Effects of alkaline solution concentration and deposition temperature on the film thickness, crystal structure, and ferroelectric properties were investigated for epitaxial PbTiO₃ films grown on (100)_cSrRuO₃//(100)SrTiO₃ substrates by hydrothermal method. The (001)-oriented PbTiO₃ films were grown at 150 °C with KOH solution concentrations from 1 to 10 mol/L. Potassium incorporation was not detected for the films deposited at 10 mol/L. c-axis lattice parameters continuously decreased with the increase in the KOH solution concentration. A 250-nm-thick film grown at 8 mol/L showed lattice parameters expected from bulk PbTiO₃ and showed good ferroelectric properties; a remanent polarization and an average coercive field (average of plus and minus coercive field, E_c) of about 74 µC/cm² and about 173 kV/cm, respectively. Film thickness was successfully increased by raising the deposition temperature to 200 °C at 8 mol/L. Lattice parameters were almost unchanged by changing the deposition temperature. The relationship between E_c and the film thickness (t) agreed with the semiempirical scaling law (E_c ∼ t^−2/3). These data suggest that the increase in the deposition temperature is one of the effective ways to deposit thick PbTiO₃ films.

Acceptor–donor codoping is gaining attention as a novel method for enhancing the intrinsic dielectric permittivity of TiO₂ ceramics. Aiming at the application of TiO₂ in high-voltage multilayer ceramic capacitors, this study investigated the dielectric properties and breakdown strength of TiO₂ ceramics doped with equimolar amounts of Al³⁺ and Nb⁵⁺ ions at a total doping concentration of up to 10.0 mol %. The dielectric permittivity was not increased by the codoping, but gradually decreased with increasing the codoping concentration. Meanwhile, a small amount of Al–Nb codoping (up to 2.0 mol %) suppressed dielectric loss at both room and elevated temperatures. The dielectric breakdown field was also significantly improved at up to 5.0 mol % codoping concentrations. The effects of Al–Nb codoping on the dielectric properties and breakdown strength are discussed in terms of microstructure, defect concentration, and mobility of charged oxygen vacancies.

In thin epitaxial films of Ruddlesden–Popper oxides [A_n+1B_nO_3n+1 (n = 1, 2, …)], the formation of intergrowth phases with varying n values is commonly observed as lattice impurity defects. These intergrown impurities often exert a pronounced influence on film properties, prompting extensive investigations into formation mechanisms for better control. In this study, we demonstrate that the intergrowth of an A₃B₂O₇-structured impurity with n = 2 is notably influenced by the thickness of epitaxial thin-film growth of a Ruddlesden–Popper oxide Ca₂RuO₄ with n = 1 in solid-phase epitaxy. Employing structural analyses via X-ray diffraction and scanning transmission electron microscopy, we discovered an unusual intensification of A₃B₂O₇ intergrowth within Ca₂RuO₄ thin films with reduced thicknesses of ≤25 nm. We further validated the significant impact of this atypical intergrowth on the electrical transport properties of Ca₂RuO₄ thin films through resistivity–temperature measurements. The source of the anomalous thickness-dependent behavior in A₃B₂O₇ intergrowth was discussed based on the insights from the structural characterizations.

HfO₂-based ferroelectrics have attracted much attention as key materials for silicon process compatible ferroelectric memories. The ferroelectric phase of HfO₂-based materials is a metastable orthorhombic phase whose potential for stabilization by chemical doping with various elements has been investigated. Recently, it was reported that a nanolaminate film composed of HfO₂ and ZrO₂ films have a ferroelectric phase. However, the stabilization mechanism of the ferroelectric phase is not completely understood. In this study, HfO₂–Y₂O₃ nanolaminate films were fabricated by pulsed laser deposition (PLD) and investigated the ferroelectricity of HfO₂–Y₂O₃ nanolaminate thin films. HfO₂–Y₂O₃ nanolaminate films, where Y₂O₃ is a paraelectric material, provide greater internal stress compared to HfO₂–ZrO₂ nanolaminate films. The crystal structure of HfO₂–Y₂O₃ nanolaminate films was evaluated by X-ray diffraction (XRD) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) measurements. The results of the HAADF-STEM observations revealed that the orthorhombic phase is stabilized by applying internal stress into HfO₂ in the nanolaminate film with Y₂O₃. The ferroelectricity of HfO₂–Y₂O₃ nanolaminate films was elucidated by carrying out piezoelectric response microscopy (PFM). The coercive electric field value shows a dependence on the thickness of each layer. We demonstrated the importance of the stress engineering in HfO₂ ferroelectricity, as it serves not only to stabilize the ferroelectric properties but also to regulate the coercive field.

The effect of post heat treatment on the crystal structure, Curie temperature, and ferroelectric properties of hydrothermally grown epitaxial films of (1 − x)(Bi,K)TiO₃–xCaTiO₃ (x = 0–0.26) was investigated. The out-of-plane c-axis lattice parameter of the post-heat-treated films decreased compared to that of the as-deposited films for all x values. It decreased with the increase in x value in the single-phase region (x = 0–0.12). However, it became almost constant for the film above x = 0.18, coexisting region with the second phase. The solubility limit of CaTiO₃ in (Bi,K)TiO₃ remained almost unchanged after the heat treatment. In the single-phase region, the phase transition temperature from ferroelectric to paraelectric phase (T_c) decreased from 780 °C for the film with x = 0 to 490 °C for the film with x = 0.12. The heat treatment caused a significant decrease in remnant polarization (P_r) and coercive field (E_c) values, and almost eliminated the imprint of P-E hysteresis loops became almost zero by this heat treatment. These results demonstrate that the crystal structure and ferroelectric properties of these films were altered by the post heat treatment.

Lead-free ferroelectric and piezoelectric ceramics, (Bi_0.5Na_0.5)TiO₃ [BNT] have a low depolarization temperature T_d of ∼180 °C in ordinary fired (OF) process. To solve this problem, we focused on two effects to increase T_d: compositing ZnO and the quenching effect. T_d is elevated by dipole pinning or increasing of lattice distortion, respectively. In this study, ZnO–BNT ceramics composites with controlled ZnO content (ZnO 100x, 100x = 0, 10, 20, 30, and 40 mol %) were prepared to increase T_d. To further increase the T_d, their composites were sintered through a quenching process. We examined the elevation of T_d and electrical properties of that quenched ZnO–BNT ceramics composites. As a result, the T_d of OF-ZnO100x increased by about 20 °C with increasing the compositing ZnO content. However, the T_d of quenched ZnO100x increased uniformly to 215 °C, regardless of the compositing ZnO content. This indicates that quenching is more effective than compositing ZnO in elevation of T_d. Quenching is the dominant effect that contributes to the overall ZnO–BNT ceramics composites, whereas compositing ZnO is effective in the vicinity of ZnO grains, that is, it is a local and limited effect. Therefore, the T_d of the ZnO–BNT ceramics composites could not be further increased by the quenching effect. In other words, dipole pinning and quenching were not synergistically effective.

Small-signal piezoelectric characteristics in pseudo-cubic BiFeO₃–BaTiO₃-based ceramics are rarely studied. In this study, the xMnO₂-doped 0.67BiFeO₃–0.33BaTiO₃ (BF33BT–xMn, x = 0.0, 0.2, 0.4, and 0.6 wt %) ceramics are investigated. Small and large amounts of Mn ions play different roles in resistivity, with the highest resistivity observed at a MnO₂ content of 0.2 wt %. The examination of poling conditions revealed the impact of electric field strength and poling time on impedance phase θ. The piezoelectric constant (d₃₃) and electromechanical coupling factor (k₃₃) increase with impedance phase θ, while the mechanical quality factor (Q_m) value remains unchanged. Our finding indicates that poled BF33BT–0.2Mn ceramic reaches an impedance phase θ of 73°, k₃₃ of 0.45, Q_m of 41.0, and d₃₃ × Q_m value of 3856 pC/N. This study contributes to further research on the high-power piezoelectric characteristics of lead-free BiFeO₃–BaTiO₃-based ceramics.

The effect of alternating current (AC) plus direct current (DC) poling conditions such as an AC field amplitude, an AC field frequency, an AC field cycle number, and poling temperature (T_p) on a piezoelectric constant (d₃₃) was investigated for barium titanate (BT) ceramics. It was found that the AC poling with an AC field amplitude (zero-peak value) of 35 kV/cm, a frequency of 0.1 Hz, an AC field cycle number of 5, T_p of 131 °C followed by 30 kV/cm DC poling and cooling with an application of a DC field of 15 kV/cm maximized the d₃₃ value to 328 pC/N. This result could be attributed to the reduced domain size of the ceramics.

Lead-free 〈110〉 grain-oriented tetragonal 0.50BNT–0.50BT ceramics were fabricated by the reactive-templated grain growth (RTGG) method with plate-like H_1.08Ti_1.73O₄·nH₂O (HTO) templates. A high relative density of 97 % was obtained by sintering at 1200 °C for 2 h. An orientation degree (F₁₁₀) of 63 % was obtained by the Lotgering method. Domain engineering treatment above their Cuire temperature (T_c) was conducted under optimized alternating current (AC) poling and direct current (DC) poling conditions. With optimized conditions, the piezoelectric coefficient d₃₃ values measured by a d₃₃ meter were 82 pC/N for AC poling and 58 pC/N for DC poling, respectively. The nearly 40 % increase in d₃₃ could be attributed to the fine domain structures induced by the optimized AC conditions.

We fabricated 0.3BaTiO₃–0.1Bi(Mg_0.5Ti_0.5)O₃–0.6BiFeO₃ (0.3BT–0.1BMT–0.6BF) ceramics prepared using powders synthesized using the citrate method (CM) and solid state method (SSM) and investigated their microstructure and electric properties. Compared with SSM, CM reduced calcination temperature by 300 °C to synthesize 0.3BT–0.1BMT–0.6BF and reduced sintering temperature by 100 °C to densify the ceramics with a relative density of more than 95 %. The reduced calcination and sintering temperatures with CM were due to a smaller calcined powder. Improved ferroelectric and piezoelectric properties were observed for the ceramics prepared using this method. This could be attributed to reduced defects associated with the suppression of Bi volatilization.

In this study, the photoluminescence wavelength and decay constant of Cr:Gd₃Ga₅O₁₂ garnet oxide were modified by substituting Gd³⁺/Ga³⁺ cations with a Ca²⁺/Si⁴⁺ cation pair. The primary objective of this study was to shift the emission wavelength to the near-infrared (NIR) region and accelerate decay of Cr:Gd₃Ga₅O₁₂ for its potential use as a scintillation detector in medical applications. The incorporation of Ca²⁺ and Si⁴⁺ ions in site of Gd³⁺/Ga³⁺ ions resulted in a significant redshift in the photoluminescence wavelength of Cr³⁺ ions and an acceleration of decay. The results clearly demonstrated that Ca²⁺/Si⁴⁺ cation pair substitution is a clearly effective strategy for modulating the optical properties of Cr:Gd₃Ga₅O₁₂, with the potential to develop a NIR garnet scintillator.

This study focuses on the growth of transparent single crystals of Li₂CaSiO₄ (LCS) doped with Ce as neutron scintillator candidates. LCS is a light material with a high melting point and chemical stability; thus, LCS is applicable for use in high-flux neutron environments. However, the growth of transparent single crystals from the melt has been difficult owing to the phase transitions. In this study, the LiF/LiCl eutectic was used as a flux with a lower melting point and less decomposition than LiCl, and Ce was selected as the luminescent center. The phase diagram of the LiF/LiCl-LCS system was determined, and the conditions for crystal growth were optimized, consequently, transparent non-doped LCS and Ce:LCS crystals were successfully obtained. The alpha-ray-excited scintillation spectrum and scintillation decay time of Ce:LCS were measured in a feasibility study for neutron detection. The results showed that Ce:LCS has potential as a neutron scintillator, with a significantly fast decay time of 28.46 ns.

We study the valences of dopants in BaTiO₃ at room temperature based on a thermodynamic model combined with first-principles simulations. Here we assume that cations and anions are frozen-in at different temperatures during a sintering process. We calculated the defect concentration at two different temperatures (sintering temperature and anion freeze-in temperature) to eventually obtain defect concentrations at the room temperature. Our first-principles results show that variable valence dopants exhibit different valences depending on the simulation temperature.

The structural and property control of copper oxide thin films driven by vacuum ultraviolet irradiation at room temperature in air was determined in this study. Cu₂O(110) thin films were epitaxially grown as precursor crystals on MgO(100) substrates using pulsed laser deposition. The Cu₂O thin films were subsequently irradiated with vacuum ultraviolet light (λ = 172 nm) using a xenon excimer lamp for up to 120 min. Structural analyses by in-plane X-ray diffraction, reciprocal space mapping, and reflection high-energy electron diffraction observation indicated the phase transition of Cu₂O to CuO crystals in the near-surface area, maintaining the oriented structure. The bi-epitaxial relationships were as follows: CuO(010)[100] // Cu₂O(110)[001] // MgO(100)[011] and CuO(010)[001] // Cu₂O(110)[110] // MgO(100)[011]. Significant improvement of conductivity in the order of 10³ was demonstrated, that the resistivity decreased from ∼2.1 × 10³ Ω cm to ∼1.2 Ω cm after irradiation of 120 min. The present light-driven formation of epitaxial heterostructures of both Cu₂O and CuO p-type semiconductors will be useful for the development and property control of devices.

The electrophoresis of particles by applying an electric field to a colloidal suspension allows the simultaneous manipulation and shaping of particles. A film forming process utilizing the electrophoretic phenomenon of charged particles in a liquid is called “electrophoretic deposition (EPD)” and is used for powder coating. The main difference between the EPD process and other colloidal processes is the coexistence of electrode reactions, i.e., electrochemical reactions that occur at the interface between the electrode and the electrolyte solution. The applied electric field in the EPD method plays an important role not only in the electrophoresis of charged particles but also in controlling the coagulation mechanism and film quality. This paper introduces the principle of the EPD process considering the electrode reaction and an example of the functional film formation process.

Zinc silicate (ZS), which occurs naturally as willemite, is primarily used as a ceramic pigment and finds wide applications as a fluorescent material and in nonlinear voltage resistor devices. In recent years, ZS has been studied as a coating material for the electrode surfaces of air batteries and as a negative electrode material for Li-ion batteries. In this study, we investigated its suitability as an anode material in aqueous Zn secondary batteries. Conventional ZnO dissolves and precipitates as zincate ions in alkaline electrolytes, leading to dendritic growth and passivation. By utilizing ZS as the anode material, we observed significant improvements in cycle characteristics. Furthermore, substituting a portion of the ZS with P resulted in increased battery capacity and confirmed the reversible dissolution–precipitation reactions of Zn ions during charging and discharging.

Ferroelectricity in hafnium dioxide (HfO₂)-based thin films originates from a metastable orthorhombic phase. Therefore, the formation of other phases, such as monoclinic (m) phase, should be avoided. Here, grain-size effects on transformations to the m phase in zirconium-doped HfO₂ nanoparticles were examined. X-ray diffraction and electron microscopy revealed that only a fraction transformed into the m phase via 900-°C annealing and the m-phase particles exceeded 50 nm. Thus, grain size alone was a critical factor to avoid forming the m phase.

To investigate the anisotropy of Ca²⁺ conduction, the b-axis-aligned CaAl₄O₇ (space group C2/c) polycrystal was prepared by colloidal processing under high magnetic field of 12 T followed by sintering at 1773 K for 2 h. The textured polycrystal was characterized by X-ray diffractometry and impedance spectroscopy with respect to the grain alignment direction, which was parallel to the applied magnetic field. The texture fraction of {0k0}, expressed as the Lotgering factor f_0k0, was 0.63. The 〈101〉 directions of individual crystal grains were randomly distributed around the grain-alignment direction of the polycrystal. The conductivities perpendicular (σ_⊥) and parallel (σ_∥) to the grain alignment direction were compared with the conductivity (σ_random) of random grain oriented polycrystal between 773 and 1073 K. The σ_⊥, ranging from 3.09 × 10⁻⁷ to 1.80 × 10⁻⁵ S cm⁻¹, showed the highest value at each temperature, followed by σ_random and σ_∥ in that order. The results have confirmed for the first time the anisotropy of Ca²⁺ conduction and strongly supported the preferential conduction in the 〈101〉 direction predicted in the literature by the bond valence method.

The c-axis-aligned polycrystalline lanthanum silicate oxyapatite (LSO) was prepared by the reactive diffusion between random grain oriented La₂SiO₅ polycrystal and [SiO + 1/2O₂] gases at 1873 K for 10 h. Based on the grain sizes, aspect ratios, elongation directions, and c-axis orientations for the constituent LSO crystals, the polycrystalline microtexture was classified into three regions denoted by I, II, and III. Region I was located on the innermost side of the pellet sample, region III was situated on the surface, and region II was positioned between the regions I and III. The region I with the layer thickness of approximately 140 µm was composed of relatively large crystal grains with their elongation directions along the c-axes. The individual crystal grains were aligned almost along their c-axes, with their a-axis directions being randomly oriented around the grain alignment direction. Although this region exhibited the highest orientation compared to the other two regions, there were also significantly lower orientation grains present, accounting for approximately 6.1 % of the total. If the formation of these grains that reduce the degree of orientation can be prevented, a higher degree of orientation can be achieved for the region I. On the other hand, there were much smaller crystal grains in the regions II and III. The LSO polycrystal with the region II showed the lowest orientation degree among those of the three regions. The region III consisted of grains with a relatively high degree of c-axis orientation and elongated along the c-axis. The differences in microtexture between the three regions would be attributed to the distinct growth behaviors of the constituent LSO crystal grains.

We have explored ultra-high refractive index over 2.05 in multi-component borate glasses with large optical bandgap (∼3.7 eV) and high Young’s modulus (∼130 GPa). The niobate-based borate glasses commonly contain highly electronic polarized oxides like Nb₂O₅ and TiO₂, chosen from transition metal oxides. Although these glasses have a large amount of Nb₂O₅, TiO₂, and La₂O₃, they are vitrified by melting at 1300 °C. The obtained borate glasses with internal transmissions of 99 % per 10 mm-thick estimated from the optical absorption coefficient for the visible light might be used for “Augmented Reality glass” devices.

Synthesizing densified glasses with structure ordering is an important issue for the development of new optical fibers with high refractive index and low dispersion. Herein, we report on our attempt to synthesize densified silica (SiO₂) glasses by hot compression at a pressure of 7.7 GPa and temperatures above 1200 °C. We succeeded for the first time in recovering densified SiO₂ glasses compressed at 7.7 GPa and 1300 °C. Samples compressed above 1300 °C were crystallized into coesite by heterogeneous nucleation. The height of the first sharp diffraction peak in high-energy X-ray and neutron diffraction data of densified SiO₂ glasses increased with increasing temperature, indicating the evolution of intermediate-range ordering. Furthermore, the density increase of hot-compressed SiO₂ glasses was estimated by analyzing reduced pair distribution functions. We found that the SiO₂ glass compressed at 7.7 GPa and 1300 °C is by far the most densified and structurally ordered (hyperorderd) glass in the world.

This study investigates the reaction of a Zn²⁺ aqueous solution on the surface of hydroxyapatite (HAp) containing different crystallinities using X-ray absorption near edge structure (XANES). Low- and high-crystallinity HAp were prepared using the aqueous solution precipitation method, followed by calcination. Zn-K XANES revealed that Zn²⁺ is incorporated into both high- and low-crystallinity HAp via a dissolution–reprecipitation process. In addition, ZnO is formed on the surface of high-crystallinity HAp, which indicates that the surface of high-crystallinity HAp directly immobilizes Zn²⁺. This is the second example of the direct immobilization of cations after Mg²⁺. Our findings provide a better understanding of the behavior of HAp in aqueous solutions and lead to the development of environmental purification materials.

By adjusting the addition amount of Ca₃(PO₄)₂ and boron frit, the amorphous photonic crystal structural color glazes with different colors were prepared without introducing colorants with the average heating rate of 4.1 °C/min and the firing temperature of 1300 °C. Furthermore, the effects of firing temperature and heating rate on the structural colors were studied. The results indicated that the contents of Ca₃(PO₄)₂ and boron frit were 2 and 4 %, 2 and 10 %, 3 and 8 % (mass fraction), respectively, the amorphous photonic crystal structural color glazes with purple, blue, and light cyan colors were prepared. When the firing temperature decreased from 1300 to 1250 °C, the presence of residual quartz and the uneven distribution of phase separation structures increased the brightness of structural colors and decreased the color saturation. In addition, after decreasing the average heating rate from 4.1 to 2.8 °C/min, the diopside was formed in the glazes. It absorbed iron at high temperatures and transformed into green diopside. Under the coupling effect of the chemical color generated by diopside and the structural color generated by phase separation structures, a cyan color with high color saturation appeared in the glaze.

Silica (SiO₂) porous glass materials is expected as a support in environmental fields due to transparency, high surface reactivity and so on. In this study, silica porous glass with high strength was fabricated by addition of potassium carbonate (K₂CO₃) as sintering aid and mullite as reinforcing materials. At first, a sintering condition was investigated using silica disk-type samples. A brittle amorphous silica disk with low density was obtained by firing at 1200 °C. At 1400 °C, crystallization was occurred and cristobalite formed. A small amount (∼1 wt %) of K₂CO₃ was effective to densification of amorphous silica, whereas more than 1 wt % K₂CO₃ addition accelerated the crystallization of silica. This was due to the formation of liquid phase. In addition, a highly dispersed mullite to SiO₂ matrix was effective to enhance the mechanical property. The porous structure was successfully fabricated by template method using polyurethane.

The development of precious metal-free three-way catalysts presents numerous complex challenges, among which thermal aging is a significant hurdle. The effects of different thermal aging conditions, including varying heating and cooling rates, were studied using a Cu oxide catalyst supported on γ-Al₂O₃, prepared via a conventional wet impregnation method. Intriguing results emerged after subjecting the Cu/Al₂O₃ catalyst to rapid heating and cooling during thermal aging at 900 °C under 10 %H₂O/air. Notably, the integration of Cu²⁺ into γ-Al₂O₃ resulted in the preservation of a metastable low-crystalline solid with a notably higher surface area of 88 m² g⁻¹. This solid-state transformation facilitated the preferential generation of Cu⁺ species at tetrahedral Al sites during the CO and C₃H₆ oxidation process within the stoichiometric three-way catalytic reaction (NO–CO–C₃H₆–O₂). Consequently, a higher quantity of active sites for NO reduction was obtained, achieving a remarkable >70 % NO conversion at 600 °C. Conversely, subjecting the catalyst to slow heating and cooling (at a rate of 5 °C min⁻¹) during thermal aging at 900 °C resulted in a distinct outcome. This condition produced a thermodynamically favorable phase transition from the metastable phase to highly crystalline CuAl₂O₄ and α-Al₂O₃, leading to a drastic reduction in surface area (9 m² g⁻¹) and consequently, a notable decrease in NO reduction activity (<20 % NO conversion even at 600 °C). Furthermore, regardless of the heating rate employed during thermal aging, it was observed that a slow cooling rate of 5 °C min⁻¹ rendered the catalyst susceptible to phase transitions, resulting in lower surface areas and less active catalytic products.

We investigated the relationship between the self-orientation and process conditions of self-oriented LaNiO₃ (LNO) thin films fabricated by a chemical solution deposition method. The LNO thin films were deposited on Pt-coated Si, SiO₂/Si, glass, c-plane sapphire, and stainless-steel substrates, which have crystal structures different from that of LNO. Increasing the heating rate during the transition from 200 °C (pre-heating) to 380 °C (pyrolysis) enhanced the self-orientation of the resulting LNO thin films. The LNO thin films deposited on the various substrate materials exhibited (h00) self-oriented growth because Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films deposited on the self-oriented LNO thin films clearly adopted the (h00) preferential orientation. As a unique behavior of PZT/LNO bilayer structures, the PZT upper layer has stronger (h00) oriented growth than that of the underlying self-oriented LNO layer.