The dielectric behavior of perovskite-related manganese oxides with stretched bonds or multiferroic properties
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This dissertation presents two investigations into the dielectric behavior of non-d0 perovskite-related manganese oxides: the first investigation probes the unique multiferroic properties of the hexagonal-perovskite series RMn1-xGaxO3 (R = Y, Ho) and the second explores the importance of lattice stress and the effect of the metal-cation dn - character on the dielectric properties of the perovskite series SryCa1-yMn1-xBxO3-δ (B = Ti, Zr). In the hexagonal-perovskite series, doping the Mn-site with Ga increased the c lattice constant and diluted the magnetic interactions in the ab plane. The interplay of these two effects perturbed the ferrielectric, antiferromagnetic, and multiferroic interactions. The change in these interactions demonstrated chemical control of the multiferroic interactions in the hexagonal-perovskite system for the first time and highlighted the structural mechanism behind the multiferroic properties. In the second investigation, the relationship between the tensile stress on the (Mn – O) bond and the ionic dielectric constant of SryCa1-yMnO3-δ proved difficult to quantify because the small band gap and chemical activity of the Mn4+ cation made samples with y ≥ 0.5 too conducting to measure the dielectric relaxations. To explore the ionic contribution to the dielectric behavior by an alternate path, a doping scheme was devised to decrease the sample conductivity of SrMnO3-δ at the expense of some of the tensile stress on the (Mn – O) bond. Doping the Mn site with larger 4+ cations (Ti and Zr) reduced the dielectric constant; however, the reduction of the dielectric constant cannot be explained by the effect of the tensile stress on the (Mn – O) bond alone.