Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides La_xSr_3?xFe₂O_7?δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2?[])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7?δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.     

Microstructure and Magnetic Properties of Metastable RFeO3 (R: Rare‐earth element) Formed from Undercooled Melt

Containerless levitation technique, where the undercooling can be treated as one of the major thermodynamic parameters, was used to study the influence of oxygen partial pressure () on the microstructure and physical properties of rare‐earth orthoferrites RFeO₃ (where R = Rare‐earth element) in the ranges from 10⁵ to 10^?1 Pa. The microstructure of the as‐solidified samples changed into orthorhombic RFeO₃ (o‐RFeO₃), metastable hexagonal RFeO₃ (h‐RFeO₃), and Fe²⁺‐containing RFe₂O₄ and a new metastable R₃Fe₂O₇ phases with decreasing . The effect of on the magnetic properties was indicated as that the saturation magnetization gradually increased for R = La to Yb and decreased for R = Lu with decreasing due to the formation of metastable and magnetic phases such as Fe₃O₄ and Fe.     

New Lead‐free (1−x)BaTiO3–xBi(Mg1/2Zr1/2)O3 Solid Solution with Morphotropic Phase Boundary and Diffuse Phase Transition

A new lead‐free perovskite solid solution (1?x)BaTiO₃–xBi(Mg_1/2Zr_1/2)O₃ with morphotropic phase boundary (MPB) has been developed, and its structural and dielectric properties have been investigated. Rietveld structural analysis of the X‐ray diffraction data suggest a composition‐dependent tetragonal (P4mm) to cubic () phase transition with an intermediate, phase coexistence region, demarcating the MPB. The compositions with x ≤ 0.05 are tetragonal in the P4mm space group and the compositions with x ≥ 0.25 are cubic in the space group. Coexistence of monoclinic phase (space group Cm) with tetragonal/cubic phase (space group P4mm/) is observed in the MPB region for the compositions with 0.10 ≤ x ≤ 0.22. The temperature dependence of permittivity exhibits a nonrelaxor type diffuse phase transition for all the compositions across the MPB.     

The Orientation Dependence of the Photochemical Activity of α‐Fe2O3

The orientation dependence of the visible‐light stimulated photochemical reduction of aqueous Ag⁺ on polycrystalline hematite Fe₂O₃ was determined by observing the relative amount of reduced Ag⁰ on crystals of known surface orientation over all possible orientations. The results show that surfaces oriented within 20° of the () plane are the most active and surfaces close to the (0001) plane are the least active. A strong correlation is observed between the orientation‐dependent activity and the orientation‐dependent surface potential measured by scanning Kelvin probe force microscopy. Surfaces near the most active () plane and the least active (0001) plane are, respectively, at the most positive and the least positive ends of the potential range among all grains. The trends in activity are concluded to arise from a combination of internal fields associated with surface charges and bulk transport effects.     

First‐principles investigation of the thermodynamic stability of MB2 materials surfaces (M = Ti/Zr/Hf)

Some of the renewed interest in transition metal diborides (MB₂, M = Ti/Zr/Hf) arises from their potential use as matrices in ultrahigh‐temperature ceramic matrix composites (UHTCMCs). Crucial to the understanding of such composites is the study of the fiber/matrix interfaces, which in turn requires a deep knowledge of the surface structures and the thermodynamics of the matrix material. Here we investigate the surface stability of MB₂ compounds by first‐principles calculations. Five surfaces are stabilized when going from a M‐rich to a B‐rich environment, respectively (0001)_M, (100)_M, (101)_B(M), (113)_M and (0001)_B, with the highly stable (100)_M, (101)_B(M) and (113)_M surfaces being discussed here for the first time. The mechanism behind the surface stability is analyzed in terms of cleavage energy, surface strain and surface bonding states. Our results provide important information for a better understanding of the most likely surfaces exposed to the fibers in UHTCMCs, thereby for the construction of reliable interfaces and ultimately UHTCMCs models.     

Crystal structure,defect relaxation,and microwave dielectric properties of Ba[(Mg1/3Nb2/3)1−xHfx]O3 solid solutions

Ba[(Mg_1/3Nb_2/3)_1?xHf_x]O₃ (BMNH, x = 0.05, 0.1, 0.15, 0.2) solid solutions were prepared via the solid‐state reaction method. The effect of BaHfO₃ on the crystal structure, microwave dielectric performance, and defect relaxation behavior of Ba(Mg_1/3Nb_2/3)O₃ (BMN) were studied. BaHfO₃ additions degraded the sintering activity of BMN powder, requiring a high sintering temperature (T_s) ~ 1650°C; but it could be effectively improved by a prolonged sintering process at a lower T_s of 1600°C. The well‐sintered BMNH ceramics (1600°C for 30 h) possessed a high densification >96%, and exhibited cubic perovskite structures without 1:2 cation ordering. Once doped with Hf, the low‐temperature relaxation in dielectric spectroscopy and thermally stimulated depolarization current (TSDC) for pure BMN disappeared, further indicating such relaxation is related to cation‐ordered structure. Oxygen vacancies, namely showing in‐grain and across‐grain‐boundary relaxation of ‐related defects, were the main defect types in BMNH. The concentrations of in‐grain decreased as x increased, which is beneficial to BMNH to maintain high Q × f values of 69 400‐73 000 GHz. Accompanied by a high ε_r of 33.27‐33.59 and a low τ_f of +13.6 to +20.7 ppm/°C, these materials have a good potential for applications in microwave components and devices.     

Optimized Growth of Heteroepitaxial (111) NiFe2O4 Thin Films on (0001) Sapphire with Two In‐Plane Variants via Chemical Solution Deposition

(111)‐oriented epitaxial thin films of nickel ferrite (NFO) are grown on c‐plane sapphire [α‐Al₂O₃(0001)] substrates using a chemical solution deposition technique. The processing conditions, including pyrolysis and annealing temperatures, are varied to achieve a film that shows maximum texture and epitaxy. It is shown that increasing the pyrolysis temperature to 400°C and decreasing the annealing temperature to 750°C for 10 min result in the highest degree of texture in the films. Lower film thickness also leads to a higher degree of texture. Microstructural studies confirm an in‐plane epitaxial relationship between the (111) NFO film and the (0001) Al₂O₃ substrate in two variants, [110]_NFO || or .     

Effect of interfacial energy on microstructure of a directionally solidified Al2O3/YAG eutectic ceramic

An Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic was prepared with a c‐axis sapphire seed by an optical floating zone furnace. The crystallographic relationships in the initial and the steady growth sections of the as‐grown eutectic ceramic were investigated by electron backscattering diffraction and transmission electron microscopy. The corresponding results were: <111 > {101}Y₃Al₅O₁₂ || <0001 > {}Al₂O₃ and <110 > {}Y₃Al₅O₁₂ || < > {0001}Al₂O₃, respectively. The steady orientation of the latter one shows smaller planar disregistry. Interfacial strain played the decisive role in affecting the solidification behavior of the Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic. The stability of interfaces with minimum interfacial strain and better ionic charge balance in irregular microstructure prevail upon the constraint of the seed. These results might cast light for the interfacial design of the Al₂O₃/Y₃Al₅O₁₂ binary eutectic ceramic.     

Oxygen Nonstoichiometry and Thermodynamic Quantities of La2Ni0.95Al0.05O4.025 + δ

The oxygen nonstoichiometry of La₂Ni_0.95Al_0.05O_4.025 + δ (LNAO) is measured as a function of oxygen partial pressure (pO₂) and temperature by coulometric titration method and thermogravimetric analysis (TGA) in 800°C–1000°C temperature range and 10^?16‐1 atm pO₂ range. The partial molar quantities for mixing of oxygen were calculated and results were compared with the literature results on La₂NiO_4 + δ (LNO). The variation in activity coefficient of holes versus oxygen nonstoichiometry illustrated an early positive deviation of the activity coefficient of holes from unity, leading to  ≈  7 at δ  ≈  0.08, which was lower than the literature value of  ≈  14 for La₂NiO_{4  +  δ} at δ  ≈  0.08, indicating lesser deviation of LNAO from ideal solution behavior. The effective mass of holes () was 1.02–1.21 times the rest mass (m_o), which indicated the band‐like conduction and allowed the effect of the small degree of polaron hopping to be ignored. The comparison of oxygen nonstoichiometry and partial molar quantities showed that incorporation of interstitial oxygen is less favorable in LNAO in comparison to LNO.     

High‐Performance Small‐Amount Fe2O3‐Doped (K,Na)NbO3‐Based Lead‐Free Piezoceramics with Irregular Phase Evolution

[(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃ + x mol% Fe₂O₃ (KNLNST + x Fe, x = 0~0.60) lead‐free piezoelectric ceramics were prepared by conventional solid‐state reaction processing. The effects of small‐amount Fe₂O₃ doping on the microstructure and electrical properties of the KNLNST ceramics were systematically investigated. With increasing Fe³⁺ content, the orthorhombic‐tetragonal polymorphic phase transition temperature (T_O‐T) of KNLNST + x Fe ceramics presented an obvious “V” type variation trend, and T_O‐T was successfully shifted to near room temperature without changing T_C (T_C = 315°C) via doping Fe₂O₃ around 0.25 mol%. Electrical properties were significantly enhanced due to the coexistence of both orthorhombic and tetragonal ferroelectric phases at room temperature. The ceramics doped with 0.20 mol% Fe₂O₃ possessed optimal piezoelectric and dielectric properties of d₃₃ = 306 pC/N, k_p = 47.0%, = 1483 and tan δ = 0.023. It was revealed that the strong internal stress in the KNLNST + x Fe ceramics with higher Fe³⁺ contents (x = 0.40, 0.60) stabilized the orthorhombic phase, leading to the irregular “V” type rather than the usually observed monotonic phase transition with composition change in the ceramics.     

Effect of Oxygen Vacancy on Electrical Property of Acceptor Doped BaTiO3–Na0.5Bi0.5TiO3–Nb2O5 X8R Systems

In this study, we reported a new BaTiO₃–Na_0.5Bi_0.5TiO₃–Nb₂O₅–Mn₂O₃/Fe₂O₃/Co₃O₄/In₂O₃ X8R system with high dielectric constant (>2100) at room temperature. The impacts of oxygen vacancy ( ) on dielectric, electrical conductivity, and ferroelectric properties were systematically studied. The Curie point is largely depended on the concentration, which can be confirmed by the dielectric behavior and A_1g octahedral breathing modes in Raman spectrum. In addition, the activation energy of diffusion is greatly reduced with the increase in concentration. It was found that the remnant polarization and coercive field were both decreased with increasing concentration, due to the facilitated defect dipoles reorientation and domain switching.     

Direct Evidence for the Modulation Caused by Ti Substitution of Ta in a (Ta2O5)0.92(TiO2)0.08 Ceramic by Analytical Electron Microscopy

The high‐temperature tetragonal structure of Ta₂O₅ was observed in a Ta₂O₅ ceramic sample doped with 8 at.% TiO₂. Direct evidences for the modulation along [] direction caused by Ti replacing Ta are obtained based on atomic‐resolution high‐angle annular dark‐field lattice imaging and electron energy loss spectroscopy analysis. The atomic mechanism of Ti substitution of Ta is explicitly described based on the structure model along [110] direction projection.     

Long‐Range Ordered Structure of Ti–B–C–N in a TiB2–TiCxN1−x Eutectic Composite

A TiB₂–TiC_xN_1?x quasi‐binary eutectic composite, with a rod‐like faceted texture and a long‐range ordered structure of Ti–B–C–N, was prepared by the arc‐melting of TiB₂, TiC, and TiN powders. Hexagonal single‐crystalline TiC_xN_1?x rods were grown in a single‐crystalline TiB₂ matrix with a crystal orientation relationship of TiB₂ //TiC_xN_1?x and TiB₂ [0001]//TiC_xN_1?x [111]. A long‐range ordered structure of Ti–B–C–N was formed by the intermixing of the coherent interplanar spacings of seven TiB₂ (0001) and nine TiC_xN_1?x (111) planes.     

ZrB2–ZrCxN1−x Eutectic Composites Produced by Melt Solidification

Ceramic eutectics are naturally occurring in‐situ composites and can offer superior mechanical properties. Here, ZrB₂–ZrC_xN_1?x quasi‐binary ceramic eutectic composites were produced by arc‐melting a mixture of ZrB₂, ZrC, and ZrN powders in an N₂ atmosphere. The arc‐melted ZrB₂–ZrC_xN_1?x composites containing 50 mol% of ZrB₂ (irrespective of the ZrC/ZrN ratio) showed rod‐like eutectic structures, where ZrC_xN_1?x single‐crystalline rods were dispersed in the ZrB₂ single‐crystalline matrices. Multiple orientation relationships between the ZrC_xN_1?x rods and the ZrB₂ matrices were observed, and one was determined as ZrB₂ {} //Zr_xN_1?x {111} and ZrB₂ < > //ZrC_xN_1?x < > . The rod‐like eutectic composites had higher hardness than the hypo‐ and hypereutectic composites and the 50ZrB₂–40ZrC–10ZrN (mol%) eutectic composite showed the highest Vickers hardness (H_v) of 19 GPa.     

Defect‐driven evolution of piezoelectric and ferroelectric properties in CuSb2O6‐doped K0.5Na0.5NbO3 lead‐free ceramics

Defect greatly affects the microscopic structure and electrical properties of perovskite piezoelectric ceramics, but the microscopic mechanism of defect‐driven macroscopic properties in the materials is not still completely comprehended. In this work, K_0.5Na_0.5NbO₃+x mol CuSb₂O₆ lead‐free piezoelectric ceramics were fabricated by a solid‐state reaction method and the defect‐driven evolution of piezoelectric and ferroelectric properties was studied. The addition of CuSb₂O₆ induces the formation of dimeric (DC1) and trimeric (DC2) defect dipoles. At low doping concentration of CuSb₂O₆ (0.5‐1.0 mol%), DC1 and DC2 coexist in the ceramics and harden the ceramics, inducing a constricted double P‐E loop and high Q_m of 895 at x=0.01. However, DC2 becomes more dominant in the ceramics with high concentration of CuSb₂O₆ (≥1.5 mol%) and thus leads to softening behavior of piezoelectricity and ferroelectricity as compared to the ceramic with x=0.01, giving a single slanted P‐E loop and relatively low Q_m of 206 at x=0.025. All ceramics exhibit relatively high d₃₃ of 106‐126 pC/N. Our study shows that the piezoelectricity and ferroelectricity of K_0.5Na_0.5NbO₃ ceramics can be tailored by controlling defect structure of the materials.     

Decisive role of mixed‐valence structure in colossal dielectric constant of LaFeO3

The role of mixed‐valence structure in colossal dielectric constant (CDC) behavior has been investigated in LaFeO₃ ceramics by tuning the ratio of Fe²⁺/Fe³⁺ through substituting Al for Fe. The ratio of Fe²⁺/Fe³⁺ is decreased gradually from 1.0 to 0.0 by increasing the concentration of Al³⁺. Two clear‐cut correlations have been found: (i) the relationship between the CDC behavior and the ratio of Fe²⁺/Fe³⁺ follows an exponential function and (ii) the activation energy of the polaron relaxation is proportional to , where is the intrinsic dielectric constant. These findings underscore the role of the mixed‐valence structure in CDC behavior and suggest that adjusting the mixed‐valence structure through doping/alloying can be a promising strategy to achieve superior CDC behavior in transition‐metal oxides.     

Diffusion Kinetics of Indium in TiO2 (Rutile)

This work determines the self‐diffusion coefficients of indium in TiO₂ single crystal (rutile). Diffusion concentration profiles were imposed by deposition of a thin surface layer of InCl₃ on the TiO₂ single crystal and subsequent annealing in the temperature range 1073–1573 K. The diffusion‐induced concentration profiles of indium as a function of depth were determined using secondary ion mass spectrometry (SIMS). These diffusion profiles were used to calculate the self‐diffusion coefficients of indium in the polycrystalline In₂TiO₅ surface layer and the TiO₂ single crystal. The temperature dependence of the respective diffusion coefficients, in the range 1073–1573 K, can be expressed by the following formulas: and The obtained activation energy for bulk diffusion of indium in rutile (316 kJ/mol) is similar to that of zirconium in rutile (325 kJ/mol). The determined diffusion data can be used in selection of optimal processing conditions for TiO₂–In₂O₃ solid solutions.     

Growth of diopside crystals in CMAS glass‐ceramics using Cr2O3 as a nucleating agent

CaO–Al₂O₃–MgO–SiO₂ (CAMS)‐based glass‐ceramics were prepared using body crystallization method. Adding Cr₂O₃ into the ceramics not only effectively lowered the crystallization temperature, but also led to significant grain refinement of diopside that crystallized in the CAMS glass‐ceramic after crystallization treatment at 900°C for 2 hours. Experimental work verified that the epitaxial growth of the diopside on the spinel particles, which formed during nucleation treatment when fabricating the glass‐ceramics, facilitated the heterogeneous nucleation of diopside on the spinel and refined the diopside. In addition, two energetically favored crystallographic orientation relationships between the epitaxial growth diopside and spinel were experimentally observed. They are //[001]_diopside,////(200)_diopside and //[101]_diopside, (311)_spinel//. These two novel results can be potentially used to develop new glass‐ceramic materials with improved performance.     

Anisotropic domain switching in Pb(Mg1/3Nb2/3)O3‐0.30PbTiO3 single crystals with rhombohedral structure

Anisotropic domain switching paths in [001]‐, [011]‐, and [111]‐poled Pb(Mg_1/3Nb_2/3)O₃‐0.30PbTiO₃ single crystals were studied by in situ polarized light microscopic driven by an antiparallel electric field. Orientation‐dependent electric field induced polarization and strain behaviors were investigated systematically. For [001]‐oriented crystals, only one‐step 71° switching occurred during the domain switching process, resulting in the appearance of stripe domain walls whose traces on (001) plane were along 45° or 135° with respect to [100] direction. But for [011]‐oriented samples, a two‐step 71° switching was observed during 109° switching and the projections of formed twin domain walls on the (011) plane are along 35.3° or 144.7° with respect to [01] direction. Moreover, a three‐step 71° switching was found during 180° switching in [111]‐oriented samples. It was demonstrated by the produced domain walls whose projections on the (10) plane are along 35.3°, 90° or 160.6° with respect to [11] direction. The energetically motivated mechanism based on multistep polarization switching process was also proposed to explain the anisotropic domain switching paths. Our results provided a visualized observation on the ferroelectric domain switching process and also laid the solid foundations for controlling polarization order parameter in ferroelectric single crystals.     

Effects of K2O Evaporation on the Structural Properties of KSbO3 Compounds

A specimen having a stoichiometric composition of KSbO₃·(KSb) calcined at 800°C has an R rhombohedral structure (RS), and changes to a Pn cubic structure (CS) when calcined at 1100°C. Finally, a <111>‐oriented rhombohedral phase is formed in the specimen calcined at 1230°C. K/Sb ratio decreases from 1.0 in RS, 0.93 in CS, and finally to 0.85 in <111>‐oriented rhombohedral phases. On the other hand, a specimen having a K‐excess composition of K_1.1SbO₃ calcined at 800°C shows a RS that is maintained in the K‐excess specimen calcined at 1230°C. The composition of these specimens is very close to KSb. Therefore, the RS with a space group of R is a stable form of KSbO₃. The formation of Pn cubic and <111>‐oriented R phases can be explained by the evaporation of K₂O during the calcination process at temperatures above 1100°C.