BaTiO₃-based ferroelectric family is one of the most studied ferroelectric materials for tunable microwave devices. However, a good tunable material should exhibit low dielectric losses at a high operating frequency and high stability of the dielectric tunability over a wide temperature range, which are conflicting parameters. This study aims to experimentally demonstrate the effects of thickness and temperature on the tunable dielectric properties of epitaxial Ba(Zr_0.2Ti_0.8)O₃ [BZT] films deposited on (100) SrTiO₃ [STO] substrates by the pulsed laser deposition. The crystal structure and electrical properties of the BZT films with different film thicknesses were investigated. Polarization–electric field curves and relative dielectric permittivity–electric field loops (ε_r–E) were measured to characterize the effect of film thickness on the dielectric tunability of the BZT films. Further, the ε_r–E loop was also characterized as a function of temperature. The coercive field decreased with increasing film thickness above 430 nm. The tunability values in the BZT film show little variations with temperature, indicating good temperature stability from −100 to 100 °C. These results show that the BZT film is a promising material for tunable capacitors.

Titanite-type oxides, ABMO₅, have been recently recognized as a novel vein of antiferroelectric (AFE) materials due to their unique crystal structure including an antiparallel arrangement of polar chains. This study presents the results of synthesis, structure analysis, and dielectric measurement of titanite-type solid solutions (Ca,Pb)TiGeO₅. With the Pb composition increasing, structural symmetry at room temperature changes from monoclinic to triclinic. We identified the crystal structure of PbTiGeO₅ by Rietveld refinement similar to that of triclinic titanite-type CaZrGeO₅. In addition, the AFE phase transition temperature is decreased by Pb substitution. These indicate the competition between triclinic and AFE distortions, in other words, lone-pair electrons on Pb²⁺ ions and the second-order Jahn–Teller effect on the Ti⁴⁺ ions.

Thin films of all-proportional PZT system, i.e., Pb(Zr_yTi_1.00−y)O₃ with y = 0–1.00, were deposited by hydrothermal deposition with a low reaction temperature of 150 °C. The PZT films were grown epitaxially on the surface of (100)SrTiO₃:La substrates via a microwave-assisted hydrothermal reaction using various input ratios of precursor chemicals x = [ZrOCl₂]/([ZrOCl₂] + [TiO₂]), which included more Zr ions (or less Ti ions) with a higher y value than that of the input ratio x. Dielectric and ferroelectric parameters for the resulting films, i.e., the relative dielectric constant (ε_r) and remanent polarization (P_r), exhibited maxima at the certain Zr/Ti ratio y = ∼0.60 though these parameters were relatively lower than those of the conventional films.

We prepared (001) oriented Cd_0.9Mg_0.1O thin films on soda-lime glass substrates using pulsed laser deposition (PLD). Use of the (001) oriented Cd_0.9Mg_0.1O thin film as a buffer layer was attempted to prepare VO₂(R) (rutile) thin film. Because the lattice mismatch between the a-axis of VO₂(R) (a = 0.4555 nm) and Cd_0.9Mg_0.1O (a = 0.4651) is slight (−2.06 %), we expected a (001) oriented VO₂(R) thin film on Cd_0.9Mg_0.1O buffered glass. The results indicate (110) orientation, which is likely attributable to the influence of the surface energy of VO₂(R). In the rutile structure, the (001) plane is unstable because the coordination number of the cations at the outermost surface is low. In contrast, the (110) plane has the lowest surface energy. Therefore, the surface energy of VO₂(R) is considered to exert a stronger effect than the interaction between the substrate and the thin film. The temperature dependence of resistivity of VO₂ thin films with thickness deposited on Cd_0.9Mg_0.1O buffered glass substrates revealed that no metal–insulator transition was observed for thinner VO₂ thin film (thickness less than 59 nm). For large thickness (greater than 82 nm), then a clear metal–insulator transition was observed. This phenomenon was explained in terms of stress and oxygen vacancy. A similar result was obtained from measurement of the temperature dependence of IR transmittance with thickness. For 94-nm-thick VO₂ thin film, IR transmittance change as high as 53.9 % was obtained.

This study reports on the dielectric, piezoelectric, and microstructural properties of Pb(Mg_1/3Nb_2/3)O₃–0.305PbTiO₃ (PMN-PT) single crystals (SCs) prepared by the one-charge Bridgman method, focusing on the effects of varying AC cycles, 1 to 1600, of alternating current poling (ACP) on the piezoelectric properties. Optimal properties were obtained at a low electric field of 3 kVrms/cm with 80 ACP cycles of a 1.0 Hz bipolar sine wave, resulting in a dielectric constant (ε^T₃₃/ε₀) of 14700, a piezoelectric constant d₃₃ of 4200 pC/N and a low dielectric loss of 0.24 %. These superior properties are among the best reported to date for piezoelectric SCs whose phase change temperatures exceed 75 °C. Scanning electron microscopy analysis revealed significant microstructural changes, including the formation of finely striped 1 to 4 µm 109° domain wall patterns (with an average width of 1.5 µm), which developed with increasing ACP cycles. A significant decrease in piezoelectric properties—low ε^T₃₃/ε₀ of 9500 and d₃₃ of 2500 pC/N—were observed after 800 cycles, correlating with the microstructural change. The study demonstrates that precise control of ACP conditions is essential to achieve the desired piezoelectric properties in PMN-PT SC materials. These findings provide critical insights for optimizing the properties of PMN-PT SCs and their potential for high-performance piezoelectric applications.

Tin-doped indium oxide (ITO) thin films are used extensively, particularly in flat-panel displays and photovoltaic cells, because of their exceptional electrical conductivity and high visible-light transmittance. However, ITO is expensive because one of its components is In, which is a rare metal, making a stable supply of ITO difficult to attain. The objective of the present study was to reduce the use of In in ITO by combining it with Al₂O₃, which is relatively inexpensive and highly transparent, and exhibits high mechanical strength. The resulting ITO–Al₂O₃ composite thin films were fabricated by aerosol deposition. The resultant films exhibited a resistivity of 4 × 10⁻³ Ω cm and a visible-light transmittance of 87 % at a wavelength of 600 nm, which are similar to the values for ITO films deposited by the same AD method. The resistivity remained almost constant with increasing volume fraction of Al₂O₃ in the composite thin film until the volume fraction reached 66 %, beyond which a notable increase in resistivity was observed. An increase in the gas flow rate resulted in a reduction in resistivity, with a strong correlation observed between the resistivity and the crystallite size determined by the Williamson–Hall method. Elemental analysis of the cross-section of a composite thin film indicated the presence of In and Al. The distribution of In was found to become more widespread and denser with increasing gas flow rate, indicating densification of the film. The visible-light transmittance exhibited a minimal dependence on the Al₂O₃ volume fraction.

The dependence of ferroelectric properties on composition and film thickness was investigated for CeO₂–HfO₂ films with various ratios x = Ce/(Hf + Ce), deposited on polyethylene terephthalate coated with an amorphous indium tin oxide layer (amorphous ITO/PET) substrates without substrate heating. Additionally, the dependence of composition on piezoelectric properties was examined. X-ray diffraction (XRD) analysis showed that the films consisted of a tetragonal phase (T.) and/or orthorhombic (O.) phases with (111) and {100} out-of-plane orientations for the films with thicknesses ranging from 440 to 910 nm at x = 0.23. In addition, the remnant polarization (P_r) increased from 6.0 to 11.7 µC/cm² with the increase in the film thickness. To investigate the composition dependence, CeO₂–HfO₂ films with a thickness of 800–1000 nm and x values of 0.17, 0.18, 0.23, and 0.28 were prepared. XRD showed (111) and {100} orientations in all films. Clear P-E hysteresis loops and switching currents were observed for x = 0.18 and 0.23, with P_r exceeding 10.0 µC/cm², even in films deposited on amorphous ITO/PET without substrate heating. Moreover, these films exhibited piezoelectric properties consistent with P-E loops, showing their potential for flexible piezoelectric applications.

This study investigates the influence of synthesis conditions on electrode performance by growing TiNb₂O₇ (TNO) single crystals, conducting single-crystal X-ray structural analysis, and comparing the resulting structures. The findings revealed that the site preferences of Ti⁴⁺ and Nb⁵⁺ within the five octahedral sites (M1–M5) remained unaffected by the synthesis atmosphere at low temperatures below 800 °C. However, an increase in synthesis temperature from 800 °C onwards enhanced the site preference, particularly at the M1 and M5 sites, displaying significant changes. Based on the anisotropy of ion conduction and the conduction mechanism of TNO, it is proposed that ion conduction is facilitated by the positioning of Ti⁴⁺ at the M1 site, which is characterized by many shared edges within the tunnel structure and plays a critical role in enabling Li-ion conduction. While the site selectivity of Ti⁴⁺ and Nb⁵⁺ remained constant across synthesis atmospheres, the observed increase in the lattice constant and electronic conductivity contributed to the charge–discharge characteristics. In contrast, the sample synthesized at 800 °C, which exhibited the highest charge–discharge performance among the temperature-varied samples, had the highest occupancy rate of Ti⁴⁺ at the M1 site. This occupancy is hypothesized to underpin its superior electrochemical performance.

(Bi_0.5Na_0.5)TiO₃ [BNT] ceramics can be sintered at low temperatures of 940 °C with CuO addition. This is believed to reduce Bi³⁺ volatilization and improve the quality of BNT-based ceramics by lowering the number of oxygen vacancies. However, there is no direct evidence that low-temperature sintering prevents the formation of oxygen vacancies. In this study, the formation of oxygen vacancies by ¹⁸O tracer diffusion and the additive effect of CuO on this formation in BNT ceramics were examined using secondary ion mass spectrometry (SIMS). In addition, Nb₂O₅, which acts as a donor, was added to the CuO-doped BNT ceramics. The formation of oxygen vacancies was discussed, and the electrical properties were clarified. As a result, the volume diffusion coefficients D of the BNT ceramics with 0.5 wt % CuO (Cu0.5) were 2.0 × 10⁻¹¹ cm²/s. This value is equivalent to pure BNT, which means that for BNT ceramics, CuO acts as an acceptor, suggesting that BNT ceramics still contain many oxygen vacancies. On the other hand, the D of ¹⁸O in Cu0.5 added 0.4 wt % Nb₂O₅ was 3.5 × 10⁻¹⁵ cm²/s, indicating that the formation of oxygen vacancies is suppressed. Moreover, Nb₂O₅ enhanced the poling treatment and the coercive field E_c decreased, indicating a softening trend.

The influence of A-site substitution in pyrochlore-type neodymium zirconate with smaller rare earth ions on dielectric properties was investigated. First-principles calculations for [Nd₁₆Zr₁₆O₅₆] and [Nd₁₅MZr₁₆O₅₆] (M = Sm, Gd, Y, Er, or Sc) predicted that Gd, Y, and Er have two types of six equivalent stable positions displaced from the original Nd position, generating a dipole moment. In addition, the energy barrier between these stable positions is below 10⁻² eV, suggesting a potential enhancement of the dielectric constant. To validate the impact of substitution on dielectric properties, (Nd_1−xGd_x)₂Zr₂O₇ (NGZO, x = 0.00, 0.0625, 0.25, 0.50, 0.75, 1.00) and (Nd_1−xEr_x)₂Zr₂O₇ (NEZO, x = 0.00, 0.0625, 0.15, 0.25, 0.50, 0.75, 1.00) were synthesized using the solid-state reaction method. X-ray diffraction confirmed the pyrochlore structure for NGZO (except at x = 1.00) and NEZO (for x ≤ 0.25). The lattice constant followed Vegard’s law within the compositional range of the pyrochlore phase. The dielectric constant slightly increased with increasing x (≤0.25) in NGZO, implying the contribution of dipole moments between Gd and its surrounding O ions.

Alternating current poled (ACP) (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (PMN–xPT) single crystals (SCs) exhibited 8–49 % higher piezoelectric strain constant length extensional mode (d₃₁) at room temperature than direct current poled (DCP) SCs. However, thermal stability of these d₃₁ was 13 to 69 °C lower for ACP SCs than for DCP SCs, indicating lower thermal stability. These results were obtained for four rhombohedral compositions of PMN–xPT SCs (x = 0.26, 0.295, 0.305, and 0.33) prepared by the one-charge Bridgman method. Microstructures observed by scanning electron microscopy showed that the average 109° domain walls width (ADW) gradually increased proportionally with increasing PT from PMN–0.26PT to PMN–0.33PT, with the highest d₃₁ (−1700 pC/N) in the PMN–0.305PT SC. The PMN–0.26PT and PMN–0.33PT SCs showed almost the same piezoelectric properties, but their microstructures, ADW, and thermal stability were clearly different, indicating that ADW is not the only primary factor determining high piezoelectric properties. These experimental results suggest that accurate control of PMN–xPT SC composition, SC growth method, and optimal ACP condition setting are essential to simultaneously satisfy excellent piezoelectric properties and thermal stability after ACP.

Solid electrolyte Ta-substituted Li₇La₃Zr₂O₁₂ (LLZTO) epitaxial thin films were grown using LiOH-flux liquid phase epitaxy at 600 °C under the flow of N₂ and H₂O vapor mixture. Prior to the liquid phase epitaxy, amorphous La–Zr–Ta–O thin films were deposited on garnet-type Ga₃Gd₅O₁₂ (GGG) substrates by dynamic aurora pulsed laser deposition for the thickness limitation and uniform film formation. Obtained thin films after liquid phase epitaxy, include Li content by X-ray photoemission spectroscopy. The epitaxial growth of LLZTO thin films on GGG(001), (110) and (111) substrates were confirmed by pole figures. The thin films had relatively low out-of-plane orientation degree despite liquid phase method. The surface observation reveals that the thin film surfaces consist of radially spreading regions with about 5–20 µm, which implies that the growth condition in this study was at higher supersaturation degree.

The discovery of elastic deformation induced mechanoluminescence (ML) has sparked significant research interest in developing materials capable of emitting both visible and invisible light for sensing and lighting applications across nano- to macro-scale systems. Among these, ML materials with near-infrared luminescence have garnered particular attention for medical and biological applications, as the scattering and absorption of biological tissues were reduced in near-infrared light. Zinc oxide (ZnO) is a promising candidate due to its high biocompatibility. Recent density functional theory (DFT) calculations have revealed that co-doping ZnO with Li and Nd leads to crystal softening, suggesting enhanced deformation susceptibility and improved luminescence efficiency. In this study, ZnO co-doped with Li and Nd was successfully synthesized, and near-infrared ML from a ZnO-based host material has been demonstrated for the first time.

The photoluminescence properties of solid solutions of YNbO₄, GdNbO₄, and LuNbO₄ were investigated to determine the rare earth composition that maximizes the intrinsic luminescence intensity. All samples exhibited a monoclinic β-fergusonite-type structure without impurity phases. The intrinsic luminescence intensity followed the order of GdNbO₄, YNbO₄ and LuNbO₄, with GdNbO₄ showing significantly low intensity due to concentration quenching and self-absorption by Gd³⁺. Among the solid solutions, (Y,Lu)NbO₄ showed higher luminescence intensity than LuNbO₄, with Y_0.75Lu_0.25NbO₄ exhibiting the highest intensity, approximately 42 % higher than LuNbO₄. Fluorescence lifetime measurements of (Y,Lu)NbO₄ showed single-component decay curves with lifetimes ranging from 3.3 to 4.3 µs, with no significant differences or correlation with luminescence intensity.

Mechanoluminescent materials exhibit light emission in response to mechanical stimuli. Its intensity is expected to be correlated with the local symmetry around the luminescent center. This study aimed to quantitatively investigate the correlation between the D index, representing the distortion of the oxide ion coordination polyhedron around the luminescence center, and mechanoluminescence (ML) intensity, focusing on four representative ML materials. The materials studied, including SrAl₂O₄:Eu (SAO), Sr₃Sn₂O₇:Sm (SSS), (Li,Na)NbO₃ (LNNO), and Sr₃Sn₂O₇:Nd (SSN), exhibited visible and near-infrared ML. For these materials, the average D index, considering the occupancy and multiplicity of luminescent center, was determined from the Rietveld analysis of X-ray powder diffraction patterns, and a quantitative comparison was made with ML intensity. A quantitative correlation between the D index and ML intensity was observed within the same host crystal structure. This demonstrates that the D index is an effective indicator in material development to enhance ML intensity. The study revealed that when multiple host ion sites are available for the luminescence center, preferentially substituting the sites with a lower D index is crucial. This research provides valuable insights for developing high-performance ML materials.

First-principles and molecular dynamics simulations are used to understand the stability and defect state of vanadium ions at BaTiO₃ grain boundaries. Qualitative analysis is performed based on two approaches to approximate polycrystalline grain boundaries. First, stable grain boundaries are modeled to evaluate the defect formation energies of grain boundary sites and bulk sites. Second, amorphous BaTiO₃ is used to analyze the relationship between defect structure and stability of vanadium. The electronic states of vanadium atoms in the amorphous state are statistically analyzed to understand the effect of complex structures such as grain boundaries. These results suggest that vanadium acts as a donor at grain boundaries.

The optimization of kaolinite [Al₂Si₂O₅(OH)₄] as a raw materials for solid-state reactions plays a crucial role in the formation of high-quality ceramics. The earliest discover in this field was the production of thin-walled yet remarkably strong Chinese porcelain, which primarily made from kaolinite-rich clay. Despite kaolinite’s poor plasticity and low dry strength, its exceptional hand formability was achieved through the practice of slurrying and storing of clay with urine in large pits in traditional Chinese porcelain-making techniques. This “ageing” process, along with a focus on “kaolinite’s layered structure”, led to the intercalation of urea—a component produced by urine decay during “ageing”—between kaolinite layers, inducing their expansion. As a result, rigid kaolinite platelets were transformed into more thinner ones, and some forming wrinkled aggregates, ultimately enhancing plasticity and facilitating precise hand shaping. When used as a raw material, kaolinite undergoes calcination in the presence of other compounds, initiating solid-state reactions that contribute to ceramic formation. In industrial applications, where cost-effectiveness is prioritized, these reactions are carried out in the presence of by-products. Additionally, its layer expansion has never been optimized for solid-state reactions. Thus, kaolinite’s layered structure should be further “harnessed” in ceramics research to maximize its potential for future applications. This perspective also offers a pathway to elucidating kaolinite’s solid-state reactions, particularly that through the solid-state reactions of CaAl₂Si₂O₈ and BaAl₂Si₂O₈ polymorphs. In this review, recent studies—beginning with the first major optimization of kaolinite’s layered structure for solid-state reactions in 2021, following its foundational discovery in 1963—are introduced. Kaolinite intercalation chemistry emerges as a promising approach to optimizing kaolinite as a raw material for solid-state reactions, with particular attention given to layer expansion to disrupt the stacking order, the role of impurities in reaction dynamics, and the adsorption at edge surfaces, as well as the influence of morphology and crystallinity.

Lead-free (1 − x)(Bi_0.5Na_0.5)TiO₃–xBaTiO₃ (abbreviated as BNT–BT, x = 0.05–0.55) ceramics were synthesized by solid-state reaction, and their crystal structures, dielectric and ferroelectric properties were investigated in detail. For poling treatment, a special poling strategy was conducted, which is applying a direct current electric field above the Curie (or phase transition) temperature on the BNT–BT ceramics. The Curie (or phase transition) temperatures of BNT–BT samples were determined by temperature dependence of polarization versus electric field measurement. The piezoelectric coefficient d₃₃ of poled BNT–BT samples were calculated from their resonance curves. Among them, the 0.875BNT–0.125BT was considered an interesting composition due to the relatively high phase transition temperature (>200 °C), and it showed a d₃₃ value of 88.6 pC/N and a coupling factor k₃₃ of 38.8 % after poling above its phase transition temperature. This work may contribute to optimizing the poling treatment for BNT–BT system.

Single phase NaLuO₂, which crystallizes in the α-NaFeO₂-type structure (trigonal, space group R3m), was synthesized by heating a mixture of Na₂O and Lu₂O₃ at 1150 °C. The crystal structure of this oxide was analyzed using the Rietveld method for the powder X-ray diffraction pattern, and the oxygen atom coordinate [z = 0.2397(3)] was refined. Mixtures of NaLuO₂ and europium monoxide with a composition NaLuO₂–xEuO (x = 0.0, 0.01, 0.1, 1.0 mol %) were heated at 900 °C for 12 h in a N₂ atmosphere. The obtained samples were Eu²⁺ and Eu³⁺-doped NaLuO₂ with trace amounts of Lu₂O₃. The emission spectrum peak due to the 5d → 4f transition of Eu²⁺ occurred at a wavelength of 617 nm under the excitation of a 442 nm blue light, and the full width at half maximum of the spectrum was 80 nm. Intense emission peaks due to the f-f transition of Eu³⁺ were observed at approximately 590 nm when excited by a 230 nm ultraviolet light.

A self-cleaning flat transparent thin photocatalyst film was prepared on a bare soda-lime glass by a simple method using niobium alkoxide solution, which is a common coating solution for the sol–gel method. The film consisted of crystalline NaNbO₃ and Na₂Nb₂O₆·H₂O phases. It was suggested that NaNbO₃ and Na₂Nb₂O₆·H₂O were directly formed between the soda-lime glass and the niobium alkoxide coating solution during the heat treatment. Under UV irradiation, the film surface exhibited low photocatalytic oxidation activity and excellent photo-induced hydrophilicity. The hydrophilic state of the sample was maintained for 1 month in the dark, while the hydrophilicity of TiO₂ sample prepared by a sol–gel method was decreased within 5 days in the dark. Additionally, the surface demonstrated excellent underwater oil repellency toward n-hexadecane and oleic acid and the ability to remove the adsorbed oily contaminant in water. These properties were also superior to those of the TiO₂ surface.

Chitosan/silica organic–inorganic hybrid gas barrier membranes were prepared by sol–gel method on plastic films using tetramethoxysilane, 3-glycidoxypropyltrimethoxysilane, chitosan, and cross-linked structures of chitosan were introduced by cross-linking reaction using citric acid (CA) and malic acid (MA). Water vapor transmission rate (WVTR) of the membranes was evaluated by dish method. Oxygen permeation through the membranes was also measured by a variable-pressure method. WVTR of the hybrid layers were lower than that of poly(vinylidene chloride) (PVDC) and oxygen permeability coefficients of the hybrid layer was small and about one-third to half of that of PVDC. These properties were thought to be due to well dispersion of inorganic segments (silica) and organic segments (chitosan) at molecular level and the formation of cross-linked structures in the hybrid. Pencil hardnesses (750 g load) of the hybrid layers coated on polyethylene terephthalate (PET) films were HB. These values were higher than that of PET film (B). Light transmittance of the chitosan/silica organic–inorganic hybrid gas barrier membranes (CA100 and MA110) coated on PET film were higher than PET film. The hybrid gas barrier membranes are transparent, rigid, flexible and have excellent water vapor and oxygen barrier properties.

Design of Mino ware porcelain clay is undertaken to solve a multi-objective optimization problem of achieving balance among physical properties such as plasticity, pyroplastic deformation, firing color, linear shrinkage, and the coefficient of linear thermal expansion, and raw material cost. The designer seeks a Pareto solution through repeated empirical trial and error. For this study, we attempted to solve this problem using Monte Carlo simulation (MCS). An MCS computer program was created that uses the pyroplastic deformation number obtained by finite element analysis as an indicator function, implements machine learning, and interprets and executes three commands that convey the designer’s intentions. The MCS effectiveness was confirmed by examining the number of Pareto solutions satisfying the four objectives of plasticity, pyroplastic deformation, firing color, and cost using a porcelain clay design that is actually used in Mino ware. When the number of pseudorandom numbers generated by the MCS was set as 2¹³ per batch and 10 samples of 10 batches were run, the arithmetic means of the Pareto solution numbers were 13–1158, depending on the porcelain clay design and command combination.

Ethylene glycol (EG) was added to an aqueous solution of nickel nitrate and that of nickel and lanthanum nitrates, and Ni/Al₂O₃ and Ni–La/Al₂O₃ catalysts were prepared by an impregnation method using the respective aqueous solution and commercial alumina. It was shown, by CO chemisorption, TEM, and STEM-EDS mapping, that finer nickel particles were formed by EG, and improved Ni dispersion and decreased distribution width of nickel diameter profile were also obtained. Owing to the fine nickel particles, carbon formation during CH₄–CO₂ reforming was suppressed. It was speculated that EG bound and/or interacted with surface hydroxyl groups and coordinately unsaturated Al³⁺ sites of alumina, thereby aggregation of nickel and lanthanum during the decomposition of the nitrates was suppressed due to the physical interfering by EG.

In this research, the synthesis of needle-like aragonite, which is one of CaCO₃ polymorphs was conducted to utilize as a filler after CO₂ fixation to CaCO₃ in the CaCl₂–Na₂CO₃–H₂O reaction system without additives. Influences of temperature and solution concentration of reaction on aragonite particle size were investigated. When the concentration was 0.05 mol·dm⁻³, aragonite single phase was synthesized at or above 60 °C. On the other hand, calcite and vaterite coexisted in the main component of aragonite phase at 0.01 mol·dm⁻³ and 80 °C. From SEM observation, obtained aragonite shape was needle-like, and its particle size was from 4.3 to 8.8 µm when reaction conditions were from 60 to 85 °C and 0.01 to 0.05 mol·dm⁻³. Aragonite length was tended to be large with increasing the reaction temperature and decreasing the reaction concentration.