A Synchrotron X-ray Study of a Solid-Solid Phase Transition in a Two-Dimensional Crystal
A Synchrotron X-ray Study of a Solid-Solid Phase Transition in a Two-Dimensional Crystal
复制标题
二维晶体中固-固相变的同步加速器 X 射线研究
作者：
S. Wolf;E. Landau;M. Lahav;L. Leiserowitz;M. Deutsch;K. Kjaer;J. Als
报道了有序朗缪尔单分子膜中相变的分子水平的测量和解释。手性(S)-[CF3-(CF2)9-(CH2)2-OCO-CH2-CH(NH3+)Co2-]单分子膜在水上的表面压力(π)与分子面积的关系图表明，在πS=每米25毫牛顿时，斜率发生了变化。掠入射X射线衍射和镜面反射率测量表明，πS发生了固-固相变。在低压下的衍射图显示了两个等强度的衍射峰，晶格间距d分别为5.11和5.00埃；这两个衍射峰在π≥πS中结合在一起。与衍射数据相吻合的结构模型表明，在π>；πS，分子堆积在二维晶体中，分子垂直排列。在π<；πS，有一种16��7�的分子倾斜。独立的X射线反射率数据产生26��7�的倾斜度。伴随着倾斜，衍射数据表明了从六方晶格到扭曲六方晶格的转变。由于-(CF2)9CF3部分采用螺旋构象，因此六方排列受到青睐。压缩到每米70毫牛顿会产生结晶度增加、相干长度超过1000埃的晶胞。
A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (π) versus molecular area of a monolayer of chiral (S)-[CF3-(CF2)9-(CH2)2-OCO-CH2-CH (NH3+)CO2-] over water shows a change in slope at about πs= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at πs. The diffraction pattren at low pressures reveals two diffraction peaks of equal intensities, with lattice spacings d of 5.11 and 5.00 angstroms; these coalesce for π ≥πs. Structural models that fit the diffraction data show that at π> πs the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At π < πs there is a molecular tilt of 16 � � 7 �. Independent x-ray reflectivity data yield a tilt of 26 � � 7�. Concomitant with the tilt, the diffraction data indicate a transition from a hexagonal to a distorted-hexagonal lattice. The hexagonal arrangement is favored because the -(CF2)9CF3 moiety adopts a helical conformation. Compression to 70 millinewtons per meter yields a unit cell with increased crystallinity and a coherence length exceeding 1000 angstroms.