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Structure refinement of sub-cubic-mm volume sample at high pressuresby pulsed neutron powder diffraction:application to brucite in an opposedanvil cellTakuo Okuchia, Naotaka Tomiokaa, Narangoo Purevjava, Jun Abeb,Stefanus Harjob & Wu Gongb

a Institute for Study of the Earth's Interior, Okayama University,Misasa, Tottori 682-0193 Japanb J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki319-1195, JapanPublished online: 29 Apr 2014.

To cite this article: Takuo Okuchi, Naotaka Tomioka, Narangoo Purevjav, Jun Abe, Stefanus Harjo &Wu Gong (2014) Structure refinement of sub-cubic-mm volume sample at high pressures by pulsedneutron powder diffraction: application to brucite in an opposed anvil cell, High Pressure Research:An International Journal, 34:2, 273-280, DOI: 10.1080/08957959.2014.909931

To link to this article: http://dx.doi.org/10.1080/08957959.2014.909931

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High Pressure Research, 2014Vol. 34, No. 2, 273–280, http://dx.doi.org/10.1080/08957959.2014.909931

Structure refinement of sub-cubic-mm volume sample at highpressures by pulsed neutron powder diffraction: application to

brucite in an opposed anvil cell

Takuo Okuchia∗, Naotaka Tomiokaa, Narangoo Purevjava, Jun Abeb†, Stefanus Harjob

and Wu Gongb

aInstitute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193 Japan;bJ-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

(Received 7 February 2014; final version received 26 March 2014)

Neutron powder diffraction measurements of 0.9 mm3 of mixture of deuterated brucite and pressuremedium were conducted at pressures to 2.8 GPa, using an opposed anvil cell and a medium-resolutiondiffractometer at Japan Proton Accelerator Research Complex pulsed neutron source. Spurious-freediffraction patterns were successfully obtained and refined to provide all structural parameters includ-ing Debye–Waller factors. Tilting of hydroxyl dipoles of brucite toward one of the three nearest-neighboroxygen anions was confirmed to be substantial at pressure as low as 1.5 GPa. By this application, technicalfeasibility to analyze such a small sample has been newly established, which would be useful to extendthe applications of neutron diffraction at high pressures.

Keywords: pulsed neutron powder diffraction; structure refinement; hydrogen

1. Introduction

Structure refinement by neutron powder diffraction is one of the most versatile methodologywhich has been widely applied for solid-state physics and chemistry, materials science, as wellas mineralogical researches at high pressures.[1] It has been effectively applied when tens ofcubic millimeters of sample volume are available to be compressed in suitable large-volumeapparatuses.[2] On the other hand, high pressure apparatuses with smaller sample capacities weremuch less popular to be effectively coupled with structure refinement studies. Previous worksusing diamond and sapphire anvil cells have achieved remarkable progresses in magnetic neu-tron diffraction studies including those at very high pressures, as represented by Goncharenkoet al.[3,4] The diamond anvil cell has been also recently applied for structure analysis of icesat very high pressures using neutron powder diffraction, which indicates the good potential ofsmall-volume cells for the lattice structure refinements when it is coupled with the strong pulsedsource.[5,6] In addition to these works, we have recently established a capacity-increased sap-phire anvil cell design having very high neutron transparency, which was effectively coupledwith a medium-resolution powder diffractometer installed at Japan Proton Accelerator Research

∗Corresponding author. Email: okuchi@misasa.okayama-u.ac.jp†Present Address: Research Center for Neutron Science and Technology, CROSS-Tokai, Tokai, Ibaraki 319-1106, Japan.

© 2014 Taylor & Francis

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Complex (J-PARC) to obtain a high-quality pattern best suitable for lattice structure refinement.[7]In this paper, we further applied this scheme for the structure refinement of even much smallersample with sub-cubic-millimeter volume. By this application, technical feasibility to analyze asmall sample has been solidly established, which would be useful to extend the use of neutronpowder diffraction at high pressures into even a wider range of research topics.

2. Experimental

2.1. Overview

Atomic-scale structures around hydrogen atoms in hydrous minerals may significantly changewith increasing pressure, which affect thermodynamic stability, optical properties (Raman, IR,etc.), as well as transport phenomena of the relevant minerals. To clarify these structure changesaround hydrogen occurring in a prototypical hydrous mineral, brucite (magnesium hydroxide) wasselected to be studied here.[8,9] In order to quantitatively evaluate the actual structural differencesbetween ambient and high pressure conditions, the sample was measured at both conditions by theidentical neutron spectrometer and optics, which were previously established to be best suitablefor the pulsed powder diffraction measurements using the opposed anvil cell.

2.2. Sample preparation

Fully deuterated brucite powder with the stoichiometry of Mg(OD)2 and with appropriate grainsize for neutron diffraction (ca. 5–10 µm) was synthesized from the MgO reagent and D2O liquidin an autoclave at 240◦C and 40 MPa. The synthesized powder was dried in a vacuum and thenevaluated by powder X-ray diffraction and micro-Raman spectroscopy (Figure 1). It was confirmedthat neither additional phase nor light hydrogen (protium) was contaminated within the sample;its negligible Raman intensity of OH stretching vibration was the established evidence for >99%of deuteration.[10,11] Then it was charged into the cell along with small amounts of deuteratedglycerin pressure medium and some ruby spheres with ∼10 µm in diameter. While the anvilswere uniaxially loaded together to compress the sample between them, the liquid glycerin worksto alleviate the stress in the sample chamber.

Figure 1. Raman spectrum of the synthesized Mg(OD)2. The OD vibration at 2693cm−1 was dominant, while the OHvibration at 3649cm−1 was negligible.

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1

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Figure 2. Schematic cross-section of the cell design. (1) Body, (2) screw plate, (3) tungsten carbide pusher plate, (4)piston, (5) piston-side anvil bases (inner part made of stainless steel), (6) cylinder-side anvil bases (inner part made ofstainless steel), (7) single-crystal diamond anvils, (8) Ti–Zr gasket, and (9) M10 set screws (all made of stainless steel).The parts without material descriptions were made of beryllium–copper alloy.

Figure 2 shows the diagram of the cell used for the current study. We adopted single-crystaldiamond anvils having 2 mm flat culet in diameter, about 6 mm in girdle diameter and about4 mm in height to compress the sample. Geometrical axes of these anvils were close to thecrystallographic [100] direction of diamond, which were therefore approximately parallel to theincident neutron beam path. Our other available choices include sapphire,[7] moissanite andpolycrystalline diamond,[12,13] which were more effectively used at lower or higher pressureregimes than the current experimental conditions. The sample and pressure medium were filledtogether into a cylindrical hall of 1.2 mm in diameter and 0.8 mm in thickness prepared withina titanium–zirconium “null” alloy gasket with 11.6 mm in diameter and 4.8 mm in thickness ofits outermost rim. About 0.9 mm3 volume of the sample–medium mixture was then compressedbetween the two diamond anvils. The fully hardened Ti–Zr alloy is sufficiently strong to confinethe sample while it is free from induction of any spurious Bragg reflection during the beamexposure.[1,7]

2.3. Neutron diffraction measurements

Diffraction measurements were conducted at the engineering materials diffractometer “TAKUMI”installed at the BL19 port in J-PARC Materials and Life Science Experimental Facility (MLF).[14]At TAKUMI, two detector banks cover 90 ± 15◦ horizontal and ±16◦ vertical scattering anglesfrom the sample position to provide the resolution of !d/d ∼ 0.3% in its medium-resolutionmode. This mode provides not only sufficient resolution along with symmetric peak shapes, butalso moderately strong neutron beam intensity. The opposed anvil cell has two wide openingsfor the diffracted beams to make full use of the two detector arrays at TAKUMI. Time-of-flightpath of the instrument is L1 = 40 m before the sample and L2 = 2 m after the sample to thedetector arrays, and the pulse repetition rate was 25 Hz in the single frame mode, where the rangeof d-spacing was set between 0.7 and 3.0Å through the tuning of the chopper rotation phase.

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This range was selected to cover all the important diffraction peaks of brucite except for the001 reflection occurring at ∼4.7Å. While this reflection at the largest d-value is still desirableto be detected, it is possible only when 12.5 Hz double frame mode was adopted where all thereflection intensities must become half of the 25 Hz mode, which are fatal for the analysis of thesmall sample.

For the measurements both at ambient and at high pressure conditions, a one-dimensionalneutron-focusing device [15] was installed in the incident beam path at the position closest possibleto the sample position. Using this device, the beam intensity at the center of the sample positionwas increased by about factor 1.5–2.5 with increasing d-value, while the resolution at largerd-values was gradually degraded along with more effective focusing. Radial collimators of 2 mmgauge volume were also set to both of the two detector banks, which worked effectively toshield spurious neutrons coming from outside of the sample position. The sample at ambientcondition was measured in a standard vanadium container with 6 mm in diameter. The sample athigh pressures in the opposed anvil cell was measured along with the multiple collimator systemwhich was directly equipped to the cell.[7] The collimated beam passed through the cylinder-sideanvil, diffracted by the high pressure sample, passed through the Ti–Zr gasket, and finally arrivedat the detector arrays. In addition to the multiple collimator system, we covered the whole cell bycadmium foils except for the openings for the diffracted beam. The beam exposure times were 2 hfor the sample at ambient condition, 8 h at 1.5 GPa and 9 h at 2.8 GPa pressures, respectively. Theproton beam power was 210 kW at ambient condition and 280 kW at high pressures. The samplepressures were optically determined by a ruby fluorescence spectrometer system available on siteat J-PARC MLF, along with a typical measurement error of ±0.1 GPa.

2.4. Attenuation correction

For each measurement, two-dimensional intensity profiles were obtained from the detector arrays,which were raw intensity data as a function of time-of-flight (corresponding to d-value) anddetector positions. The high pressure profiles were corrected by removing all the overlappedsingle-crystal diffraction spots of diamond anvils at their corresponding d-values.Then no spuriousspots or peaks remained in the high pressure profile to provide absolutely pure sample powderpattern. These corrected profiles, and also that at ambient pressure, were all laterally integratedinto one-dimensional patterns as a function of the d-value.

While attenuation through the diamond single-crystal reflections at their specific wavelengthscould induce some glitches in the sample powder patterns, it did not seem to affect the currentresults. To conform the possible occurrence of such glitches, we have carefully analyzed theintensities of sample peaks as a function of diffraction angle 2θ , as shown in the inset of Figure 3(b).The inset shows the normalized intensity of 110 reflection of brucite observed at d = 1.56Å at2.8 GPa pressure, as the function of 2θ which covered the neutron wavelengths between 1.9and 2.5Å along with the Bragg’s law. As shown, we did not observe any glitches. Like this,other major diffraction peaks and the baselines in the two observed patterns did not involve theglitches. This may be because of the much smaller sample pressure compared with the elasticmodulus of diamond, where its strain remained insignificant. Then, diffraction conditions foralmost-unstrained single-crystal diamond are only satisfied within a very narrow width which istoo small to be observed with the current wavelength resolution. We note that glitches shouldbe observed at higher pressures where the strain of diamond increases to expand the wavelengthwidth of the diffraction condition.

The attenuation correction for the high pressure patterns through the beam path within theTi–Zr gasket (about 4 mm, based on the gasket geometry) was calculated and applied with ourexperimentally calibrated equation µ = 0.013λ + 0.023(mm−1), which is in good agreement with

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2q [deg]

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2.8 GPa

Figure 3. Time-of-flight neutron powder diffraction patterns of fully deuterated brucite at (a) 1.5 GPa and (b) 2.8 GPapressures. The observed intensities after background subtraction are shown with points, and the calculated profiles areshown with solid lines. Fitting residual is shown at the bottom of each profile. It was confirmed that all the observed peakscame from the sample. The inset diagram in (b) shows the normalized intensity of 110 reflection as the function of 2θ .

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the reported value.[1]A standard intensity profile of bare vanadium cylinder with 2 mm in diameterand 3 mm in height was separately measured and applied for the normalization of the high pressureprofiles, while that with 3 mm in diameter and 3 mm in height was also measured and applied forthe ambient pressure profile.

We note that neither empty cell profile nor vanadium profile confined within the cell is requiredto be measured in these correction procedures, as in the previous case.[7]

3. Results and discussion

Figure 1 shows the obtained powder diffraction patterns of Mg(OD)2 at high pressures alongwith corresponding profiles of their refinements. In spite of the small sample volume, qualityof these patterns obtained at high pressures was really satisfactory and suitable for the structureanalyses. The peak resolutions were !d/d = 0.3% at d < 2Å and 0.5% at d = 2.6Å at ambientcondition (not shown), while they become !d/d = 1.0% at d < 2Å and 1.3% at d = 2.6Å afterthe compression to 2.8 GPa. Degradation of the resolution after the compression was substantial,while it is still sufficient to resolve all the significant reflections as shown in Figure 1.

The structure refinements were conducted using the ‘Z-Rietveld’code developed at J-PARC forneutron time-of-flight powder diffraction studies.[16] Table 1 shows the results. Brucite structureat the ambient condition belongs to the P3̄m1 space group, which consists of stacked paral-lel sheets made of octahedrons of magnesium cations and oxygen anions, where hydrogen (ordeuterium) is attached with each oxygen anion to form two-dimensional triangular hydrogen lat-tices along both side of each magnesium–oxygen sheet. All these dipoles are directed normalto the magnesium–oxygen sheet along with the asymmetric “riding” motion, which were pre-viously analyzed by single-crystal or powder neutron diffraction studies.[17–20] In our resultsat the ambient condition (left two columns of Table 1), two structure models with 6i and 2d

Table 1. Refined structure parameters of Mg(OD)2.

P (GPa) 10−4 10−4 1.5 1.5 2.8 2.8model D at 6i D at 2d D at 6i D at 2d D at 6i D at 2d

a (Å) 3.14616(2) 3.14614(2) 3.1273(7) 3.1270(1) 3.1142(1) 3.1142(4)c (Å) 4.7540(1) 4.7540(1) 4.6571(22) 4.6575(9) 4.5954(6) 4.5954(13)V (Å3) 40.753(1) 40.753(1) 39.444(11) 39.441(4) 38.596(3) 38.596(6)z for O 0.2193(6) 0.2203(7) 0.223(4) 0.219(4) 0.229(2) 0.230(2)

x for D 0.3669(18) 0.3333 0.387(5) 0.3333 0.386(3) 0.3333z for D 0.4158(6) 0.4165(6) 0.421(4) 0.415(2) 0.431(2) 0.433(2)D…Da 0.317(17) 0.50(4) 0.49(3)

Uiso for Mgb 1.1(2) 1.0(1) 3.1(7) 2.6(4) 1.2(3) 0.5(4.0)

Uiso for Ob 1.3(1) 1.4(1) 1.6(5) 1.9(3) 0.7(2) 0.0(4.0)Uiso for Db 2.3(2) 1.7(7) 1.5(4)U11 for Db 4.3(2) 8.6(1.1) 5.5(4.0)U33 for Db 2.2(1) 0.1(8) 0.6(4.0)Preferred

orientationc1.008(5) 1.026(7) 1.012(21) 1.098(34) 0.914(12) 0.923(21)

Rwp (%) 9.54 9.46 2.96 2.96 3.18 3.19

Rp (%) 7.00 6.98 2.37 2.37 2.57 2.58Re (%) 8.26 8.26 2.76 2.76 3.10 3.10

Note: The structure models are with the P3̄m1 space group. D at 6i (x, 2x, z); D at 2d (1/3, 2/3, z); Mg at (0,0,0); and O at (1/3, 2/3, z).aDistance between the split D sites inÅ.bUnits in ×10−2 Å2.cThe parameter of the March–Dollase function.

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deuterium Wyckoff positions were equally reasonable, where the assumed Debye–Waller factorswere isotropic for D at the 6i model positions (Uiso) and anisotropic for D at the 2d model positions(U11 along the a-axis and U33 along the c-axis). The former 6i model parameters were totallyconsistent with those recently evaluated by powder diffraction at a strong reactor source,[11]where the d-value range was wider (0.6 to 6.4Å at 10◦ ≥ 2θ ≥ 132◦ with λ = 1.1176Å) and theresolution was higher (!d/d = 0.2%) than the current condition. Their structure parameters fordeuterium at 6i positions in Mg(OD)2 at ambient temperature were x = 0.363(5), z = 0.4145(8)

and Uiso = 2.6(2) × 10−2, which was almost identical with our results. From this consistency, itwas demonstrated that the current neutron optics and correction procedures were entirely appro-priate for the structure refinements of crystals having the lattice parameters of the order of brucite.Particularly, differences of the covered d-value ranges and resolution between our and the previ-ous works did not disturb the consistency, which supports the reliability of our refinement resultsfor high pressure patterns.

The high pressure structure parameters were all reasonably refined with D at 6i model (Table 1).On the other hand, one of the Debye–Waller factor, U33 for D, became too small with D at 2dmodel at 1.6 GPa. At 2.8 GPa, some of the other Debye–Waller factors with D at the 2d modelbecame also too small and their standard deviations were too large to allow meaningful discussion,suggesting that this model is less appropriate to describe the high pressure structures. It waspreviously reported by neutron powder diffraction that with increasing pressure, distance betweenthe adjacent magnesium–oxygen sheets decreases to cause a substantial tilting of the hydroxyldipoles in brucite.[19,20] Such tilting was considered as an evidence of emerging interlayerhydrogen bonding between each dipole and one of the three nearest-neighbor oxygen anions inthe adjacent layer. Our observed distance between the three-split deuterium sites reaches 0.50Åat 1.5 GPa, which is substantially larger than the reported distance of 0.37Å at 1.9 GPa in theprevious study.[19] Thus, it has been demonstrated that the tilting of OD dipoles actually occursat a much lower pressure than that given by the previous researches, which was 5.3 GPa oreven higher.[19,20] The split-site distance of 0.50Å at 1.5 GPa was also unambiguously largerthan that observed for Mg(OD)2 at ambient pressure and high temperature; the maximum in thatcondition is 0.37Å at 583 K, indicating the limit without hydrogen bonding where thermal motionis extremely active.[21] We therefore conclude that, as suggested by polarized IR spectroscopyof brucite in diamond anvil cell,[8,22] the emergence of interlayer hydrogen bonding in bruciteat this moderate pressure regime has been confirmed.

Acknowledgements

The machine time for pulsed neutron diffraction was provided through the J-PARC MLF user program (No. 2012A0059and 2012B0016). This work was supported by JSPS grant-in-aid for Scientific Research (No. 23340161, 23540558 and26287135).

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