Synthesis, structures and properties of new double perovskite oxides

Double perovskite oxides have attracted much interest since the discovery of spin-polarized conduction and large low field magnetoresistances in Sr₂FeMoO₆.  This behaviour exists to high temperatures, making Sr₂FeMoO₆ and related materials candidates for application in data storage devices. This thesis describes the synthesis, structures and properties of some new double perovskite oxides. These can be divided into the (LaM)BNbO₆ (M = Ca, Sr, Ba; B = Mn, Co) and LaSrFeNbO₆ materials that have one magnetic transition metal (TM) cation, and the (La_1+xM_1-x)CoRuO₆ (M = Ca, Sr; -0.25 £ x £ 0.25 x = 1) materials that have two magnetic TM cations. The (LaM)CoNbO₆ double perovskites crystalize with the monoclinic P2_1/2 (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, which have rocksalt ordered arrangements of Co²⁺ (3d⁷) and Nb⁵⁺ (4d⁰), with almost no Co/Nb inversion (<4%). All three phases are anti-ferromagnetic with Neel temperatures of 17 K (M = Ca), 16 K (M = Sr) and 10K (M = Ba). The magnetic ordering is described by the magnetic propagation vector k = ( ½ 0 ½ ) for all materials. For M = Ca and Sr, two “non-collinear” and two “collinear” magnetic structures are found. These are experimentally indeterminate because of the small monoclinic angle (b = 89.9⁰).  The M = Ba material has a collinear antiparallel magnetic structure. The saturated cobalt moment decreases from 2.97(2) m_B to 2.52 (3) m_B to 1.85 (5) m_B for M = Ca, Sr and Ba, respectively. The frustration due to competing 90⁰ exchange interactions. The (LaK)MnNbO₆ materials also have the monoclinic P2₁/n (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, with <3% inversion of Mn²⁺ (3d⁵) and Nb⁵⁺ (4d⁰). All materials are antiferromagnetic with Neel temperatures of 9 K (M = Ca), 8 K (M = Ba) and 6.5 K (M = Ba). The magnetic Bragg reflections are indexed by k = (0 ½ ½ ), and the possible magnetic models are identical to those for (LaM)CoNbO₆. The manganese saturation moment decreases from 4.77(3) m_B (M = Ca) to 4.43 (4) m_B (M = Sr) to 41.9 (4) m_B (M = Ba). (LaSr)FeNbO₆ has the monoclinic P2₁/n superstructure with 14% inversion of Fe²⁺ (3d⁶) and Nb⁵⁺ (4d⁰). This material is antiferromagnetic (T_N = 22 K), and has a ( ½ 0 ½) magnetic propagation vector (m_Fe = 3.45(3) m_B). The lattice parameters, antisite disorder, Neel temperatures and resistivities of the (La_1-xM_1-x)CoRuO₆ double perovskites are found to vary in a systematic manner. The Co/Ru antisite disorder is minimal for x = 0, showing that this disorder is driven by the mixed Co or Ru electronic states at x ≠ 0 rather than by La/M cation mixing. The x = 0 materials have T_N = 96K (M = Ca) and T_N = 85 K (M = Sr), and are variable range hopping semiconductors. Hole doping )x < 0) results in a rapid reduction in the resistivity while the Neel temperature increases only moderately, whereas electron doping (x > 0) results in a significant reduction in T_N but has little effect on the resistivity. X-ray absorption near edge spectroscopy shows that for x < 0, the Co²⁺ states are selectively hope doped while for x > 0 only the Ru⁵⁺ states are electron doped, which is in keeping with the strongly asymmetric doping response.