Year

2015

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The synthesis, crystal structures, ordering, magnetic, optical, water oxidizing, and electronic properties of RTiNO2 (R = La - Nd) have been investigated. Neutron powder diffraction indicates that CeTiNO2 and PrTiNO2 crystallize with orthorhombic Pnma symmetry (Ce: a = 5.5580(5), b = 7.8369(7), and c = 5.5830(4) Å; Pr: a = 5.5468(5), b = 7.8142(5), and c = 5.5514(5) Å) as a result of a-b+a- tilting of the titanium octahedra. Careful examination of the NPD data, confirms the absence of long range anion order in both compounds, while apparent superstructure re ections seen in transmission electron microscopy provide evidence for short range anion order. Inverse susceptibility plots reveal that the RTiNO2 (R = Ce, Pr, and Nd) compounds are paramagnetic with Weiss constants that vary from -28 to -42 K. Effective magnetic moments for RTiNO2 (R = Ce, Pr, and Nd) are 2.43 µB, 3.63 µB, and 3.47 µB, respectively, in line with val- ues expected for free rare-earth ions. Deviations from Curie-Weiss behavior at low temperatures are driven by magnetic anisotropy, spin-orbit coupling, and crystal field effects. CeTiNO2, PrTiNO2, and NdTiNO2 have band gaps that fall in the range of 2.0 - 2.1 eV, very similar to LaTiNO2, which enables them to absorb a significant fraction of the visible spectrum. Photocatalytic oxygen evolution studies under visible light irradiation in the presence of a sacrificial electron acceptor (Ag+) show that the activity of NdTiNO2 (16 µmol/g/h) is comparable to that of LaTiNO2 (17 µmol/g/h), while PrTiNO2 (11 µmol/g/h) and CeTiNO2 (5 µmol/g/h) have activities that are only 65% and 30% that of LaTiNO2. X-ray photoelectron spectroscopy measurements reveal the presence of partially occupied f-orbital states that lie in the band gap for CeTiNO2, and near the valence band maximum for PrTiNO2. As evidenced by time resolved infrared kinetic decay, these localized f-orbital states act as electron-hole recombination centers that inhibit the photocatalytic activities of both compounds. NdTiNO2, where the f-orbital energies fall below the valence band maximum, does not suffer from this effect. The synthesis, crystal structures, ordering, magnetic, and optical properties of A3V4(PO4)6 (A = Mg, Mn, Fe, Co, and Ni) have also been explored. Combined synchrotron and neutron powder diffraction indicates that A3V4(PO4)6 (A = Mg, Mn, Fe, Co, and Ni) crystallize with triclinic P1 symmetry. Lattice parameters expand as expected with successive ionic radii increases, and cation disorder scales as A- and B-site ionic radii converge. DC magnetic susceptibility measurements suggest that all compounds with A = transition metal are antiferromagnetic and have complex magnetic structures. Effective magnetic moments for A3V4(PO4)6 (A = Mg, Mn, Fe, Co, and Ni) are 5.16 µB, 11.04 µB, 10.08 µB, 9.76 µB, and 7.96 µB, respectively, in line with calculated values for high spin transition metal ions. d-d transitions drive the colors of the compounds which are sequentially light lime green, brown, brown, green, and tan.

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