Proton dynamics in lithium-ammonia solutions and expanded metals.
Proton dynamics in lithium-ammonia solutions and expanded metals.
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锂氨溶液和膨胀金属中的质子动力学。
DOI：
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发表时间：
2006
影响因子：
4.4
通讯作者：
F. Fernandez
中科院分区：
文献类型：
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作者：
Helen Thompson;N. Skipper;Jonathan C. Wasse;W. Spencer Howells;M. Hamilton;F. Fernandez
准弹性中子散射已被用来研究质子动力学的锂-氨系统中的浓度为0，4，12，和20摩尔%的金属（MPM）在液体和固体（膨胀金属）相。在230 K时，在均匀液态下，质子自扩散系数首先随金属浓度的增加而增加，从纯氨中的5.6 × 10（-5）cm ~ 2·s（-1）增加到12 MPM时的7.8 × 10（-5）cm ~ 2·s（-1）。在较高浓度下，我们注意到在20 MPM（饱和）时，浓度小幅下降至7.0 × 10（-5）cm 2 s（-1）。这些结果是一致的NMR数据，并可以解释在电子和离子溶剂化的竞争影响。在饱和时，溶液冻结以形成一系列组成为Li（NH3）4的多孔金属化合物。在熔点以上，在100 K时，我们能够将我们的数据拟合到跳跃扩散模型，平均跳跃长度（l）为2.1 A，停留时间（tau）为3.1 ps。该模型给出的扩散系数为2.3 × 10 ~（-5）cm ~ 2·s ~（-1）。在固相I（立方晶系，从88.8到82.2 K稳定）中，我们发现质子仍在进行这种跳跃扩散，l=2.0 A，τ =3.9 ps，扩散系数为1.8 × 10（-5）cm 2 s（-1）。这种运动让位于固相IIa（从82.2到69 K）和IIb（从69到25 K稳定）中的纯定域旋转。我们发现，在第IIa和第IIb阶段，旋转相关时间（tau（rot））分别为2.0和7.3 ps量级。这些值可以与在150 K下τ（rot）约为2.4 ps的固体氨中的旋转模式进行比较。
Quasielastic neutron scattering has been used to study proton dynamics in the system lithium-ammonia at concentrations of 0, 4, 12, and 20 mole percent metal (MPM) in both the liquid and solid (expanded metal) phases. At 230 K, in the homogenous liquid state, we find that the proton self-diffusion coefficient first increases with metal concentration, from 5.6x10(-5) cm2 s(-1) in pure ammonia to 7.8x10(-5) cm2 s(-1) at 12 MPM. At higher concentrations we note a small decrease to a value of 7.0x10(-5) cm2 s(-1) at 20 MPM (saturation). These results are consistent with NMR data, and can be explained in terms of the competing influences of the electron and ion solvation. At saturation, the solution freezes to form a series of expanded metal compounds of composition Li(NH3)4. Above the melting point, at 100 K, we are able to fit our data to a jump-diffusion model, with a mean jump length (l) of 2.1 A and residence time (tau) of 3.1 ps. This model gives a diffusion coefficient of 2.3x10(-5) cm2 s(-1). In solid phase I (cubic, stable from 88.8 to 82.2 K) we find that the protons are still undergoing this jump diffusion, with l=2.0 A and tau=3.9 ps giving a diffusion coefficient of 1.8x10(-5) cm2 s(-1). Such motion gives way to purely localized rotation in solid phases IIa (from 82.2 to 69 K) and IIb (stable from 69 to 25 K). We find rotational correlation times (tau(rot)) of the order of 2.0 and 7.3 ps in phases IIa and IIb, respectively. These values can be compared with a rotational mode in solid ammonia with tau(rot) approximately 2.4 ps at 150 K.