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Dynamical properties and temperature induced molecular disordering of LiBH_4 and LiBD_4

机译：LiBH_4和LiBD_4的动力学性质和温度诱导的分子无序

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We report on neutron powder-diffraction experiments, inelastic incoherent neutron-scattering experiments, and density-functional calculations on dynamics, order and disorder properties of LiBH_4 and LiBD_4. From refinement of LiBD_4 structure at 10 and 302 K, we found an almost ideal tetrahedral geometry of BD_4 ions (difference between shortest and longest interatomic distances is less than 4% for B-D bond, and less than 3% for D-D bond), close to the calculated geometry. A quantitative agreement was found between experimental and calculated anisotropic temperature factors of individual atoms. For phonon energies < 15 meV, the pho-non density of states of LiBH_4 in the low-temperature phase depends quadratically on the phonon energy while for the high-temperature phase a linear dependence is observed, revealing a high lattice anharmonicity in the high-temperature phase. Moreover, an increased phonon density of states at low energies in the high-temperature phase compared to the low-temperature phase give a direct evidence for disorder in the high-temperature phase of LiBH_4 of the hydrogen sublattice which can originate from orientational disorder of BH_4 units. Potential energy landscape for rotation of BH_4 indicates that fairly localized minima and barriers higher than 0.6 eV exist in the low-temperature phase, i.e., ordered BH_4 ions. The high-temperature structure shows shallow barriers of ～0.2 eV without distinct energy minima, i.e., orientation of a single BH_4 unit cannot be precisely defined. This corroborates the large thermal displacements observed in diffraction studies and high disorder of BH_4 ions deduced from experimental partial phonon density of states in the high-temperature phase.
机译：我们报告了中子粉末衍射实验，非弹性非相干中子散射实验以及LiBH_4和LiBD_4动力学，有序和无序特性的密度泛函计算。通过在10和302 K下对LiBD_4结构进行细化，我们发现BD_4离子几乎是理想的四面体几何形状（最短和最长原子间距之间的差异对于BD键小于4％，对于DD键小于3％），接近计算的几​​何。在单个原子的实验和计算的各向异性温度因子之间找到了定量协议。对于声子能量<15 meV，低温相中LiBH_4的光子非密度密度二次依赖于声子能量，而对于高温相，则观察到线性相关性，这表明高相中的高晶格非谐性温度阶段。此外，与低温相相比，高温相在低温下处于低能量状态的声子密度增加，直接证明了氢亚晶格LiBH_4在高温相中的无序现象可能源于BH_4的取向无序。单位。 BH_4旋转的势能图表明低温阶段存在相当局部的极小值和高于0.6 eV的势垒，即有序BH_4离子。高温结构显示了约0.2 eV的浅势垒，没有明显的能量最小值，即无法精确定义单个BH_4单元的方向。这证实了在衍射研究中观察到的大的热位移，以及由高温相中的实验部分声子密度引起的BH_4离子的高无序性。

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