Journal of the Ceramic Society of Japan

Journal of the Ceramic Society of Japan
vol. 133, no.6, 2025

◆Full paper◆

Low oxygen potential synthesis of W-type strontium ferrite SrFe₁₈O₂₇ in a closed atmosphere

Shinji Nakai, Takeshi Waki, Yoshikazu Tabata and Hiroyuki Nakamura

235

W-type strontium ferrite SrFe₁₈O₂₇ (SrW) has the largest room-temperature (RT) magnetization among iron-based non-substituted hexagonal ferrites and is a candidate for high-magnetization materials. However, the expected properties have not been achieved because, in most cases, the synthesized samples contain secondary phases or have non-stoichiometric compositions, indicating that the phase stability of SrW is not well understood. Single-phase SrW is known to be stable only at low oxygen potentials, but the stable temperature range remains unknown. In this study, we applied the sealed-tube technique to investigate the phase stability and decomposition reaction of SrW and found that SrW was formed as the predominant phase (the ratio of 97–98 wt %) when synthesized in a closed low oxygen potential atmosphere at temperatures only above 1448 K, below which M-type ferrite (SrFe₁₂O₁₉) and magnetite (Fe₃O₄) were formed instead. In contrast, once formed, SrW does not decompose by post-annealing at temperatures below the phase boundary temperature for a sufficiently long time, indicating that the decomposition reaction of SrW as a metastable phase is slow enough to ensure the availability of relatively pure SrW at RT using the sealed-tube technique.

◆Express letter◆

Formation of triclusters in silica melt under high pressure

https://doi.org/10.2109/jcersj2.25042

Shino Hayafune, Tatsuya Sakamaki, Haruki Ichikawa, Yohei Onodera, Shinji Kohara and Akio Suzuki

242

Silica (SiO₂) is the major glass-forming material, and the structures of silica glass and melt have been extensively studied using X-ray and neutron diffraction techniques. The diffraction data of silica glass and melt show the first sharp diffraction peak (FSDP) at Q ∼ 1.5 Å⁻¹, which is a signature of intermediate-range order. In this study, we performed classical molecular dynamics (MD) simulations at 2000 K and 5 GPa to understand the behaviour of the diffraction peak associated with the modification of intermediate-range order. The high-pressure melt data obtained show the decrease in the height of FSDP with a shift of the peak position to the high-Q side in X-ray diffraction data, although the average coordination number of four was maintained. In addition, we observed the formation of OSi₃ triclusters, which share an oxygen atom with a SiO₄ tetrahedron. This unusually dense packed atomic arrangement is the result of high pressure and is associated with the very sharp principal peak observed at Q ∼ 3 Å⁻¹ in the O–O partial structure factor derived by MD simulation.

◆Full papers◆

Rapid fluorination using polyvinylidene-difluoride (PVDF) for the synthesis of oxyfluoride Bi₂VO₅F₁₋_x

https://doi.org/10.2109/jcersj2.25035

Kengo Oka, Hiroko Mizutani, Ibuki Sumitani, Joichiro Murasaki, Naoki Noma and Mitsunobu Iwasaki

246

The topotactic fluorination of Bi₄V₂O₁₁ to Bi₂VO₅F₁₋_x using polyvinylidene difluoride (PVDF) was investigated. The reaction proceeded rapidly at a moderate temperature of 400 °C. In situ XRD-DSC measurement revealed that fluorination initiated above 270 °C and reached near completion by 352 °C within minutes, beyond which prolonged heating led to sample decomposition. These results underscore the feasibility of achieving topotactic fluorination at moderate temperatures with rapid ionic kinetics in ionic conductors, presenting a promising strategy for synthesizing novel mixed-anion compounds via soft chemistry.

Photoluminescence and scintillation properties of Tb-doped SrTa₂O₆ single crystals

https://doi.org/10.2109/jcersj2.25026

Yuta Tominaga, Takumi Kato, Akihiro Nishikawa, Keiichiro Miyazaki, Daisuke Nakauchi, Noriaki Kawaguchi and Takayuki Yanagida

250

We fabricated 0.1, 0.3, 1.0, and 3.0 % Tb-doped SrTa₂O₆ single crystal samples using the floating zone method to evaluate their photoluminescence and scintillation properties. The XRD measurement results showed no impurity phases and retained a single-phase structure for all samples. In addition, no clear peak shift was observed, which was thought to be due to a small amount of Tb³⁺ substituted on the Sr²⁺ site or the substitution of Tb⁴⁺ with a close ionic radius on the Ta⁵⁺ site. The photoluminescence measurements of all the samples confirmed luminescence due to the 4f–4f transitions of Tb³⁺ ions at an excitation wavelength of 280 nm, and the maximum internal quantum yield of 25.5 % was observed in the 0.1 % sample. The scintillation spectra were obtained under X-ray irradiation, and several emission peaks were observed in all the samples, originating from 4f–4f transitions of Tb³⁺ ions. The maximum scintillation integral intensity was obtained in the 0.3 % sample, which was about 47 % of the values of BGO set to 100 %.

Thermal expansion behavior of sodalite cage in cubic Ca₈[Al₁₂O₂₄](SO₄)₂

https://doi.org/10.2109/jcersj2.25018

Kana Kanefusa, Maria Zapata, Daisuke Urushihara, Toru Asaka, Yassine El Khessaimi, Youssef El Hafiane, Agnès Smith and Koichiro Fukuda

255

The thermal expansion behavior of the sodalite cage [Al₁₂O₂₄] in cubic ye’elimite, Ca₈[Al₁₂O₂₄](SO₄)₂ with space group I−43m and Z = 1, was investigated using high-temperature X-ray powder diffraction. A split-atom model previously reported for 1073 K accurately represented the disordered crystal structures at 773–973 K. This model is characterized by the splitting of the O atoms coordinating to the S atom around the triad axes within the [Al₁₂O₂₄] cage structure. As the temperature increased from 773 to 1073 K, both the cage and the unit cell expanded steadily and continuously by 0.255 %, with a coefficient of linear thermal expansion of 8.5 × 10⁻⁶ K⁻¹. Two distinct thermal expansion processes were observed above and below approximately 923 K for the [Al₁₂O₂₄] cage. Between 773 and 873 K, the values of quadratic elongation and bond angle variance, which describe the distortion of the [AlO₄] tetrahedron, increased continuously by 0.16 and 11.74 %, respectively, leading to a cage expansion of 0.073 %. Below approximately 923 K, the Al–O–Al angle of the apex-sharing [AlO₄] tetrahedra remained nearly constant. However, as the temperature increased to 1073 K, this angle increased by 0.77 %, contributing to a further 0.182 % expansion of the cage. Thus, the total expansion of 0.255 % (= 0.073 + 0.182 %) for the [Al₁₂O₂₄] cage was primarily due to the distortion of the [AlO₄] tetrahedron below approximately 923 K and the mutual rotation of the apex-sharing [AlO₄] tetrahedra above this temperature. This study identified, for the first time, the switching between these two distinct processes in the thermal expansion behavior of sodalite cages.

◆Note◆

Dielectric breakdown behavior of oxynitride glass

https://doi.org/10.2109/jcersj2.25022

Yuki Nakashima, Hirokazu Katsui, Kiyoshi Hirao, You Zhou, Tatsuki Ohji, Norimitsu Murayama and Manabu Fukushima

264

This study presents the first direct measurement of the dielectric breakdown strength (DBS) of oxynitride glass, providing new insights into its electrical insulation properties. Oxynitride glass was prepared using a blend of Si₃N₄, SiO₂, Y₂O₃, and MgO powders and formed into thin glass substrates of varying thicknesses. The DBS measurement was conducted by using circular column shape and McKewon electrodes. DBS measurements revealed that the oxynitride glass exhibited consistently slightly lower DBS values than Si₃N₄ sintered bodies across all tested thicknesses. This finding suggests that DB in Si₃N₄ predominantly occurs within the grain boundary glass phase. These results, aligning with previous studies on the dielectric breakdown mechanisms of silicon nitride ceramics, confirmed the critical role of the grain boundary glass phase in determining their electrical insulation performance.

◆Announcement◆

Call for a Guest Editor for the Feature

https://doi.org/10.2109/jcersj2.133.A6-1

A6-1