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.
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