Vacancy Ordering Research Articles

The effect of oxygen impurities on the stability and structural properties of vacancy-ordered and -disordered ZrC x.

Theoretical calculations predict several long-range ordered sub-stoichiometric zirconium carbide phases to be stable at low temperature, rather than a random (disordered solution) distribution of vacancies. However, experimental synthesis of vacancy-ordered phases is extremely challenging and not all predicted phases have been experimentally observed. It has been hypothesised that the inevitable oxygen contamination in experimental samples may affect the vacancy ordering. In this work, the stability and structural properties of the vacancy-ordered and vacancy-disordered phases are investigated as a function of oxygen defect concentration using first-principles calculations. The observed trends are explained in terms of changes to the local bonding in the presence of varying amounts of oxygen and vacancies. It is found that the relative stability of the ordered phases (compared to the disordered phase at the same composition) decreases as oxygen concentration increases, and some vacancy-ordered phases are destabilised by the level of oxygen impurities found in experimental samples. This suggests that oxygen contamination is a contributing factor to the challenge of synthesising ordered zirconium carbides, and gives insight that may assist fabrication in the future. The volume of all ZrCx (x ≤ 1) phases was found to decrease with increasing oxygen concentration, which can be attributed to the different ionocovalent nature of the C–Zr and O–Zr bonds. The volume of the vacancy-ordered phases within the expected oxygen solubility limit is greater than the disordered phase of the same composition, which is explained in terms of the relative bond strengths surrounding different vacancy distributions.

Ordered Oxygen Vacancies in the Lithium-Rich Oxide Li4CuSbO5.5, a Triclinic Structure Type Derived from the Cubic Rocksalt Structure.

Li-rich rocksalt oxides are promising candidates as high-energy density cathode materials for next-generation Li-ion batteries because they present extremely diverse structures and compositions. Most reported materials in this family contain as many cations as anions, a characteristic of the ideal cubic closed-packed rocksalt composition. In this work, a new rocksalt-derived structure type is stabilized by selecting divalent Cu and pentavalent Sb cations to favor the formation of oxygen vacancies during synthesis. The structure and composition of the oxygen-deficient Li4CuSbO5.5□0.5 phase is characterized by combining X-ray and neutron diffraction, ICP-OES, XAS, and magnetometry measurements. The ordering of cations and oxygen vacancies is discussed in comparison with the related Li2CuO2□1 and Li5SbO5□1 phases. The electrochemical properties of this material are presented, with only 0.55 Li+ extracted upon oxidation, corresponding to a limited utilization of cationic and/or anionic redox, whereas more than 2 Li+ ions can be reversibly inserted upon reduction to 1 V vs Li+/Li, a large capacity attributed to a conversion reaction and the reduction of Cu2+ to Cu0. Control of the formation of oxygen vacancies in Li-rich rocksalt oxides by selecting appropriate cations and synthesis conditions affords a new route for tuning the electrochemical properties of cathode materials for Li-ion batteries. Furthermore, the development of material models of the required level of detail to predict phase diagrams and electrochemical properties, including oxygen release in Li-rich rocksalt oxides, still relies on the accurate prediction of crystal structures. Experimental identification of new accessible structure types stabilized by oxygen vacancies represents a valuable step forward in the development of predictive models.

Suppressing the P2 − O2 phase transformation and Na+/vacancy ordering of high-voltage manganese-based P2-type cathode by cationic codoping

High-voltage and low-cost manganese-based P2-type oxides show real promise as promising cathode for sodium-ion batteries (SIBs). But the P2 − O2 phase transformation and Na+/vacancy ordering results in the inferior structural stability and Na+ diffusion coefficient, which further leads to rapid decay of capacity and poor rate capability. Herein, in consideration of the synergetic effects of dual cationic doping, electrochemically inactive Li+ and active Co3+ codoping are proposed to solve the above issues. The novel two-step doping strategy, Co doping during synthesis of precursors via coprecipitation reaction followed by Li doping during solid-state reaction, are rationally developed. As anticipated, the Li/Co codoped P2-type oxide exhibits the absence of P2 − O2 phase transformation and Na+/vacancy disordering, which gives rise to an outstanding cycling stability (86.7% capacity retention within 100 cycles at 0.1C) and high-rate capability (reversible capacity of 109 mAh g−1 even at 10C). In addition, the full-cells composed of the codoped P2-type positive and hard carbon negative show high energy-density, good lifespan and high-rate property. This proposed cationic codoping provides an effective and scalable tactics for modulating the structural properties of high-voltage P2-type cathodes for advanced SIBs.

Insights into the enhanced structure stability and electrochemical performance of Ti4+/F− co-doped P2-Na0.67Ni0.33Mn0.67O2 cathodes for sodium ion batteries at high voltage

P2-Na0.67Ni0.33Mn0.67O2 is considered as a promising cathode material for sodium-ion battery (SIBs) because of its high capacity and discharge potential. However, its practical use is limited by Na+/vacancy ordering and P2-O2 phase transition. Herein, a Ti4+/F− co-doping strategy is developed to address these issues. The optimal P2-Na0.67Ni0.33Mn0.37Ti0.3O1.9F0.1 exhibits much enhanced sodium storage performance in the high voltage range of 2.0–4.4 V, including a cycling stability of 77.2% over 300 cycles at a rate of 2 C and a high-rate capability of 87.7 mAh g−1 at 6 C. Moreover, the P2-Na0.67Ni0.33Mn0.37Ti0.3O1.9F0.1 delivers reversible capacities of 82.7 and 128.1 mAh g−1 at −10 and 50 °C at a rate of 2 C, respectively. The capacity retentions over 200 cycles at −10 °C is 94.2%, implying more opportunity for practical application. In-situ X-ray diffraction analysis reveals that both P2-O2 phase transitions and Na+/vacancy ordering is suppressed by Ti4+/F− co-doping, which resulting in fast Na+ diffusion and stable phase structure. The hard carbon//P2-Na0.67Ni0.33Mn0.37Ti0.3O1.9F0.1 full cell exhibits a high energy density of 310.2 Wh kg−1 and remarkable cyclability with 82.1% retention after 300 cycles at 1 C in the voltage range of 1.5–4.2 V. These results demonstrate that the co-doping Ti4+/F− is a promising strategy to improve the electrochemical properties of P2-Na0.67Ni0.33Mn0.67O2, providing a facile tactic to develop high performance cathode materials for SIBs.

Synthesis of O3-Na<sub>0.8</sub>Fe<sub>0.6</sub>Mn<sub>0.3</sub>O<sub>2</sub> Positive Electrode and Its Application to Sodium Ion Batteries

Herein, we report precise variation of Fe and Mn constituent in the sodium magnate layered cathodes with the compositions of Na0.8Fe0.4Mn0.5O2, Na0.8Fe0.5Mn0.4O2, Na0.8Fe0.6Mn0.3O2, Na0.8Fe0.6Mn0.4O2, Na0.8Fe0.7Mn0.4O2, Na0.9Fe0.6Mn0.3O2 in order to attain a high performing cathode. Based on this transition metal stoichiometry, an interesting sodium magnate combination of Na0.8Fe0.6Mn0.3O2, with O3-type crystal phase, possess R3m space group along with the superior electrochemical behavior is obtained. On charge-discharge capacities in the range of 2.0-3.8 V at 0.1 C, it shows the comparatively higher performance of the first and the second charge capacity of 115 and 180 mAhg-1 and discharge capacity of 184 and 181 mAhg-1, respectively. The best sample was then compared with the closely related Na0.8Fe0.6Mn0.4O2, Na0.9Fe0.6Mn0.3O2 combination in terms of valence ratio and influence of excess sodium for the structure robustness, stability along with purity. The best sample with the composition Na0.8Fe0.6Mn0.3O2 does not show detectable impure phase while Na0.8Fe0.6Mn0.4O2 and Na0.9Fe0.6Mn0.3O2 shows a tendency of P-type (Cmca space group) behavior with 30.8% and 32.8%, respectively. The enhancement of iron constituent increases not only the performances but also the stabilization of sodium vacancy ordering and substitution of Mn with a substantial reduction of Jahn-Teller distortion, mounting biphasic characteristics and high peak intensity of 41.5 °.

Boosting the Redox Kinetics of High‐Voltage P2‐Type Cathode by Radially Oriented {010} Exposed Nanoplates for High‐Power Sodium‐Ion Batteries

High‐voltage P2‐type cathode with large capacity, high air stability, and low cost has attracted extensive attention. However, large ionic radius of Na+ and Na+/vacancy ordering result in low reaction kinetics and poor high‐rate capability. Herein, cobalt‐doped hierarchical structure assembled by radially oriented {010} exposed nanoplates is developed. With radially oriented grains infiltrating from surface to interior, all the {010} exposed surfaces are electrochemically active planes, and Na+ ions diffuse directly into electrolyte without passing through grain boundaries, building up 3D transfer channels. The trivalent cobalt substitution suppresses unwanted Na+/vacancy ordering and increases energy barrier of the phase transition from P2‐ to O2‐type oxide. These synergetic effects remarkably boost the redox kinetics of high‐voltage P2‐type cathode. Consequently, a large reversible capacity of 112 mAh g−1 is achieved even at 20 C, indicating excellent high‐rate capability. The absence of P2–O2 phase transition also gives rise to superior cycling stability, maintaining a capacity retention of 85% after 200 cycles. In addition, full cells composed of this hierarchical P2‐type cathode and hard carbon anode deliver high energy‐ and power‐densities. These achievements offer a new insight into boosting redox kinetics of P2‐ and O3‐type layered cathodes for high‐power sodium‐ion batteries.

Biodegradable and Excretable 2D W1.33 C i-MXene with Vacancy Ordering for Theory-Oriented Cancer Nanotheranostics in Near-Infrared Biowindow.

MXenes, a new class of two‐dimensional (2D) nanomaterials, have shown enormous potential for biological applications. Notably, the development of 2D MXenes in nanomedicine is still in its infancy. Herein, a distinct W1.33C i‐MXene with multiple theranostic functionalities, fast biodegradation, and satisfactory biocompatibility is explored. By designing a parent bulk laminate in‐plane ordered (W2/3Y1/3)2AlC ceramic and optionally etching aluminum (Al) and yttrium (Y) elements, 2D W1.33C i‐MXene nanosheets with ordered divacancies are efficiently fabricated. Especially, theoretical simulations reveal that W1.33C i‐MXene possesses a strong predominance of near‐infrared (NIR) absorbance. The constructed ultrathin W1.33C nanosheets feature excellent photothermal‐conversion effectiveness (32.5% at NIR I and 49.3% at NIR II) with desirable biocompatibility and fast degradation in normal tissue rather than in tumor tissue. Importantly, the multimodal‐imaging properties and photothermal‐ablation performance of W1.33C‐BSA nanosheets are systematically revealed and demonstrated both in vitro and in vivo. The underlying mechanism and regulation factors for the W1.33C‐BSA nanosheets‐induced hyperthermia ablation are also revealed by transcriptome and proteome sequencing. This work offers a paradigm that i‐MXenes achieve the tailoring biomedical applications through composition and structure design on the atomic scale.

Reversible OP4 phase in P2–Na2/3Ni1/3Mn2/3O2 sodium ion cathode

P2–Na2/3Ni1/3Mn2/3O2 as a promising cathode material for sodium ion batteries (SIBs) has attracted much attention owing to high capacity and wide operation voltage. However, it suffers from inferior rate and cycling performances due to P2–O2 phase transition and Na+/vacancy ordering during charge/discharge process. Herein, a Fe/Ti co-substitution strategy is successfully utilized to mitigate these issues. Notably, as probed by In-situ X-ray diffraction, the substitution of Ni with Fe significantly stabilizes the interlayer framework by impeding slab glide during deep Na+ extraction, resulting in reversible P2-OP4 phase transition with smaller volume change of host structure compared to P2–O2 transition. Meanwhile, synchrotron X-ray diffraction shows that, the Na+/vacancy ordering is greatly suppressed by further replacing Mn with Ti, which breaks down the long-range ordering of Ni/Mn distribution, promoting Na ion transport. In addition, the charge compensation mechanism of Fe/Ti co-substituted P2–Na0.67Ni0.23Fe0.1Mn0.57Ti0.1O2 (NFMT) revealed by Ex-situ X-ray absorption spectroscopy demonstrates that the incorporation of Fe3+/4+ redox couple largely improve the working voltage. Consequently, the obtained NFMT cathode delivers a high energy density over 420 Wh/kg within 4.3–2.6 V, accompanied with remarkably enhanced kinetics and capacity retention. Such co-substitution strategy provides a promising approach to develop high-voltage and cycle-stable cathode materials for SIBs.

Vacancy ordered phases of nonstoichiometric hafnium carbide from evolutionary crystal structure predictions

Carbides of IV and V group transitional metals with B1 structure have large homogeneity regions due to high amounts of vacancies in the carbon sublattice. Ordering of the vacancies leads to formation of new phases with different compositions and crystal structures. In this work, we performed a theoretical search for the ordered phases in hafnium carbide using evolutionary algorithm USPEX for crystal structure prediction. Four thermodynamically stable compounds with stoichiometries Hf3C2, Hf4C3, Hf7C6 and Hf9C8 as well as near-to-equilibrium variants Hf9C7, Hf5C4, and Hf6C5 have been found in the composition range HfC0.67-HfC1.0. All the stable and meta-stable phases are formed by vacancy ordering in the carbon sublattice of B1 structure. We also described possible ordered structures for hypothetical compositions Hf10C7, Hf7C5, Hf8C7 and Hf10C9. This allowed us to investigate the effect of vacancy concentration on the structural and mechanical properties. The calculations have demonstrated the ordered vacancies with a concentration of less than approximately 11% do not deteriorate the mechanical properties of hafnium carbide. The hardness of ordered phases with vacancy concentrations less than 16.7% is higher as compared to the defect-free HfC.

Magnetic structure and exchange interactions in pyrrhotite end member minerals: hexagonal FeS and monoclinic Fe7S8

Iron mono-sulphides, or pyrrhotites, are minerals present in the Earth’s crust and mantle as well as major magnetic constituents of several classes of meteorites, thus are of interest to a wide range of disciplines including geology, geophysics, geochemistry, and material science. Despite displaying diverse magnetic properties as a result of iron vacancy ordering, the underlying exchange mechanism has not been quantified. This study presents an examination of the electronic and magnetic properties for the two pyrrhotite group end members, hexagonal FeS and monoclinic Fe7S8 (4C superstructure) by means of density functional theory coupled with a Heisenberg magnetic model. The easy magnetization axes of FeS and Fe7S8 are found to be positioned along the crystallographic c-direction and at an angle of 56° to the c-direction, respectively. The magnetic anisotropy energy in Fe7S8 is greatly increased as a consequence of the vacancy framework when compared to FeS. The main magnetic interaction, in both compounds, is found to be the isotropic exchange interaction favouring antiferromagnetic alignment between nearest-neighbouring spins. The origin of the exchange interaction is elucidated further following the Goodenough–Kanamori–Anderson rules. The antisymmetric spin exchange is found to have a minor effect in both compounds. The theoretical findings presented in this work thus help to further resolve some of the ambiguities in the magnetic features of pyrrhotites.

Неоднорідне впорядкування кисневих вакансій в ітрій-барієвому купраті

Неоднорідне впорядкування кисневих вакансій в ітрій-барієвому купраті

Probing the effect of Mg doping on triclinic Na2Mn3O7 transition metal oxide as cathode material for sodium-ion batteries

Triclinic Na2Mn3O7 has been identified as a promising material for high-capacity sodium-ion batteries. However, the knowledge on the effect of doping of metal ions and structural transformations of Na2Mn3O7 during dis(charge) is limited. Integration of alkali metal-ions, specially Mg2+ can enhance the electrochemical properties in transition metal oxides. Herein, a series of Mg2+ doped triclinic Na2Mn3O7 cathode materials was explored for the first time. Electrochemical analysis revealed that Mg2+ improves specific capacities, and rate capabilities. Ex situ X-ray diffraction (XRD) and Galvanostatic charge discharge cycling (GCD) showed that the triclinic phase reversibly converts into two monoclinic phases at high Na+ insertion levels. Na+ extraction at high potentials is supported by another biphasic region which converts to a major triclinic phase at the end of the charge. GCD, cyclic voltammetry (CV) and ex situ X-ray absorption spectroscopy (XAS) documented that the capacity mainly evolved through a Mn4+/3+ redox couple and a reversible O2-/n− redox reaction. CV and Galvanostatic intermittent titration techniques (GITT) showed that Mg2+ reduces the Na+-vacancy ordering and improves the Na+ diffusion. The 2 mol.% Mg-doped material exhibited a high specific capacity of 143 mAh/g after 30 cycles and a rate capability of 93 mAh/g (at 500 mA/g). GCD analysis demonstrated that O2-/n− redox is remarkably stable up to at least 90 cycles. Full cells made using the 0.5 mol.% Mg-doped material displayed a promising discharge specific capacity of 80 mAh/g. The effects of cation doping into the complex crystal structures, phase transformations during Na+ de(intercalation) and the importance of O2-/n− redox for achieving high capacities were uncovered. The findings of this work will guide the design of novel cathode materials for sodium-ion batteries.

Thermoelectric properties and band structures of vacancy-containing Sn-based clathrates K8Sn44−xGex

The type-I clathrate K8Sn44 showed a change in the arrangement of Sn vacancies around 320 ​K. Below the temperature, it exhibited a 2 ​× ​2 ​× ​2 superstructure of the conventional type-I clathrate with ordered Sn vacancies; above it, the ordering disappeared. K8Sn42Ge2 and K8Sn40Ge4 showed no structural transition and no ordering of Sn vacancies. The thermoelectric and transport properties of K8Sn44 changed at the transition temperature. The vacancy-disordered K8Sn44 in the high-temperature phase had large carrier concentrations by one order of magnitude, low carrier mobility, and large effective masses, compared to that in the low-temperature phase. The band structure of K8Sn44 in the high-temperature phase contains localized states, probably due to disordered Sn vacancies, whereas that in the low-temperature phase has no localized states. The high-temperature phase, containing localized states, is expected to provide higher thermoelectric performance because of its larger density-of-states effective mass. Also, the room-temperature lattice thermal conductivity of K8Sn44 was lower than those of the related clathrates Rb8Sn44 and K8Ga8Sn38. It may be related to the framework's Sn-weighted thermal vibration modes of the guest atom K, in addition to the complexity of its crystal structure with disordered vacancies.

Rational design of Na0.67Ni0.2Co0.2Mn0.6O2 microsphere cathode material for stable and low temperature sodium ion storage

Layered transition metal oxides with a classic composition of Na0.67Ni0.33Mn0.67O2 have attracted extensive attention due to their high specific capacity and suitable working voltage. However, its inherent structural defects can lead to a prone to phase change upon charging/discharging and cause degradation of electrochemical performance. In this work, a suitable Co-substituted Na0.67Ni0.2Co0.2Mn0.6O2 microsphere is designed and synthesized. The introduction of cobalt can effectively increase the interlayer spacing of transition metal oxides (TMO2), resulting in a faster Na+ diffusion kinetics. Such a structure enables the charge compensation from different redox couples (Ni2+/3+/4+, Co3+/4+ and Mn3+/4+), leading to a high specific capacity. Furthermore, the drawbacks of P2-O2 phase transition and Na+/vacancy ordering can be simultaneously restrained by a suitable content of Co substitution, which is confirmed by both cyclic voltammetry test and in-situ X-ray diffraction (XRD). The optimized Na0.67Ni0.2Co0.2Mn0.6O2 demonstrates a superior long-term cycle stability with a capacity retention rate of 83% over 200 cycles at 2 C. The electrochemical performance under extreme low-temperature (−40 °C) of this Na0.67Ni0.2Co0.2Mn0.6O2 material has also been explored for the first time, and the DNa+ values of this material can be maintained decently within a range from 10−11 to 10−10 cm2s−1. A high reversible discharge capacity of 132.2 mAh g−1 at 0.2 C is achieved at −40 °C, which is 80% of that at room temperature, indicating a satisfactory sodium storage performance at extremely low temperature.

Structural, Magnetic and Catalytic Properties of a New Vacancy Ordered Perovskite Type Barium Cobaltate BaCoO2.67

A new vacancy ordered, anion deficient perovskite modification with composition of BaCoO2.67 (Ba3Co3O8□1) has been prepared via a two‐step heating process. Combined Rietveld analysis of neutron and X‐ray powder diffraction data shows a novel ordering of oxygen vacancies not known before for barium cobaltates. A combination of neutron powder diffraction, magnetic measurements, and density functional theory (DFT) studies confirms G‐type antiferromagnetic ordering. From impedance measurements, the electronic conductivity of the order of 10−4 S cm−1 is determined. Remarkably, the bifunctional catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is found to be comparable to that of Ba0.5Sr0.5Co0.8Fe0.2O3–y , confirming that charge‐ordered anion deficient non‐cubic perovskites can be highly efficient catalysts.

New Insights into the Bulk and Surface Defect Structures of Ceria Nanocrystals from Neutron Scattering Study

Neutron diffraction and pair distribution function studies coupled with Raman spectroscopy have successfully unraveled the detailed oxygen defect structures of ceria nanocubes and nanorods. Two types of defect sites are revealed for the ceria nanocrystals: surface and bulk defects. It is proposed that the surface oxygen defects in both types of CeO2 nanocrystals are predominantly the partially reduced Ce3O5+x, with the bulk defect structures dominated by interstitial Frenkel-type oxygen vacancies. Ceria nanorods possess much higher concentration of surface oxygen defects relative to the nanocubes, albeit with only slightly higher concentration of bulk Frenkel-type oxygen vacancies. Upon annealing the nanorod sample at 600 °C under vacuum (∼10–4 to 10–5 mbar), a partially reduced ceria phase with long-range oxygen vacancy ordering (Ce3O5+x) has been observed experimentally for the first time. This intriguing observation that surface defect phases can take on ordered defect sublattices under certain conditions is of great value in understanding the temperature-dependent catalytic performance of ceria nanocrystals. Furthermore, a drastic decrease of the surface vacancies in the ceria nanocrystals is observed upon exposure to SO2, especially for the nanorods, a likely origin for the sulfur poisoning effect on ceria-based materials. This study suggests that tailoring surface morphology is a promising strategy to control defect properties of ceria nanomaterials. It also provides fundamental insights to stabilize surface oxygen defects in CeO2 nanocrystals to achieve high redox performance under corrosive environments such as under SO2/SOx exposure.

Robust Narrow-Gap Semiconducting Behavior in Square-Net La3Cd2As6

ABSTRACT: Narrow-gap semiconductors are sought-after materials due to their potential for long-wavelength detectors, thermoelectrics, and more recently non-trivial topology. Here we report the synthesis and characterization of a new family of narrow-gap semiconductors, $R$$_{3}$Cd$_{2}$As$_{6}$ ($R=$ La, Ce). Single crystal x-ray diffraction at room temperature reveals that the As square nets distort and Cd vacancies order in a monoclinic superstructure. A putative charge-density ordered state sets in at 279~K in La$_{3}$Cd$_{2}$As$_{6}$ and at 136~K in Ce$_{3}$Cd$_{2}$As$_{6}$ and is accompanied by a substantial increase in the electrical resistivity in both compounds. The resistivity of the La member increases by thirteen orders of magnitude on cooling, which points to a remarkably clean semiconducting ground state. Our results suggest that light square net materials within a $I4/mmm$ parent structure are promising clean narrow-gap semiconductors.

Impact of Solution Chemistry on Growth and Structural Features of Mo-Substituted Spinel Iron Oxides

The effect of crystallizing solution chemistry on the chemistry of subsequently as-grown materials was investigated for Mo-substituted iron oxides prepared by thermally activated co-precipitation. In the presence of Mo ions, we find that varying the oxidation state of the iron precursor from Fe(II) to Fe(III) causes a progressive loss of atomic long-range order with the stabilization of 2-4 nm particles for the sample prepared with Fe(III). The oxidation state of the Fe precursor also affects the distribution of Fe and Mo cations within the spinel structure. Increasing the Fe precursor oxidation state gives decreased Fe-ion occupation and increased Mo-ion occupation of tetrahedral sites, as revealed by the extended X-ray absorption fine structure. The stabilization of Mo within tetrahedral sites appears to be unexpected, considering the octahedral preferred coordination number of Mo(VI). The analysis of the atomic structure of the sample prepared with Fe(III) indicates a local ordering of vacancies and that the occupation of tetrahedral sites by Mo induces a contraction of the interatomic distances within the polyhedra as compared to Fe atoms. Moreover, the occupancy of Mo into the thermodynamic site preference of a Mo dopant in Fe2O3 assessed by density functional theory calculations points to a stronger preference for Mo substitution at octahedral sites. Hence, we suggest that the synthetized compound is thermodynamically metastable, that is, kinetically trapped. Such a state is suggested to be a consequence of the tetrahedral site occupation by Mo ions. The population of these sites, known to be reactive sites enabling particle growth, is concomitant with the stabilization of very small particles. We confirmed our hypothesis by using a blank experiment without Mo ions, further supporting the impact of tetrahedral Mo ions on the growth of iron oxide nanoparticles. Our findings provide new insights into the relationships between the Fe-chemistry of the crystallizing solution and the structural features of the as-grown Mo-substituted Fe-oxide materials.

Coupling between interfacial strain and oxygen vacancies at complex-oxides interfaces

The complex-oxides interfaces hold rich physics that have resulted in the emergence of various novel functional properties. While strain engineering has been widely used to induce many properties over the past decade, more recently the role of oxygen vacancies has increasingly drawn wider attention. In particular, research has revealed that there exists a strong coupling between strain and oxygen vacancy formation energy. This coupling can be used to alter oxygen vacancy concentration at interfaces, thereby opening another degree of freedom to control interfacial properties. In this review, we highlight recent works that have interrogated the connection between coupling and the emerging interfacial properties. The coupling has not only been used to selectively create oxygen vacancies at specific crystallographic oxygen sites but has also been used to manipulate ordering of oxygen vacancies near interfaces. In addition, recent studies have extended the existing connection between strain and octahedra distortion to oxygen vacancies, where the role of vacancies in the properties emerging due to octahedra distortion is now being unveiled. Finally, we discuss recent data-science efforts in the design and discovery of complex oxides and property prediction.

Resistance fluctuations in superconducting K x Fe2 – y Se2 single crystals studied by low-frequency noise spectroscopy* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774303 and 11574373) and Joint Fund of Yunnan Provincial Science and Technology Department, China (Grant No. 2019FY003008).

Low-frequency resistance noise spectroscopy is applied to investigate bulk single crystals of the intercalated iron-selenide K x Fe2 – y Se2 superconductors with different iron vacancy orders. Based on a generalized fluctuation model, the well-observed resistance hump above 100 K is interpreted as an insulator–metal phase transition with a characteristic transition energy of 0.1–0.6 eV, indicating a highly inhomogeneous energy distribution configuration. In the superconducting transition regime, we find that the normalized resistance noise scales with resistance R excellently as SR /R 2 ∝ R l rs with the noise exponent l rs ≈ 1.4. With reduced iron vacancy disordering in enhanced superconductivity K x Fe2 – y Se2 crystals, the level of resistance fluctuations is greatly suppressed, suggesting a geometrical phase transition for conduction channel, which is directly related to the microstructure of the crystals.