The Critical Role of Stereochemically Active Lone Pair in Introducing High Temperature Ferroelectricity.

In this paper, a comparative structural, dielectric, and magnetic study of two langasite compounds Ba3TeCo3P2O14 (absence of lone pair) and Pb3TeCo3P2O14 (Pb2+ 6s2 lone pair) have been carried out to precisely explore the development of room temperature spontaneous polarization in the presence of a stereochemically active lone pair. In the case of Pb3TeCo3P2O14, mixing of both Pb 6s with Pb 6p and O 2p helps 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 the Ba3TeCo3P2O14 compound remains in a nonpolar structure due to the absence of any such effect. Consequently, polarization measurement under varying electric fields confirms room temperature ferroelectricity for Pb3TeCo3P2O14, which was not the case for Ba3TeCo3P2O14. A detailed study was carried out to understand the microscopic mechanism of ferroelectricity, which revealed the exciting underlying activity of a polar TeO6 octahedral unit as well as Pb-hexagon.