CrO4 distortion-driven ferroelectric order in (R,Y)CrO4(R=Sm,Gd,and Ho): A new family of multiferroics

We report a new multiferroic family of $(R,\mathrm{Y}){\mathrm{CrO}}_{4}(R=\mathrm{Sm},\mathrm{Gd},\text{and} \mathrm{Ho})$. The ferroelectric ordering temperatures are observed at much higher temperatures than their corresponding magnetic ordering temperatures for all the members. The values of electric polarization ($P$) are significantly high, which vary from $\ensuremath{\sim}217$ to $\ensuremath{\sim}640\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{C}/\mathrm{m}}^{2}$ for a 4 kV/cm poling field. The large value of $P\phantom{\rule{0.16em}{0ex}}(\ensuremath{\sim}590\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{C}/\mathrm{m}}^{2})$ for ${\mathrm{YCrO}}_{4}$ compared to the smallest $P$ value in the series for ${\mathrm{GdCrO}}_{4}$ signifies that the magnetic rare-earth element does not directly influence the $P$ value. Our careful analysis of the low-temperature synchrotron diffraction studies confirms that the distortion of ${\mathrm{CrO}}_{4}$ tetrahedra directs the $P$ value. The structural transformation from the centrosymmetric $I{4}_{1}/amd$ structure to a noncentrosymmetric $I\overline{4}2d$ space group at the onset of polar order is found to be correlated with the appearance of spontaneous electric polarization.