Yang L, Tulk C A, Klug D D, Moudrakovski I L, Ratcliffe C I, Ripmeester J A, Chakoumakos B C, Ehm L, Martin C D, Parise J B
Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Proc Natl Acad Sci U S A. 2009 Apr 14;106(15):6060-4. doi: 10.1073/pnas.0809342106. Epub 2009 Mar 30.
Atoms and molecules <0.9 nm in diameter can be incorporated in the cages formed by hydrogen-bonded water molecules making up the crystalline solid clathrate hydrates. For these materials crystallographic structures generally fall into 3 categories, which are 2 cubic forms and a hexagonal form. A unique clathrate hydrate structure, previously known only hypothetically, has been synthesized at high pressure and recovered at 77 K and ambient pressure in these experiments. These samples contain Xe as a guest atom and the details of this previously unobserved structure are described here, most notably the host-guest ratio is similar to the cubic Xe clathrate starting material. After pressure quench recovery to 1 atmosphere the structure shows considerable metastability with increasing temperature (T <160 K) before reverting back to the cubic form. This evidence of structural complexity in compositionally similar clathrate compounds indicates that the reaction path may be an important determinant of the structure, and impacts upon the structures that might be encountered in nature.
直径小于0.9纳米的原子和分子可以纳入由构成结晶固体笼形水合物的氢键水分子形成的笼中。对于这些材料，晶体结构通常分为3类，即2种立方晶型和1种六方晶型。在这些实验中，一种以前仅在理论上已知的独特笼形水合物结构在高压下被合成，并在77K和环境压力下回收。这些样品包含氙作为客体原子，此处描述了这种以前未观察到的结构的细节，最值得注意的是主客体比与立方氙笼形起始材料相似。在压力骤冷恢复到1个大气压后，该结构在转变回立方晶型之前，随着温度升高（T<160K）显示出相当大的亚稳定性。在组成相似的笼形化合物中这种结构复杂性的证据表明，反应路径可能是结构的一个重要决定因素，并影响自然界中可能遇到的结构。
