The critical role of stereochemically active lone pair in introducing high temperature ferroelectricity

Abstract: 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.