X-ray Diffraction Pattern of Quasi-liquid Layer on Ice Crystal Surface

The existence of a quasi-liquid layer on the surface of ice crystals at temperatures not far belaw O°C is widely accepted on the basis of many experimental and theoretical studies (e.g., 1-4). However, there has been no experimental study including the direct information on, the structure of the quasi-liquid layer. It is therefore highly desirable to obtain structural data by X-ray diffraction method. In this study, we investigated the dependence on temperature of the X-ray diffraction pattern of the surface of ice crystals and obtained direct evidence that there was no long range order of the structure in the quasi-liquid layer on ice surface near O°C. In general, it is difficult to obtain clear X-ray diffraction pattern of thin film on a substrate. We get only obscure diffraction pattern by usual 29/9 scan diffractometer, because the diffraction pattern of thin film is disturbed by strong diffracted beam from the substrate crystal. So, usual diffractometer was remodeled for the study of thin films (Rigaku TFD System). To increase an intensity of diffracted X-ray from the thin film, we kept an incidental angle at a fixed value, e-g., 2 degree, during measurement, and scaned only X) axis. We used a Soller slit and a platy graphite monochromator, to increase angular resolution of diffracted X-ray and signal to noise ratio, respectively. The specimen holder which contained ice crystals was mounted on the diffractometer. We can control both surface temperature of ice crystal and supersaturation independently. We used both ice single crystals and plycrystals whose surface was mthened by microtame. The diffraction pattern of ice crystal surface under the equilibrium condition was measured between -0.5 and -lO°C with the use of CuKdl X-rays. Figure 1 shows an example of X-ray diffraction patterns of the plycrystal ice surface obtained at -7.3, -2.1 and -l.l°C. There are sane diffraction 1 . 3 " ~ peaks of ice Ih at -7.3OC (Fig. la), even though the pattern does not coincide with ideal powder diffraction pattern, because grain sizes are large about 2 mn in diameter. It is noted that intensity . .. . . of some peaks of the ice Ih at -2.1°C (Fig. lb) are . . , . . , . . ~ . , . . . . . . . . . . . ~ . ~ . . , . . . . , . , . . ~ 2 . I 'c relatively weak in cumparison with those of -7.3OC b (la). These results are interpreted as follows. The thickness of the quasi-liquid layer increases with increasing temperature (2-4) . Therefore, the I I diffraction of the ice Ih are weakened by the thicker quasi-liquid layer on ice surface at -2.1°C. In Fig. lc, the pattern at -l.l°C .contains a diffraction halo centered at 28 N26O in addition to the weak reflexes of the ice Ih, which clearly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . / . I -c C J'+ indicates that there is no long range order of the ' io' " ' i0 do " structure in the quasi-liquid layer. This is a first clear evidence that the so-called 28 "quasi-liquid layer" is a liquid layer. Again the Fig. 1 diffraction peaks of the ice Ih are more weakened ampared with Fig. lb. Same results were obtained when single crystals were used. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19871105