped samples. From these results, ball milling under dry conditions followed by wet conditions not only allowed a longer milling process but also generated a small fraction of pore which was a part of the reduction in thermal conductivity. Especially, the advantage of the existence of Sn and Se point defects and nanosized grains from this work will be escalated when it was applied to prepare materials that have high power factor values.

Hidden role of intrinsic Sb-rich nano-precipitates for high-performance Bi2-xSbxTe3 thermoelectric alloys

Nanostructuring is a universal strategy to improve the figure of merit (zT) of thermoelectric (TE) materials by the substantial decrease of lattice thermal conductivity due to the intensive scattering of phonons at interfaces. However, nanostructured bismuth antimony telluride alloys with foreign nanosized phases usually suffer from the degradation of carrier mobility at incoherent interfaces. The results reported here describe an approach to generate intrinsic Sb-rich precipitates that have a synchronal effect on micro-grained Bi0.5Sb1.5Te3 alloy. The designed formation of nanosized Sb-rich precipitates by dissolving excess Pb into Bi0.5Sb1.5Te3 matrix intensifies the phonon scattering and modulates the carrier transport simultaneously. By employing density functional theory calculation, transmission electron microscope and atom probe tomography, the role of Pb substituting for Sb site on the evolution of Sb-rich nanophase is clarified and the compositions and crystal structures of the precipitated Sb-rich phases are analyzed, revealing various interface structures. The optimized Bi0.5Sb1.5Te3 + 0.22 wt.% Pb sample shows a maximum zT value of 1.32 at 400 K with an outstanding average zT value of 1.2 over 300 K to 500 K. This work identifies the hidden role of intrinsic Sb-rich precipitates in Bi0.5Sb1.5Te3 alloys and provides an effective way beyond nanostructuring to develop high-performance bismuth antimony telluride thermoelectric alloys.

Effects of Ge Doping on the Charge Transport and Thermoelectric Properties of Permingeatites Cu3Sb1−yGeySe4

Permingeatites Cu3Sb1−yGeySe4 (0 ≤ y ≤ 0.14) were synthesized by mechanical alloying and hot pressing. The charge-transport parameters (Hall coefficient, carrier concentration, mobility, and Lorenz number) and thermoelectric properties (electrical conductivity, Seebeck coefficient, power factor, thermal conductivity, and figure of merit) were examined with respect to the Ge doping level. A single permingeatite phase with a tetragonal structure was obtained without subsequent heat treatment, but a small amount of the secondary phase Cu2GeSe3 was found for the specimens with y ≥ 0.08. All hot-pressed compacts exhibited a relative density of 97.5%–98.3%. The lattice constants of the a-axis and c-axis were decreased by the substitution of Ge at the Sb sites. As the Ge content increased, the carrier concentration increased from 5.2 × 10<sup>18</sup> to 1.1 × 10<sup>20</sup> cm<sup>−3</sup>, but the mobility decreased from 92 to 25 cm<sup>2</sup>·V<sup>−1</sup>·s<sup>−1</sup>. The Lorenz number of the undoped Cu3SbSe4 implied a non-degenerate semiconductor behavior, ranging from (1.57–1.56) × 10<sup>−8</sup> V<sup>2</sup>·K<sup>−2</sup> at 323–623 K. The thermoelectric figure of merit was 0.39 at 623 K, resulting from a power factor of 0.49 mW·m<sup>−1</sup>·K<sup>−2</sup> and a thermal conductivity of 0.76 W·m<sup>−1</sup>·K<sup>−1</sup>. However, the Lorenz numbers of the Gedoped specimens indicated degenerate semiconductor characteristics, increasing to (1.63–1.94) × 10<sup>−8</sup> V<sup>2</sup>·K<sup>−2</sup> at 323–623 K. The highest thermoelectric figure of merit of 0.65 was at 623 K for Cu3Sb0.86Ge0.14Se4, resulting from the significantly improved power factor of 0.93 mW·m<sup>−1</sup>·K<sup>−2</sup> and the thermal conductivity of 0.89W·m<sup>−1</sup>·K<sup>−1</sup>. As a result, the thermoelectric properties were remarkably enhanced by doping Ge into the Sb sites of the permingeatite.

Site Mixing for Engineering Magnetic Topological Insulators

The van der Waals compound, MnBi$_2$Te$_4$, is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. With higher concentrations of same types of magnetic defects, the isostructural compound MnSb$_2$Te$_4$ is a better model system for a systematic investigation of the connections among magnetic, topology and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb$_2$Te$_4$. Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site-mixing of MnSb$_2$Te$_4$ crystals depend on the growth parameters, which can lead to $\approx$40\% of Mn sites occupied by Sb and $\approx$15\% of Sb sites by Mn in as-grown crystals. Single crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn-Sb site-mixing favors a FM interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion required for a nontrivial topology. Our results suggest a long range magnetic order of Mn ions sitting on Bi sites in MnBi$_2$Te$_4$. The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topological layers, including the entire family of MnTe(Bi$_2$Te$_3$)$_n$, its Sb analogs and their solid solution.

Theoretical study on water behavior on the copper surfaces.

We calculated the adsorption of H, O, OH, and H2O and the dissociation of H2O molecule on the Cu(111), Cu(100), and Cu(110) surfaces using density functional theory. H, O, and OH tend to adsorb stably at the highly coordinated dh and h sites on the Cu(111) and Cu(100) surfaces. OH and H tend to adsorb on sb site on the Cu(110) surface. The more charge transfer of the adsorbed substance, the more stable the adsorption. The dissociation product is O+H on the Cu(111) surface, while the dissociation product is OH+H on the Cu(100) and Cu(110) surfaces. Due to the different geometric structures of initial state (IS), transition state (TS), and final state (FS) in the dissociation reaction, the dissociation of water on the copper surface does not establish a linear Brønsted-Evans-Polanyi (BEP) relationship. These results provide theoretical support for the understanding of the interaction between water and metals as well as the behavior of water molecules.

Search for a possible flexible-to-rigid transition in models of phase change materials

Structural models of the prototypal phase change material Ge-Sb-Te are generated from first-principles molecular dynamics simulations with a particular attention to the ${\mathrm{Ge}}_{x}{\mathrm{Sb}}_{x}{\mathrm{Te}}_{100\ensuremath{-}2x}$ join. Constraint counting algorithms permit us to analyze the obtained amorphous networks within the framework of topological constraint (rigidity) theory. A flexible-to-rigid transition is found at $x\ensuremath{\simeq}8.5$% which satisfies the Maxwell isostatic criterion and is characterized by an important topological disorder with large amounts of mixed geometries and miscoordinations. The angular constraint count furthermore leads to the identification of tetrahedral Ge sites whose population decreases with growing Ge/Sb content and is about 55% for ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. Sb sites change from a dominant pyramidal geometry typical of Group V chalcogenides to a defect octahedral one which is reminiscent of the crystalline polymorph of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$.