Chemical-pressure-driven orthorhombic distortion and significant enhancement of ferroelectric polarization in Ca1−xLaxBaCo4O7 ( x≤0.05 )

We report significant correlation of the multiferroic order and ferroelectric polarization to the orthorhombic structural distortion for ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_x{\mathrm{BaCo}}₄{\mathrm{O}}₇$ ($x\ensuremath{\le}$ 0.05). Analysis of the synchrotron diffraction studies reveal that La doping increases considerable structural distortion, which is associated with the increase of multiferroic ordering temperature and electric polarization. Intriguingly, the value of polarization increases remarkably to $\ensuremath{\approx}385\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}²$ ($x=0.05$) from $\ensuremath{\approx}150\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}²$ ($x=0$) for a 3 kV/cm poling field. Synchrotron diffraction studies in magnetic field provides an important clue, where structural distortion provides more impact on the polarization value than the contribution from the change in unit cell volume. Geometric magnetic frustration holds the key for the occurrence of the structural distortions, around which multiferroic ordering takes place for ${\mathrm{CaBaCo}}₄{\mathrm{O}}₇$. Our work thus highlights crystal structural distortion as a rich playground for tuning multiferroic order as well as polarization value.