Bond strengthening in dense H2O and implications to planetary composition Zachary M Grande,y Chenliang Huang,z Dean Smith,y Jesse S Smith,{ John H Boisvert,z Oliver Tschauner,x Jason H Steffen,z and Ashkan Salamat∗,y yDepartment of Physics and Astronomy and HiPSEC, University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA zDepartment of Physics and Astronomy, University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA {HPCAT, X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA xDepartment of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA E-mail: [email protected] Abstract mantle a potential long-term reservoir of ancient water. H2O is an important constituent in plane- The pressure-temperature phase diagram of tary bodies, controlling habitability and, H2O exhibits remarkable polymorphism, with in geologically-active bodies, plate tec- as many as 18 phases currently reported.1,2 At tonics. At pressures within the interior low pressures, this complexity arises from steric of many planets, the H-bonds in H2O col- rearrangements of hydrogen-bonded molecules, lapse into stronger, ionic bonds. Here we while the H–O–H bond angle and length re- present agreement between X-ray diffrac- main almost constant. H-bonds are established tion and Raman spectroscopy for the through correlated disorder of the protons be- transition from ice-VII to ice-X occurring tween adjacent oxygen atoms such that, at each at a pressure of approximately 30.9 GPa moment, two protons and one oxygen form an 3 by means of combining grain normaliz- H2O molecule. Ice structures generally exhibit ing heat treatment via direct laser heat- network-like topologies similar to those of sil- ing with static compression. This is ev- ica and silicates.4 The behavior of condensed idenced by the emergence of the charac- H2O phases (ices) is dominated by this H-bond teristic Raman mode of cuprite-like ice- network. Under the conditions found in the X and an abrupt 2.5-fold increase in bulk interior of Earth and many other planets, the arXiv:1906.11990v1 [cond-mat.mtrl-sci] 27 Jun 2019 modulus, implying a significant increase H-bonds in ice are gradually replaced by ionic in bond strength. This is preceded by a bonds in ice-X.5–7 transition from cubic ice-VII to a struc- At room temperature, ice-VII becomes the ture of tetragonal symmetry, ice-VIIt at stable solid phase of H2O at pressures above 5.1 GPa. Our results significantly shift 2.7 GPa.8 The subsequent transition into ice- the mass/radius relationship of water- X has been observed in spectroscopic measure- rich planets and define a high-pressure ments and inferred from structural data, but limit for release of chemically-bound wa- there has been no consensus between stud- ter within the Earth, making the deep ies. Estimates of the transition pressure range 1 from 40 GPa to above 120 GPa.6,7,9–17 However, treated pattern. the existence of a molecular-to-ionic transition above 40 GPa has been observed at tempera- tures beyond the melting curve.18,19 Results Bond states in soft molecular compounds are A single-phase sample of powdered ice-VII strongly affected by non-hydrostatic stress at is achieved by heating at the first appear- high pressure conditions. H O is especially sus- 2 ance of its coexistence with ice-VI, as con- ceptible to this since the use of a pressure trans- firmed with XRD measurements (See Meth- mitting medium is inhibited due to the forma- ods and Supplementary Information). Begin- tion of hydrates and clathrates. The result- ning at 2.7 ± 0.4 GPa, we unambiguously index ing distortions caused by non-hydrostatic com- and refine the phase as cubic ice-VII. Above pression are further exacerbated by the het- 5.1 ± 0.5 GPa, we observe deviations in the peak erogeneous nucleation of ice-VII within ice- positions and profiles both before and – more VI, which yields large crystalline domains and clearly – after heat treating. Specifically, we ob- causes significant anisotropic effects at grain serve splitting between the (2 0 0)/(0 0 2) which boundaries.20 are not accommodated by the cubic ice-VII To minimize these effects, we heat ice sam- (P n3m) structure. Figure1b shows the Bragg ples at high pressure using a CO laser and al- 2 feature at ∼ 14.5◦, where these deviations are low them to cool to ambient temperature. The most pronounced. Here, we find significant im- cooling rate is slower than rapidly quenching, provements in the Rietveld refinement when which can potentially trap internal stresses, and modelling with a tetragonal sub-group of cubic is faster than annealing, which typically results ice-VII (P 4 =nnm), and name this tetragonal in enlarged domains (see Methods and Supple- 2 phase ice-VII . mentary Information), and an analogy can be t Structural anomalies have been reported in made with metallurgical normalization.21 the 10–14 GPa regime,9,12,14,22 and are at- There are several benefits of this heat treat- tributed to a proton-disordered ice-VII0, but ment: anisotropic strain within both the sam- these claims have lacked spectroscopic evidence. ple and the Au pressure marker are relieved, One such anomaly has been suggested to result minimizing deviatoric stress for more accurate from a tetragonal distortion.12 We show that volume-pressure measurements; the recrystal- this transition from ice-VII to ice-VII is ac- lization of the ice produces a powdered sam- t companied by a 2.18 ± 0.01% volume collapse ple with nanoscopic domains in random orien- of the unit cell at this pressure (Supplementary tation (Supplementary Video); and provides a Information). We only confirm a transition in direct, localized method of heating. The re- the oxygen sublattice to a tetragonal symmetry duced domain size and their random orienta- and cannot comment on its relation to proton- tions yield well-resolved Debye-Sherrer rings for disordered ice-VII0, as the few observations of an extensive q-range (Figure1a), making our the (1 1 1) diffraction peak were dominated by data suitable for Rietveld powder X-ray diffrac- the background in our XRD patterns. tion analysis (Figure1b). The powdered na- We further examine the symmetry of the unit ture of the sample also reduces its susceptibility cell above ∼ 5 GPa, by combining structural to further strain as compression continues, de- refinements with a Bayesian model comparison spite the uniaxial nature of the DAC, allowing algorithm (see Methods).23 Figure1c shows the full structural refinement. Data that are not log Bayes factors comparing cubic and tetrag- heat treated display significantly fewer diffrac- onal models for a selection of points over the tion features and typically exhibit multi-grain pressure range of our data. A cubic model is spots or highly textured rings with significant favoured for pressures below 5 GPa, indicated peak broadening from deviatoric strain, shown by Bayes factors below unity. Meanwhile, the in Figure1a where the FWHM of the (1 1 0) tetragonal model is clearly preferred between peak improves from 0.24 to 0.088◦ 2θ in the heat 2 5 and 30 GPa, shown by values much larger nated by a feature at 280 cm−1 which is fit to than unity. Above 30 GPa, the cubic model a single mode and a weaker mode at 211 cm−1 becomes increasingly viable – consistent with (Figure3a and b inset). Due to the close sim- our Reitveld refinements, as well as previous ilarities between the Raman spectra of proton- diffraction-based studies.11,13 We find no static disordered ice-VII and its proton-ordered ana- displacement of the O-sublattice in tetragonal logue, ice-VIII, these modes have previously ice-VIIt, suggesting that the gradual softening been assigned to the analogous translational- of the O–H vibrational mode over its stability vibration modes of ice-VIII, B1g and A1g, re- range5–7 is solely related to the weakening of spectively.15,25 We also observe a very weak the H-bond. mode which is not reported previously near We perform a similar Bayesian analysis com- 160 cm−1 (Supplementary Information). Fea- paring a single-phase equation of state (EOS) tures were also observed in the 500 cm−1 to −1 to a three-phase EOS and find that a three- 800 cm range corresponding to the known Eg 15 phase model is required to reproduce our data. and B2g rotational modes. In doing so, we fit a three-phase P -V Vinet Beginning at approximately 5.0 GPa, the EOS to the data using Markov Chain Monte dominant feature near 280 cm−1 displays an in- Carlo (MCMC). This model includes two tran- creasingly asymmetric profile requiring multiple sition pressures as fitting parameters as well as modes to reproduce the peak profile (Figure3a two parameters, β and γ, to model pressure- and b inset). This asymmetry arises due to the dependent systematic uncertainties (Supple- appearance of new lattice modes (Figure3b in- mentary Information). The uncertainties for set) which is consistent with a lowering of sym- data that were not heat treated are adjusted metry from the cubic P n3m space group to the by the function, σ = σ0(β + γP ) (orange er- tetragonal P 42=nnm space group. This asym- ror bars in Figure2a), while heat treated data metric profile continues with further compres- (where distortions in our Au pressure marker sion and at approximately 21.1 GPa, the peak are relieved) use their nominal error, σ0. The profile becomes increasingly symmetric (Fig- results of our three-phase fit of the P -V EOS ure3b), mirroring the results obtained from the and the transition pressures between the phases Bayesian model comparison of our XRD data is shown in Figure2a.