Mechanism of ferroelectricity in half-doped manganites with pseudocubic and bilayer structure

By means of density-functional simulations for half-doped manganites, such as pseudocubic Pr0:5Ca0:5MnO3and bilayer PrCa2Mn2O7, we discuss the occurrence of ferroelectricity and we explore its crucial relation to the crystal structure and to peculiar charge/spin/orbital ordering effects. In pseudocubic Pr 0:5Ca0:5MnO3, ferroelectricity is induced in the Zener polaron type structure, where Mn ions are dimerized. In marked contrast, in bilayer PrCa2Mn2O7, it is the displacement of apical oxygens bonded to either Mn3+or Mn 4+ions that plays a key role in the rising of ferroelectricity. Importantly, local dipoles due to apical oxygens are also intimately linked to charge and orbital ordering patterns in MnO2planes, which in turn contribute to polarization. Finally, an important outcome of our work consists in proposing Born effective charges as a valid mean to quantify charge disproportionation effects, in terms of anisotropy and size of electronic clouds around Mn ions. © 2013 The Physical Society of Japan.