  To investigate the possible origin and mechanism of ferroelectricity in
polycrystalline spin ices Ho2Ti2O7 and Dy2Ti2O7 a detailed dielectric study has
been performed. Experimental finding suggests that both materials have two
prominent diffuse ferroelectric phase transitions around 90K and 36K. These
transitions are distinctly generated by the lattice distortions at the oxygen
sites as confirmed by triggered distortions and order of activation energy. Due
to the incompatibility of the gyrotropic order with any phonon mode at the
Brillouin zone center, observed diffuse ferroelectric phase transitions can
have only an electronic origin. Through magnetic susceptibility and previously
reported spin relaxation behavior it has been concluded that single ion
anisotropy has thermal variation, due to which orientation of rare earth
magnetic moment from isotropic non-Ising to Ising spin along local <111>axis
takes place. This spin orientation distinctly distorting the both oxygen sites
of the structure reflects in the form of diffuse ferroelectric phase
transitions.
