Tuning structural and physical properties via A-site doping in perovskite-type transition metal oxides
Somaily, Hamoud H.
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This thesis investigates the structure-property relationship of two important classes of transition metal oxides (the perovskite-type A-site substituted titanates (Sr[sub 1-x-y]Ca[sub x]Nd[sub y])TiO[sub 3] and manganites (Sr[sub 1-x]Ba[sub x])MnO[sub 3]). A thorough evaluation is provided of their potential for prospective technological applications in heat recycling and information technology by examining the thermoelectric and multiferroic prop- erties, respectively. In the titanate compounds, we doped on the A-site with small rare earth ions in order to generate mixed valent transition metals to increase band filling while the Ca doping maintained fixed levels of distortions. In the case of the manganites, A-site Sr ions were substituted with large Ba ions for the purpose of increasing the materials strain and to promote ferroelectricity. Crystal structure was investigated using high-resolution neutron powder diffraction as a function of temperature and Nd/Ba doping. In the titanates, two series were synthesized and designed to have a nominally constant tolerance factor at room temperature. We determine the room temperature structures as tetragonal I4/mcm and orthorhombic Pbnm for the Sr-rich and Ca-rich series, respectively. Three low temperature orthorhombic structures, Pbnm, Ibmm and Pbcm were also observed for the Sr-rich series; whereas, the symmetry of the Ca-rich series remained unchanged throughout the full measured temperature range. Thermoelectricity in ternary (Sr[sub 1-x-y]Ca[sub x]Nd[sub y])TiO[sub 3] perovskites was investigated. The double substitution at the A-site maintained a fixed crystal distortion while Nd3+ doping modified the electronic properties of the materials via increased band filling. Unique compositions of cations allowed for increased A-site atomic mass disorder and the lattice thermal conductivity was significantly suppressed to values as low as ~ 1.5 W/K.m in some samples, approaching amorphous Silicon limit. Charge doping via balanced formation of Ti^3+ at the B-site has transformed materials into n-type semi-conductors. I examined the range of applicability of various conduction models, viz., variable range hopping, semiconductor-type conductivity across band gap, and small polaron hopping for the best description of the temperature variation of measured resistivity. We succeeded in achieving a relatively high figure of merit ZT=0.07 at ~ 400 K in the Sr-rich Sr[sub 0.76]Ca[sub 0.16]Nd[sub 0.08]TiO[sub 3] composition which is comparable to that of the best n-type TE SrTi[sub 0.80]Nb[sub 0.20]O[sub 3] oxide material reported to date. With an enhanced Seebeck coefficient at elevated temperatures and reduced thermal conductivity, we predict that Sr[sub 0.76]Ca[sub 0.16]Nd[sub 0.08]TiO[sub 3] and similar compositions have the potential to become some of the best materials in their class of thermoelectric oxides. We also report the structure-property phase diagram of unique single-ion type-1 multiferroic pseudocubic Sr1-xBaxMnO3 perovskites. Employing a specially designed multi-step reduction-oxidation synthesis technique, we have synthesized previously unknown Sr[sub 1-x]Ba[sub x]MnO[sub 3] compositions in their polycrystalline form with a significantly extended Ba solubility limit that is only rivaled by a very limited number of crystals and thin films grown under non-equilibrium conditions. Understanding the multiferroic interplay with structure in Sr[sub 1-x]Ba[sub x]MnO[sub 3] is of great importance as it opens the door wide to the development of newer materials from the parent (AA')(BB')O3 system with enhanced properties. To this end, using a combination of time-of-flight neutron and synchrotron x-ray scattering techniques, we determined the exact structures and quantified the Mn and oxygen polar distortions above and below the ferroelectric Curie temperature TC and the Neel temperature TN. In its ferroelectric state, the system crystalizes in the noncentrosymmetric tetragonal P4mm space group which gives rise to a large electric dipole moment PS, in the z-direction, of 18.4 and 29.5 microC/cm2 for x = 0.43 and 0.45, respectively. The two independently driven ferroelectric and magnetic order parameters are single-handedly accommodated by the Mn sublattice leading to a novel strain-assisted multiferroic behavior in agreement with many theoretical predictions. Our neutron diffraction results demonstrate the large and tunable suppression of the ferroelectric order at the onset of AFM ordering and confirm the coexistence and strong coupling of the two ferroic orders below T[sub N]. The refined magnetic moments confirm the strong covalent bonding between Mn and the oxygen anions which is necessary for stabilizing the ferroelectric phase.