LaNi_(5)固溶碳杂质存在形式研究

Study on the Existence of Impurity C in LaNi

含碳杂质的LaNi_(5)在吸放氢过程中容易产生气体杂质CH_(4),进而会影响H_(2)的纯度。为认识LaNi_(5)中的固溶碳杂质的氢化行为,采用第一性原理首先对LaNi_(5)中固溶碳杂质的存在形式进行了研究。计算结果表明,理想LaNi_(5)晶体中固溶的碳原子稳定存在于2d(5个Ni原子),3f(2个La原子和4个Ni原子)间隙位置,且当碳原子存在于2d位置时更稳定,此时碳原子和镍原子间存在一定的离子键合。计算得到碳原子在3f位置向2d位置扩散的能垒为0.628 eV,表明常温下碳原子并不能从3f向2d位置扩散。考虑到实际晶体不可避免具有缺陷,进一步研究了含空位缺陷的LaNi_(5)中的碳原子的存在形式。研究发现LaNi_(5)中易形成Ni_(2c)空位,且空位中存在两个碳原子时较稳定,此时两个碳原子间存在共价作用,并与周围镍原子存在离子间的相互作用。

The LaNi_(5)containing impurity C was prone to generate impurity CH_(4) during use.In order to understand the hydrogenation behavior of the impurity C in LaNi_(5),the first principles were used to study the existence of impurity C in LaNi_(5).The calculation results show that the carbon atoms in the ideal LaNi_(5)crystal are stable at the 2 d(5 Ni atoms)and 3 f(2 La atoms and 4 Ni atoms)sites,and are more stable when the carbon atoms are in the 2 d site.At this time,there is a certain ionic bond between carbon and nickel atoms.The energy barrier for the diffusion of carbon atoms from the 3 f site to the 2 d site is 0.628 eV,indicating that the carbon atoms cannot diffuse from the 3 f site to the 2 d site at room temperature.Considering that the actual crystal inevitably has defects,the existence of carbon atoms in LaNi_(5)containing vacancy defects was studied.The results show that Ni_(2c)vacancies are easy to form in LaNi_(5),and the presence of two carbon atoms in the vacancies is relatively stable.At this time,there is a covalent interaction between the two carbon atoms,and ion interaction between the carbon atom and the surrounding nickel atoms.