PHYSICAL REVIEW B 99, 174201 (2019) Search for a ferroelectrically ordered form of ice VII by neutron diffraction under high pressure and high electric field R. Yamane, 1 , * K. Komatsu, 1 H. E. Maynard-Casely, 2 S. Lee, 2 N. Booth, 2 and H. Kagi 1 1 Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan 2 Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, 2234 New South Wales, Australia (Received 30 July 2018; revised manuscript received 17 March 2019; published 3 May 2019) Neutron diffraction experiments of ice VII at pressures up to 6.2 GPa and 10.2 kV/mm were conducted to investigate the potential ferroelectrically ordered structure of ice VII induced by a high electric field. To accomplish this, we developed a high-pressure cell assembly to allow for powder neutron diffraction to be con- ducted under both high-pressure and high-electric field conditions. However, the subsequent observed diffraction patterns taken in situ at these conditions do not show sufficient evidence of the proposed ferroelectrically ordered structure of ice VII. We estimate the degree of the hydrogen ordering in ice VII under the applied conditions from high-pressure P-E loop measurements, and discuss the future possibilities of detecting the ferroelectrically ordered structure. DOI: 10.1103/PhysRevB.99.174201 A single water molecule has an electric dipole moment; the dielectric properties derived from this dipole moment are a relatively unexplored parameter in the investigations of water ice’s many polymorphs (e.g., [1–3])—18 phases are known to date [4]. The 18 polymorphs are classified into hydrogen disordered and ordered phases, and this classification directly relates to the degree of ordering of dipole moments. The majority of ordered ice polymorphs have a nonpolarized structure, in other words, their structures possess a center of symmetry. The exceptions to this are ice XI (the ordered form of ice Ih) [5] and ice XV (the ordered form of ice VI though its structure is still under debate [3,6–8]). The structure of ice VII includes disordered hydrogen atoms within the structure (space group Pn 3m) shown in Fig. 1 and has long been considered as a single phase in the wide stability region, from 2 to 60 GPa at room tempera- ture. However, several anomalous phenomena are observed between 10 and 15 GPa by a number of different methods, and these anomalies have caused questions for the single-phase scenario of ice VII. For example, Pruzan et al. reported that a width of a Raman-scattering peak, corresponding to a sym- metric vibration mode of a water molecule, has a minimum at around 10 GPa [9]. Somayazulu et al. showed a peak splitting in an x-ray-diffraction pattern at 14.8 GPa [10]. Fukui et al. reported that x-ray-induced dissociation yield of ice VII has a maximum at 14 GPa from x-ray Raman spectra [11]. The anomalies even extend into the physical properties: Okada et al. show that a pressure dependence of electric conductivity of ice VII has a maximum at 12 GPa from the impedance measurements [12]. More recently, Noguchi et al. showed a maximum in self-diffusion coefficients using micro-Raman spectroscopy at around 11 GPa [13]. Although previous stud- ies suggested a possibility that a pressure-induced partial * yamane@eqchem.s.u-tokyo.ac.jp ordering occurs at around 10–15 GPa [9,10], no comprehen- sive interpretation for the experimental anomalies has been found. The ordered form of ice VII is ice VIII, which has an antiferroelectrically ordered structure (called “AFE structure” hereafter, and its crystal structure is shown in Fig. 1, space group I 4 1 /amd ). The AFE structure has been considered as a candidate for the partially ordered structure in ice VII (if ice VII is really partially ordered). However, Caracas and Hemley [15] have proposed a different scheme for the partial ordering of ice VII by using density functional theory calcula- tions. They proposed that a ferroelectrically ordered structure (called “FE structure” hereafter, and its crystal structure is shown in Fig. 1, space group P4 2 nm) could occur in ice VII at a bulk scale under high-electric field. Their calculations show that the AFE structure is slightly more stable than the FE structure, but the energy difference between AFE and FE structures was so small (∼10 meV/molecule [15]) that the FE structure could be stabilized under an external electric field. Their calculation also shows that the energy difference slightly decreases from 12.3 meV/molecule at 2 GPa to 11.7 meV/molecule at 10 GPa with increasing pressure. The energy difference between FE and AFE is almost comparable below 10 GPa; it will be predicted, from the estimated volume fraction of the FE structure at pressure lower than 10 GPa, how much the ferroelectrical ordering is induced by high elec- tric field in ice VII above 10 GPa. In this study, high-pressure and high electric-field neutron experiments were conducted at up to 6.2 GPa with overcoming some technical difficulties as mentioned later. If the FE structure is found experimentally, the discovery will contribute not only for the understanding of the anomalies observed in ice VII but also in the knowledge of how orientations of water molecules affect ice structure, stability, and other physical properties. Neutron diffraction experiments under high pressure and high electric field would be the most direct method to find 2469-9950/2019/99(17)/174201(5) 174201-1 ©2019 American Physical Society