Mn掺杂的Na_xCoO_2多晶制备与电输运性质研究

Synthesis and research of electronic transport on Mn substituted Na_xCoO_2 polycrystalline

采用固相反应法制备了沿(00l)面高度择优取向的γ相NaxCoO2(x=0.5+δ,0.7)和Na0.5+δCo0.95Mn0.05O2系列多晶,且微量Mn掺杂阻碍了晶粒的长大。77～300K电阻率-温度曲线表明,NaxCoO2为金属的电输运行为,在相同烧结工艺下,Mn掺杂导致Na0.5+δCo0.95Mn0.05O2电阻率有很大提高且为绝缘体行为,但经920℃/10h进一步烧结后,转变为电阻率大大降低的金属行为。分析认为,Mn4+的3d轨道交叠能带e′g+a1g的电子数比Co3.5+少,使得载流子数量降低;并且Mn4+的t2g轨道的电子最高占据能级比Co3.5+低,使电子在Mn4+和Co3+/Co4+之间跃迁时存在能隙,从而电子跃迁几率降低,载流子局域化。晶界增多导致的晶界散射,也增加了电阻率。

The NaxCoO2（x=0.5＋δ,0.7） and Na0.5＋δCo0.95Mn0.05O2 polycrystals are of γ-phase with highly preferential c-orientation,which were prepared by solid-state reaction;Mn-doping hinders the grain growth.The curves of resistivity vs temperature showed that NaxCoO2 present metallic behavior,the Na0.5＋δCo0.95Mn0.05O2 present insulating behavior.We considered that the electrons of the 3d bands substituted by Mn4＋ are less than the electrons of Co3.5＋,which reduces the carriers in overlap band e′g＋a1g of t2g orbital and increases the resistivity.In addition,the highest orbital level of Mn4＋ t2g electrons occupied is lower than that of Co3.5＋ t2g electrons does,which results in energy gap of electrons jumping,reduces the jumping probability and increases the resistivity.The grain boundary scattering also increases the resistivity.