A large amount of water exists in the interior of solid Earth as a form of hydroxyl ions or hydrogen in lattice defects. These minerals can play essential role as a host for H2O transportation into the crust and mantle of the Earth. Therefore, it is important to investigate high-pressure responses of these minerals and to clarify the behavior of hydrogen. Among recent high-pressure studies of hydrous minerals, an attention has been focused mostly on layered silicates and hydroxides. As the simplest model of hydrous phases, brucite-type hydroxides M(OH)2 (M = Mg, Ca, Mn, Co, Ni etc.), which have hydrogen bonds in their layered structure, show diverse responses to pressure depending on cation size. These results will provide significant insights for understanding the high-pressure physical properties of hydrogen bond in solids for both geo-science and materials science.

In this thesis, high-pressure behaviors of Ca(OH)2, portlandite, as shown in Fig. 1, is focused on. To date, many studies have clarified the existence of pressure-induced phase transitions and/or amorphization of portlandite using variety of high-pressure cells, analytical methods and pressure-transmitting media (e.g., Meade and Jeanloz 1990; Nagai et al., 2000; Ekbundit et al., 1996; Catalli et al., 2008). However, there remain some difficulties in clarifying the phase transition at around 6-8 GPa, since the hydrostaticity of the applied pressure sensitively affects on this transition. Neutron diffraction is a powerful tool to clarify the crystal structures including hydrogen positions. In portlandite, however, no data on the structure of the high-pressure phase (High-P phase) has been reported in previous neutron diffraction studies (Pavese et al., 1997; Xu et al., 2007). In other words, the nature of the transition and the crystal structure of the High-P phase of portlandite remain unknown.

The main purposes of the present study are: (i) to clarify the pressure response of portlandite including the behavior of hydrogen bonds, (ii) to identify the crystal structure of the High-P phase of portlandite, and (iii) to improve experimental techniques for neutron diffraction measurements under high pressure to determine the hydrogen position in the High-P phase. This thesis consists of three following chapters.

In chapter 2, the pressure responses of portlandite were studied by single-crystal Raman and IR spectra, and powder XRD was investigated using diamond anvil cells (DACs). All the experiments were conducted under highly hydrostatic condition using a helium pressure-transmitting medium. The H-D isotope effect on the pressure response was also examined because deuterated samples are required in neutron diffraction studies to avoid large incoherent scattering from hydrogen. Moreover, similar experiments were repeated using a 4:1 methanol-ethanol pressure medium (Met/EtOH) to clarify the effect of the hydrostaticity on the pressure-induced phase transitions.

A reversible pressure-induced phase transition at around 6 GPa and room temperature was observed. The small H-D isotope effect was observed on the phase transition pressures (and partial amorphization); the transition pressure of Ca(OH)2 is slightly lower than that of Ca(OD)2 as shown in Fig. 2. In contrast, no isotope effect was found on the volume and axial compression of portlandite, as shown in Fig. 3. These results suggest that the H-D isotope effect is limited in the local environment surrounding H(D) atoms. All the Raman and IR spectroscopic peaks disappeared at pressures greater than about 20 GPa, whereas the XRD peaks of the High-P phase were observed even at 28 GPa. This suggests that long-range ordering of Ca and O atoms is preserved even after the nature of the O-H bond has changed probably by the partial amorphization of H-sublattice.

In Chapter 3, in-situ single crystal X-ray diffraction studies are presented to clarify the crystal structure of the High-P phase of portandite. In order to overcome the difficulties in previous studies, some new experimental techniques were developed for single crystal X-ray diffraction measurements using DACs. Figure 4 shows a diamond anvil cell, which was improved in this study from "Radial-DAC" to have a wider aperture angle and lower background so as to obtain more reflections from the single crystal samples. Based on the clarified crystal structure of the High-P phase as shown in Fig. 5, the phase transition mechanism of portlandite was discussed. The phase transition is accompanied by the geometrical change in hydrogen bonds. Hydrogen atoms are expected to locate on at least two positions, which can be geometrically stable for hydrogen bonds. The identified crystal structure of the High-P phase will provide a valuable clue to determine H positions and state of hydrogen bonds in future neutron diffraction studies.

In Chapter 4, developments of a high-pressure experimental technique were described for studying the phase transition of portlandite using neutron diffraction. A newly designed cell-assembly was tested using a Paris-Edinburgh (P-E) press (Fig. 6 left), so as to increase the high-pressure capability and to obtain the high-intensity diffraction data. This will allow us to perform more reliable neutron diffraction measurements under higher pressures. In order to extend the pressure range of the P-E press without reducing the sample volume, following technical developments were made (Fig. 6 right): (1) New anvils were designed with a wide conical aperture to get higher signal intensity, (2) A hybrid gasket was developed by TiZr and Al-alloy for reducing absorption of neutron beam, (3) A new anvil with an optical window was designed for monitoring ruby fluorescence spectra to determine pressure and evaluate the hydrostaticity in a sample chamber. Pressure generation tests were repeated by measuring the electrical resistance of Bi and ruby fluorescence spectra at the same time. In-situ synchrotron XRD experiments were also carried out using NaCl pressure marker at Photon Factory-Advanced Ring (PF-AR), KEK. At present, maximum pressure of 13 GPa was achieved as shown in Fig. 7.

The performance of this newly developed system was tested at J-PARC using a pulsed neutron source. As a result, 2.5-3 times higher intensities were obtained compared with those of original "toroidal anvil" designs with the same initial sample volume using Pb as standard material. A preliminary high pressure experiment on powder Ca(OD)2 was carried out using the new cell at J-PARC. The neutron diffractions derived from the High-P phase of portlandite were observed for the first time as shown in Fig. 8. Further experiments will allow us to investigate structural information of hydrogen bond in portlandite and in various solid materials.

For summary of this study, high-pressure behaviors of portlandite were investigated from IR and Raman spectroscopic methods, and X-ray diffraction measurements. The crystal structure of the High-P phase and the phase transition mechanism of portlandite were clarified. New experimental techniques for neutron diffraction were developed. Future neutron diffraction measurements at J-PARC using the developed cell-assembly will play important roles to clarify the high-pressure behaviors of hydrogen bonds in various hydrogen-bearing materials.

Fig. 1. (Left) Crystal structure of portlandite at ambient pressure. (Right) Partial structure of hydrogen bonding around the rectangle in the left figure. Solid lines indicate possible hydrogen bonds between H and O. Dotted lines indicate repulsive H...H interactions.

Fig. 2. IR spectra of Ca(OH)2 (left) and Ca(OD)2 (right) single crystals in helium in the regions of OH and OD stretching vibration modes. The shaded region indicates the phase transition toward the High-P phase.

Fig. 3. Pressure dependences in (a) Unit cell volume and (b) the ratio of unit cell parameters of the a axis, c axis for powder Ca(OH)2 and Ca(OD)2 in various pressure-transmitting media. Open and closed symbols represent the data taken on compression and decompression, respectively. The three line plots from other previous studies are shown for comparison.

Fig. 4. Measurement geometry of single-crystal X-ray diffraction using the improved Radial-DAC.

Fig. 5. Crystal structure of the High-P phase of portlandite at 8.5 GPa, from the perspectives of (left) the a-axis, and (right) the b-axis. Blue and red spheres indicate Ca and O atoms, respectively.

Fig. 6. Design of the new cell assembly loaded into a P-E press. (1) TiZr encapsulate and ring gasket, (2) Ni-binded WC anvil, (3) ruby chip, (4) moissanite anvil, (5) Al-alloy (A6061) disk, (6) SNCM439 support ring, (7) Ni-binded WC support ring, (8) Optical fiber.

Fig. 7. Pressure generation curves for various types of anvils.(inset) Culet shapes of (a) Toroidal and (b) Non-Toroidal anvils.

Fig. 8. Neutron diffraction pattern of Ca(OD)2 at 9.1 GPa. Red tick marks and arrows indicate the diffraction peak positions derived from the High-P phase.

本論文は、含水鉱物の中でも最も基本的な物質である水酸化カルシウムに着目し、高圧下での結晶構造の変化や水素結合の圧力応答について、多角的な実験に基づいた詳細なデータから記述しており、全5章から構成される。

第1章では、導入として、含水鉱物が地球内部への水の循環や地球内部ダイナミクスに果たす重要な役割が述べられ、さらに高圧下での含水鉱物の構造変化と物性との関連について過去の研究がレビューされている。層状構造をもつ金属水酸化物は、金属元素によって高圧下での構造変化の挙動が異なる。なかでも水酸化カルシウムは、室温下約20 GPaでのアモルファス化のほか、約6 GPaで構造未解明の高圧相に相転移し、他の金属水酸化物とは異なる性質を持つことが知られている。本章では水酸化カルシウムの高圧下での挙動を解明することの重要性と、本論文における研究内容の位置づけが述べられている。

第2章では、水酸化カルシウムの圧力応答と圧力誘起相転移への同位体効果について述べられている。ヘリウムを圧力媒体とした準静水圧条件下での赤外吸収スペクトル、ラマンスペクトル、X線回折その場観察から、約6 GPaで水酸化カルシウムは高圧相へ相転移することが明らかになった。この相転移はこれまで実験条件によって再現されないこともあったが、それが静水圧条件によるものであることも明らかにした。さらに約20 GPaでは水素原子周辺が部分的にアモルファス化する現象が観察され、これらの変化が可逆的であることも示された。さらにこれらの相転移に、HとDとの間で同位体効果があることも本研究によって初めて明らかになり、圧力誘起相転移が水素原子周辺から起こっていることが確認された。

第3章では、これまで未解明であった水酸化カルシウムの高圧相の結晶構造について述べられている。より広い開口角でX線回折を観測することが可能となるようダイヤモンドアンビルセルの改良を施し、高圧下での単結晶X線回折の解析から、高圧相の空間群ならびに格子定数を決定した。理論計算で推定した構造をもとに、結晶構造を推定し、高圧下で測定された粉末X線パターンに基づいて構造パラメーターの最適化を行い、水酸化カルシウムの高圧相の結晶構造を決定した。さらに得られた結晶構造に基いて、高圧下での水酸化カルシウムの相転移のメカニズムを提案した。

第4章では、高圧下で中性子回折を測定するための高圧セルの開発について述べられている。世界の中性子実験施設で汎用されているParis-Edinburghセルをベースに、新たにアンビル形状を最適化し、信号強度を倍以上に増加することに成功した。さらにアンビル内にSiCの光学窓を入れることで、これまで光学的に内部を観察することが出来なかったParis-Edinburghセル内の光学スペクトル測定を可能にした。ここで開発された新しい高圧セルは、今後の高圧下中性子回折実験に広く応用されていくことが予想される。

第5章は以上の研究成果のまとめである。

本研究により、きわめて単純な構造でありながら、高圧下で起こる相転移について未解明な点が多かった水酸化カルシウムについて、ヘリウムを圧力媒体として準静水圧条件における高圧下その場観察により、いくつもの新しい知見が得られた。特に本研究で明らかになった、水酸化カルシウムの高圧相転移における同位体効果や高圧相の構造解明は、層状金属水酸化物の圧力応答の本質的に理解を深める新たな描像を与えるものである。このような新規な圧力応答メカニズムの提案とその科学的意義を提示した本論文の内容は高く評価できる。

本論文第2章の主要部分は、Journal of Physics: Conference Series誌とPhysics and Chemistry of Minerals誌に公表済みである。いずれにおいても、論文提出者が主体となって実験および解析を行っており、その寄与が十分であるので、学位論文の一部とすることに何ら問題ないと判断する。

以上の理由から、論文提出者 飯塚理子に博士(理学)の学位を授与できると認める。