Formation of local electric dipoles with no unique polar axis in Tb3 Fe5 O12

Using neutron scattering and the pair-density-function analysis, we investigated, the local atomic structure of the ferrimagnetic dielectric Tb3 Fe5 O12 garnet. Pronounced magnetic diffuse scattering is observed at high temperatures that gradually decreases with cooling as the Tb spins take on an ordered structure at about 50 K. In the temperature range where the dielectric (ε) constant is enhanced, a volume striction is observed from the diffraction measurements under a magnetic field that couples the magnetic response to the dielectric properties. At the same time, large oxygen displacements, on the order of 0.1-0.2 Å are observed starting first at 90 K that persist up to 550 K, resulting in the formation of electric dipoles. However, they are not ordered and the lack of a unique polar axis in the hyperkagome structure may be linked to the absence of a polarized phase in this system. If it were present, the system could have a high ferroelectric transition temperature.