Crystal structure and properties of perovskite-type rubidium niobate, a high-pressure phase of RbNbO3.

We synthesized a perovskite-type RbNbO3 at 1173 K and 4 GPa from non-perovskite RbNbO3 and investigated its crystal structure and properties towards ferroelectric material design. Single-crystal X-ray diffraction analysis revealed an orthorhombic cell in the perovskite-type structure (space group Amm2, no. 38) with a = 3.9937(2) Å, b = 5.8217(3) Å, and c = 5.8647(2) Å. This non-centrosymmetric space group is the same as the ferroelectric BaTiO3 and KNbO3 but with enhanced distortion. Structural transition from orthorhombic to two successive tetragonal phases (Tetra1 at 493 K, Tetra2 at 573 K) was observed, maintaining the perovskite framework before reverting to the triclinic ambient phase at 693 K, with no structural changes between 4 and 300 K. The first transition is similar to that of KNbO3, whereas the second to Tetra2, marked by c-axis elongation and a significant cp/ap ratio jump (from 1.07 to 1.43), is unique. This distortion suggests a transition similar to that of PbVO3, where an octahedron's oxygen separates along the c-axis, forming a pyramid. Ab initio calculations simulating negative pressure like thermal expansion predicted this phase transition (cp/ap = 1.47 at -1.2 GPa), aligning with experimental findings. Thermal analysis revealed two endothermic peaks, with the second transition entailing a greater enthalpy change and volume alteration. Strong second harmonic generation signals were observed across Ortho, Tetra1, and Tetra2 phases, similar to BaTiO3 and KNbO3. Permittivity increased during the first transition, although the second transition's effects were limited by thermal expansion-induced bulk sample collapse. Perovskite-type RbNbO3 emerges as a promising ferroelectric material.

Atomic-Level Changes during Electrochemical Cycling of Oriented LiMn2O4 Cathodic Thin Films.

Spinel LiMn2O4 is an attractive lithium-ion battery cathode material that undergoes a complex series of structural changes during electrochemical cycling that lead to rapid capacity fading, compromising its long-term performance. To gain insights into this behavior, in this report we analyze changes in epitaxial LiMn2O4 thin films during the first few charge-discharge cycles with atomic resolution and correlate them with changes in the electrochemical properties. Impedance spectroscopy and scanning transmission electron microscopy are used to show that defect-rich LiMn2O4 surfaces contribute greatly to the increased resistivity of the battery after only a single charge. Sequences of {111} stacking faults within the films were also observed upon charging, increasing in number with further cycling. The atomic structures of these stacking faults are reported for the first time, showing that Li deintercalation is accompanied by local oxygen loss and relaxation of Mn atoms onto previously unoccupied sites. The stacking faults have a more compressed structure than the spinel matrix and impede Li-ion migration, which explains the observed increase in thin-film resistivity as the number of cycles increases. These results are used to identify key factors contributing to conductivity degradation and capacity fading in LiMn2O4 cathodes, highlighting the need to develop techniques that minimize defect formation in spinel cathodes to improve cycle performance.

Chemical design of a new displacive-type ferroelectric.

Since the discovery of the ferroelectric perovskite-type oxide BaTiO3 in 1943, numerous materials have been surveyed as candidates for new ferroelectrics. Perovskite-type materials have played a leading role in basic research and applications of ferroelectric materials since the last century. Experimentalists and theoreticians have developed a new materials design stream for post-perovskite materials. In this stream, we have mainly focused on the role of covalency in the evolution of ferroelectricity for displacive-type ferroelectrics in oxides. This perspective surveys the following topics: (1) crossover from quantum paraelectric to ferroelectric through a ferroelectric quantum critical point, (2) the role of cation-oxygen covalency in ferroelectricity and the crossover to quantum paraelectric in perovskite-type compounds, (3) off-center-induced ferroelectricity in perovskites, (4) second-order Jahn-Teller effect enhancement of ferroelectricity in lithium-niobate-type oxides, (5) the presence of four ferroelectric phases and structural transitions of phases of AFeO3 with decreasing radius of A (A = La-Al), (6) tetrahedral ferroelectrics of perovskite-related Bi2SiO5 and wurtzites, (7) a rare type of polarization switching system in which the coordination number of ions in κ-Al2O3 systems changes between 4 and 6, and (8) lone-pair-electron-induced ferroelectrics in langasite-type compounds.

Oxide-ion diffusion in brownmillerite-type Ca2AlMnO5+δ from first-principles calculations.

Oxide-ion diffusion pathways in brownmillerite oxides Ca2AlMnO5 and Ca2AlMnO5.5 are systematically investigated using first-principles calculations. These structures reversibly transform into each other by oxidation and reduction. We examine oxide-ion migration in Ca2AlMnO5 and Ca2AlMnO5.5 using the nudged elastic band method. In the reduced structure (Ca2AlMnO5), oxide-ion migration through a vacancy channel is found to have the lowest migration energy barrier, at 0.58 eV. The migration energy barrier of the second-lowest energy path, perpendicular to the vacancy channel, is found to be 0.98 eV. In the oxidized structure (Ca2AlMnO5.5), oxide-ion migration within AlO6 layers has migration energy barriers of 0.55 eV and 0.56 eV in the [100] and [001] directions, respectively. Oxide-ion migration perpendicular to the AlO6 layer has a migration energy barrier of 1.33 eV, suggesting that oxide-ion diffusion in the [010] direction is difficult even at elevated temperature. These results indicate that diffusion in the reduced phase is predominantly one-dimensional whereas it is two-dimensional in the oxidized phase.

Revealing Spatial Distribution of Al-Coordinated Species in a Phase-Separated Aluminosilicate Glass by STEM-EELS.

Structure determination of glass remains an important issue in glass science. The electron microscope with its high spatial resolution and versatile functions has been widely applied to observe phase separation and structural heterogeneity in glass. While elemental analysis such as energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) may provide local compositional information with nanometer-scale resolution, structural information in a glass network cannot be directly obtained. Here, a novel way to probe local coordination is employed using electron energy loss fine structure (ELNES) in the scanning transmission electron microscope (STEM). The method is demonstrated in a phase-separated aluminosilicate glass with multiple Al-coordinated species. With the support of ab initio calculation, two exciton-like peaks in the Al L2,3-edge at around 77 and 80 eV are attributed to 4-fold and 5,6-fold Al excitations, respectively. Mapping of the relative intensity ratio for two peaks in a phase-separated microstructure reveals a heterogeneous distribution of highly coordinated Al species in real space. The finding is in agreement with previous MD simulation that 5- and 6-fold Al species are favored to form in the Al-rich phase. This work has demonstrated that complex network structure within the phase-separated region can now be studied via STEM-EELS.

Quantitative prediction of grain boundary thermal conductivities from local atomic environments.

Quantifying the dependence of thermal conductivity on grain boundary (GB) structure is critical for controlling nanoscale thermal transport in many technologically important materials. A major obstacle to determining such a relationship is the lack of a robust and physically intuitive structure descriptor capable of distinguishing between disparate GB structures. We demonstrate that a microscopic structure metric, the local distortion factor, correlates well with atomically decomposed thermal conductivities obtained from perturbed molecular dynamics for a wide variety of MgO GBs. Based on this correlation, a model for accurately predicting thermal conductivity of GBs is constructed using machine learning techniques. The model reveals that small distortions to local atomic environments are sufficient to reduce overall thermal conductivity dramatically. The method developed should enable more precise design of next-generation thermal materials as it allows GB structures exhibiting the desired thermal transport behaviour to be identified with small computational overhead.

Density functional study of the phase stability and Raman spectra of Yb2O3, Yb2SiO5 and Yb2Si2O7 under pressure.

The phase stability and Raman spectra of Yb2O3, Yb2SiO5 and Yb2Si2O7 under hydrostatic pressure are investigated using density functional theory calculations. The calculated energies of polymorphs of each compound show that the stable phases at zero pressure, viz., C-type Yb2O3, X2-Yb2SiO5 and ß-Yb2Si2O7, exhibit a pressure-induced phase transition as compressive pressure increases, which is consistent with available experimental data. The theoretical Raman spectra at zero pressure are in good agreement with experimental results for the stable phases and can be used to identify each polymorph. Although the calculated pressure dependence of Raman peak positions of C-type Yb2O3 is overestimated compared to available experimental data, piezospectroscopic coefficients extracted from Raman peaks of X2-Yb2SiO5 and ß-Yb2Si2O7 suggest that Raman spectroscopy can be used to measure stresses and strains in Yb silicates. Normal mode analyses reveal that characteristic Raman peaks of Yb silicates at frequencies above 600 cm-1 are strongly associated with vibrations of Si-O bonds in SixOy tetrahedral units.

ZnTaO2N: Stabilized High-Temperature LiNbO3-type Structure.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO2N that crystallizes in a centrosymmetric (R3̅c) high-temperature LiNbO3-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO3-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order-disorder mechanism of the phase transition. It is found that the closed d-shell of Zn2+, as well as the high-valent Ta5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO2N coordination. Interestingly, only 3% Zn substitution for MnTaO2N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

Impact of local strain on Ti-L2,3 electron energy-loss near-edge structures of BaTiO3: a first-principles multiplet study.

Identification of local strains is crucial because the local strains largely influence the ferroelectric property of BaTiO3. The effects of local strains induced by external pressures on the Ti-L2,3 electron energy-loss near-edge structure (ELNES) of BaTiO3 were theoretically investigated using first-principles multiplet calculations. We revealed that the effects appear in the position of the spectral threshold, namely the spectrum shifts to lower and higher energy sides by the tensile and compressive pressures, respectively. We concluded that conventional ELNES observations can identify only large strains induced by -10 GPa, and 0.1 eV energy resolution is required to identify ±2% of strains.

First-principles calculations of lithium-ion migration at a coherent grain boundary in a cathode material, LiCoO(2).

Results of theoretical calculations are reported, examining the effect of a coherent twin boundary on the electrical properties of LiCoO(2) . This study suggests that internal interfaces in LiCoO(2) strongly affect the battery voltage, battery capacity, and power density of this material, which is of particular concern if it is used in all-solid-state Li-ion batteries.