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J. Phys. Chem. B 2000, 104, 12102-12104
Thermal Properties of Metastable Ice XII Ingrid Kohl, Erwin Mayer, and Andreas Hallbrucker* Institut fu¨ r Allgemeine, Anorganische und Theoretische Chemie, UniVersita¨ t Innsbruck, A-6020 Innsbruck, Austria ReceiVed: August 31, 2000; In Final Form: October 19, 2000
Samples of ice XII containing varying amounts of high-density amorphous water (HDA) were prepared by pressurizing hexagonal ice up to 1.8 GPa at 77 K, and they were characterized by X-ray diffraction and differential scanning calorimetry (DSC). Only exotherms were observed in the DSC curves on heating at a rate of 10 K min-1 up to the melting point: conversion of HDA to low-density amorphous water (LDA) at 124 K, LDAfcubic ice transition at 166 K, and at 155 K from ice XII to cubic ice. Thus, unlike ice VIII, ice XII converts directly into cubic ice, and not via LDA. The enthalpy of the ice XIIfcubic ice transition is -1.27 ( 0.05 kJ mol-1. From comparison with the reported enthalpy value of the ice Vfcubic ice transition of -0.915 kJ mol-1 determined at similar temperature (Handa, Y. P.; Klug, D. D.; Whalley, E. Can. J. Chem. 1988, 66, 919), we conclude that ice XII is metastable with respect to ice V in the investigated temperature range.
Introduction Lobban et al.1 recently reported a new phase of ice, called ice XII, which formed at 260 K on compression of water within the domain of ice V at a pressure of 0.55 GPa. Ice XII “contains only seven- and eight-membered rings and is the first example of a 4-connected net of this type”.2 It has a density similar to that of metastable ice IV which also occurs in this pressure range within the stability region of ice V, and is also likely to be metastable with respect to the less-dense ice V.1 Subsequently Koza et al.3 reported formation of ice XII as an incidental product in the preparation of high-density amorphous water (HDA) at 77 K on compression of hexagonal ice (ice Ih) up to 1.8 GPa, by following the recipe described first by Mishima et al.4,5 In the study of Koza et al., the relative amounts of HDA and ice XII were scattered more or less randomly, and the decisive conditions favoring ice XII or HDA formation were not clear. Similar findings were reported afterward by Hallbrucker at the “Metastable Water” Meeting.6 In retrospect Bragg preaks of ice XII had been observed and reported before in samples prepared on compression of ice Ih for formation of HDA7-9 and attributed to “some metastable form of highpressure crystalline ice,”9 but they were not assigned to a new phase if ice. Koza et al.10 further reported that ice XII can form on compression of ice Ih between 77 and ≈150 K, and they proposed a “second regime of metastability” of ice XII in order to account for its unexpected formation at low temperatures. Here we report a study of ice XII samples containing varying amounts of HDA by differential scanning calorimetry (DSC) and X-ray diffraction. The comparison of exothermic heat effects on phase transition to cubic ice (ice Ic) shows that ice XII must be metastable with respect to ice V. We further show that ice XII converts directly into ice Ic, and not via some amorphous phase. Experimental Section Ice XII samples containing varying amounts of HDA were prepared by compression of ice Ih in a piston-cylinder apparatus
with an 8 mm diameter piston at 77 K up to 1.8 GPa. The samples used for Figures 2 and 3 were obtained by compression with a hand-operated press, for the sample used for Figure 1 a computerized “universal testing machine” (Zwick, Model BZ100/TL3S) was used with a compression rate of 7000 N min-1. Samples were recovered after compression from the piston-cylinder apparatus under liquid N2. A differential scanning calorimeter (Model DSC-4, PerkinElmer) with a self-written computer program was used. After heating each sample up to the melting point and recording its DSC scan, a second heating scan of now ice Ih was recorded and subtracted as a baseline from the first scan. The DSC scans were recorded on heating at a rate of 10 K min-1. Between 10 and 22 mg of sample were transferred under liquid N2 into steel capsules with screwable lids. The mass of the sample was obtained via the melting endotherm of ice, by using the value of 6.012 kJ mol-1 as heat of melting. The DSC instrument was calibrated with cyclopentane. Thermal lag is negligible for the heating rate of 10 K min-1 used in this study. X-ray diffractograms were recorded on a diffractometer in θ-θ geometry (Siemens, model D 5000, Cu-KR), equipped with a low-temperature camera from Paar. The sample plate was in horizontal position during the whole measurement. Installation of a “Goebel mirror” allowed to record small amounts of sample without distortion of the Bragg peaks. Results and Discussion Figure 1a shows the X-ray diffractogram of a sample of ice XII recorded at 93 K. The d spacings are about the same as those reported by Lobban et al.1 and by Koza et al.3 The DSC scan of another sample from the same batch obtained on heating at 10 K min-1 is shown as Figure 1b. Three exothermic effects are observable, with peak minimum temperatures at 124, 155, and 166 K (marked). Another sample of ice XII containing much more HDA is shown in Figure 2, and the comparison with Figure 1 allows unambiguous assignment of the exothermic effects. Figure 2a shows its X-ray diffractogram, and Figure 2b its DSC scan. In Figure 2a the broad feature is characteristic
10.1021/jp003151x CCC: $19.00 © 2000 American Chemical Society Published on Web 12/05/2000
Letters
Figure 1. (a) X-ray diffractogram (Cu-KR) of an ice XII sample containing a small amount of HDA. The sample was recovered after compression under liquid N2 and recorded at 93 K. (b) DSC scan of a sample from the same batch used for (a) and recorded on heating at 10 K min-1. For Figures 1-3: In the X-ray diffractograms, peaks marked with an asterisk are from ice Ih, and bands marked as + are from the sample holder.
for HDA.4,5,7,8 In Figure 2b three exothermic effects are observable at the same temperatures as those in Figure 1b, but their relative intensities differ. The thermal effects observed on heating at 124 and 166 K are the HDAfLDA conversion and phase transition of LDAfice Ic.11,12 Their relative intensities are much higher than those in the DSC scan of Figure 1b which is consistent with the larger amount of HDA in the sample used for Figure 2. We note that the exotherm at 124 K from the HDAfLDA conversion has lower relative intensity than that reported by Handa et al.11 We attribute the reduced intensity to partial conversion of HDA to LDA during thermal equilibration of our DSC instrument at 93 K. Thus, the only thermal effect attributable to ice XII is the exothermic peak with a peak minimum temperature of 155 K and an onset temperature of 151 K. For the DSC scan shown in Figure 1b, the total mass of the sample of 8.796 mg was obtained from the area of the melting endotherm for a value of 6.012 kJ mol-1 (not shown).13 The enthalpy of the ice XIIfice Ic transition was obtained after determining the amount of HDA in the sample. The literature value for the LDAfice Ic transition is between -1.363 and -1.425 kJ mol-1 (ref 10) and, by using the mean value of -1.388 kJ mol-1, the minor exothermic effect from LDAfice Ic transition in Figure 1a amounts to 0.720 mg LDA (or 8.2 wt %). Thus, the exothermic feature in Figure 1a
J. Phys. Chem. B, Vol. 104, No. 51, 2000 12103
Figure 2. (a) X-ray diffractogram of a sample containing comparable amounts of ice XII and HDA recorded at 93 K. (b) DSC scan of a sample from the same batch used for (a) and recorded on heating at 10 K min-1.
from the ice XIIfice Ic transition is caused by 8.076 mg sample, and its ∆H value is -1.27 kJ mol-1. Estimation of the error of (0.05 kJ mol-1 is obtained from evaluation of 10 further DSC measurements, by using samples from four different preparations. We have not attempted to correct this value for the small amount of ice Ih in the samples (see Figure 1a). Therefore the ∆H value is a lower-bound value, but it is expected to be low by a few percent only. We emphasize that the small amount of 8.2 wt % HDA is not discernible in the X-ray diffractogram of Figure 1a even at high gain. Thus, DSC is a more sensitive method for detecting minor amounts of HDA than X-ray diffraction. We have further investigated in detail whether ice XII transfroms first into amorphous water (LDA?) and subsequently into ice Ic. Such an effect can be difficult to detect, and the transformation of ice VIII into LDA reported by Klug et al.14 is a good example. However, the DSC scans of ice samples containing mainly ice XII did not show any thermal features in addition to those already discussed above. In particular, an endothermic feature indicative of either a proton order-disorder effect as in ice V15 or a glassfliquid transition of an amorphous phase16 could not be observed, even at high gain. We further studied the ice XIIfice Ic by X-ray diffraction at increasing temperatures (see Figure 3). The diffractograms are characteristic for transition of ice XII into ice Ic, and not for additional intermediate formation of an amorphous phase. The broad features developing with formation of ice Ic are a property of this crystalline phase.17-22 Thus, unlike the conversion of
12104 J. Phys. Chem. B, Vol. 104, No. 51, 2000
Letters to ice Ic occurs,1,15 and thus order-disorder effects do not have to be taken into account. It follows from the larger ∆H value of -1.27 kJ mol-1 obtained for the ice XIIfice Ic transition that ice XII must be metastable with respect to ice V in the investigated temperature range. Lobban et al.1 already pointed out that ice XII (and ice IV) “are likely to be metastable with respect to the less-dense ice V”. Note Added in Proof: After submission of the revised manuscript, we have started to investigate the effect of annealing ice XII, and observed in the DSC scan new features below the phase transition to cubic ice. Independent study of the effect of annealing by X-ray diffraction ascertained that ice XII does not transform into an amorphous phase before transition to cubic ice, and that the features seen in the DSC scan therefore must have a different origin. These studies will be reported separately. Acknowledgment. We are grateful for financial support by the “Forschungsfo¨rderungs-fonds” of Austria (Project No. P13930-PHY). References and Notes
Figure 3. Conversion of ice XII to cubic ice as seen in X-ray diffractograms of an ice XII sample recorded on heating in steps from 113 to 183 K. The sample was heated to increasingly higher temperatures, held at each temperature for 5 min, and cooled to 93 K for recording its diffractogram. Note the absence of broad features attributable to formation of an amorphous water phase.
ice VIII into ice Ic via LDA shown by Klug et al.14 in their Figure 1, we cannot observe the simultaneous coexistence of ice XII and LDA in our X-ray diffractograms. According to our Figure 3, ice XII transforms into ice Ic between 128 and 138 K. If ice XII were to transform first into an amorphous phase and then into ice Ic, then the conversion of the amorphous phase into ice Ic would have to be very rapid to become unobservable in our measurements. This is unlikely because in this temperature range the amorphous forms of water are known to transform only slowly into ice Ic. We thus conclude that, according to our measurements, ice XII converts directly into ice Ic, and not via an amorphous phase. Handa et al.15 have studied ice V by calorimetry and determined the ∆H value of the ice Vfice Ic transition as -0.915 ( 0.045 kJ mol-1. This transition occurs at similar temperature as the ice XIIfice Ic transition. Both ice V and ice XII are proton disordered at the temperature where transition
(1) Lobban, C.; Finney, J. L.; Kuhs, W. F. Nature 1998, 391, 268270. (2) O’Keeffe, M. Nature 1998, 392, 879. (3) Koza, M.; Schober, H.; To¨lle, A.; Fujara, F.; Hansen, T. Nature 1999, 397, 660-661. (4) Mishima, O.; Calvert, L. D.; Whalley, E. Nature 1984, 310, 393395. (5) Mishima, O.; Calvert, L. D.; Whalley, E. Nature 1985, 314, 76-78. (6) Hallbrucker, A. “Pure ice XII by pressurizing hexagonal ice at 77 K”; Metastable Water, 1999, International Bunsen Discussion Meeting, Schloss Nordkirchen, Germany. (7) Bosio, L.; Johari, G. P.; Teixeira, J. Phys. ReV. Lett. 1986, 56, 460463. (8) Bizid, A.; Bosio, L.; Defrain, A.; Oumezzine, M. J. Chem. Phys. 1987, 87, 2225-2230. (9) Bellissent-Funel, M.-C.; Teixeira, J.; Bosio, L. J. Chem. Phys. 1987, 87, 2231-2235. (10) Koza, M. M.; Schober, H.; Hansen, T.; To¨lle, A.; Fujara, F. Phys. ReV. Lett. 2000, 84, 4112-4115. (11) Handa, Y. P.; Mishima, O.; Whalley, E. J. Chem. Phys. 1986, 84, 2766-2770. (12) Floriano, M. A.; Handa, Y. P.; Klug, D. D.; Whalley, E. J. Chem. Phys. 1989, 91, 7187-7192. (13) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1987, 91, 503-505. (14) Klug, D. D.; Handa, Y. P.; Tse, J. S.; Whalley, E. J. Chem. Phys. 1989, 90, 2390-2392. (15) Handa, Y. P.; Klug, D. D.; Whalley, E. Can. J. Chem. 1988, 66, 919-924. (16) Johari, G. P.; Hallbrucker, A.; Mayer, E. Nature 1987, 330, 552553. (17) Dowell, L. G.; Rinfret, A. P. Nature 1960, 188, 1144-1148. (18) Arnold, G. P.; Finch, E. D.; Rabideau, S. W.; Wenzel, R. G. J. Chem. Phys. 1968, 49, 4365-4369. (19) Kuhs, W. F.; Bliss, D. V.; Finney, J. L. J. Phys. Colloq., C1 1987, 48, 631-636. (20) Mayer, E.; Hallbrucker, A. Nature 1987, 325, 601-602. (21) Jenniskens, P.; Banham, S. F.; Blake, D. F.; Coustra, M. R. S. J. Chem. Phys. 1997, 107, 1232-1241. (22) Kohl, I.; Mayer, E.; Hallbrucker, A. PCCP 2000, 2, 1579-1586.
