J. Phys. Chem. B 1997, 101, 6177-6179
6177
Thermally-Stimulated Depolarization Studies of the Ice XI-Ice Ih Phase Transition Susan M. Jackson*,† and R. W. Whitworth School of Physics and Space Research, The UniVersity of Birmingham, Birmingham B15 2TT, U.K. ReceiVed: October 11, 1996; In Final Form: February 19, 1997X
Thermally-stimulated depolarization measurements on single-crystal specimens of KOH-doped ice have shown that the crystal structure of ice XI has ferroelectric ordering parallel to its c-axis. The charge-release peak identified with the detransformation of ice XI to ice Ih is produced at the 72 K transition temperature. This temperature is independent of experiment parameters, such as the magnitude of the original polarizing field, the temperature at which the ice had been transformed in this field, and the length of time for which the ice had been subjected to these conditions. The size of the charge release depends on the conditions of transformation but is always smaller than that predicted for full ferroelectric ordering.
Introduction The disorder in the orientations of the molecules in pure ice Ih persists down to very low temperatures. Doping with OHions catalyzes reorientation of the molecules, so that below 72 K the ordered structure known as ice XI can be obtained.1,2 There has been much speculation over the crystal structure of this ordered ice. Results from dielectric experiments3,4 and neutron diffraction studies5,6 had indicated that the ordering could be ferroelectric. The technique of thermally-stimulated depolarization (TSD) was adopted to test this, and our measurements have been interpreted as showing that ice XI is ferroelectrically-ordered parallel to its c-axis. This is consistent with the orthorhombic Cmc21 model of the ice XI structure, which is polar and which is also supported by more recent neutron diffraction experiments.7 Our preliminary results, together with their interpretation, have already been published.8 More detailed experiments reported here investigated the use of different strengths of polarizing field, variations on our standard conditions for producing ice XI, and cases where the field was applied at different stages of the transformation. Technique and Background When ice is cooled in an applied electric field, electrical polarizations develop, which become frozen-in below certain temperatures. On reheating the specimen after removal of the field, the frozen-in polarizations are lost, causing electrical currents to flow. Contributions from different depolarization processes at different temperatures are manifested as discrete TSD current peaks. TSD work on pure and doped ice started more than 30 years ago.9,10 Ice Ih exhibits a distinct TSD peak at 220 K and another at about 100 K. The high-temperature peak results from the release of trapped space charges.11 The lower-temperature peak is attributed to relaxation of frozen-in orientation polarization of molecules at the glass transition.12 Past claims of ferroelectricity in ice9 arose through misinterpretation of these effects. However, when KOH-doped ice Ih is cooled through the ice XI transition temperature (at 72 K), a large spontaneous polarization can develop because ice XI has a polar crystal * To whom correspondence should be addressed. † Current address: The Institute of Physics, 76 Portland Place, London W1N 4AA, U.K. Email:
[email protected]. X Abstract published in AdVance ACS Abstracts, June 1, 1997.
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structure. The charge release on subsequent heating through the transition results from the sudden loss of this polarization. Experimental Detail The eight single-crystal specimens of KOH-doped ice used for the TSD work were cut from crystals grown by a modified Bridgman method3 from 0.055 M KOH solution. These crystals also provided specimens for dielectric and neutron scattering studies.4,7 The TSD specimens were plates of dimensions 20 × 14 × 2 mm, with gold-leaf electrodes applied to their large faces. These specimens were cooled through the transition in an applied electric field and underwent a sequence of nucleation and annealing to convert them to ice XI. A high-voltage supply was used to generate the electric field across the ice. After removal of the field, the subsequent depolarization current was measured with an electrometer as the specimens were warmed back through the transition. Application of voltages of between 100 and 700 V (corresponding to applied fields of 40-200 V mm-1) resulted in depolarization currents ranging from 10-10 to 10-7 A. The electrometer output was fed to an X-t recorder. The total charge release was given by the time-integrated current. The specimen temperature was measured using a diode thermometer attached to the brass block on which our specimens were mounted. Frequent sampling and recording of time, temperature, and depolarization current values were achieved using a computer LabView system. The fast heating rates used in the early experiments gave rates of temperature variation of 1-3 K/min. Later work covered a narrower temperature range and did so in more detail by using heating rates that kept the temperature rise below 1 K/min. This avoided rapid depolarization and allowed closer study of the main TSD peak. General Results The TSD evidence for ferroelectric ordering along the crystallographic c-axis was obtained by comparing measurements on two specimens cut in perpendicular orientations from the same crystal. The specimen cut as a plate normal to the c-axis of the crystal showed a large charge release around 72 K, whereas the plate cut in the perpendicular geometry released only 1% of that charge and did so over a broad temperature range, typical of a paraelectric effect. Both specimens had been kept at the nucleation temperature of 63 K for 25 h and annealed at 69 K for a further 70 h, with an applied field of 210 V mm-1. © 1997 American Chemical Society
6178 J. Phys. Chem. B, Vol. 101, No. 32, 1997 The magnitude of the charge release for the first specimen is compatible with a ferroelectric interpretation. These results were presented in ref 8. The same main features have been observed for other crystals. There were a number of general results from the work: (1) The ferroelectric effect was only present parallel to the c-axis of the ice. (2) For zero applied field no TSD current peak was seen, even at the most sensitive setting of the electrometer (10-15 A). This is not to say that the transformed ice does not adopt ferroelectric ordering in the absence of an applied field. It just means that the summed contributions of signals over the bulk ice average to zero. This is consistent with the transformed ice being made up of domains with two opposing orientations. The null signal was also evidence that the experiment was not affected by thermoelectric effects arising from temperature gradients within the system. (3) Identical repeat runs made on a single specimen indicated that the error limit on the reproducibility of charge release was of the order of 10%. (4) It was noticed that later runs on a particular specimen took significantly longer times to fully depolarize than the first few runs, although the runs were similar in all other respects. This raises the question of whether crystals become in some way “damaged” by repeated transformation. (5) No visible signal was seen when ice that had already been polarized was held below 72 K and left for several hours without thermal stimulation. Main Studies The studies described in this paper involved only specimens for which the applied field was parallel to the c-axis. The aim was to study the properties of the ferroelectric peak. A series of measurements, for which the only parameter changed was the size of the applied field, were made on a single specimen. In each measurement the ice was cooled in the field and held for 10 h at a single sub-72 K temperature. At low values of applied field, the depolarization charge was seen to increase with the polarizing field, but there was leveling-off for field strengths approaching 200 V mm-1. The increase with field represents an increasing proportion of domains oriented with the field, but the leveling-off occurs at polarizations an order of magnitude less than that for complete orientation parallel to the c-axis of the ice. For a study of the effect of different nucleation temperatures, several TSD runs were made on the same specimen. For each run, a standard field of 210 V mm-1 was applied at 74 K and the specimen was fast-cooled to a chosen temperature, where it was held for 24 h before measurement. Runs were made for temperatures from 74 down to 57 K, at 2 K intervals, subject to systematic errors of (1 K. The results of the study are shown in Figure 1. When the ice had been polarized at or above the transition temperature, the temperature at which the depolarization current peaked varied according to the temperature at which the ice had been polarized. However, for polarization below the transition, the maximum release of charge always occurred at the same temperature, between 72 and 74 K. The shapes of these two types of peak, in Figure 2, are different. The current peak observed for sub-72 K polarization was decidedly asymmetric, with a steeper leading edge and a shallower falling edge. This was a general feature for all samples studied. Calorimetric experiments2,13 give values of the change in entropy at the transition. On the basis of these values, the latent heat for one of the reasonably active crystals having undergone
Jackson and Whitworth
Figure 1. Variation of the temperature of maximum current release as a function of the temperature at which the ice had previously been polarized.
Figure 2. TSD current peaks for different polarization temperatures. The shape of the TSD peaks is seen to be very different depending on whether the ice has been polarized above (73.5 K) or below (69 and 65 K) the ice Ih-ice XI transition temperature. When the ice has not undergone transformation, the “peak” is smooth, but for the transformed ice it is pronounced and decidedly skew.
Figure 3. Charge release as a function of the polarization temperature.
a typical nucleation and annealing would be about (140 ( 30) J mol-1. Assuming a specific heat13 of Cp ) 12.4 J mol-1 K-1, this would correspond to a temperature change of (11 ( 3) K in the region of Tc. Therefore, there will be a pause in the temperature rise of the specimen as the ice passes though the transition. The effect might manifest itself as peak shapes of the kind observed. Our measured temperature is the temperature of the block on which the specimen is mounted, not the temperature of the specimen itself, and this will continue to drift upward, as observed. The change in the charge release as a function of the polarization temperature is shown in Figure 3. It is clear that nucleation does not start to become effective until the temperature has been lowered to about 63 K. Moreover, temperatures below 59 K are unable to support successful nucleation. These observations give an insight into the optimizing of the trans-
Depolarization Studies of Ice XI-Ice Ih Transition formation which would be difficult to obtain from dielectric or neutron diffraction studies. In general, it was observed that the longer periods of nucleation and annealing produced larger depolarization currents. However, time constraints prevented many studies of this type. Long polarization times, in excess of 50 h, at single temperatures gave very little enhancement over shorter polarization times, of around 25 h. The same trends were observed for the cases where the polarization time was the sum of a nucleation and an annealing part at two different temperatures. The use of a nucleation and annealing sequence to transform the ice is more successful than simply holding the ice at a single temperature. Studies were also made with different heating rates for the same specimen. The overall shape of the TSD spectrum was not greatly affected, though the height of the observed peak varied because faster heating released larger currents. Within the levels of reproducibility, the amount of charge released was independent of the heating rate. Most importantly, the position of the peak did not shift in temperature when the heating rate was varied. This is evidence that we have a thermodynamic transition and not a dielectric relaxation effect. The TSD spectrum was radically altered by applying the electric field at different temperatures and at different times during the polarization process. In the earliest studies the field was applied at high temperature (i.e. above 100 K) before cooling down to the region below 72 K. Three TSD peaks were produced on reheating. In addition to the large peak at 72-74 K, there was a small bump around 110 K and a shallow rise at a still higher temperature. These are consistent with the TSD observations by Johari and Jones11 of peaks at 110 and 159 K for pure H2O ice Ih. When the field was applied only just above the 72 K transition temperature, these extra peaks did not appear in the resulting TSD spectrum. The nucleation and annealing schemes were investigated further by looking at the effect of applying the field only for certain times during the nucleation and annealing periods. Later studies also included switching the polarization of the field in mid-experiment. TSD measurements take so long that it was
J. Phys. Chem. B, Vol. 101, No. 32, 1997 6179 not possible to try out all the different schemes of interest. Ideally one would like to have very long nucleation and annealing times to transform as much of the ice as possible, but this was not practically feasible. There is opportunity for further study in this area. Summary The TSD measurements on KOH-doped ice showed a single well-defined current-release peak at the temperature of the ice XI-ice Ih transition. The temperature at which the peak appeared was independent of the conditions of polarization, i.e. the magnitude of the polarizing field, the temperature at which the ice was transformed in this field, and the length of time for which this was done. It was also independent of both the heating rate and the length of time taken for the ice to depolarize. The phenomenon is therefore intrinsic to the ice XI-ice Ih transition. Acknowledgment. The authors acknowledge the support of the Engineering and Physical Sciences Research Council of the U.K., through a research grant and a studentship for S.M.J. References and Notes (1) Kawada, S. J. Phys. Soc. Jpn. 1972, 32, 1442. (2) Tajima, Y.; Matsuo, T.; Suga, H. J. Phys. Chem. Solids 1984, 45, 1135. (3) Oguro, M.; Whitworth, R. W. J. Phys. Chem. Solids 1991, 52, 401. (4) Jackson, S. M. Ph.D. Thesis, The University of Birmingham 1996. (5) Howe, R.; Whitworth, R. W. J. Chem. Phys. 1989, 90, 4450. (6) Oguro, M.; Whitworth, R. W.; Wilson, C. C. In Physics and Chemistry of Ice; Maeno, N., Hondoh, T., Eds.; Hokkaido University: Sapporo, Japan, 1992; p 14. (7) Jackson, S. M.; Nield, V. M.; Whitworth, R. W.; Oguro, M.; Wilson, C. C. J. Phys. Chem. 1997, 101, 6142. (8) Jackson, S. M.; Whitworth, R. W. J. Chem. Phys. 1995, 103, 7647. (9) Dengel, O.; Eckener, U.; Plitz, H.; Riehl, N. Phys. Lett. 1964, 9, 291. (10) Bishop, P. G.; Glen, J. W. In Physics of Ice; Riehl, N., Bullemer, B., Engelhardt, H., Eds.; Plenum: New York, 1969; p 492. (11) Mascarenhas, S.; Arguello, C. J. Electrochem. Soc. 1968, 115, 386. (12) Johari, G. P.; Jones, S. J. J. Chem. Phys. 1975, 62, 4213. (13) Yamamuro, O.; Oguni, M.; Matsuo, T.; Suga, H. J. Phys. Chem. Solids 1987, 48, 935.
