碱金属配位硼氢化物相变行为的模拟研究

Alkali complex borohydrides for MBH4(M=Li, Na, K, Rb, Cs) are one kind of hydrogen-storage materials with vast prospect of applications. Their phase transitions can be triggered by heat and stress. The transition mechanisms and influence of additions of alkali halide on the transition temperature have not well been understood. Especially, there exist disagreeements between the geometrical structures at higher temperature or pressure and those of theoretical predictions based on first-principles calculations, and these problems have not been explained so far. We intended to systematically search a series of energetically lower isomers with genetic algorithm and particle-swarm-optimization algorithm etc, and then calculate static energy, vibrational entropy, free energy and free enthalpy of their supercells with DFT-D and quantem Monte Carlo theory to determine the most stable structures of high temperature or pressure phase through the curve of free energy---volume or free enthalpy---volume/pressure. The transition paths from high to low temperature/pressure phases can be defined with the potential surface of the supercell in the course of supercell distortions, reconstruction of chemical bonds, and atomic movements etc. and then as the circumstantial evidence to support the structures of high temperature or pressure phases we predicted. The origin of the influence of the additions of alkali halide on decrease of transition temperature can be explained through the free energy difference between the high and low temperature phase as well as change of chemical bonding, etc. parameters.

碱金属配位硼氢化物MBH4(M=Li, Na, K, Rb, Cs)是具有广阔应用前景的储氢材料。它们可以发生热诱发和压力诱发相变，各晶相之间的相变过程以及碱金属卤化物添加对相变温度的影响还未得到很好地理解。尤其是硼氢化锂热诱发相变导致的β相结构以及压力诱发导致的高压相结构与基于第一性原理理论计算所得并不一致，这个差异至今还没有得到很好地解释。本项目拟采用遗传算法和粒子群算法等技术手段系统性地寻找一系列能量较低的异构体，使用第一性原理DFT-D和量子蒙特卡罗方法计算研究超晶胞静态能量、振动熵、自由能和自由焓，通过自由能随温度变化和自由焓随体积/压强变化曲线分别确定高温和高压相稳定结构；通过超晶胞畸变、化学键重组和原子迁移等过程中势能面和势垒高度等参数确定高低温和高低压相变路径以及反过来佐证高温/压相结构；从高低温相之间自由能差和化学键变化等角度解释碱金属卤化物添加导致相变温度降低的起因。

碱金属配位硼氢化物MBH4(M=Li, Na, K, Rb, Cs)是具有广阔应用前景的储氢材料。它们可以发生热诱发和压力诱发相变，各晶相之间的相变过程以及碱金属卤化物添加对相变温度的影响还未得到很好地理解。尤其是硼氢化锂热诱发相变导致的β相结构以及压力诱发导致的高压相结构与基于第一性原理理论计算所得并不一致，这个差异至今还没有得到很好地解释。. 本项目采用遗传算法和粒子群算法等技术手段系统性地寻找一系列能量较低的异构体，使用第一性原理DFT-Tkatchenko-Scheffler和量子蒙特卡罗方法计算研究超晶胞静态能量、振动熵、自由能和自由焓，通过自由能随温度变化和自由焓随体积/压强变化曲线分别确定高温和高压相稳定结构；通过超晶胞畸变、化学键重组和原子迁移等过程中势能面和势垒高度等参数确定高低温和高低压相变路径以及旁证高温/压相结构。计算表明：LiBH4中BH4绕着[001]/[110]/[-110]以及各种轴向的转动势垒和Li/B原子移动势垒较低，LiBH4在高温下可以沿着某些轴自由转动到相变所需各种角度。在LiBH4高温相中，B-H四面体绕着[001]或者[110]等轴自由转动。随着温度降低，BH4四面体转动到低温相结构对应角度时，Li离子向上移动直至到达B-H四面体附近。这样的形成低温相过程，势垒最低也就是最可能相变路径。NaBH4/KBH4/RbBH4热诱发和压力诱发相变路径与NaBH4热诱发机制差别不大，都是属于无序-有序相变。BH4四面体在常温常压下无序转动和振动，随着压力增大或者降温，转动逐渐减弱最终停留在低压相结构。碱金属配位氢化物中阳/阴离子的离子半径比对高温相结构有序产生重要影响。当阳/阴离子半径之比在0.32<ratio<0.712之间时，阳离子和氢组成的四面体就会在高温相出现无序取向，导致整体结构一定程度的无序性。