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Shuya Sato, [Masashi Miyakawa](https://orcid.org/0000-0002-0838-8156), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Yohei Onodera](https://orcid.org/0000-0002-3080-6991), Koji Ohara, Kazutaka Ikeda, Naoto Kitamura, Yasushi Idemoto, [Shinji Kohara](https://orcid.org/0000-0001-9596-2680)

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[Synthesis of hyperordered permanently densified silica glasses by hot compression above the glass transition temperature](https://mdr.nims.go.jp/datasets/bf73bd9f-1b2d-4224-aff5-468d3ca1dad3)

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Synthesis of hyperordered permanently densified silica glasses by hot compression above the glass transition temperatureFULL PAPERSynthesis of hyperordered permanently densified silica glassesby hot compression above the glass transition temperatureShuya Sato1,2, Masashi Miyakawa3, Takashi Taniguchi3, Yohei Onodera2, Koji Ohara4,Kazutaka Ikeda5, Naoto Kitamura6, Yasushi Idemoto6 and Shinji Kohara2,1,³1Graduate School of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278–8510, Japan2Center for Basic Research on Materials, National Institute for Materials Science, 1–2–1 Sengen, Tsukuba, Ibaraki 305–0047, Japan3Research Center for Materials Nanoarchitectonics, National Institute for Materials Science,1–1 Namiki, Tsukuba, Ibaraki 305–0044, Japan4Faculty of Materials for Energy, Shimane University, 1060 Nishikawatsu-cho, Matsue 690–8504, Japan5Neutron Industrial Application Promotion Center, Comprehensive Research Organization for Science and Society (CROSS),162–1 Shirakata, Tokai, Ibaraki 319–1106, Japan6Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278–8510, JapanSynthesizing densified glasses with structure ordering is an important issue for the development of new opticalfibers with high refractive index and low dispersion. Herein, we report on our attempt to synthesize densifiedsilica (SiO2) glasses by hot compression at a pressure of 7.7GPa and temperatures above 1200 °C. We succeededfor the first time in recovering densified SiO2 glasses compressed at 7.7GPa and 1300 °C. Samples compressedabove 1300 °C were crystallized into coesite by heterogeneous nucleation. The height of the first sharp diffractionpeak in high-energy X-ray and neutron diffraction data of densified SiO2 glasses increased with increasingtemperature, 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 SiO2glass compressed at 7.7GPa and 1300 °C is by far the most densified and structurally ordered (hyperorderd)glass in the world.Key-words : Densified glass, Hot compression, Structure, X-ray diffraction, Neutron diffraction[Received February 19, 2024; Accepted May 10, 2024; Published online June 7, 2024]1. IntroductionThe theory of glass transition1) is one of the importanttopics in glass science, and unraveling the glass structure isthe first step to understanding the origin of glass transition.Silica (SiO2) is a prototypical network-forming oxidematerial, and the SiO2 glass has been the most intensivelystudied glass from ambient to extreme conditions such ashigh temperatures2),3) and high pressures,4)–25) since liquidSiO2 is a “strong liquid26)”, which results in a glass-forming material. In particular, a permanently densifiedSiO2 glass is an essential scientific target to understand“polyamorphism27)–29)” and the formation of a “perfectglass30)–32)”.The synthesis of a permanently densified glass at hightemperatures and pressures was reported by Mackenzie in1963.33),34) Inamura et al. reported the structure and dy-namics of a cold-compressed densified glass analyzed byneutron diffraction,11),35) Raman spectroscopy,11) and neu-tron inelastic scattering.35) They confirmed that a networkcomprising SiO4 tetrahedra with the corner sharing of oxy-gen atoms is stable up to ³20% densification. The modi-fication of an intermediate-range structure can be ascribedto the decrease in cavity volume associated with the modi-fication of intermediate-range ordering. They also ascribedthe modification to the suppression of low-energy dynam-ics on the basis of their analysis of the behavior of neutrondynamical structure factors. Their conclusion is supportedby a recent study by Wakabayashi et al.36) using Ramanspectroscopy and X-ray diffraction. For in situ measure-ments, Sato and Funamori17),20) reported the results of theirX-ray diffraction measurements at pressures up to 100GPaat room temperature (RT) and they observed the trans-formation from SiO4 tetrahedra to SiO6 octahedra at 35–40GPa. Zeidler et al. pointed out the role of SiO5 polyhedrain tetrahedral-to-octahedral transformation on the basis ofthe results of neutron diffraction and classical moleculardynamics (MD) simulation.21) Conversely, the structuralmodification of SiO2 glasses at high pressures and hightemperatures is not so well understood owing to the lack ofexperimental information.The most important work was conducted by Inamuraet al. at high pressures up to 9.9GPa, in which the modi-³ Corresponding author: S. Kohara; E-mail: KOHARA.Shinji@nims.go.jpJournal of the Ceramic Society of Japan 132 [7] 427-433 2024DOI https://doi.org/10.2109/jcersj2.24013 JCS-Japan©2024 The Ceramic Society of Japan 427This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.24013https://creativecommons.org/licenses/by/4.0/fication of the first sharp diffraction peak (FSDP) by in-creasing temperature up to 800 °C was observed.16) Theypredicted the formation of a stable structure at high pres-sures, but they were unable to recover the glass underambient conditions for consistency. Trachenko and Doveperformed classical MD simulation under high pressuresand high temperatures and observed rebonding during den-sification.12) They also found an analog of the “reversibilitywindow” observed in chalcogenide glasses.13) Huang andKieffer studied amorphous–amorphous transitions by clas-sical MD simulation and discussed thermomechanicalanomalies14) and the densification limit.15) However, themodification of the FSDP induced by a high temperatureand a high pressure, and the amorphous–amorphous tran-sition are still unsolved issues.Guerette et al. reported the fabrication of densifiedglasses at 1100 °C and up to 8GPa.37) The density of theglasses they densified at 1100 °C and 8GPa is ³25%larger than that of a pristine glass. Furthermore, a Young’smodulus increase of ³71% relative to that of the pristineglass under ambient condition was achieved. Althoughthey carried out X-ray diffraction measurements and MDsimulation, structural differences between hot- and cold-compressed glasses are still unclear. Accordingly, it is im-portant to clarify the structure of SiO2 glasses synthesizedabove the glass transition temperature Tg of ³1180 °C todevelop methods of synthesizing perfect glasses.30)–32)Previous studies on densified SiO2 glasses have beenreviewed in detail by Kapoor et al.38)Onodera et al. have recently reported the structure anddensity of SiO2 glasses recovered after hot compression ata pressure of 7.7GPa and temperatures up to 1200 °C.39)They observed FSDP evolution in the X-ray diffractiondata of glasses compressed at 7.7GPa and a temperaturehigher than 400 °C. The sharpest FSDP was observed inthe X-ray diffraction data of glasses compressed at 1200 °Cand 7.7GPa. They also prepared a densified glass by coldcompression at a pressure of 20GPa and RT. Although thedensities of those two glasses are the same (hot-compressedglass, 2.72 g/cm3; cold-compressed glass, 2.71 g/cm3),FSDP evolution is only observed in the X-ray diffractiondata of the hot-compressed glass. In addition, the hot-compressed glass was stable for at least 1.5 years underambient conditions, whereas the cold-compressed glassshowed a reduction in density by 2.8% after 1.5 years,suggesting that a permanently densified SiO2 glass can beobtained only by hot compression.In this article, we report on the synthesis of densifiedSiO2 glasses by hot compression at a pressure of 7.7GPaand temperatures above 1200 °C because samples com-pressed at 7.7GPa and above 1200 °C were found to crys-tallize by heterogeneous nucleation in a previous study.39)The structure of densified SiO2 glasses was probed byhigh-energy X-ray diffraction and neutron diffraction mea-surements in conjunction with density analysis employingthe reduced pair distribution function.2. ExperimentsDensified glasses were prepared from cylindrical (5.0mm diameter, 1.8mm thick) synthetic fused SiO2 (ES,Tosoh Corp.). The prepared glass sample were designatedaccording to their processing conditions, e.g., 1300 °C/7.7GPa refers to the glass sample recovered after hot com-pression at 1300 °C and 7.7GPa. The glasses were syn-thesized at a pressure of 7.7GPa and temperatures up to1500 °C using a belt-type high-pressure apparatus (FB30H,NIMS)40) which can generate pressures up to about 8GPa.Since the applied pressure value was confirmed by thephase transition of Bi at 7.7GPa, the pressure at synthesiswas set to 7.7GPa. A schematic of the apparatus is shownin Fig. 1(A). Because the apparatus compresses a sam-ple through its upper and lower anvils, it has excellent108642018012060012001000800600400200AnvilCylinderAnvil5 mm(A)(C)(B)Pressure (GPa)Time (min)Temperature (C)PristineHot-compressed SiO2 glass(1300 C/7.7 GPa)Crystallized SiO2 glassFig. 1. Synthesis of densified SiO2 glass. (A) Configuration of belt-type high-pressure apparatus. (B) Synthesisconditions. (C) Synthesized samples.Sato et al.: Synthesis of hyperordered permanently densified silica glasses by hot compression above the glass transitiontemperatureJCS-Japan428hydrostatic pressure and can retrieve relatively large sam-ples for a high-pressure generator. To remove surface im-purities that cause heterogeneous nucleation, the startingglass was soaked in hydrofluoric acid and hydrochloric acidfor 10min each, dried, and then sonicated with acetone for10min to clean its surface. The cleaned glass was encap-sulated in a copper sleeve, tantalum foil, and graphite lid.The copper sleeve and tantalum foil, which forms theinterface with the glass, are surface-treated by the sameprocess as the starting glass. The synthesis conditions areshown in Fig. 1(B). The glass was compressed at a pressureof 7.7GPa and then heated from 25 to 300 °C within 10minand from 300 °C to target temperatures of 400, 1200, 1300,1400, and 1500 °C within ³1min. The target temperaturewas maintained for 30min before the sample was cooled to300 °C within 5min and from 300 °C to RT within 10min.Finally, the pressure was slowly released and the samplewas recovered. The picture of the pristine glass, the glassdensified at 1300 °C and 7.7GPa, and the crystallized sam-ple is shown in Fig. 1(C). It can be seen that the densifiedglass shows internal cracks whereas the pristine glass istransparent.High-energy X-ray diffraction experiments were carriedout on the BL04B241) beamline at SPring-8 (Hyogo, Japan)using the diffractometer dedicated to disordered materials.The incident X-ray energy was 61.24 keV. The diffractionpatterns of glasses were measured in transmission geom-etry. The intensity of incident X-rays was monitored in anAr-filled ionization chamber, and the scattered X-rays weredetected by four CdTe detectors and three Ge detectors.A vacuum chamber was used to suppress air scatteringaround a sample. The raw data obtained were corrected forpolarization, absorption, and background signals, and thecontribution of Compton scattering was subtracted usinga standard data analysis program.42) Neutron diffractionmeasurements were conducted using on a NOVA diffrac-tometer43) installed on the BL21 beamline of the Materialsand Life Science Facility (MLF) at the J-PARC spallationneutron source (Ibaraki, Japan). The wavelength range ofthe incident neutrons was 0.12¡ < ­ < 8.3¡. The glasssample was transferred into a V–Ni null alloy cell with anouter diameter of 6.0mm and a thickness of 0.1mm. Eachsample held in a V–Ni cell, the empty container, the emptyinstrument, and a vanadium standard for normalizationpurposes were measured. The scattering intensity observedin the sample was corrected for the background and atten-uation signals of the sample and cell, and then normalizedby the incident beam profile. The corrected data sets werenormalized to obtain the Faber–Ziman44) total structurefactor S(Q). The reduced pair distribution function G(r)was obtained by a Fourier transform of S(Q),GðrÞ ¼ 2³Z QmaxQminQ½SðQÞ � 1� sinðQrÞdQ: ð1Þ3. Results and discussionFigure 2(A) shows the X-ray and neutron total structurefactors, S(Q) of the pristine glass and a series of hot-compressed SiO2 glasses compressed at 7.7GPa and dif-ferent temperatures. Note that the X-ray S(Q) are displacedupward by 1.5 for clarity. We can see the typical structurewith three peaks in neutron diffraction data: Q1 (FSDP), Q2[principal peak (PP)], and Q3. The FSDP is an importantpeak for discussing the formation of intermediate order-ing45)–48) in SiO2 glass, which reflects the periodicity ofboundaries between successive cages in the networkformed by connected regular SiO4 tetrahedra with sharedoxygen atoms at the corners associated with the formationof a ring structure and a cavity. The most striking featureof the S(Q) of hot-compressed glasses is that the height ofthe FSDP in the X-ray and neutron S(Q) decreases at400 °C and increases at a temperature higher than 400 °C.The height of the FSDP reaches a maximum at 1300 °C inthe X-ray S(Q), whereas that of the neutron S(Q) does notchange so significant, suggesting that the difference arisesfrom the different weighting factors of each atomic corre-lation (Si–Si, Si–O, and O–O) for X-rays and neutrons.The second peak, PP, can be observed in only neutrondiffraction data, which becomes sharp with increasingtemperature (density) at 7.7GPa. This feature is consistentwith that in in situ high-pressure neutron diffraction datameasured at RT reported by Zeidler et al.,21) indicating thatthe PP reflects the increased packing fraction of oxygen321020151050 Pristine 400 °C/7.7 GPa 1200 °C/7.7 GPa 1300 °C/7.7 GPa 1400 °C/7.7 GPa 1500 °C/7.7 GPaX-rayNeutronS(Q)Temperature ( C)Correlation length (Å)Q (Å−1)(A)(B)13121110987150010005000 Onodera et al.39) This studyFig. 2. (A) The X-ray and neutron total structure factors, S(Q)of the pristine glass and hot-compressed SiO2 glasses. X-ray S(Q)are displaced upward by 1.5 for clarity. (B) Correlation length2³/¦QFSDP extracted from the X-ray total structure factors, S(Q)of the pristine glass and hot-compressed SiO2 glasses.Journal of the Ceramic Society of Japan 132 [7] 427-433 2024 JCS-Japan429atoms49) occupying the corner of SiO4 tetrahedra. Neutrondiffraction data of the samples synthesized at tempera-tures above 1400 °C show a Bragg peak at approximatelyQ = 2¡¹1, whereas X-ray diffraction data do not showany Bragg peaks. This is considered to be due to smallamounts of crystals formed in the samples. Since the beamsize in high-energy X-ray diffraction experiments issmaller than that in neutron diffraction experiments, thediffraction data from only a glassy part was observed in theX-ray diffraction measurements.The correlation length represented by 2³/¦QFSDP,where ¦QFSDP is the full width at half-maximum of theFSDP, was extracted from the X-ray S(Q) of the pristineglass and a series of hot-compressed SiO2 glasses using aLorenzian function [Fig. 2(B)]. The correlation length fordensified SiO2 glasses decreases at 400 °C and then in-creases with processing temperature, reaching a maximumfor 1300 °C/7.7GPa, which is consistent with the behaviorof the height of the FSDP. The correlation lengths for400 °C/7.7GPa and 1200 °C/7.7GPa show good agree-ment with those reported by Onodera et al.39) This behavioris in consistent line with that of the peak position observedat E ³ 10 eV of reflectance spectra in the vacuum ultra-violet region reported by Masuno et al.,50) although therefractive index monotonically increases in hot-compressedglasses. Furthermore, a correlation length of 13.0¡ wasobtained for 1300 °C/7.7GPa, which is larger than that for1200 °C/7.7GPa reported by Onodera et al.,39) demon-strating that the densified SiO2 glass synthesized at1300 °C/7.7GPa is by far the most structurally ordered(hyperordered) glass in the world. Conversely, it is foundthat the correlation length decreases above 1400 °C. Thereare two possible reasons for this. The first is the possibilityof melting. However, the melting point is approximately2700 °C at 7.7GPa, according to the phase diagram of theglass at high pressures and high temperatures reported byZhang et al.,51) which excludes melting as a reason. Thesecond possibility is the effect of glass transition. Bianchiet al. demonstrated that the glass transition temperatureof polyvinyl chloride tends to increase with increasingpressure.52) Although we cannot definitively confirm thatthe trends for polyvinyl chloride and SiO2 glass are thesame, the glass transition temperature of SiO2 may haveincreased at 7.7GPa.Figure 3 shows the X-ray reduced pair distributionfunctions G(r) of the pristine glass and a series of hot-compressed SiO2 glasses. G(r) is obtained from the mea-sured S(Q) via Fourier transform, and can be described as:GðrÞ ¼ 4³rμ½gðrÞ � 1�; ð2Þwhere r is the distance in real space, μ is the averagenumber density of atoms, and g(r) is the pair distributionfunction that describes the probability of finding an atomicpair separated by a certain distance r. The nearest-neighborSi–O correlation peak is observed at approximately 1.6¡.The G(r) in the shorter distance region is given as:GðrÞ ¼ �4³rμ: ð3ÞThe average number density of atoms μ can be calculatedby fitting the slope of G(r) in the shorter distance regionusing ¹4³rμ.53),54) The obtained average number densityof each sample was converted to the density d as:d ¼ μMSiO2NA; ð4Þwhere MSiO2 is the molar weight of SiO2 and NA is theAvogadro constant. The ¹4³rμ values obtained by fittingthe slope of G(r) of the pristine and 1300 °C/7.7GPa sam-ples are shown in the inset of Fig. 3. By comparing theslopes of the pristine glass and 1300 °C/7.7GPa, we cansee that the slope is steeper for the hot-compressed glass,indicating that the glasses are densified by hot compres-sion. This behavior is also consistent with the shift of theposition of the FSDP of S(Q). The densities obtained bypycnometry and neutron/X-ray G(r) fitting are shown inTable 1. Since the samples synthesized at temperaturesabove 1400 °C show a Bragg peak, as shown in Fig. 2(A),the calculated density may be estimated to be higher owingto the locally crystallized area. Therefore, the density wascalculated for the samples synthesized below 1300 °C. Thediscrepancy between the densities obtained by pycnometryand G(r) fitting for the pristine, 400 °C/7.7GPa, and1200 °C/7.7GPa samples are within 5%.Figure 4 shows the densities of the glasses synthesizedat 7.7GPa and different temperatures together with thoseof a series of crystalline phases. The density of the glasscompressed at 7.7GPa rapidly increases up to 600 °C andthen gradually increases with further increase in process-ing temperature. This behavior is completely different6543210-1-2109876543210Si–OO–OSi–SiG(r)–4rρr (Å) Pristine 400 °C/7.7 GPa 1200 °C/7.7 GPa 1300 °C/7.7 GPa 1400 °C/7.7 GPa 1500 °C/7.7 GPaFig. 3. The X-ray reduced pair distribution functions, G(r) ofthe pristine glass and hot-compressed SiO2 glasses.Table 1. Densities of the pristine glass and hot-compressed SiO2glassesPycnometryDensity (g/cm3)Neutron G(r) X-ray G(r)Pristine 2.20 2.17 2.20400 °C/7.7GPa 2.5439) 2.48 2.451200 °C/7.7GPa 2.7239) 2.74 2.761300 °C/7.7GPa — 2.79 2.821100 °C/8.0GPa 2.7437) — —Sato et al.: Synthesis of hyperordered permanently densified silica glasses by hot compression above the glass transitiontemperatureJCS-Japan430from that in cold-compressed glasses reported by Sato andFunamori, in which the density changed monotonicallywith increasing pressure.18) The density of the 1300 °C/7.7GPa sample of approximately 2.8 g/cm3 is higher thanthat of the 1100 °C/8GPa glass synthesized by Gueretteet al. (2.74 g/cm3).37) Thus, the densified SiO2 glass ob-tained at 1300 °C/7.7GPa could be considered the mostdensified SiO2 glass, which should have the highest refrac-tive index.51) Note that glass crystallizes into coesite whenthe temperature is greater than 1200 °C/7.7GPa, as shownin our previous study.39)Figure 5 shows the density dependence of the positionof the FSDP in the X-ray S(Q). The position of the FSDPshifts with increasing temperature (density) up to 1200 °Cand does not shift above 1300 °C. Onodera et al. reportedthat the cavity volume ratio of the SiO2 glass synthesizedat 1200 °C/7.7GPa was reduced to 6%,39) as determinedby the cavity volume analysis of the MD-RMC model.This behavior indicates that a marked reduction in cavityvolume is induced by the hot compression of SiO2 glass.The nonshifting of the FSDP may be due to the reductionin cavity volume.Figures 6(A) and 6(B) show the X-ray and neutron totalcorrelation functions, T(r) of the pristine glass and a seriesof hot-compressed SiO2 glasses synthesized up to 1300 °C/7.7GPa. The peaks observed at 1.61, 2.65, and 3.08¡ canbe ascribed to Si–O, O–O, and Si–Si correlations, respec-tively. As can be seen in the figure, the contrast betweenthe X-ray and neutron diffraction data is excellent. The O–O correlation peak is prominent in neutron diffraction datasince neutrons are sensitive to oxygen atoms, whereas theSi–Si correlation peak is clearly detected in X-ray datasince X-rays are sensitive to heavy elements. The area ofthe Si–O correlation peak was almost the same for allsamples. Since this area reflects the coordination numberof O atoms around a Si atom, the same area means that thecoordination number of all samples is identical. Since theSi–O coordination number in SiO2 glass is four, the area ofthe peak in all samples was calculated by curve fitting andcomparing it with the peak area of the sample before com-pression.55) The Si–O coordination numbers derived fromthe experimental T(r) are shown in Table 2. The Si–Ocoordination number of four was obtained in all glasses,indicating that a SiO4 tetrahedral network is stable in aα-QuartzCoesiteα-CristobaliteTemperature ( C)Density (g/cm3 )2.92.82.72.62.52.42.32.2120010008006004002000Guerette et al.37)Onodera et al.39)This studyFig. 4. Density of the glasses synthesized at 7.7GPa and dif-ferent temperatures.1.91.81.71.61.52.82.72.62.52.42.32.2Pristine400 °C/7.7 GPa600 °C/7.7 GPa39)800 °C/7.7 GPa39)1000 °C/7.7 GPa39)1200 °C/7.7 GPa1300 °C/7.7 GPaDensity (g/cm3)QFSDP(Å−1)Fig. 5. The density dependence of the position of FSDP for thepristine glass and hot-compressed SiO2 glasses.10864201098765432101086420109876543210 Pristine 400 °C/7.7 GPa 1200 °C/7.7 GPa 1300 °C/7.7 GPaSi–OO–OSi–Sir (Å)T(r)Si–OO–OSi–Sir (Å)T(r)(A)(B)X-rayNeutronFig. 6. Total correlation functions, T(r) of the pristine glass andhot-compressed SiO2 glasses obtained from (A) X-ray and (B)neutron diffraction.Table 2. Si–O coordination numbers of the pristine glass andhot-compressed SiO2 glassesNeutron T(r) X-ray T(r)Pristine 4.0 4.0400 °C/7.7GPa 4.0 3.91200 °C/7.7GPa 4.0 3.91300 °C/7.7GPa 4.0 3.9Journal of the Ceramic Society of Japan 132 [7] 427-433 2024 JCS-Japan431series of densified glasses, which agrees well with otherdensified glasses.11),37) In contrast, the O–O and Si–Si cor-relation peaks become broad with increasing density.4. ConclusionsWe investigated the densified SiO2 glasses synthesizedby hot compression. Although local crystallization hasbeen observed at temperatures above 1300 °C at 7.7GPa,we succeeded in synthesizing the glasses without crystal-lization up to temperatures of 1300 °C/7.7GPa and foundthat SiO2 glass synthesized at 1300 °C/7.7GPa is by farthe most structurally ordered (hyperordered) SiO2 glass inthe world on the basis of the analysis of the FSDP of thestructure factor S(Q) obtained from X-ray and neutron dif-fraction measurements. The structural ordering decreasesat temperatures above 1400 °C/7.7GPa, which may bedue to glass transition. The density obtained from thereduced pair distribution function G(r) suggests that thedensity of the SiO2 glass synthesized at 1300 °C/7.7GPais higher than 2.75 g/cm3, which is the highest densityamong the samples synthesized thus far. At 7.7GPa, thesynthesis temperature of the SiO2 glass with the highestdensity and structural ordering was 1300 °C. The effects ofthese structures on mechanical and optical properties56)and relationship with synthesis conditions will be inves-tigated as a next step.Acknowledgements This work was supported by JSTSPRING, Grant Number JPMJSP2151 (to S.S.), Grant-in-Aidfor Transformative Research Areas (A) Hyper-Ordered Struc-ture, Grant Numbers 20H05878 (to S.K.), 20H05880 (toN.K.), and 20H05881 (to Y.O. and S.K.), for ScientificResearch on Innovative Areas, Grant Number 19H05790 (toM.M.) and World Premier International Research CenterInitiative (WPI), MEXT, Japan.The X-ray diffraction measurements were conducted atSPring-8 with the approval of Japan Synchrotron RadiationResearch Institute (JASRI) (Proposal No.: 2022A1002). Theneutron diffraction experiments at the MLF of the J-PARCwere performed under a long-term proposal (Proposal No.2022L0202). Discussions with Profs. M. Ono and A. Masuno,and Drs. H. Yusa, H. Masai, H. Segawa, and Y. Shuseki aregratefully appreciated. We thank M. 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