Photon Factory Activity Report 2014 #32 (2015) B

BL-18C/2012G509

Phase change of D2O ice VII at approximately 11 GPaHirokazu Kadobayashi, 1,* Hisako Hirai,1 Michika Ohtake,2 Yoshitaka Yamammoto,2Satoshi Nakano,3 and Takumi Kikegwa4
1Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan
2
National Institute of AIST, Tsukuba, 305-8569, Japan
3
National Institute for Material Science, Tsukuba, Ibaraki 305-0044, Japan
4Photon Factory, Tsukuba 305-0801, Japan
1 Introduction
The H2O ice exists in a wide variety of forms.
Twenty different stable and metastable forms have beenreported as a function of pressure, temperature, and P–Tpath for their formation. In addition, many novel
properties such as symmetrization of hydrogen bonds athigh pressure, proton ordering at low temperature, andsuperionic property at high-P and high-T have been
investigated. Whereas, at a moderate pressure of 10 GPa,
where ice VII and VIII are stable above and below
approximately 270 K, respectively, new phase changes
and property changes have been uncovered for both ices[e.g.1-3]. For example, Pruzan et al. attributed theirRaman data to partial proton ordering in ice VII atapproximately 13 GPa even at room temperature [1]. Xray study of H2O ice VIII by Yoshimura et al. suggesteda phase change at 14 GPa from the change in axial ratio,
a/c, versus pressure [2]. According to high-resolution
XRD, radial XRD, and single crystal XRD studies bySomayazulu et al., a phase transition to a low symmetry
structure was clearly detected for H2O ice at 14 GPa atroom temperature [3]. As described above, the existence
and nature of this phase change have become
increasingly well understood by many studies. In thisstudy, high pressure X-ray experiments were performed
on deuterated water (D2O) and light water (H2O), forcomparison, to clearly detect the phase change and tofurther the understanding of the nature of the phasechange occurring near 10 GPa [4, 5].
2 Experiment
A lever-and-spring type diamond anvil cell (DAC)
was used. Pressure measurements were carried out using
the ruby fluorescence method and diamond Raman
method. Super pure water (milli-Q) and deuterium oxide(100.0 % D) were used. The pressure range of in-situXRD for D2O water was 2.0 to 60 GPa, and that for H2O
water was 2.0 to 21 GPa. All experiments were performedat room temperature, 297 K. Three sample-preparation
methods were tried. Among them, the best one was asfollows: the gasket hole was filled with deuterated water,
and the entire DAC was rapidly cooled in a liquid nitrogenbath, then the pressure was loaded up to 2 GPa. The
powder patterns obtained Fig.
exhibited1:very smooth andhomogeneous diffraction lines along Debye rings. In orderto reduce strain due to non-hydrostatic stress, heating

treatments were given for D2O samples prepared. The
heating was performed at pressures from 8 to 35 GPa byheating the entire DAC in an oven at 373, 423, and 473 Kfor 2 to 4 hours. Samples were characterized by XRD at
the BL-18C beamline of the Photon Factory of KEK. The
diffraction peaks obtained were fitted using the fitting
software, Fityk.
3 Results and Discussion
Below 11 GPa, the diffraction lines were exactlyindexed as 110, 200, 211, and 220 of cubic ice VII. At
approximately 11 GPa, diffraction lines began to split.
Especially in the XRD patterns of the samples prepared by
the best method, clear splitting was observed up to 40 GPa,
which was the highest pressure limit for these samples.
The 110, 211, and 220 diffraction lines clearly split,
whereas the 200 line did not split. Since such splitting
could have been derived from strain due to uniaxial
compression, a heating treatment of the samples wereperformed at pressures before and after splitting from 8 to35 GPa to reduce such effects. After heating treatment,splitting diffraction lines sharpened and the splitting itselfclearly remained. The effect of non-hydrostatic stresses
should have been reduced because of the observed peak
sharpening. Therefore, the observed splitting was inferredFig.1 Representative XRD patterns before and aftersplitting. The wavelength of X-ray is 0.06198 nm.

Photon Factory Activity Report 2014 #32 (2015) B

to be intrinsic phenomenon, i.e. the lowering of thesymmetry occurred for D2O samples. These splitting
diffraction lines were indexed as the 110 and 101, 211 and112, and 220 and 202 lines of a tetragonal structure. The
magnitude of split increased with increasing 2θ. Fig. 1
shows representative XRD patterns before splitting at 10.3
GPa and after splitting at 25.6 GPa. The inserted figures
display peak separation results by the peak fitting software.
The 111 diffraction line was not observed due to weakintensity, and the 200 line did not split though the 110,
211, and 220 lines split. The existence or absence ofsplitting for the cubic lines in the present results agreed
well with the previous study for H2O ice [3].
Although the structure and symmetry of thetransformed phase could not be determined, an attempt
to calculate the volume of the low symmetry phase wasmade, assuming that the phase had a tetragonal structure.
Changes in the lattice volume calculated with pressures
up to 40 GPa for D2O ice and those up to 20 GPa for
H2O ice, respectively, are shown in Fig. 2. The greysolid curve in Fig. 2 indicates the pressure versus
volume curve (P-V curve) of ice VII reported as anentirely cubic structure by Hemley et al. [6]. Below 10GPa, the volumes calculated as an exact cubic structurein this study closely coincided with their curve [6]. The
volumes calculated as a tetragonal structure above 11
GPa showed values slightly smaller than those reportedby that study, which may be conceivable because thelow symmetry structure is a high-pressure phase. The
black solid curve indicates a P-V curve presently
estimated as a tetragonal structure. As for lattice volumeand transition pressure, there was no difference between
H2O and D2O ice in this study.
The bulk modulus K0 and K0’ of the cubicstructure below 10 GPa were calculated as 22.7 GPa and3.45, respectively, using the Birch–Murnaghan equationof state. The value of 22.7 GPa was close to the value of
23.7 GPa reported by Hemley et al. [6], while those for atetragonal structure above 11 GPa were 26.2 GPa and3.82, respectively. The absolute value of bulk modulus,
26.2 GPa, might not be exact; however, the transformed
structure might have a substantially stiffer structure witha bulk modulus larger than that of the cubic structure.
As described above, previous comprehensive
studies have argued the existence of a phase transition
and/or property change at pressure range between 11 and
14 GPa for ice VII and ice VIII. In the present study, thetransition of D2O ice VII to a low symmetry structure atapproximately 11 GPa was clearly observed by XRD
study. Thus, in this study, the additional evidence for theexistence of a phase change for D2O ice VII wasprovided; however, the reason for the formation of thelow symmetry structure was not inferred.
The low symmetry structure of D2O ice wasretained at least up to 40 GPa in the present study.
Somayazulu et al. reported that the low symmetry
structure of H2O ice was maintained before hydrogenbond symmetrization transition near 60 GPa [3]. They
explained the transition as ferroelastic with the appearanceof the spontaneous strain, and proposed that a change in

Fig.2: Changes in lattice volumes with pressure for cubic
ice VII and the low symmetry ice VII. Solid grey curve
indicates the P-V curve determined as an entirely cubic
structure by Hemley et al. [6]. Solid black curve indicatesa P-V curve calculated by assuming that the lowsymmetry phase has a tetragonal structure. Error bars are
hidden by individual marks.
the character of proton order/disorder is associated withthe phase transition [3]. Pruzan et al. also explained thephase transition as a phenomenon relating to protonorder/disorder behaviour based on the change in theFWHM of OD-modes [1]. These studies suggested theinvolvement of proton behaviour with the phase transition.
A recent neutron diffraction study demonstrated that
protonic species were localized on octahedral interstitialsites of oxygen sublattice above about 13 GPa, however,the form did not exhibit low symmetry but cubic one [7].
In any case, the neutron study pointed out that one shouldtake into account protonic species existing at sites and/orstates different from conventional network structure of ice
VII even at moderate pressures. Further experimental andtheoretical studies are required to elucidate the nature ofphase transition at about 11 GPa and to understand still
hidden properties of ice VII.
References[1] Ph. Pruzan et al., Europhys. Lett. 13, 81 (1990).
[2] Y. Yoshimura et al., J. Chem. Phys. 124, 02450(2006).
[3] M. Somayazulu et al., J. Chem. Phys. 128, 064510(2008).
[4] H. Kadobayashi et al., Journal of Physics: Conference
Series 500, 182017 (2014).
[5] H. Hirai et al., High Pressure Research 34, 289 (2014).
[6] R. J. Hemley et al., Nature 330, 737 (1987).
[7] M. Guthrie et al., PNAS 110, 10552 (2013).
* kadobayashi@sci.ehime-u.ac.jp