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論文

High-temperature short-range order in MnRhSi

山内宏樹; Sari, D. P.*; 渡邊功雄*; 安井幸夫*; Chang, L.-J.*; 近藤啓悦; 伊藤孝; 石角元志*; 萩原雅人*; Frontzek, M. D.*; et al.

Communications Materials (Internet), 1, p.43_1 - 43_6, 2020/07

https://doi.org/10.1038/s43246-020-0042-1

中性子とミュオンの相補利用により、720Kまでの高温での短距離磁気秩序現象をMnRhSiで発見した。

論文

Horizontal line nodes in SrRuO proved by spin resonance

飯田一樹*; 古府麻衣子; 鈴木雄大*; 村井直樹; 河村聖子; 梶本亮一; 稲村泰弘; 石角元志*; 長谷川舜介*; 増田隆嗣*; et al.

Journal of the Physical Society of Japan, 89(5), p.053702_1 - 053702_5, 2020/05

https://doi.org/10.7566/JPSJ.89.053702

被引用回数：2 パーセンタイル：27.71(Physics, Multidisciplinary)

We investigated the low-energy incommensurate (IC) magnetic fluctuations in SrRuO by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $hbar omega_{rm res}0.56$ meV centered at a characteristic wavevector $Q_{rm res}=(0.3,0.3,0.5)$ . The resonance energy corresponds well to the superconducting gap 2 = 0.56 meV estimated by the tunneling spectroscopy. The spin resonance shows the modulation with a maximum at around . The modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional and sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of SrRuO. We also discuss the implications on possible superconducting order parameters.

論文

Magnetically induced electric polarization in Ba $_{3}$ Fe $_{2}$ O $_{5}$ Cl $_{2}$ with tunable direction in three dimensions

阿部伸行*; 塩澤俊介*; 松浦慧介*; 佐賀山基*; 中尾朗子*; 大原高志; 徳永祐介*; 有馬孝尚*

Physical Review B, 101(18), p.180407_1 - 180407_5, 2020/05

https://doi.org/10.1103/PhysRevB.101.180407

被引用回数：0 パーセンタイル：100(Materials Science, Multidisciplinary)

The correlation between magnetism and electric polarization in a chiral insulating magnet Ba $_{3}$ Fe $_{2}$ O $_{5}$ Cl $_{2}$ has been investigated. The temperature dependence of magnetic susceptibility shows an anomaly at 564 K, suggesting the onset of antiferromagnetic order of Fe $^{3+}$ moments. Electric polarization appears in a magnetic field below TN due to the spin-direction-dependent metal-ligand hybridization. The direction of the electric polarization can be controlled in three-dimensional space by changing the direction of an external magnetic field. This compound also shows an antiferromagnetic-to-weak-ferromagnetic transition accompanied by a structural phase transition at 140 K.

論文

Successive phase transitions in $R_{3}$ Ir $_{4}$ Sn $_{13}$ (: La and Ce) investigated using neutron and X-ray diffraction

中里晟也*; 岩佐和晃*; 橋本大輔*; 塩澤真未*; 桑原慶太郎*; 中尾裕則*; 佐賀山基*; 石角元志*; 大原高志; 中尾朗子*; et al.

JPS Conference Proceedings (Internet), 30, p.011128_1 - 011128_6, 2020/03

https://doi.org/10.7566/JPSCP.30.011128

Successive phase transitions of $R_{3}$ Ir $_{4}$ Sn $_{13}$ (: La and Ce) were studied using neutron and X-ray diffraction techniques. A semimetal Ce $_{3}$ Ir $_{4}$ Sn $_{13}$ undergoes three phase transitions: an antiferromagnetic ordering characterized by a propagation vector $q_{rm M}$ = (1/2, 1/2, 2/5) below 0.6 K, a structural transformation with $q_{rm S2}$ = (1/4, 1/4, 1/4) at 2.0 K, and another structural transformation with $q_{rm S1}$ = (1/2, 1/2, 0) above room temperature. La $_{3}$ Ir $_{4}$ Sn $_{13}$ was confirmed to be a superconductor below 2.5 K under the $q_{rm S1}$ = (1/2, 1/2, 0) structure that also appears above room temperature.

論文

Crystal structure and cation distribution of the X-type hexaferrite Sr $_{2}$ Co $_{2}$ Fe $_{28}$ O $_{46}$

駒淵舞*; 漆原大典*; 浅香透*; 福田功一郎*; 大原高志; 宗像孝司*; 石川喜久*

Journal of the Physical Society of Japan, 89(3), p.034601_1 - 034601_5, 2020/03

https://doi.org/10.7566/JPSJ.89.034601

被引用回数：0 パーセンタイル：100(Physics, Multidisciplinary)

The crystal structure of an X-type hexaferrite Sr $_{2}$ Co $_{2}$ Fe $_{28}$ O $_{46}$ was investigated by the X-ray and neutron diffraction methods for a single crystal. Sr $_{2}$ Co $_{2}$ Fe $_{28}$ O $_{46}$ has the crystal structure described as a space group () and its lattice constants are = 5.9165(2) ; and = 84.1395(33) ; at 843 K. Co ions are almost localized in the octahedral site () in the middle of S $^{*}$ S $^{*}$ blocks. In addition, the adjacent octahedral and tetrahedral sites to site contain the significant number of Co $^{2+}$ .

論文

Magnetic, thermal, and neutron diffraction studies of a coordination polymer; bis(glycolato)cobalt(II)

中根僚宏*; 米山翔太*; 兒玉健*; 菊地耕一*; 中尾朗子*; 大原高志; 東中隆二*; 松田達磨*; 青木勇二*; 藤田渉*

Dalton Transactions (Internet), 48(1), p.333 - 338, 2019/01

https://doi.org/10.1039/C8DT04358B

被引用回数：1 パーセンタイル：72.9(Chemistry, Inorganic & Nuclear)

The two-dimensional quadratic lattice magnet, bis(glycolato)cobalt(II) ([Co(HOCH $_{2}$ CO $_{2}$ ) $_{2}$ ]), showed anti-ferromagnetic ordering at 15.0 K and an abrupt increase in magnetisation at H = 22 600 Oe and 2 K, thereby acting as a metamagnet. Neutron diffraction studies suggested that the magnetic moment vectors of the Co(II) ions had an amplitude of 3.59 mB and were not aligned in a fully antiparallel fashion to those of their neighbours, which caused canting between the magnetic moment vectors in the sheet. The canting angle was determined to be 7.1deg. Canting induced net magnetisation in the sheet, but this magnetisation was cancelled between sheets. The magnetisations in the sheets were oriented parallel to the magnetic field at the critical magnetic field.

論文

High-energy spin fluctuation in low-T $_{c}$ iron-based superconductor LaFePO $_{0.9}$

石角元志*; 社本真一; 樹神克明; 梶本亮一; 中村充孝; Hong, T.*; Mutka, H.*

Scientific Reports (Internet), 8, p.16343_1 - 16343_6, 2018/11

https://doi.org/10.1038/s41598-018-33878-x

被引用回数：2 パーセンタイル：60.51(Multidisciplinary Sciences)

磁気揺らぎはBCS理論に従わない非従来型超伝導で最も可能性のある機構として重要な役割を担うと考えられてきた。今回、これまで磁気ゆらぎが見つかっていない5Kという低い超伝導転移温度の鉄リン系超伝導体LaFePO $_{0.9}$ で、30-50meVの高エネルギーで磁気ゆらぎを見つけた。これはこれまで考えられてきた高エネルギー磁気揺らぎで、高い超伝導転移温度になるという考えとは矛盾することから、超伝導機構の理解に向けて重要な発見である。

論文

Thermal behavior, structure, dynamic properties of aqueous glycine solutions confined in mesoporous silica MCM-41 investigated by X-ray diffraction and quasi-elastic neutron scattering

吉田亨次*; 井上拓也*; 鳥越基克*; 山田武*; 柴田薫; 山口敏男*

Journal of Chemical Physics, 149(12), p.124502_1 - 124502_10, 2018/09

https://doi.org/10.1063/1.5039892

被引用回数：1 パーセンタイル：89.91(Chemistry, Physical)

異なる幾つかの、グリシン濃度, pH、および充填率(=グリシン溶液の質量/MCM-41の乾燥質量))をパラメーターとして、メソポーラスシリカ(MCM-41)に閉じ込められたグリシン水溶液の示差走査熱量測定、X線回折および準弾性中性子散乱(QENS)を305-180Kの温度範囲で実施して、グリシン水溶液の熱的挙動, 構造、および動的特性に対する閉じ込め効果を検討した。

論文

Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

根本文也*; 古府麻衣子; 長尾道弘*; 大石一城*; 高田慎一; 鈴木淳市*; 山田武*; 柴田薫; 上木岳士*; 北沢侑造*; et al.

Journal of Chemical Physics, 149(5), p.054502_1 - 054502_11, 2018/08

https://doi.org/10.1063/1.5037217

被引用回数：6 パーセンタイル：37.89(Chemistry, Physical)

Alkyl-methyl-imidazolium ionic liquids CmimX (: alkyl-carbon number, X: anion) have short-range layer structures consisting of ionic and neutral (alkylchain) domains. To investigate the temperature dependences of the interlayer, interionic group, and inter-alkylchain correlations, we have measured the neutron diffraction (ND) of CmimPF ( = 16, 9.5, 8). The quasielastic neutron scattering (QENS) of C16mimPF was also measured to study the dynamics of each correlation. C16mimPF shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at $T_{rm c}$ = 394 K. C8mimPF exhibits a glass transition at $T_{rm g}$ = 200 K. C9.5mimPF has both transitions at $T_{rm c}$ = 225 K and $T_{rm g}$ = 203 K. In the ND experiments, all samples exhibit three peaks corresponding to the correlations mentioned above. The widths of the interlayer peak at ca. 0.2 $AA$^{-1}$$ changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 $AA$^{-1}$$ exhibited a small jump at $T_{rm c}$ . The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at $T_{rm g}$ . The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at $T_{rm c}$ , while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes are characteristic of the L-LC transition in imidazolium-based ionic liquids.

論文

High oxide-ion conductivity in Si-deficient La $_{9.565}$ (Si $_{5.826}$ [] $_{0.174}$ )O $_{26}$ apatite without interstitial oxygens due to the overbonded channel oxygens

藤井孝太郎*; 八島正知*; 日比野圭祐*; 白岩大裕*; 福田功一郎*; 中山享*; 石沢伸夫*; 花島隆泰*; 大原高志

Journal of Materials Chemistry A, 6(23), p.10835 - 10846, 2018/06

https://doi.org/10.1039/C8TA02237B

被引用回数：11 パーセンタイル：29.53(Chemistry, Physical)

Apatite-type rare earth silicates are attractive materials with exhaust application such as solid-oxide fuel cells, due to its extremely high oxide-ion conductivity below 600 $^{circ}$ C. Interstitial (excess) oxygens have been believed to be responsible for the high conductivity in apatite-type materials. On the contrary, the present study clearly reveals the presence of Si vacancies [] in La-rich La $_{9.565}$ (Si $_{5.826}$ []] $_{0.174}$ )O $_{26}$ instead of the interstitial oxygens, by single-crystal neutron and X-ray diffraction analyses, density measurements and electronic calculations.

論文

Magnetic-ordering propagation vectors of terbium hexaboride revisited

岩佐和晃*; 伊賀文俊*; 茂吉武人*; 中尾朗子*; 大原高志

Journal of the Physical Society of Japan, 87(6), p.064705_1 - 064705_5, 2018/06

https://doi.org/10.7566/JPSJ.87.064705

被引用回数：0 パーセンタイル：100(Physics, Multidisciplinary)

The magnetic-ordered state of a single-crystal sample of TbB $_{6}$ has been studied with the high-precision neutron diffraction instrument SENJU installed at BL18 of the Materials and Life Science Facility in J-PARC. We observed new Bragg-reflection spots characterized by a propagation vector $q_{rm M2}$ = (1/4, 1/4, 0) below the Nel temperature $T_{rm N}$ 20 K, in addition to the $q_{rm M1}$ = (1/4, 1/4. 1/2) reported in the previous literatures. Because X-ray Thomson scattering measurements do not detect the $q_{rm M2}$ superlattice reflections at 13 K, $q_{rm M2}$ is a new magnetic-ordering propagation vector. The magnetic multi-q structure of TbB $_{6}$ is similar to that of CeB $_{6}$ , whereas the ratio of the $q_{rm M2}$ amplitude to that of qM1 is smaller than in CeB $_{6}$ . The combination of $q_{rm M1}$ and $q_{rm M2}$ is expected to assist the $q_{rm L1}$ = (1/2, 0, 0) lattice modulation.

論文

Dynamic of organic species in organo-clay/polypropyrene composite by quesi-elastic neutron scattering

福嶋喜章*; 山田武*; 田村堅志*; 柴田薫

Applied Clay Science, 155, p.15 - 19, 2018/04

https://doi.org/10.1016/j.clay.2017.12.041

被引用回数：3 パーセンタイル：61.51(Chemistry, Physical)

ジオクタデシルジメチルアンモニウムイオン(DODA $^{+}$ )/ポリプロピレン複合体のために交換されたフルオロマイカ(ME100)カチオンのダイナミクスは、XRDおよびDSC以外に準弾性中性子散乱(QENS)によって分析された。低いQ=2.75nm $^{-1}$ のDODA-ME100のQENSスペクトルは、DODA $^{+}$ の融点である445Kより高い温度でも変化しなかった。この結果は、硬質シリケート層とDODA $^{+}$ とME100との間の強い静電相互作用のために、層間距離における長距離(2nm)の分子運動が制限されることを示唆している。弾性強度スキャンの結果は、445Kにおける溶融状態においても、複合体中のポリマー鎖の動きが少し制限されていることを示唆した。QENSは、複合材料を研究するための有用なツールの1つであることが期待される。

論文

Liquid-like thermal conduction in intercalated layered crystalline solids

Li, B.; Wang, H.*; 川北至信; Zhang, Q.*; Feygenson, M.*; Yu, H. L.*; Wu, D.*; 尾原幸治*; 菊地龍弥*; 柴田薫; et al.

Nature Materials, 17(3), p.226 - 230, 2018/03

https://doi.org/10.1038/s41563-017-0004-2

被引用回数：47 パーセンタイル：3.08(Chemistry, Physical)

As a generic property, all substances transfer heat through microscopic collisions of constituent particles. A solid conducts heat through both transverse and longitudinal acoustic phonons, but a liquid employs only longitudinal vibrations. As a result, a solid is usually thermally more conductive than a liquid. In canonical viewpoints, such a difference also serves as the dynamic signature distinguishing a solid from a liquid. Here, we report liquid-like thermal conduction observed in the crystalline AgCrSe $_{2}$ . The transverse acoustic phonons are completely suppressed by the ultrafast dynamic disorder while the longitudinal acoustic phonons are strongly scattered but survive, and are thus responsible for the intrinsically ultralow thermal conductivity. This scenario is applicable to a wide variety of layered compounds with heavy intercalants in the van der Waals gaps, manifesting a broad implication on suppressing thermal conduction. These microscopic insights might reshape the fundamental understanding on thermal transport properties of matter and open up a general opportunity to optimize performances of thermoelectrics.

論文

Mn $_{2}$ VAl Heusler alloy thin films; Appearance of antiferromagnetism and exchange bias in a layered structure with Fe

土屋朋生*; 小林亮太*; 窪田崇秀*; 斉藤耕太郎*; 小野寛太*; 大原高志; 中尾朗子*; 高梨弘毅*

Journal of Physics D; Applied Physics, 51(6), p.065001_1 - 065001_7, 2018/02

https://doi.org/10.1088/1361-6463/aaa41a

被引用回数：2 パーセンタイル：74.71(Physics, Applied)

Mn $_{2}$ VAl Heusler alloy films were epitaxially grown on MgO(100) single-crystal substrates by ultra-high-vacuum magnetron sputtering. A2- and L21-type Mn $_{2}$ VAl order was controlled by the deposition temperature. A2-type Mn $_{2}$ VAl films showed no spontaneous magnetization, while L21-type Mn $_{2}$ VAl films showed ferrimagnetic behaviour with a maximum saturation magnetization of 220 emu/cm $^{3}$ at room temperature (RT). An antiferromagnetic reflection was observed with neutron diffraction at RT for an A2-type Mn $_{2}$ VAl film deposited at 400 $^{circ}$ C. A bilayer sample of the antiferromagnetic A2-type Mn $_{2}$ VAl and Fe showed an exchange bias of 120 Oe at 10 K.

論文

Experimental investigation of the glass transition of polystyrene thin films in a broad frequency range

井上倫太郎*; 金谷利治*; 山田武*; 柴田薫; 深尾浩次*

Physical Review E, 97(1), p.012501_1 - 012501_6, 2018/01

https://doi.org/10.1103/PhysRevE.97.012501

被引用回数：4 パーセンタイル：34.19(Physics, Fluids & Plasmas)

本研究では、非弾性中性子散乱(INS),誘電緩和分光法(DRS),熱膨張分光法(TES)を用いたポリスチレン薄膜の過程を調べた。DRSおよびTES測定は、フィルムの厚さとともにガラス転移温度( $T_{rm g}$ )の低下を示した。一方、INS測定ではTgの上昇が認められた。この矛盾を解明するために、我々は、DRSとTESによって測定された過程のピーク周波数( $f_{rm m}$ )の温度依存性を調べた。実験では、測定周波数領域で膜厚が減少するにつれてピーク周波数( $f_{rm m}$ )が増加することが明らかになった。この測定結果は、膜厚に伴う観察されたTgの減少と一致する。INSとDRSまたTESとの間の過程の説明の相違は、不透明な壁効果のために、膜厚および移動度の低下による見掛けの活性化エネルギーの低下に起因すると考えられる。

論文

Spin resonance in the new-structure-type iron-based superconductor CaKFe $_{4}$ As $_{4}$

飯田一樹*; 石角元志*; 永井佑紀; 吉田紘行*; Christianson, A. D.*; 村井直樹; 川島健司*; 吉田良行*; 永崎洋*; 伊豫彰*

Journal of the Physical Society of Japan, 86(9), p.093703_1 - 093703_4, 2017/09

https://doi.org/10.7566/JPSJ.86.093703

被引用回数：18 パーセンタイル：12.86(Physics, Multidisciplinary)

The dynamical spin susceptibility in the new-structure iron-based superconductor CaKFe $_{4}$ As $_{4}$ was investigated by a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at Q $_{rm res}$ = 1.17(1) vector in the tetragonal notation, evolves below Tc. The characteristic energy of the spin resonance E $_{rm res}$ = 12.5 meV is smaller than twice the size of the superconducting gap (2). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations which take into account the different superconducting gaps on the different Fermi surfaces. Our INS and PRA studies demonstrated that the superconducting pairing nature in CaKFe $_{4}$ As $_{4}$ is the s symmetry.

論文

Vibrational states of atomic hydrogen in bulk and nanocrystalline palladium studied by neutron spectroscopy

古府麻衣子; 橋本直樹*; 秋葉宙*; 小林浩和*; 北川宏*; 飯田一樹*; 中村充孝; 山室修*

Physical Review B, 96(5), p.054304_1 - 054304_7, 2017/08

https://doi.org/10.1103/PhysRevB.96.054304

被引用回数：7 パーセンタイル：55.89(Materials Science, Multidisciplinary)

中性子分光法を用いて、バルク及びナノ結晶パラジウム中の水素原子の振動状態を、広いエネルギー領域(meV)について調べた。バルクのパラジウム水素化物では、水素の振動励起は量子調和振動子(QHO)で大まかに記述された。一方、直径8nmのナノ結晶パラジウム水素化物では、付加的な振動励起が80meV以上のエネルギー領域に観測された。この付加的な振動励起のエネルギーと強度はQHOでは説明できず、非調和性の強いトランペット型ポテンシャルにおける振動と記述された。この付加的な励起は、表面効果により安定化されたサブ表面(表面下2-3層)の正四面体サイトに配置された水素原子の振動だと考えられる。本研究は、金属ナノ粒子中の水素の振動を明瞭に検知した初めての実験的研究である。

論文

Non-aqueous selective synthesis of orthosilicic acid and its oligomers

五十嵐正安*; 松本朋浩*; 八木橋不二夫*; 山下浩*; 大原高志; 花島隆泰*; 中尾朗子*; 茂吉武人*; 佐藤一彦*; 島田茂*

Nature Communications (Internet), 8, p.140_1 - 140_8, 2017/07

https://doi.org/10.1038/s41467-017-00168-5

被引用回数：5 パーセンタイル：64.38(Multidisciplinary Sciences)

Orthosilicic acid (Si(OH) $_{4}$ ) and its small condensation compounds are among the most important silicon compounds but have never been isolated, despite the long history of intense research due to their instability. These compounds would be highly useful building blocks for advanced materials if they become available at high purity. We developed a simple procedure to selectively synthesize orthosilicic acid, its dimer, cyclic trimer, and tetramer, as well as appropriate conditions to stabilize these species, in organic solvents. Isolation of orthosilicic acid, the dimer and the cyclic tetramer as hydrogen-bonded crystals with tetrabutylammonium halides and of the cyclic trimer as solvent-containing crystals was achieved. The solid-state structures of these compounds were unambiguously clarified by single crystal X-ray diffraction analysis and also by neutron diffraction study for orthosilicic acid. Based on these results, we also succeeded in developing a more practical synthetic procedure for high concentrations of stable orthosilicic acid stably in organic solvents via a simple hydrolysis of tetraalkoxysilanes.

論文

Polar rotor scattering as atomic-level origin of low mobility and thermal conductivity of perovskite CH $_{3}$ NH $_{3}$ PbI $_{3}$

Li, B.; 川北至信; Liu, Y.*; Wang, M.*; 松浦直人*; 柴田薫; 河村聖子; 山田武*; Lin, S.*; 中島健次; et al.

Nature Communications (Internet), 8, p.16086_1 - 16086_9, 2017/06

https://doi.org/10.1038/ncomms16086

被引用回数：35 パーセンタイル：9.55(Multidisciplinary Sciences)

Perovskite CH $_{3}$ NH $_{3}$ PbI $_{3}$ exhibits outstanding photovoltaic performances, but the understanding of the atomic motions remains inadequate even though they take a fundamental role in transport properties. Here, we present a complete atomic dynamic picture consisting of molecular jumping rotational modes and phonons, which is established by carrying out high-resolution time-of-flight quasi-elastic and inelastic neutron scattering measurements in a wide energy window ranging from 0.0036 to 54 meV on a large single crystal sample, respectively. The ultrafast orientational disorder of molecular dipoles, activated at approximately 165 K, acts as an additional scattering source for optical phonons as well as for charge carriers. It is revealed that acoustic phonons dominate the thermal transport, rather than optical phonons due to sub-picosecond lifetimes. These microscopic insights provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized.

論文

Bulk moduli and equations of state of ice VII and ice VIII

Klotz, S.*; 小松一生*; 鍵裕之*; Kunc, K.*; 佐野亜沙美; 町田真一*; 服部高典

Physical Review B, 95(17), p.174111_1 - 174111_7, 2017/05

AA2017-0082.pdf:0.79MB

https://doi.org/10.1103/PhysRevB.95.174111

被引用回数：14 パーセンタイル：23.13(Materials Science, Multidisciplinary)

重水素化した氷VII相及びVIII相の圧縮挙動を、93-300K、2-13.7GPaの温度圧力領域にわたって、高圧中性子散乱によって調べた。その結果から、正確な体積弾性率B ${}_{0}$ ,その圧力微分B' ${}_{0}$ 及び常圧下での体積V ${}_{0}$ を含む状態方程式を決定した。このように決めた状態方程式は、過去のX線データと比べて氷VIIの安定領域のほほ全域を、また氷VIIIに関しては、約13GPaまでをカバーしている。両者の圧縮挙動に関して、低圧域では区別できないが、7GPa以上の圧力では氷VIIは予想以上に固くなることが分かった。これは、今回の圧力温度領域において過去の氷VIIIの研究[D.D. Klug et al., Physical Review B, 70, 144113 (2004)]で報告されている異常なフォノン硬化と関係しているかも知れない。