Structures and Dynamics of Amorphous Clathrate Hydrates

在本项目开始时，我们已经开发了一条气相沉积线，用于低温制备非晶样品。我们还对用于气相沉积样品中子原位散射测量的低温恒温器进行了重大改造，使其可用于卢瑟福-阿普尔顿实验室的MARI光谱仪。利用上述装置制备了Xe、SF_6、CD_4、Ar等气体的非晶态笼形水合物，并在10 ~ 200k的温度范围内进行了原位中子散射测量。使用的中子能谱仪有高能加速器研究组织（KEK）的HIT（中子衍射）、SWAN（小角中子散射）和LAM-D（非相干非弹性中子散射），以及英国Rutherford-Appleton实验室（RAL）的MARI（高强度相干非弹性中子散射）。中子衍射和小角散射数据表明，增加客体含量和客体尺寸有利于氢键的形成和水分子局部笼状结构的形成。这是由于氢盐水化作用。此外，无定形包合物水合物在加热到150k左右时转变为包合物水合物晶体（不是冰和客体晶体）。非相干非弹性散射表明，上述氢键的形成降低了低能声子的激发强度。相干非弹性散射在非晶态冰和非晶态Xe包合物水合物中表现出准色散声子激发，表明非晶态氢键体系中存在准布里渊区。因此，我们制备了新的材料，非晶笼形水合物，并利用各种中子散射方法系统地研究了它们的结构和动力学。

At beginning of the present project, we have developed a vapor-deposition line for preparation of the amorphous samples at low temperature. We have also made major modification on a cryostat for in situ neutron scattering measurements of vapor-deposited samples to use it on MARI spectrometer of Rutherford-Appleton Laboratory.We have prepared amorphous clathrate hydrates of Xe, SF_6, CD_4, Ar gases by using the above apparatus and performed in situ neutron scattering measurements in a temperature range 10-200K. The used neutron spectrometers are HIT (neutron diffraction), SWAN (small-angle neutron scattering), and LAM-D (incoherent inelastic neutron scattering) of High Energy Accelerator Research Organization (KEK), and MARI (high-intensity coherent inelastic neutron scattering) in Rutherford-Appleton Laboratory (RAL, UK).Neutron diffraction and small-angle scattering data revealed that increasing guest content and/or guest size facilitates formation of hydrogen-bonds and resultant local cage like structure of water molecules. These are due to hydroforbic hydration effect. Furthermore, amorphous clathrate hydrates transformed to clathrate hydrate crystals (not ice and guest crystals) on heating the samples up to around 150 K. Incoherent inelastic scattering revealed that above hydrogen-bond formation decreases the intensity of the low-energy phonon excitations. Coherent inelastic scattering showed the quasi-dispersive phonon excitation in amorphous ice and amorphous Xe clathrate hydrate, indicating the existence of quasi-Brillouin zone in amorphous hydrogen-bonded systems.Thus, we have prepared new materials, amorphous clathrate hydrates, and investigated their structures and dynamics systematically by using various neutron scattering methods.