立体化学活性孤对电子在引入高温铁电性中的关键作用
The critical role of stereochemically active lone pair in introducing high temperature ferroelectricity
摘要：
本文对两种硅酸镧系化合物Ba $_3$ TeCo $_3$ P $_2$ O $_{14}$（无孤对电子）和Pb $_3$ TeCo $_3$ P $_2$ O $_{14}$（Pb $^{2+}$ 6 $s^2$孤对电子）的结构、介电和磁性进行了比较研究，以精确地探索存在立体化学活性孤对电子时室温自发极化的发展.在$_3$的情况下，Pb 6PDMATHABMR与Pb 6PDMATHABMS和O2 $p$的混合有助于孤对电子具有立体化学活性.该立体化学活性孤对电子在晶胞内带来大的结构畸变并产生极性几何形状，而Ba $_3$ TeCo $_3$ P $_2$ O $_{14}$化合物由于不存在任何此类效应而保持非极性结构。因此，在变化的电场下的极化测量证实了Pb $_3$ TeCo $_3$ P $_2$ O $_{14}$的室温铁电性，这不是Ba $_3$ TeCo $_3$ P $_2$ O $_{14}$的情况。通过对铁电性微观机理的深入研究，揭示了TeO $_6$八面体单元和Pb-六方体单元的激发活性。
英文摘要：
In this paper a comparative structural, dielectric and magnetic study of two langasite compounds Ba$_3$TeCo$_3$P$_2$O$_{14}$ (absence of lone pair) and Pb$_3$TeCo$_3$P$_2$O$_{14}$ (Pb$^{2+}$ 6$s^2$ lone pair) have been carried out to precisely explore the development of room temperature spontaneous polarization in presence of stereochemically active lone pair. In case of Pb$_3$TeCo$_3$P$_2$O$_{14}$, mixing of both Pb 6$s$ with Pb 6$p$ and O 2$p$ help the lone pair to be stereochemically active. This stereochemically active lone pair brings a large structural distortion within the unit cell and creates a polar geometry, while Ba$_3$TeCo$_3$P$_2$O$_{14}$ compound remains in a nonpolar structure due to the absence of any such effect. Consequently, polarization measurement under varying electric field confirms room temperature ferroelectricity for Pb$_3$TeCo$_3$P$_2$O$_{14}$, which was not the case of Ba$_3$TeCo$_3$P$_2$O$_{14}$. Detailed study was carried out to understand the microscopic mechanism of ferroelectricity which revealed the exciting underlying activity of poler TeO$_6$ octahedral unit as well as Pb-hexagon.
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