Cation Disorder Research Articles

Using pressure to unravel the structure–dynamic-disorder relationship in metal halide perovskites

The exceptional optoelectronic properties of metal halide perovskites (MHPs) are presumed to arise, at least in part, from the peculiar interplay between the inorganic metal-halide sublattice and the atomic or molecular cations enclosed in the cage voids. The latter can exhibit a roto-translative dynamics, which is shown here to be at the origin of the structural behavior of MHPs as a function of temperature, pressure and composition. The application of high hydrostatic pressure allows for unraveling the nature of the interaction between both sublattices, characterized by the simultaneous action of hydrogen bonding and steric hindrance. In particular, we find that under the conditions of unleashed cation dynamics, the key factor that determines the structural stability of MHPs is the repulsive steric interaction rather than hydrogen bonding. Taking as example the results from pressure and temperature-dependent photoluminescence and Raman experiments on MAPbBr_3 but also considering the pertinent MHP literature, we provide a general picture about the relationship between the crystal structure and the presence or absence of cationic dynamic disorder. The reason for the structural sequences observed in MHPs with increasing temperature, pressure, A-site cation size or decreasing halide ionic radius is found principally in the strengthening of the dynamic steric interaction with the increase of the dynamic disorder. In this way, we have deepened our fundamental understanding of MHPs; knowledge that could be coined to improve performance in future optoelectronic devices based on this promising class of semiconductors.

Protective and ion conductive: Ni-rich cathode with enhanced electrochemical performance via dual-modification

The advantages of affordable price and large capacity have made LiNixCoyMn1−x−yO2 (NCM) become a prominent topic in contemporary energy research. However, problems such as severe cation mixing and surface side reactions have critically hindered its commercial utilization. Herein, we propose a strategy of bulk-to-surface, namely Mo doping combined with LiBO2 coating, to synthetically boost the property of NCM. Considering that Mo6+ (0.62 Å) could reduce cationic disorder, whose bond energy is stronger than M–O, it contributes greatly to the stabilization of internal structure. What’s more, H3BO3 not only consumes surface residual alkali, but also produces LiBO2 with good ionic conductivity, which can promote the migration rate of Li+. Density functional theory (DFT) calculations and first principles elucidated the feasibility of doping-to-coating. In addition, structural evolution implied that the improved cyclic stability is due to the effective inhibition of serious structure evolution in the modified material. The discovery provides a novel thought for further commercialization of NCM.

Interplay of Static and Dynamic Disorder in the Mixed-Metal Chalcohalide Sn2SbS2I3.

Chalcohalide mixed-anion crystals have seen a rise in interest as "perovskite-inspired materials" with the goal of combining the ambient stability of metal chalcogenides with the exceptional optoelectronic performance of metal halides. Sn2SbS2I3 is a promising candidate, having achieved a photovoltaic power conversion efficiency above 4%. However, there is uncertainty over the crystal structure and physical properties of this crystal family. Using a first-principles cluster expansion approach, we predict a disordered room-temperature structure, comprising both static and dynamic cation disorder on different crystallographic sites. These predictions are confirmed using single-crystal X-ray diffraction. Disorder leads to a lowering of the bandgap from 1.8 eV at low temperature to 1.5 eV at the experimental annealing temperature of 573 K. Cation disorder tailoring the bandgap allows for targeted application or for the use in a graded solar cell, which when combined with material properties associated with defect and disorder tolerance, encourages further investigation into the group IV/V chalcohalide family for optoelectronic applications.

Low-cost, ecofriendly, and large-scale synthesis of nanostructured Co1−xMnxFe2O4 microgranules with enhanced magnetic performance by chemical spray drying processing

We report, for the first time, the low-cost, eco-friendly, and large-scale synthesis of spray-drying production of Mn-substituted CoFe2O4 (Co1−xMnxFe2O4; x = 0.0–0.2; CMF) microgranules and their structural, morphological, and magnetic properties and performance characteristics in detail. The comprehensive study explored the intimate relationships between cation disorder or inversion degree due to Mn substitution in CoFe2O4 (CF) microgranules and corresponding changes in the structural and magnetic properties. Crystal structure and morphology studies indicate the formation of spherical shape, single cubic mixed inverse spinel structure of all the CMFO- microgranules with a size variation in the range of ⁓6.5–7.5 µm. Small angle X-ray scattering analyses indicate that the nanostructured CMF microgranules exhibit a virtually hard-sphere-like interaction. It is concluded that distortions are related to Co ions at octahedral locations due to Mn substitution in all materials. Raman spectroscopic studies, which corroborate with other structural studies, also reveal that replacing Co with Mn increases the degree of inversion in the cubic inverse spinel structure. Divalent (Co2+, Mn2+) and trivalent (Fe3+, Mn3+) cations are distributed differently across tetrahedral and octahedral sites. In addition to large-scale chemical synthesis, our results demonstrate the enhanced magnetic saturation from 82.27 to 86.18 emu/gm and 77.05–79.87 emu/gm for Co0.9Mn0.1Fe2O4 at 10 K and 300 K, respectively. In order for the proposed architectural design to serve as a crucial building block for a wide range of technological applications and be applicable to a large class of spinel ferrites, we present our attempt to draw the necessary fundamental scientific explanation from the trends in the local structural parameters and magnetic characteristics of CMF nanomaterials.

Mitigation of Thermal Instability for Electrical Properties in CaZrO3-Modified (Na, K, Li) NbO3 Lead-Free Piezoceramics.

Lead-free ceramics 0.96(Na0.52K0.48)0.95Li0.05NbO3-0.04CaZrO3 (NKLN-CZ) are prepared by using the solid-state procedure and two-step synthesis technique. The crystal structure and thermal stability of NKLN-CZ ceramics sintered at 1140-1180 °C are investigated. All the NKLN-CZ ceramics are ABO3-type perovskite phases without impure phases. With the increase in sintering temperature, a phase transition occurs in NKLN-CZ ceramics from the orthorhombic (O) phase to the concomitance of O-tetragonal (T) phases. Meanwhile, ceramics become dense because of the presence of liquid phases. In the vicinity of ambient temperature, an O-T phase boundary is obtained above 1160 °C, which triggers the improvement of electrical properties for the samples. The NKLN-CZ ceramics sintered at 1180 °C exhibit optimum electrical performances (d33 = 180 pC/N, kp = 0.31, dS/dE = 299 pm/V, εr = 920.03, tanδ = 0.0452, Pr = 18 μC/cm2, Tc = 384 °C, Ec = 14 kV/cm). The relaxor behavior of NKLN-CZ ceramics was induced by the introduction of CaZrO3, which may lead to A-site cation disorder and show diffuse phase transition characteristics. Hence, it broadens the temperature range of phase transformation and mitigates thermal instability for piezoelectric properties in NKLN-CZ ceramics. The value of kp for NKLN-CZ ceramics is held at 27.7-31% (variance of kp < 9%) in the range from -25 to 125 °C. The results indicate that lead-free ceramics NKLN-CZ is one of the hopeful temperature-stable piezoceramics for practical application in electronic devices.

In-doping induced resonant level and thermoelectric performance enhancement in n-type GeBi2Te4 single crystals with intrinsically low lattice thermal conductivity

Resonant level, the impurity level located inside the valence/conduction band, is an effective strategy for decoupling the electrical conductivity and Seebeck coefficient and enhancing the thermoelectric power factor. In this work, we show that In doping induces resonant level and enhances the thermoelectric performance of GeBi2Te4, a promising n-type thermoelectric material with layered structure. Single crystals grown by Bridgman method were used in this study to take the advantage of its anisotropic transport properties. An ultralow lattice thermal conductivity of ∼0.51 W m−1 K−1 at 323 K was found along the c-axis of GeBi2Te4, driven by intrinsic cation disorder and the low phonon group velocity from the relatively weak chemical bonds as well as the strong lattice anharmonicity caused by hierarchical bond orders. Both enhanced electrical conductivity and Seebeck coefficient were observed in the In-doped samples, indicating that In-doping not only increases the carrier concentration, but also introduces the resonant level that was further confirmed by the first-principles calculation. A maximum zT of ∼0.42 at 523 K and an average zT of 0.36 over 323 to 573 K were achieved in Ge0.97In0.03Bi2Te4 crystal sample, ∼100% and ∼89% enhancement compared to those of pristine single crystal sample, respectively. This study demonstrates another example of resonant level for thermoelectric performance enhancement, which may be applied to other related materials.

Cation-Disorder-Enhanced Unintentional Doping in MgSnN2

${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$ has recently attracted significant interest as a promising candidate for applications in green light-emitting diodes and photovoltaic absorbers. However, the experimentally synthesized ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$ samples suffer from a high concentration of unintentionally doped electrons, and their origin is not fully understood yet. By performing first-principles calculations, we investigate the properties of intrinsic point defects and oxygen impurities in cation-ordered and -disordered wurtzite ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$. It is found that the cation antisite defect ${\mathrm{Sn}}_{\mathrm{Mg}}$ is the predominant donor defect, contributing an electron concentration as high as ${10}^{17}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ to cation-ordered ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$. However, this value is 2--3 orders of magnitude lower than experimental observations (${10}^{19}\ensuremath{-}{10}^{20}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$). We further show that cation disorder significantly decreases the formation energies of defects, leading to a much higher electron concentration (${10}^{19}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$) than in the case of the cation-ordered phase. The reduced formation energy can be understood by the energy gain due to the creation of energetically favorable local motif structures after the formation of defects. Hence, reducing the degree of cation disorder is beneficial for decreasing the densities of defects and self-doped carriers.

Synergetic effects of Na+-Mo6+ co-doping on layered Li1.2Mn0.54Ni0.13Co0.13O2 cathode for high performance lithium-ion batteries

The Li-rich Mn-based material, with high energy density, less toxicity and low cost, is regarded as one of the potential cathode candidates for the next generation lithium-ion batteries (LIBs). However, some issues need to be addressed before large-scale commercialized application, such as capacity attenuation, low initial coulomb efficiency and poor rate performance, caused by the structure defects and oxygen escape from the crystal lattice at high voltage. Here, the Na+ and Mo6+ dopants are introduced to Li1.2Mn0.54Ni0.13Co0.13O2 by sol-gel method, with slight Li+ displaced by Na+ and tiny Mo6+ substituting transition metal ions, respectively. The doping effects on the Li layer spacing, Li/Ni mixing and oxygen vacancy defects have been examined. Regarding the doped samples, the valence states of transition metal elements and electrochemical performance are systematically studied. The Na+ and Mo6+ co-doping not only expands the lithium layer spacing and reduces cationic disorder to promote the Li+ diffusion and enhance the rate capability, but also stabilizes the oxygen skeleton, inhibits oxygen loss, and suppresses the migration of transition metal ions to improve the cyclic performance. As a result, Li1.18Na0.02(Mn0.54Ni0.13Co0.13)0.98Mo0.02O2 delivers 122.8 mAh.g−1 at 5 C and 262.9 mAh.g−1 at 0.2 C, retaining 90% capacity after 150 cycles.

Cation disorder and the charge ordering transition temperature in La1/3(Sr1-xCax)2/3FeO3−δ investigated by mechanical spectroscopy

The lattice effect on the charge ordering in La1/3Sr2/3FeO3−δ is addressed by calcium ion substitution. Cation disorder effect due to size differences between A-site La3+, Sr2+, and Ca2+ has been found in La1/3(Sr1-xCax)2/3FeO3−δ perovskites. Below room temperature, a charge ordering transition has been detected by temperature dependent resistivity, differential scanning calorimeter, and mechanical spectroscopy. The charge ordering transition temperature TC shows a clear dependence on the variance of the A-site cation radius distribution. This result shows an important role of local lattice distortions on the charge ordering transition. As a comparison, octahedral titling transitions above room temperature were identified by mechanical spectroscopy, and the tilting transition temperatures are sensitive to the average cation radius. Finally, the implication of the cation disorder effect is discussed by a comparison of the case in manganates.

Mechanism for the Combined Li–Na Ionic Conductivity in Sugilite (Fe2Na2K[Li3Si12O30])-Type Compounds

This study explains the ionic conductivity in the mineral sugilite (idealized formula: Fe2Na2K[Li3Si12O30]) by resolving the dynamic disorder of both Li and Na cations using synchrotron X-ray single-crystal diffraction from 298 K to 1023 K. Non-zero anharmonic atomic displacement parameters at Na and Li sites at 1023 K adumbrated long-range charge transport routes for Li and Na cations commonly parallel to the (a–b) plane. Temperature-enhanced diffuse residuals in Fourier maps could unambiguously localize two interstitial sites suitable for Li, as well as three for Na. Each two-dimensional (2D) network of Li and Na interstitials was formed parallel to each other, providing Li and Na hopping pathways. The higher concentration of Na cations hopping in short distances of 2.0962(4)–2.3015(5) Å could be the main reason for the higher bulk conductivity values evaluated by impedance spectra of sugilite in comparison to those of its structural relatives with low Na contents, e.g., the mineral sogdianite ((Zr,Al,Fe)2Na0.36K[Li3Si12O30]). Bond valence sum landscape maps supported the critical role of dynamic disorder of Na+ over densely packed 2D interstitial networks for combined ionic conductivity along with mobile Li+ in sugilite-type compounds.

Achieving Enhanced Electrochemical Performance for Cobalt‐Free Layered Cathode Material by Aluminum Doping

AbstractWe introduce a facile strategy to greatly improve the electrochemical reversibility of cobalt‐free cathode material by doping trace quantity of aluminum (Al) in LiNi0.65Mn0.35O2. X‐ray diffraction and X‐ray photoelectron spectroscopy results reveal that Al doping reduces the cation disorder consequently and hence increases structural stability. Electrochemical measurements show that rate capability and cycling stability are remarkably enhanced by Al doping. Al‐doped LiNi0.65Mn0.35O2 exhibits the optimized electrochemical performance with a capacity retention of 94.2 % after 200 cycles at 2 C in voltage range of 2.75–4.2 V in cylindrical full cell, demonstrating that Al‐doped cathode exhibits enhanced cyclability, reduced polarization and high‐rate capability during charge/discharge process. Moreover, the full cell shows good low‐temperature performance with more than 70 % capacity retention at −20 °C. The improved performances can be ascribed to the enhanced stability in layered structure of the Co‐free material by the support of doped Al ions.

Structural studies on synthetic A2−x[M2(TeO3)3]·nH2O phases (A = Na, K, Rb, Cs; M = Mn, Co, Ni, Cu, Zn) with zemannite-type structures

During a systematic study on formation conditions of new compounds with zemannite-type structures, crystals of ten new oxidotellurate(IV) phases were grown under hydrothermal conditions, partially by employing a drastic reduction of the water content. The crystal structures of the obtained phases were determined by single-crystal X-ray diffraction. Na2[Ni2(TeO3)3]·2.5H2O, K2[Ni2(TeO3)3]·H2O, K2[Zn2(TeO3)3]·2H2O, Rb1.25[Co2(TeO3)3]·1.5H2O and Rb1.24[Mn2(TeO3)3]·2H2O exhibit a unit-cell with hexagonal symmetry (Z = 2, a ≈ 9.3 Å, c ≈ 7.7 Å), in which most of the other compounds with a zemannite-type structure are known to crystallize. Relative to this unit-cell, K2[Cu2(TeO3)3]·2H2O exhibits a twofold superstructure, K2[Co2(TeO3)3]·2.5H2O a twofold superstructure with an additional incommensurate modulation, Na2[Cu2(TeO3)3]·1.5H2O a threefold superstructure, and Rb1.5[Mn2(TeO3)3]·1.25H2O and Cs[Mn2(TeO3)3]·H2O a fourfold superstructure. Disorder of the alkali metal cations and crystal water molecules in the channels as well as variable water contents complicate modelling and structure refinement.

Unveiling the Selenization Reaction Mechanisms in Ambient Air‐Processed Highly Efficient Kesterite Solar Cells

AbstractThe selenization annealing process is vital for highly efficient kesterite solar cells. Generally, SnS is added during the selenization process, but excessive S and related defects are introduced. Meanwhile, the path of supplementing Sn has never been elucidated. Herein, in order to solve the above problems, a combination of strategies involving SnS and Sn or SnSe or SnSe2 is put forward. And the composition of the vapor inhibiting Sn loss (gaseous SnSe3) and the pathway through which SnSe3 facilitates the formation of Cu2ZnSn(SxSe1‐x)4 (CZTSSe) are clarified. When SnSe2 is added to SnS in the selenization process, grain fusion is effectively promoted. The high crystalline quality kesterite absorber makes the band bending at the GBs optimal and the interface recombination be effectively suppressed. Moreover, cation disorder is remarkably reduced. Therefore, the open‐circuit voltage (Voc) is significantly elevated from 508 to 546 mV with increased fill factor (FF) and short‐circuit current density (Jsc). A state‐of‐the‐art ambient air‐processed kesterite device with 12.89% efficiency is achieved, and the unveiled reaction mechanisms have guiding significance for further optimizing selenization atmosphere and elevating the efficiency of CZTSSe solar cells.

Mo–F Co-Doping LiNi0.83Co0.11Mn0.06O2 Stabilizes the Structure and Induces Compact Primary Particle To Improve the Electrochemical Performance

The Li+/Ni2+ cation disorder, material pulverization, surface phase transition, and transition metal (TM) ion dissolution are important issues that plague nickel-rich layered oxide cathode materials. In this paper, doping LiNi0.83Co0.11Mn0.06O2 with Mo6+ cation and F– anion effectively alleviates the above problems. The Mo6+ reduces the cations mixing, and the strong electronegativity of F strengthens the bonding with transition metal ions to resist HF corrosion. Furthermore, compact primary particles with interfused and radially aligned morphology induced by Mo–F co-doping reduce stress damage during cycling. The enhancement of electrochemical performance by Mo–F co-doping is systematically researched in terms of structure, morphology, secondary spherical pulverization, structural phase transition, and dissolution of transition metal ions.

Combinatorial Synthesis of Cation-Disordered Manganese Tin Nitride MnSnN2 Thin Films with Magnetic and Semiconducting Properties

Magnetic semiconductors may soon improve the energy efficiency of microelectronics, but materials exhibiting these dual properties remain underexplored. Here, we report the computational prediction and realization of a new magnetic and semiconducting material, MnSnN2, via combinatorial sputtering of thin films. Grazing incidence wide-angle X-ray scattering and laboratory X-ray diffraction studies show MnSnN2 exhibits a wurtzite-like crystal structure with cation disorder. This new material has a wide composition tolerance, with a single-phase region ranging from 20% < Mn/(Mn + Sn) < 65%. Spectroscopic ellipsometry identifies an optical absorption onset of 1 eV, consistent with the computationally predicted 1.2 eV bandgap. Resistivity measurements as a function of temperature support the semiconducting nature of MnSnN2. Hall effect measurements show carrier density has a weak inverse correlation with temperature, indicating that the charge transport mechanisms are more complex than in a pristine semiconductor. Magnetic susceptibility measurements reveal a low-temperature magnetic ordering transition (≈10 K) for MnSnN2 and strong antiferromagnetic correlations. This finding contrasts with bulk, cation-ordered MnSiN2 and MnGeN2, which exhibited antiferromagnetic ordering above 400 K in previous studies. To probe the origin of this difference, we perform Monte Carlo simulations of cation-ordered and cation-disordered MnSnN2. They reveal that cation disorder lowers the magnetic transition temperature relative to the ordered phase. In addition to discovering a new compound, this work shows that future efforts could use cation (dis)order to tune magnetic transitions in semiconducting materials for precise control of properties in microelectronics.

Tri(2-furyl)phosphine-induced robust interphases for durable Nickel-rich Lithium-ion batteries

Ni-rich LiNixCoyMn1−x−yO2/graphite batteries with Ni ≥ 0.8 have aroused great interest in high-energy–density Li-ion batteries. However, their practical applications still face huge challenges owing to severe capacity fading on unstable cathodic solid electrolyte interfaces (SEI). Herein, we propose an advanced electrolyte additive, tri(2-furyl)phosphine (FuP), to induce the simultaneous formation of robust SEI films on cathode and anode surfaces. This interphase can significantly improve the cyclability of commercial LiNi0.8Co0.1Mn0.1O2/graphite pouch cells at 45 °C and reduce cell impedance during cycling. The capacity retention rate of the cells with the FuP-based electrolyte can reach 90 % after 600 cycles, which is considerably better than that of baseline batteries (70 %). Mechanistic studies indicate that FuP suppresses the formation of fragile Li2CO3 to generate more stable LiF, LixPOyFz, and additional organic phosphorus species on the electrode surface, thereby preventing cation disorder and irreversible phase transitions. These results provide a facile and efficient strategy for enhancing the performance of Ni-rich Li-ion batteries.

Mechanistic Origin for High Structural Stability of Single Crystalline Nickel‐Rich Cathode Materials Via Al and Sm Co‐Doping

AbstractNickel‐rich layered oxides have attracted many attentions for their superior specific capacity and low cost, but they are subjected to fast structural degradation during cycling. Herein, the Al and Sm co‐doped LiNi0.83Co0.07Mn0.10O2 (SC‐NCM‐AS) single‐crystal is demonstrated to overcome their cycling instability issue, and its mechanistic origin for improved structural stability is investigated. It is found that soluble Al ions are homogenously incorporated in the LiNi0.83Co0.07Mn0.10O2 (SC‐NCM) lattice, while Sm ions tends to aggregate in the SC‐NCM outer surface layer. The Li/Ni cation disordering is greatly suppressed through the pillaring effect of stronger AlO bond in SC‐NCM single crystals. Sm‐concentrated outer surface layer can effectively prevent the dissolution of transition metals from SC‐NCM‐AS and inhibit undesirable side reactions induced by the organic electrolyte. This synergistic effect facilitates to suppress the formation of LiOH/Li2CO3 and oxygen vacancies, resulting in released the internal strain, decreased in‐plane transition metals migration and gliding, and eventually preventing formation of nanocracks in SC‐NCM‐AS single crystals upon cycling at high cut‐off voltage. Consequently, Al and Sm co‐doped SC‐NCM exhibits a high specific capacity of 222.4 mAh g−1 and remarkable cycling performance with a capacity retention of 91.1% for 100 cycles.

Effects of Al and Co doping on the structural stability and high temperature cycling performance of LiNi0.5Mn1.5O4 spinel cathode materials

The poor structural stability and capacity retention of the high-voltage spinel-type LiNi0.5Mn1.5O4 (LNMO) limits their further application. Herein, Al and Co were doped in LNMO materials for a more stable structure and capacity. The LNMO, LiNi0.45Al0.05Mn1.5O4 (LNAMO) and LiNi0.45Co0.05Mn1.5O4 (LNCMO) were synthesized by calcination at 900 °C for 8 h, which was called as solid-phase method and applied universally in industry. XRD, FT-IR and CV test results showed the synthesized samples have cation disordering Fd-3m space group structures. Moreover, the incorporation of Al and Co increased the cation disordering of LNMO, thereby increasing the transfer rate of Li+. The SEM results showed that the doped samples performed more regular and ortho-octahedral. The EDS elemental analysis confirmed the uniform distribution of each metal element in the samples. Moreover, the doped samples showed better electrochemical properties than undoped LNMO. The LNAMO and LNCMO samples were discharged with specific capacities of 116.3 mA·h·g−1 and 122.8 mA·h·g−1 at 1 C charge/discharge rate with good capacity retention of 95.8% and 94.8% after 200 cycles at room temperature, respectively. The capacity fading phenomenon of the doped samples at 50 °C and 1 C rate was significantly improved. Further, cations doping also enhanced the rate performance, especially for the LNCMO, the discharge specific capacity of 117.9 mA·h·g−1 can be obtained at a rate of 5 C.

Band modification towards high thermoelectric performance of SnSb2Te4 with strong anharmonicity driven by cation disorder

As a typical (IV–VI)x(V2VI3)y compound, the tetradymite-like layered SnSb2Te4-based compounds have attracted increasing attention in the thermoelectric community owing to the intrinsically low lattice thermal conductivity. Nevertheless, the effect of cations disorder on the inherent physical characteristics remains puzzling, and its inferior Seebeck coefficient is the bottleneck to achieving high thermoelectric performance. In this work, the thermoelectric properties of polycrystalline InxSn1−xSb2(Te1−ySey)4 (0 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.15) samples are comprehensively investigated. In conjunction with the calculated band structure and experimental results, the Seebeck coefficient and power factor are markedly improved after the introduction of indium and selenium, which originates from the combined effects of the emergent resonant states and converged valence bands along with optimal carrier concentration. Additionally, compared with the ordered lattice structure, the disordered cations occupancy in SnSb2Te4 further strengthens lattice anharmonicity and reduces phonon group velocity verified by first-principles calculations, securing intrinsically low lattice thermal conductivity. Finally, a record zT value of ∼0.6 at 670 K and an average zT of ∼0.4 between 320 and 720 K are obtained in the In0.1Sn0.9Sb2Te3.4Se0.6 sample, being one of the highest zT values among SnSb2Te4-based materials. This work not only demonstrates that SnSb2Te4-based compounds are promising thermoelectric candidates, but also provides guidance for the promotion of thermoelectric performance in a broad temperature range.

RIXS interferometry and the role of disorder in the quantum magnet Ba3Ti3−xIrxO9

Motivated by several claims of spin-orbit-driven spin-liquid physics in hexagonal ${\mathrm{Ba}}_{3}{\mathrm{Ti}}_{3\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{O}}_{9}$ hosting ${\mathrm{Ir}}_{2}{\mathrm{O}}_{9}$ dimers, we report on resonant inelastic x-ray scattering (RIXS) at the Ir ${L}_{3}$ edge for different $x$. We demonstrate that magnetism in ${\mathrm{Ba}}_{3}{\mathrm{Ti}}_{3\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{O}}_{9}$ is governed by an unconventional realization of strong disorder, where cation disorder affects the character of the local moments. RIXS interferometry, studying the RIXS intensity over a broad range of transferred momentum $\mathbf{q}$, is ideally suited to assign different excitations to different Ir sites. We find pronounced Ir-Ti site mixing. Both ions are distributed over two crystallographically inequivalent sites, giving rise to a coexistence of quasimolecular singlet states on ${\mathrm{Ir}}_{2}{\mathrm{O}}_{9}$ dimers and spin-orbit-entangled $j=1/2$ moments of $5{d}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{Ir}}^{4+}$ ions. RIXS reveals different kinds of strong magnetic couplings for different bonding geometries, highlighting the role of cation disorder for the suppression of long-range magnetic order in this family of compounds.