Valence Mismatch Research Articles

Repercussions of the Inner Shell Layer on the Performance of Cd-Free Quantum Dots and Their Light-Emitting Diodes.

Indium phosphide (InP) and zinc selenium tellurium (ZnSeTe) quantum dots (QDs) as less toxic alternatives have received substantial attention. The structure of QDs generally consists of a QD core, inner shell layer, and outer shell layer. We reckon that the inner shell layer, especially its components and thickness, have a significant influence on the optical and electronic performances of QDs. In this Perspective, we compare optical properties of these QDs with different inner shells and summarize how typical inner shell components and thickness influence their optical properties. The impact of the inner shell on the performance of QD light-emitting diodes (QLEDs) has also been discussed. The appropriate components and thickness of the inner shell both contribute to alleviate valence or lattice mismatch, thereby enhancing the performance of QDs. We expect that this Perspective could heighten awareness of the significance and impact of the inner shell layer in QDs and facilitate further development of QDs and QLEDs.

High Throughput Doping Study of Na-Mn-O Cathode Materials

With the large demand for energy consumption, sodium-ion batteries have been considered as an alternative to lithium-ion batteries for energy storage1. Among various cathode materials, the Na-Mn-O system is promising with its high specific capacities and excellent battery life. Herein, we applied our high throughput (HTP) system2,3 to explore Na0.66Mn0.9M0.1O2+δ (M: dopant) chemical compositions to perform fast screening of the doping effect in these materials. Using a systematic approach, we establish the relationships between the doping species, crystal structure, electrochemical performance, and air/moisture stability. In detail, 52 different dopants were utilized and prepared as precursor solutions to achieve the HTP sol-gel synthesis. HTP-XRD is performed after sintering to understand the structural changes occurring in doping. HTP-cyclic voltammetry (CV) is then performed to analyze the electrochemical properties. Additionally, to comprehend the relationship between dopants and air stability, an accelerated ageing process is carried out at various humidity levels for several weeks.In this study, we found a broad range of dopants with 20 distinct elements that can be integrated into the layered Na-Mn-O structures, including several previously unstudied dopants (Si, Sc, Ga, Rb, Rh, Cs, Re, and Tl), confirmed by XRD refinements. It's interesting to note that at 10% composition, the dopant species can significantly alter the crystal structure. For the undoped sample, a P2/P’2 biphasic material is obtained by slow cooling. However, with the presence of Li, Mg, Fe, Co, Ni, Zn, and Ga, pure P2 structures are obtained with smooth CV curves, suggesting fewer phase transitions via doping. Meanwhile, Si, B, and Mo-doped samples can introduce more distorted P’2 structures, giving more high-voltage plateaus to increase the voltage of the batteries. Nevertheless, the Rh-doped sample achieves a pure P3 structure, exhibiting smooth CV and higher voltage, but fair cycling. Tunnel structure is also found by Ti-doped samples, with an increased average discharged voltage of 2.74V (undoped:2.47V) and excellent rate performance. A series of novel cathodes yields excellent battery performance, including a high specific capacity of 200 mAh g-1 for the Rb-doped sample, and good capacity retention of 100% after 10 cycles for the Nb-doped sample (undoped showed 92% retention). Significantly, a number of doped samples exhibit high reversible anion redox which was shown to be strongly correlated with the bond valence mismatch of the M-O bonds, suggesting by tuning the polarity of M-O bonds triggered by dopants can stimulate the anionic redox behaviour. Moreover, air/moisture stability analysis is conducted for co-doped samples and found that as little as 5% Li in co-doped samples can stabilize the layered structure dramatically. Na0.66Mn0.9Li0.05Co0.05O2, for instance, substantially improves the air stability to over 99 % structure retention, compared to 0 % in the undoped P2 NMO after a one-month accelerated aging protocol. Therefore, these new insights into the impacts of doping via high throughput screening can serve as new guidelines for rational battery material design and development. Figure 1: Heat map of specific capacities in the periodic table with various dopants. The inset image demonstrates the workflow of the high throughput system.

Texture tolerance to B-site valence mismatch for [001] textured Pb97.5%Ba2.5%[(Zn1/3Nb2/3) (1-x)–Tix]O3 transparent ceramics

Transparent Pb97.5%Ba2.5% [(Zn1/3Nb2/3) (1-x)–Tix]O3 ceramics have been textured using BaTiO3 template. The influence of B-site valence mismatch between the host and the template on texture growth is investigated. The texture degree depends on the Ti4+ content in the host. The minimum Ti4+ content of 9% is necessary for texture growth. X composition in the vicinity of 9% is demonstrated at MPB where the maximum d33 and kp is 660 pC/N and 63%, and a strain as high as 0.27% at 40 kV/cm is achieved. The transmittance of textured P97.5%B2.5% (ZN–4%T) ceramic is 47% at 600 nm and exceeds 60% at the wave length of 1000–3000 nm. This research gives a better understanding of texture growth and facilitates the designs of multi-functional devices.

Colossal oxygen vacancy formation at a fluorite-bixbyite interface

Oxygen vacancies in complex oxides are indispensable for information and energy technologies. There are several means to create oxygen vacancies in bulk materials. However, the use of ionic interfaces to create oxygen vacancies has not been fully explored. Herein, we report an oxide nanobrush architecture designed to create high-density interfacial oxygen vacancies. An atomically well-defined (111) heterointerface between the fluorite CeO2 and the bixbyite Y2O3 is found to induce a charge modulation between Y3+ and Ce4+ ions enabled by the chemical valence mismatch between the two elements. Local structure and chemical analyses, along with theoretical calculations, suggest that more than 10% of oxygen atoms are spontaneously removed without deteriorating the lattice structure. Our fluorite–bixbyite nanobrush provides an excellent platform for the rational design of interfacial oxide architectures to precisely create, control, and transport oxygen vacancies critical for developing ionotronic and memristive devices for advanced energy and neuromorphic computing technologies.

Fabrication and characteristics of a new ceramic solid electrolyte LiMg2P3O10

A single phase of new compound LiMg2P3O10 is obtained by a one-step solid-state reaction. According to Rietveld analysis of XRD patterns, the monoclinic crystal structure of LiMg2P3O10 consists of MgO6 octahedron chain and shared corner PO4 tetrahedron. The chain is constructed in which lithium ions are located in a channel parallel to the b axis. Lithium ions are expected to migrate along the direction of the b axis using bond valence mismatch theory. The total electrical conductivity of dense LiMg2P3O10 ceramic at room temperature is 1.04 × 10−5 S cm−1 using an equivalent circuit model. The LiMg2P3O10 ceramics also show a large capacitance of 0.16 mF cm−2 at a low scan rate of 2 mV s−1.

Pulsed-laser deposition of InSe thin films for the detection of thickness-dependent bandgap modification

Layer-structured InSe is one of the intensively studied two-dimensional monochalcogenide semiconductors for optical and electrical devices. Significant features of the InSe device are the thickness dependent bandgap modification resulting in a peak shift of photoluminescence and a drastic variation of electron mobility. In this study, by applying the pulsed-laser deposition technique, we investigated the optical and electrical properties of c-axis oriented InSe films with the thickness varying from a few to hundred nanometers. The energy at the absorption edge systematically shifts from about 3.3 to 1.4 eV with the increasing thickness. The InSe films on Al2O3(0001) are highly resistive, while those on InP(111) are conductive, which probably originates from the valence mismatch effect at the interface. The electron mobility of the conducting charge carrier at the interface of InSe/InP is enhanced in thicker samples than the critical thickness of about 10 nm, corresponding to the bandgap modification characterized by the optical measurement. Therefore, the substrate and the film thickness are critically important factors for the materialization of InSe optical and electrical device applications.

Thiophosphates Containing Ag+ and Lone-Pair Cations with Interchiral Double Helix Show Both Ionic Conductivity and Phase Transition.

Quaternary metal thiophosphates containing second-order Jahn-Teller distorted d10 Ag+ and lone-pair cations, Ag3Bi(PS4)2 (1), Ag7Sn(PS4)3 (2), and Ag7Pb(PS4)3 (3), were obtained by solid-state synthesis. The structural frameworks of 2 and 3 feature an infinite 1-D interchiral double helix ∞1(Ag3P2S11), which is rare in inorganic compounds. Compound 3 undergoes a significant first-order structural phase transition from monoclinic to hexagonal at ∼204 °C. This can be ascribed to the significant mismatch in the expansion coefficients between Pb-S (Ag-S) and P-S bonds evaluated by bond valence theory. The three compounds are Ag+ ionic conductors, and Ag+ ion migration pathways are proposed by calculating maps of low bond valence mismatch. Moreover, the optical properties of the three compounds were studied, and electronic structure calculations were performed. The combination of second-order Jahn-Teller distorted d10 cation and lone-pair cation provides a new strategy to explore new metal thiophosphates with interesting structures and promising properties.

Average vs. local structure and composition-property phase diagram of K0.5Na0.5NbO3-Bi½Na½TiO3 system

Phase diagram of the solid solution system K0.5Na0.5NbO3- Bi½Na½TiO3 [(1-x)KNN-xBNT] has been established from dielectric permittivity measurement and structure analyses. The unit cell volumes continuously decrease depending on the composition, while the local structure maintains distortions away from the cubic average structure in the range 0.10≤x≤0.90. No clear correspondence for the temperatures of phase transition exists between structural studies and physical properties. The dielectric behavior is depicted successively from normal ferroelectric, diffuse phase transition, re-entrant-like relaxor, relaxor+dipolar glass-like relaxor, BNT-like relaxor with the increase of BNT. A comprehensive composition-property phase diagram for this system has been given to understand the various ferroelectric phenomena. The result could be mainly elucidated by the nanoclusters and the disorder driven nucleation of polar nanoregions contributed by a valence mismatch at one of the cation sites.

Analysis of Si addition on phase formation in AlCoCrCuFeNiSix high entropy alloys

In the present work, the effect of Si addition on the structure of AlCoCuCrFeNiSix alloy system is studied. The alloys were prepared by mechanical alloying in high energy ball mill. The phase formation of the present high entropy alloys were analysed by X-ray diffraction to understand the alloying behaviour. The analysis of X-ray diffraction pattern clearly shows that the addition of Si favours the formation of body centered cubic structure. The experimental results were also in accord with the theoretical prediction of structure based upon mixing entropy, atomic size mismatch, mixing enthalpy, valence electron concentration, electronegativity difference and mismatch entropy.

A Unified View of the Substitution-Dependent Antiferrodistortive Phase Transition in SrTiO3

The cubic-to-tetragonal antiferrodistortive transition at 105 K in the most widely studied perovskite, SrTiO3, is perhaps the preeminent example of a second-order structural phase transition. Extensive investigations since the 1960s have tracked the softening of the phonon mode associated with this transition, the development of octahedral rotations, and the interplay with incipient ferroelectricity and superconductivity. It is less well known that modest ionic substitutions alter the transition temperature (Ta) over a remarkable range in SrTiO3, from 0 K to above ambient. Here, we first study the thermodynamics of the transition via specific heat and determine a Ta shift of +10.9 K/atomic % in Nb-substituted crystals, the most heavily studied in terms of electronic transport. We then present the first quantitative rationalization of the trends in Ta with substitution in SrTiO3. We demonstrate that the apparently complex behavior versus tolerance factor occurs simply due to a nonlinear dependence of the rate of change of Ta with substituent concentration. We then connect this to ionic valence mismatch, using bond valence concepts to establish a new parameter, ⟨ε4⟩, which exhibits a universal linear dependence with Ta for all known substitutions. This provides the first unified view of the substituent-dependent Ta in SrTiO3, deepens our understanding of the phase transition (including a theoretical maximum in the rate of Ta suppression), and demonstrates predictive power via a simply computed parameter.

Characterization of Compound Semiconductors using Aberration-Corrected Electron Microscopy

Epitaxial growth of semiconductor materials presents many obvious challenges. In addition to preparation of the substrate surface and careful attention to the growth conditions, there are several additional obstacles for heterovalent structures based on two (or more) dissimilar materials. As well as lattice mismatch, which inevitably leads to strain and likely defect formation, valence mismatch and differences in thermal expansion, are further factors that can seriously impact the eventual epilayer quality. The transmission electron microscope has a wide range of imaging, diffraction and analytical techniques that can provide invaluable information about the often-competing effects of growth conditions and compositional differences. The development of aberration correction has enabled electron microscope information limits to be extended beyond the 1-Å resolution barrier, and makes it (almost) routine to achieve atomic-resolution imaging from many different types of materials. For elemental and compound semiconductors it is even possible to resolve the projections of individual atomic columns, often referred to as dumbbells, in the common <110> projection. Composition profiles across heterovalent interfaces, as well as bonding information at the atomic level, can also be achieved when using the probe-corrected configuration. The challenge for the electron microscopist is to utilize these enhanced capabilities to provide useful constructive feedback to the growers responsible for materials growth. In this talk, recent studies of II-VI/III-V, IV-IV, and III-nitride heterostructures using aberration-corrected electron microscopy will be described.

Interface control of bulk ferroelectric polarization.

The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.

Effect of the Amount of Vacancies on the Thermoelectric Properties of Cu&amp;ndash;Ga&amp;ndash;Te Ternary Compounds

The Cu­Ga­Te ternary compounds: Cu3Ga5Te9 ,C u 2Ga4Te7, CuGa3Te5, CuGa5Te8, and CuGaTe2 have zinc-blende or chalcopyrite structure. The compounds except for CuGaTe2 contain vacancies in the cation site due to the valence mismatch between cation and anion, and the vacancy concentration is different between these compounds. Here we investigated the effect of the amount of vacancies on the thermoelectric (TE) properties of the Cu­Ga­Te ternary compounds. At room temperature, the presence of vacancies reduced the Hall mobility (®H) and the lattice thermal conductivity (¬lat), showing that the vacancies scattered both carriers and phonons. It was found that the decreasing rate of the ®H was larger than that of the ¬lat. Therefore, the presence of vacancies degraded the TE performance of the Cu­Ga­Te ternary compounds. In other words, CuGaTe2 without vacancies would show the best TE figure of merit. The room temperature ZT values for Cu3Ga5Te9 ,C u2Ga4Te7, CuGa3Te5, CuGa5Te8, and CuGaTe2 were 2.0 © 10 ¹2 , 2.4 © 10 ¹2 , 8.9 © 10 ¹3 , 1.3 © 10 ¹2 , and 5.6 © 10 ¹3 , respectively. [doi:10.2320/matertrans.E-M2012810]

Effect of Vacancy Distribution on the Thermal Conductivity of Ga2Te3 and Ga2Se3

Our group has focused attention on Ga2Te3 as a natural nanostructured thermoelectric material. Ga2Te3 has basically a zincblende structure, but one-third of the Ga sites are structural vacancies due to the valence mismatch between Ga and Te. It has been confirmed that (1) vacancies in Ga2Te3 exist as two-dimensional (2D) vacancy planes, and (2) Ga2Te3 exhibits an unexpectedly low thermal conductivity (κ), most likely due to highly effective phonon scattering by the 2D vacancy planes. However, the effect of the size and periodicity of the 2D vacancy planes on κ has been unclear. In addition, it has also been unclear whether only the 2D vacancy planes reduce κ or if point-type vacancies can also reduce κ. In the present study, we tried to prepare Ga2Te3 and Ga2Se3 with various vacancy distributions by controlling annealing conditions. The atomic structures of the samples were characterized by means of transmission electron microscopy, and κ was evaluated from the thermal diffusivity measured by the laser flash method. The effects of vacancy distributions on κ of Ga2Te3 and Ga2Se3 are discussed.

Bond-valence Based Computational Design of High Performance Lithium Ion Battery Cathode Materials

ABSTRACTLinking the bond valence mismatch to the absolute energy scale, a generally applicable Morse-type force-field is developed and applied to study ion conduction in mixed conducting solids using both an energy landscape approach and molecular dynamics (MD) simulations. Exploring strategies to enhance the power performance of safe low cost lithium ion battery cathode materials, amblygonite-type “high voltage” cathode materials LiVPO4F and LiFeSO4F are used as examples. The amblygonite-type structure exhibits channels for low-energy migration in combination with moderate energy thresholds for "back-up" pathways in perpendicular directions mitigating the effects of channel blocking in mixed conductors with strictly one-dimensional Li+ motion.

Efficient charge carriers induced by extra outer-shell electrons in iron-pnictides: a comparison between Ni- and Co-doped CaFeAsF

A comprehensive study of the difference between CaFe1−xNixAsF and CaFe1−xCoxAsF systems has been carried out by measuring the efficient charge carrier concentration, the valence states and the superconducting phase diagram. It is found that at the same doping level, Ni doping introduces nearly twice the number of charge carriers as Co doping. However, x-ray absorption near-edge spectroscopy measurements reveal that the valence state of Fe in both systems is close to 2, indicating that there is no valence mismatch. We suggest that the charge carriers in CaFe1−xMxAsF (M=transition metal elements) are not induced by valence mismatch but come from the difference in the number of outer-shell electrons. We also suggest that with Ni and Co doping, the systems change from a multi-band material in the underdoped regions to a single-band state in the overdoped regions.

Lithium ion pathways in LiFePO4 and related olivines

Determining ion transport pathways as regions of low bond valence mismatch represents a simple, reliable way of characterizing ion transport pathways in local structure models, provided that the local structure model captures the essential structural features. The examples of LiFePO4 and other olivine-type mixed conductors discussed here demonstrate the impact of structural disorder on the ion transport pathway and mechanism. The effect of Li′Fe antisite defects on the transition from one- to two-dimensional conduction pathway dimensionality as well the possibility of heterogeneous doping of LiFePO4 by a lithium phosphate glass surface layer are discussed in detail.

Effect of periodicity of the two‐dimensional vacancy planes on the thermal conductivity of bulk Ga2Te3

AbstractGa2Te3 contains structural vacancies due to the valence mismatch between the cation and anion, and these vacancies make regularly arranged two‐dimensional (2D) planes with approximately 3.5‐nanometer intervals. Most likely due to the effective phonon scattering by the vacancy planes, Ga2Te3 exhibits an unexpectedly low thermal conductivity (κ). In the present study, the effect of the periodicity of the 2D vacancy planes on the κ of Ga2Te3 was investigated. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Universality in phase diagram of (K,Na)NbO3–MTiO3 solid solutions

Detailed phase diagrams are necessary for improving the temperature stability of piezoelectric properties of (Na,K)NbO3-based solid solutions. We report the phase diagrams of the (1−x)Na0.5K0.5NbO3–xMTiO3 (M=Pb, Ba, Sr, Ca, and Bi0.5Li0.5) solid solutions over a broad temperature range of 10 K≤T≤∼770 K. It is found that the phase diagrams show several universal characteristics: (a) Curie temperature TC-T decreases sharply at low x range; (b) the relation between TC-T and x deviates from Vegard’s law; and (c) the tetragonal-orthorhombic phase transition temperature TT-O and the orthorhombic-rhombohedral phase transition temperature TO-R are almost independent of M. We suggest that valence mismatch is the main reason for the rapid decrease in TC-T and the deviation of TC-T from Vegard’s law. It is also found that TC-T is mainly affected by the A-site cations, whereas TT-O and TO-R are mainly affected by the B-site cations.

Improved bond valence models for ion transport pathways in glasses

ABSTRACTStructure property function relationships provide valuable guidelines for a systematic development of functional materials. It is demonstrated how an augmented bond valence (BV) approach helps to establish such relationships in solid electrolytes. In principle it permits to identify mobile species, transport pathways and provides estimates for ion mobilities. In this work we discuss ion conduction pathways in glassy Lithium metasilicate as an illustrative example. The required representative local structure model is derived from Molecular Dynamics simulations, which provides the opportunity to compare the bond-valence-based predictions from a static structure model with a comprehensive analysis of a complete simulation trajectory. It is shown that understanding the bond valence mismatch as an effective Morse-type interaction opens up a way for systematically analyzing ion transport pathways and for a generally applicable method with improved reliability to predict ion transport characteristics in solid electrolytes from the structure model.