Crystal Structure of the High-Pressure Phase of Calcium Hydroxide, Portlandite: in Situ Powder and Single-Crystal X-Ray Diffraction Study

American Mineralogist, Volume 98, pages 1421–1428, 2013

Crystal structure of the high-pressure phase of calcium hydroxide, portlandite: In situ powder and single-crystal X-ray diffraction study

Riko Iizuka,1,2,3,* Takehiko Yagi,1,3 Kazuki Komatsu,2 Hirotada Gotou,1 Taku Tsuchiya,3 Keiji Kusaba,4 and Hiroyuki Kagi2

1Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8581, Japan 2Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan 3Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan 4Department of Materials Science, Nagoya University, Nagoya, 464-8603, Japan

Abstract

The crystal structure of a high-pressure phase of calcium hydroxide, Ca(OH)2 (portlandite), was clarified for the first time using the combination of in situ single-crystal and powder X‑ray diffraction measurements at high pressure and room temperature. A diamond-anvil cell with a wide opening angle and cell-assembly was improved for single-crystal X‑ray diffraction experiments, which allowed us to successfully observe Bragg reflections in a wide range of reciprocal space. The transition occurred at 6 GPa and the crystal structure of the high-pressure phase was determined to be monoclinic at 8.9 GPa and room temperature [I121; a = 5.8882(10), b = 6.8408(9), c = 8.9334(15) Å, β = 104.798(15)°]. The transition involved a decrease in molar volume by approximately 5.8%. A comparison of the

structures of the low- and high-pressure phases indicates that the transition occurs by a shift of CaO6 octahedral layers in the a-b plane along the a-axis, accompanied by up-and-down displacements of Ca atoms from the a-b plane. The crystal structure of this high-pressure phase is considered to be an intermediate state between the low-pressure phase and the high-pressure–high-temperature phase. The complicated diffraction patterns of the high-pressure phase suggest that the phase transition occurred toward three directions around the c-axis of the low-pressure phase. This explains the difficulties encountered in previous structural analyses. The present results will provide key information for discussing the behavior of hydrogen bonds in these hydrous minerals under pressure. Keywords: Portlandite, phase transition, crystal structure, high pressure, X‑ray diffraction, hydrogen bond

Introduction and Stixrude 2006). In addition to interest in the fields of mate- The pressure responses of hydrogen-bearing materials repre- rial physics and crystallography, M(OH)2 is also important for sent an interesting research topic in various fields of science. In geoscience because it is one of the simplest model structures of hydrous minerals, thereby providing insights into processes of over 20 yr, hydroxides of a divalent metal, M(OH)2 (M = Mg, Ca, water transport in subduction zones at depths ranging from the Mn, Co, Ni, Cd, etc.), with a CdI2 structure (trigonal, space group P3m1), have been investigated intensively by various methods Earth’s surface to the deep mantle. such as optical spectroscopy, X‑ray and neutron diffraction, and Ca(OH)2 (portlandite), whose cation has the largest ionic ra- numerical simulations. These hydroxides are a research target dius among these M(OH)2 hydroxides, undergoes various unique because they show complicated structural and property changes, structural changes at relatively low pressures. In early studies, it probably related to hydrogen bond interactions. In spite of their was reported that a reversible pressure-induced amorphization occurs at 11 GPa and room temperature, based on broadening simple structure, diverse behaviors in M(OH)2 have been reported under high pressure, including high stability of the starting struc- of the OH vibration mode in IR spectra (Kruger et al. 1989) and ture over a wide pressure range (Fei and Mao 1993), pressure- the disappearance of powder X‑ray diffraction (XRD) peaks induced amorphization (Meade and Jeanloz 1990; Kruger et al. (Meade and Jeanloz 1990). However, the existence of a crystal- 1989; Nguyen et al. 1997), phase transitions (Duffy et al. 1995; line phase at high pressure and high temperature [Ca(OH)2-II, Ekbundit et al. 1996; Catalli et al. 2008; Iizuka et al. 2011), hereafter “high P-T phase”] was reported at >7 GPa and >200 repulsion between hydrogen atoms within interlayers (Parise et °C, and its crystal structure was determined by an in situ XRD al. 1998, 1999), and partial (H-sublattice) amorphization (Murli study by Kunz et al. (1996). Subsequently, Leinenweber et al. et al. 2001; Shieh and Duffy 2002). Proton disorder is predicted (1997) studied this phase using neutron diffraction and clarified from theoretical simulations (Raugei et al. 1999; Mookherjee the hydrogen positions. The authors recovered the high P-T phase to ambient pressure by quenching and decompression in liquid

* E-mail: [email protected] N2, and measured the neutron diffraction at 0.1 MPa and 11 K.

0003-004X/13/0809–1421$05.00/DOI: http://dx.doi.org/10.2138/am.2013.4386 1421 1422 IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE

Even by compression at room temperature, a transition from Improvements in single-crystal high-pressure apparatus an ambient-pressure phase (hereafter “low-P phase”) into a In single-crystal X‑ray diffraction experiments, clamped diamond-anvil cells crystalline high-pressure phase (hereafter “high-P phase”) was (DACs) were used. As described above, a platelet crystal was used. In conventional observed at around 6–8 GPa (Ekbundit et al. 1996; Catalli et al. techniques, the X‑ray beam is introduced parallel to the compression axis in the DAC. However, this geometry mainly yields information on the hk0 lattice plane, 2008; Iizuka et al. 2011), instead of amorphization, when the because of the sample’s strong cleavage (001) plane, which sits on the surface of applied pressure remained hydrostatic. Although the structure of the diamond anvil. This limitation makes it difficult to obtain reflections for the this high-P phase remains unknown, spectroscopic measurements whole reciprocal lattice space. To obtain information from l ≠ 0 reflections, we suggest that the hydrogen bonds show remarkable changes at the made modifications to a Radial-DAC (Fig. 1a). The Radial-DAC was originally phase transition (Iizuka et al. 2011). Thus, the determination of developed to observe uniaxial strain and preferred orientation in the sample by introducing X‑rays perpendicular to the compression axis; consequently, the use the detailed crystal structure of this high-P phase is interesting of an X‑ray-transparent gasket is indispensable (Merkel and Yagi 2005). Beryllium from the viewpoints of both Earth science and crystallography. or a combination of boron-epoxy and kapton film has been widely used as a gasket Previous powder X‑ray diffraction studies at high pressure have material. A beryllium gasket has the disadvantage of a very strong background, revealed the difficulty encountered in clarifying the crystal struc- whereas a boron-epoxy gasket cannot maintain the liquid pressure-transmitting medium during compression. ture of the high-P phase (Catalli et al. 2008; Iizuka et al. 2011) To overcome these difficulties, we made a small hole in one of the anvils and because: (1) the high-pressure behavior of portlandite is sensitive compressed the sample in the hole with a smaller culet anvil using a stainless steel to the hydrostaticity of the applied pressure, and it is difficult (SUS) gasket, as follows. First, a small cup-shaped, flat-bottomed hole (125 µm to obtain reproducible results; (2) the low-P and high-P phases in diameter and ∼20 µm deep) was drilled in the center of the large culet (800 µm coexist over a wide pressure range and it is difficult to obtain in diameter) using an excimer laser (excitation wavelength: 248 nm of KrF). The sample was loaded into the hole, and a SUS gasket with a 175 µm diameter hole clear diffraction patterns from the single high-P phase; and (3) was placed on the sample. Then, the holes were filled with glycerin as a pressure diffraction peaks of the high-P phase become broad and weak with increasing pressure. Although previous studies reported spectroscopic measurements using single-crystal samples of portlandite (Ekbundit et al. 1996; Shinoda et al. 2000; Iizuka et al. 2011), there are no reports of single-crystal X‑ray diffraction studies, probably because the phase transition is reversible at room temperature and the high-P phase is unquenchable. In addition, even in single crystals the transition does not occur instantaneously, the low- and high-P phases coexist over a wide pressure range, and the diffraction pattern of the high-P phase is complicated, as described below. In this study, we clarified the crystal structure of the high-P phase by combining powder and single-crystal X‑ray diffraction measurements. We obtained high-quality X‑ray diffraction patterns in the wide reciprocal lattice by improving upon exist- ing experimental techniques, and analyzed the structure of the high-P phase with the aid of theoretical calculations. Based on the structure thus obtained, we discuss the structural relations of the three polymorphs of portlandite (i.e., the low-P, high-P, and high P-T phases) and the mechanism of the phase transition.

Experimental methods

Sample preparation

Powder samples of Ca(OH)2 or Ca(OD)2 were synthesized by the hydration of CaO powders (assay minimum of 99.9%; Wako Pure Chemical Industries

Ltd.) with pure water H2O (milli-Q; Nihon Millipore Ltd.) or D2O (minimum isotope purity of 99.96 at%D; Aldrich Chemical Co. Inc.) at 235 °C for 1 week. Single crystals were obtained by gradually recrystallizing the synthesized powder sample on glass plates with excess H2O or D2O in desiccators. These synthesized samples were characterized using X‑ray diffraction and Raman spectra as described in detail by Iizuka et al. (2011). Many hexagonal prismatic single crystals with a diameter of 30–300 µm were grown, and the single crystals had a strong cleavage along the (001) plane. Microscopic observations under high pressure Figure 1. (a) Schematic diagram of the newly designed cell clarified that the shape of the single crystal remained almost unchanged by the assembly for single-crystal diffraction measurements. (b) Representative phase transition, although the transition was clearly visible. The transition of the oscillation photograph of the single-crystal portlandite obtained at 8.5 single-crystal sample did not occur at once, but domains (around 10–20 µm thick) GPa using the assembly shown in a. Forty-one images taken at 90° ± along the c-axis were transformed stepwise during the pressure increase from about 6 to 8 GPa. Thus, the low- and high-P phases coexisted in this pressure 20° were combined into a single image. Most of the diffracted reflections range. In this study, therefore, single crystals of ∼50 × ∼50 × 10–20 µm3 in size in the upper hemisphere were behind the shadow of the SUS gasket. were prepared by splitting large single crystals into the desired thickness along Several diffraction lines and a big spot at the left below come from the a (001) plane, following the cleavage. gasket and the diamond anvil, respectively. IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE 1423 medium (as described below). The assembly was compressed with the smaller etveld 1969) as coded in the General Structure Analysis System (GSAS) software culet anvil (350 µm in diameter), toward which the SUS gasket gradually bent with (Larson and Von Dreele 2000). For the low-P phase, an initial structure model for increasing pressure. This enabled incident and diffracted X‑rays to pass through the the refinement was taken from Iizuka et al. (2011) and Nagai et al. (2000). For sample and the top part of the larger anvil without being disturbed by the gasket, the high-P phase, we used a new structure derived from a molecular dynamics thereby yielding clear reflections with a high signal-to-noise ratio, even from such simulation based on density functional theory (see below). a thin sample under hydrostatic conditions. This also allowed us to obtain crystallographic information in the direction perpendicular to the platelet-shaped single Results crystal. A DAC with similar X‑ray geometry was reported by Glinnemann et al. (1992). In the present design, however, X‑ray beam never hits the metal gasket Powder X‑ray diffraction directly by passing through the diamond with a hole, and we can obtain much clear diffraction patterns from the sample. In the powder X‑ray diffraction analyses, the high-P phase In most powder and single-crystal diffraction experiments, conventional started to appear above about 6 GPa. With further compression, clamped DACs with an aperture angle of ±40° were additionally used. Some the amount of the low-P phase decreased and the amount of the of the powder X‑ray diffraction experiments employed a lever-and-spring type high-P phase increased, but the overall peaks gradually became DAC. A hybrid gasket assembly was adopted, consisting of a SUS outer and a polytetrafluoroethylene (PFA) inner ring, following Komatsu et al. (2011). The broad and weak. The pressure dependence of the diffraction pat- PFA ring (100–300 µm, inner–outer diameter) was placed in a 300 µm hole of the terns has been demonstrated previously by Iizuka et al. (2011). 100 µm thick SUS gasket. The PFA ring worked to keep the sample in the center The diffraction data obtained at 8.9 GPa were used for structure of the culet during compression, and to prevent the contamination of scattering and analysis of the low-P and high-P phases, because at this pressure attenuation by the SUS gasket. Initially, a 4:1 methanol-ethanol mixture was used the diffraction pattern of the high-P phase was strong enough for as a pressure-transmitting medium to maintain hydrostatic conditions, however, the single-crystal sample was susceptible to dissolution. Moreover, fractures commonly analysis and the broadening of the diffraction was not yet severe developed in the anvil when an anvil with a hole was used. In this study, therefore, (Fig. 2). Using 14 clear diffraction lines from the high-P phase glycerin was used as a pressure medium because of its high viscosity and reduced with d-values ranging from 5.3 to 1.7 Å, we performed powder volatility compared with methanol-ethanol. The use of glycerin resulted in relatively diffraction pattern indexing to determine the unit cell. Cells with simple sample preparation. Although the freezing pressure of glycerin is reported to be around 6.5–7.0 GPa (Osakabe and Kakurai 2008; Klotz et al. 2012), it remains symmetry higher than orthorhombic were not consistent with the quite soft above the freezing pressure, and no notable difference was found in the observed diffraction for both indexing methods (DICVOL and transition behavior of portlandite when using glycerin vs. methanol–ethanol as Cell Finder). When the symmetry was lowered to monoclinic, the pressure medium. Pressure was determined by the ruby fluorescence method several possible unit cells were found, with the best fit yielding by Mao et al. (1986), or from the pressure dependence of the Raman shift of the the following cell parameters: a = 5.96, b = 6.84, c = 8.93 Å, β OH stretching peaks of portlandite by Catalli et al. (2008) and Iizuka et al. (2011). = 103.94°. However, there were other possible unit cells, and X‑ray diffraction measurements it was difficult to uniquely determine the unit cell from these The single-crystal X‑ray diffraction measurements were performed using both a analyses alone. laboratory X‑ray source and synchrotron radiation. For the laboratory measurement, we used a micro-focused X‑ray generator (MicroMax-007; Rigaku) with a Mo rotat- Single-crystal X‑ray diffraction ing target (λ = 0.7107 Å, 50 kV, 24 mA) and confocal mirror optics (Varimax-Mo; Table 1 provides a summary of the experimental conditions Rigaku), housed at the Geochemical Research Center (GCRC), the University of Tokyo. Incident X‑rays were collimated using a single pinhole collimator with a of the single-crystal diffraction analyses, which were performed diameter of 300 µm or 50 µm. Single-crystal oscillation photographs were taken parallel or normal to the c-axis. At each of the side-windows of using an imaging plate (IP) X‑ray diffractometer (R-AXIS IV++; Rigaku), with the the Radial-DAC, a set of 41 diffraction images, each covering a sample being oscillated by ±0.5° during recording of diffraction patterns. The mea- range of ±0.5°, were obtained by varying the initial angle from surement was repeated by rotating the sample stepwise at 1° increments. The Radial –20 to +20°. Figure 1b shows a combined image of 41 images at DAC used in the present study has four side-windows, each of which has ±25° openings, and measurements were performed in each window. The investigated 8.5 GPa. Data were measured through the four side-windows of pressures ranged from ambient condition up to about 9.0 GPa. A sample-to-detector distance was 150 mm, and the exposure time was 20 min for each measurement. Synchrotron X‑rays (λ ≈ 0.41 Å, ca. 30 keV) were used at the NE1 beamline of the Photon Factory, Advanced Ring (PF-AR), High Energy Accelerator Research Organization (KEK), Tsukuba. The shorter wavelength of the synchrotron X‑ray allowed us to obtain higher-index diffractions and reciprocal lattice images with less distortion. The incident X‑rays were collimated to a diameter of 75 µm, and measurements were made in the same manner as those in the laboratory experiments. The sample was oscillated by ±2.5° and measurements were repeated at 5° increments of sample rotation. The diffraction data in the pressure range of 8.0–9.0 GPa were complementally obtained from both geometry using the Radial-DAC and the conventional DAC (along the compression axis). The exposure time for each measurement was 5–10 min. The powder diffraction measurements were conducted using synchrotron X‑rays at the same beamline (PF-AR-NE1) as for the single-crystal measurements. Diffraction patterns were measured at high pressure up to above 20 GPa every 2–3 GPa (Iizuka et al. 2011). The two-dimensional diffraction image from the powder sample on an IP was converted into an intensity–2θ diffraction pattern up to 25° in 2θ using the software IPAnalyzer, as developed by Seto et al. (2010)

Figure 2. Result of the Rietveld refinement of Ca(OD)2 at 8.9 GPa. Indexing and structure refinement analyses Crosses and lines represent observed diffraction and calculated profiles, Indexing of the powder pattern was performed using two methods, DICVOL respectively. Vertical bars indicate the positions of Bragg reflections for the (Boultif and Louër 2004) and Cell Finder in the software PDIndexer (Seto et al. high- (upper) and low-P (lower) phases. The line below the profile indicates 2010). Crystal structure refinement was performed using the Rietveld method (Ri- the difference profiles between the observed and calculated patterns. 1424 IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE the Radial-DAC in the same manner, and then all the images were Analysis of crystal structure of the high-pressure phase accumulated into a single reciprocal space using CrystalClear Among the three possible space groups listed above, only software (Rigaku Corp.). After eliminating the reflections from I121(no. 5) is consistent with a reasonable arrangement of Ca and the anvils and gasket, the unit cell was derived to satisfy the re- O atoms in the observed unit cell with a chemical composition of flections from the sample, yielding the following cell parameters: Ca(OH)2. The structure was also estimated using a first-principles a = 5.8, b = 7.1, c = 9.0 Å, β = 104.0° of the monoclinic system. molecular dynamic simulation based on density functional theory This result is consistent with that obtained from powder diffrac- as our previous studies on high-pressure behavior of hydrous tion analyses at 8.9 GPa (see above). However, the orientation minerals by Tsuchiya et al. (2002, 2005). When the portlandite matrix could not be determined accurately and integrated inten- structure was compressed up to 10 GPa at 0 K, the structure was sities had a large R-factor for equivalent reflections, resulting compressed but remained basically unchanged. The temperature from the elongate nature of the diffraction spots. By including was then increased to 500 K and a stable structure was calculated the images with more reflections of lower d-values (∼1 Å) ob- at constant temperature. Some of Ca atoms shifted along the c- tained by using synchrotron X‑rays, all the reflections satisfied axis, and the Ca and O configurations changed into those quite the following selection rules for a monoclinic cell similar to the structure model of a space group I121. Using this structure as an initial structure model, further refinements of For hkl: h + k + l = 2n; 0kl: k + l = 2n; hk0: h + k = 2n; the unit-cell parameters and atomic positions were carried out h0l: h + l = 2n; h00: h = 2n; 00l: l = 2n; (1) using Rietveld analysis for the powder diffraction pattern shown 0k0: k = 2n. in Figure 2. The coexisting low-P phase was also successfully refined by means of multi-phase Rietveld analysis. The cell Given the above reflection conditions, we identified three parameters and the atomic positions (except for the positions of possible body-centered space groups: I121 (no. 5), I1m1 (no. hydrogen atoms) were clarified (Tables 2 and 3, respectively). 8), and I12/m1 (no. 12). (CIF1 available on deposit.) Table 1. Experimental conditions of single-crystal X-ray diffraction measurements Diamond-anvil cell Boehler-Almax Radial-DAC 1 Deposit item AM-13-809, CIF. Deposit items are available two ways: For a paper Pressure-transmitting medium 4:1 Methanol-ethanol Glycerin copy contact the Business Office of the Mineralogical Society of America (see X-ray source MoKα or Synchrotron (PF-AR-NE1) inside front cover of recent issue) for price information. For an electronic copy visit Wavelength (Å) 0.71069 / 0.41138 the MSA web site at http://www.minsocam.org, go to the American Mineralogist Exposure time (s) 120–240 300–600 Contents, find the table of contents for the specific volume/issue wanted, and then Oscillation angle (°) ±38 ±20 × 4 click on the deposit link there.

Table 2. Summary of Rietveld refinement of powder X-ray diffraction data of Ca(OD)2 Crystallographic data Phase Low-P High-P High P-T (This study) (This study) (Leinenweber et al. 1997) Measurement condition 8.9 GPa, 300K 0.1 MPa, 11K (synthesized at 9 GPa, 400 °C)

Space group P3m1 (no. 164) I121 (no. 5) P21/c (no. 14) Lattice parameter (Å) a 3.4631(3) 5.8882(10) 5.3979(4) b 3.4631(3) 6.8408(9) 6.0931(4) c 4.4471(8) 8.9334(15) 5.9852(4) Monoclinic angle (°) β 104.798(18) 103.581(6) 3 Unit-cell volume (Å ) Vc 46.188(10) 347.90(10) 191.34(1) Z 1 8 4

Molecular weight Mr 76.1 76.1 76.1 3 Molar volume (calculated) (cm /mol) Vm 27.8 26.18 28.8 Density (calculated) (g/cm3) d 2.74 2.91 2.64

Condition of Rietveld analysis Source Synchrotron X-ray (PF-AR NE1) Neutron Wavelength (Å) 0.41138 Exposure time (s) 600 d-range (Å) 0.9–5.9 No. data points 1024 No. parameters 28

Rp*(%) 0.47 Rwp†(%) 0.63 Re‡(%) 0.25 R (F2)§ (%) 17.4 S|| 2.09 1/2 1/2   Notes: * Ry(%) =−|(fx)|/ y ×100, † R (%) = w [y f (x)]2 / w y2 100, ‡ R = NP− × , pi∑∑ii ii wp { i i i i i i i } e (%)  2  100  ∑ iiwyi  22 2 ∑ FK0 − Fc Rwp § RF( )(%) = ×100 , || S = 2 R ∑ F0 e

where yi = observed intensity; fi(x) = calculated intensity; wi = statistical weight; N = number of data points; P = number of variables; Fo = observed structure factor; Fc = calculated structure factor; K = scale factor for which the sum extends over all the observed reflections. Calculation was made after background has been subtracted. IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE 1425

Table 3. Structure parameters of Ca(OD)2 at 8.9 GPa and 300 K Komatsu et al. (2007) reported a similar transition mecha- 2 Phase Atoms Site g x y z Uiso* ×100 (Å ) nism in gibbsite, γ-Al(OH)3, which also has a layered structure

High-P Ca1 4c 1 0.208(3) 0† 0.1980(14) 0.78(9) and transforms into a high-P phase of η-Al(OH)3 at 3 GPa. The Ca2 4c 1 0.710(2) 0.736(2) 0.1683(14) 0.78(9) O1 4c 1 0.390(6) 0.267(12) 0.137(4) 2.6(4) authors reported that the main mechanism of the phase trans- O2 4c 1 0.727(9) 0.587(4) 0.407(5) 2.6(4) formation is a layer shift of the a-b plane, which is formed by a O3 4c 1 0.560(6) –0.032(9) 0.367(4) 2.6(4) network of AlO6 octahedra. In the case of portlandite, however, O4 4c 1 0.316(6) 0.744(7) 0.079(4) 2.6(4) the transformation involved both a layer shift and vertical dis- Low-P Ca1 1a 1 0 0 0 0.78(9) placements of in-plane Ca atoms. As a result, Ca atoms shifted O2 2d 1 1/3 2/3 0.256(3) 2.6(4) closer to one of the oxygen atoms in the neighboring layer, * The isotropic atomic displacement parameters (Uiso) of Ca and O atoms between the low- and high-P phases were constrained to be the same, respectively [i.e., forming a new bond. The coordination number of Ca effectively

Uiso(CaLP) = Uiso(CaHP), Uiso(OLP) = Uiso(OHP)]. increased from six to seven at the phase transition (Table 4). † The y position of Ca of the high-P phase. Structure relations Discussion Here, the structure of the high-P phase is compared with that Crystal structure of the high-P phase of the high P-T phase [Ca(OH)2-II]. The high P-T phase was identified using powder X‑ray diffraction at 7.2 GPa and 300 °C The reversible change from the low-P phase to the high-P by Kunz et al. (1996). Subsequently, Leinenweber et al. (1997) phase at room temperature suggests that the phase transition was performed a detailed analysis of the crystal structure of the high accompanied not by significant atomic diffusion but by small P-T phase synthesized at 9 GPa and 400 °C, using powder neu- displacements of atoms. Figure 3 compares the two crystal struc- tron diffraction data obtained at 0.1 MPa and 11 K. The authors tures at 8.9 GPa, viewed from the same direction. A comparison reported a detailed structure including the hydrogen positions. of the cell parameters of the low-P and high-P phases reveals Figure 4 compares the high-P and high P-T phases viewed along the following relations the a and b directions. To enable a visual comparison, the a- and

aaHP ≈ 3;LP ≈ bbHP 2;LP (2)

ccHP ≈ 2.LP

A comparison of Figure 3a with 3b shows that the a-b plane, which is formed by a network of CaO6 polyhedrons, is preserved. However, the c-axis, which was originally perpendicular to the a-b plane, was inclined from 90 to 104.80°. At the phase transition, the a-b plane shifted along the a-axis, and Ca atoms aligned in the direction of the b-axis within the a-b plane shifted alternately up and down. The molar volume decreased by ~5.8% at 8.9 GPa.

Figure 4. Structural relation between (a) the high-P phase and (b) the high P-T phase. The crystal structure of the high P-T phase was determined at 0.1 MPa and 11 K (Leinenweber et al. 1997). For comparison, the a- and c-axes of the high P-T phase were changed from the original figure, from a( , b, c) to (a', b', c'), via the following transformation matrix:

⎛ 0 0 1 ⎞ ⎜ ⎟ (a'b'c') = (abc)⎜ 0 −1 0 ⎟ ⎝ 1 0 0 ⎠

Table 4. Ca-O interatomic distances of the two Ca sites in the high-P phase at 8.9 GPa Atoms Distance (Å) Atoms Distance (Å) Ca1-O4 2.22(5) Ca2-O4 2.18(4) Ca1-O3 2.24(3) Ca2-O4 2.25(4) Ca1-O1 2.25(7) Ca2-O3 2.34(5) Ca1-O1 2.34(6) Ca2-O3 2.35(6) Ca1-O2 2.56(5) Ca2-O2 2.55(3) Figure 3. Crystal structure of (a) the low-P phase and (b) the high-P Ca1-O4 2.63(4) Ca2-O1 2.55(3) phase of portlandite from the perspective of (upper) the a-b plane, and Ca1-O2 2.70(5) Ca2-O3 2.69(5) (lower) the b-axis. Frames indicate the unit cell of the low-P phase and Ca1-O3 3.55(6) Ca2-O3 3.70(8) the high-P phase, respectively. Note: Average of 7-bond length (Å) (left) 2.422; (right) 2.416. 1426 IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE c-axes of the high P-T phase are exchanged from the original on the hk0 reciprocal lattice can be explained by the overlap of literature (see footnote in Fig. 4). These crystal structures show reflections of the high-P phase, which was formed in three dif- similar arrangements of atoms in both phases, but each a-b plane ferent directions by the phase transition. Figure 6 shows the hk0 in the high P-T phase is more “expanded” in the c-axis direction reciprocal lattices. Using the lattice parameters of the low-P and than that in the high-P phase. This observation suggests that high-P phases listed in Table 2, the reciprocal lattice parameters –1 –1 increasing temperature enhanced the structural relaxation and were calculated to be aLP* = bLP* = 0.333 Å , cLP* = 0.225 Å , –1 resulted in further displacements of Ca atoms from the high-P γLP* = 60° for the low-P phase; and aHP* = 0.176 Å , bHP* = –1 –1 phase to the high P-T phase. 0.146 Å , cHP* = 0.115 Å , βHP* = 75.2° for the high-P phase. Although the Ca-O atomic distances of the high-P phase show Figure 6a shows these two reciprocal lattices overlapping with a wide scatter from 2.18 to 2.70 Å (Table 4), those in the high each other. Figure 6b shows three reciprocal lattices of the high-P P-T phase are much less scattered and remained in the range phase, rotated by 60° and 120° about the origin of the recipro- from 2.23 to 2.43 Å (Leinenweber et al. 1997). This finding cal lattice (black dot in the figure). Many of the lattice points indicates that with increasing temperature, Ca atoms move to of the high-P phase appear very close to those of the low-P stable position, with a more regular sevenfold coordination in phase (largest yellow dots) when the high-P phase is rotated. In the high P-T phase. To date, no study has observed the transition Figures 5b2 and 5c2, all the reflections with high intensity can from the high P-T phase to the high-P phase with decreasing be explained by these yellow dots. All the other weak reflection temperature. This suggests that the high-P phase might be a meta- spots can also be indexed by the lattice points of one of the three stable phase that exists only as an intermediate state between the rotated high-P phases. As a result, the complicated diffraction sixfold-coordinated low-P phase and the sevenfold-coordinated pattern from the high-P phase is well explained by the overlap high P-T phase. of the reciprocal lattices. In the real space, the low-P phase has trigonal symmetry and there are three equivalent directions in Microscopic observations and phase transition mechanism the a-b plane. Therefore, the layer shift of the a-b plane at the Figure 5a shows a representative oscillation X‑ray photograph transition may occur toward 0°, 120°, or 240°, with equal prob- of the single crystal, which was observed when the c-axis of ability in each case. portlandite was placed parallel to the X‑ray beam. New dif- Optical microscope observations under high pressure re- fraction spots from the high-P phase started to appear at above vealed that the crystalline shape of the low-P phase remained 6 GPa. Some of the new spots appeared very close to those of almost unchanged through the phase transition, but cracks ap- the low-P phase without any overlaps, but the intensity of these peared perpendicular to the c-axis. In addition, before the phase specific spots became much denser. Initially, strong diffraction transition, when the crystal was observed perpendicular to the spots appeared near the 100 and 010 reflections of the low-P c-axis under crossed nicols, the entire crystal showed extinc- phase (Fig. 5b1), and several weak spots appeared surrounding tion simultaneously. In contrast, after the transition, the crystal these strong spots. This complicated reflection pattern observed contained many domains of lamellae with different extinction angles. These observations are consistent with the transition mechanism described above. A reversible single-crystal to single-crystal transition was re-

ported in Zn(OH)2 at about 1.1 GPa at room temperature (Kusaba et al. 2007). In this case, the low-P phase is orthorhombic and the high-P phase is tetragonal. Because of this symmetry, there is only one possible direction for the transformation to occur when the crystal transforms from the low-P to the high-P phase, and the high-P phase retains good crystallinity. In portlandite, however, the crystallinity of the high-P phase is low because there are three possible equivalent directions for the layer shift of the a-b plane at the transition, which results in many twins in the high-P phase. As mentioned in the Introduction, brucite-type layered hydroxides show a wide range of high-pressure behaviors, and no structurally related mineral is known to have a similar phase transition to that of portlandite. In many isostructural oxides (e.g., olivine, spinel, and perovskite), the high-pressure behavior changes systematically depending on the cation size. In layered Figure 5. Typical X‑ray diffraction patterns of single crystal (a) hydroxides, however, the behavior is much more complicated before and (b, c) after the phase transition, observed parallel to the and shows greater variety, probably because the hydrogen bond c-axis. Debye rings originate from the SUS gasket. New diffraction spots appeared at around 6.3 GPa, as shown in b1. The diffraction patterns network in the structure plays an important role in controlling of a and b are from the same sample observed using a laboratory X‑ray the high-pressure behavior. In fact, it is expected that the phase source, and (c) was measured at 9.0 GPa using synchrotron X‑rays. Parts transition of portlandite is accompanied by a large geometrical b2 and c2 represent the lattice points (Fig. 6b). All the lattice points can change in hydrogen bonds, as deduced from spectroscopic mea- be explained by the overlap of three reciprocal lattices (see text). surements of OH vibration modes (Ekbundit et al. 1996; Catalli IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE 1427

(a)

010 bLP*

HP b * aLP* 100 aHP*

(b)

010 bLP*

bHP* aLP* 100 aHP*

0° 60° 120°

Figure 6. Reciprocal lattices of the hk0 plane of a. The thin-line (red) and thick-line (blue) nets represent the reciprocal lattices of the low-P and high-P phases, respectively, which are drawn so that one of the lattice points (represented by a solid black dot) coincides. (b) Reciprocal lattices of the high-P phase (denoted by purple and green rectangular nets) overlapped by rotating 60° and 120° about the center black dot, together with the reciprocal lattice of the low-P phase. The largest yellow dots represent the positions where the lattice points from the four reciprocal lattices are located very close to each other. 1428 IIZUKA ET AL.: CRYSTAL STRUCTURE OF HIGH-PRESSURE PHASE OF PORTLANDITE et al. 2008; Iizuka et al. 2011). However, little is known about under high pressure. Chemical Physics Letters, 437, 61–65. Larson, A.C., and Von Dreele, R.B. (2000) General structure analysis system the behavior of hydrogen bonds under high pressure, because (GSAS). Los Alamos National Laboratory, Report LAUR 86-748. hydrogen atoms are almost invisible to X‑rays. The present re- Leinenweber, K., Partin, D.E., Schuelke, U., O’Keeffe, M., and Von Dreele, R.B. sults of X‑ray diffraction had also difficulty in the refinement of (1997) The structure of high pressure Ca(OD)2 II from powder neutron diffraction: Relationship to the ZrO2 and EuI2 structures. Journal of Solid State the hydrogen positions. Alternatively, it is therefore important to Chemistry, 132, 267–273. clarify this behavior by using neutron diffraction analyses, and Mao, H.K., Xu, J., and Bell, P.M. (1986) Calibration of the ruby pressure gauge to the results of this paper will provide key information for such 800 kbar under quasi-hydrostatic conditions. Journal of Geophysical Research, 91, 4673–4676. detailed structure analyses, including the hydrogen positions. Meade, C., and Jeanloz, R. (1990) Static compression of Ca(OH)2 at room temperature. Geophysical Research Letters, 17, 1157–1160. Acknowledgments Merkel, S., and Yagi, T. (2005) X‑ray transparent gasket for diamond anvil cell high pressure experiments. Review of Scientific Instruments, 76, 046109. The authors are grateful to T. Okada and D. Hamane-Nishio for support with Mookherjee, M., and Stixrude, L. (2006) High-pressure proton disorder in brucite. synchrotron X‑ray diffraction experiments at PF-AR. This work was supported by American Mineralogist, 91, 127–134. JSPS through a Grant-in-Aid for Young Scientists (DC2), by Creative Scientific Murli, C., Sharma, S.M., Kulshreshtha, S.K., and Sikka, S.K. (2001) High-pressure Research (19GS0205), by Innovative Areas (No. 2002), and by the Global COE behavior of β-Ni(OH)2 – a Raman scattering study. Physica B, 307, 111–116. Programs for Chemistry Innovation and for Deep Earth Mineralogy from the Nagai, T., Hattori, T., and Yamanaka, T. 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Manuscript received October 18, 2012 Kusaba, K., Yagi, T., Yamaura, J., Miyajima, N., and Kikegawa, T. (2007) Single- Manuscript accepted April 23, 2013 crystal to single-crystal phase transition with a large deformation in Zn(OH)2 Manuscript handled by Alexandra Friedrich